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Number 12 Volume 41 BULLETIN of the A M E R I C A N ASSOCIATION OF PETROLEUM GEOLOGISTS DECEMBER, 1957 TECTONICS OF EASTERN FLANK AND FOOTHILLS OF FRONT RANGE, COLORADO 1 C. MAYNARD BOOS2 AND MARGARET FULLER BOOS' Denver, Colorado ABSTRACT The long, north-south-trending Front Range in Colorado is a complex mosaic of Precambrian crystalline rocks bordered on the eastern flank by a narrow eastward-dipping foothills belt. The sedimentary formations have a total thickness of nearly 3 miles. The Precambrian consists of high-rank metasedimentary formations, quartzites, and layered meta-igneous rocks. About one third of the area is composed of folded and faulted metamorphic formations and nearly two-thirds is occupied by at least six generations of competent granitic rocks. Small batholiths, stocks, and innumerable plutons intrude the metamorphics, constituting major, fairly rigid bulwarks of the anticlinal structure of the Range. The sedimentary formations are of every geologic age except Silurian and possibly Devonian. There are four major lithologic units: (i) older Paleozoic marine limestones and sandstones, (2) Paleozoic and Mesozoic dominantly terrestrial and littoral redbed and arkosic formations, (3) marine Cretaceous formations, and (4) Tertiary formations of continental type. Five fault-bounded, northwest-trending segments extend across the range and involve the foothills belt of the eastern flank. The segments follow structural trends established in Precambrian time and reactivated during the Laramide. The eastern margin of the Range and foothills belt is underthrust and upthrust for about 90 miles along general north-south trends. Cross folds, tear faults, grabens, horsts, en echelon faulted folds, and drape folds dislocate the foothills formations and the Precambrian rocks adjacent to the foothills belt. Fifteen structural units within the eastern foothills belt and associated Precambrian formations are described. The distinctive units are the result of strong horizontal compression from east to west during subsidence and underthrusting of the bordering basins in Laramide time, compensating uplift of the body of the Range, and minor deformation of the more plastic foothills formations. INTRODUCTION Location and topography.—The Front Range, easternmost one of the Southern Rocky Mountain system, extends south through central Colorado from the Colorado-Wyoming state line to Arkansas River (Fig. i). It is about 185 miles long and 25-45 miles wide. The Precambrian area covers approximately 6,400 1 Manuscript received, February 25, 1957; revised manuscript, June IQ, 1957. Geologic consultants, 2036 South Columbine Street, Denver, Colorado. The writers are indebted to Raymond E. Sloan, James McCullough, and Calvin Cooksey of the Carter Oil Company, Denver, for unfailing support and encouragement in 1954-1956 field work and for permission to publish. The writers thank Laurence Brundall and Don B. Gould of Geophoto Services, Denver, H. W. C. Prommel, engineer, and Thomas C. Hiestand and David M. Evans, geologists, Denver, for constructive criticism of the manuscript. 2 2603 2604 C. MAYNARD BOOS A.\'D MARGARET FULLER BOOS square miles and is more than 4 miles high. The surface of Precambrian rucks rises from approximately 8,000 feet below sea-level beneath Denver (Fogarty, 1952, PI. 6) to about 14,000 feet above sea-level in mountain peaks. The eastern foothills belt dips steeply east and forms the western flank of the Denver basin. The western margin of the Range is bordered by North Park, Middle Park, and South Park basins (Fig. 3). Medicine How Mountains, Williams River Mountains, and Vasquez Mountains branch northwest from the Front Range. The Continental Divide forms the crest of the Front Range for about 60 miles It ranges from 10,000 to 13,000 feet in altitude, slopes to about 8,000 feet at the north end, and to less than 6,000 feet at the south end. Longs Peak, Pikes Peak, and Mt. Evans, more than 14,000 feet in altitude, are slightly east of the Continental Divide. The plains on the east are 5,000-7,000 feet above sea-level. Three major erosion surfaces, about 12,000, 10,000,'and 8,000 feet altitude and a number of minor partial erosion levels are identified by Lovering (1950, p. 15). The streams of the eastern slope occupy open mature valleys in the mountain uplands and emerge from the mountains in deep canyons (PI. i, Fig. i). The steep eastern face of the Range rises from 800 to 1,500 feet above the foothills belt. Faulted, wall-like, level-topped Precambrian mountains compose the Rampart Range from South Plalte River Canyon to Colorado Springs (PI. i, Fig. 3)The foothills belt is 2-4 miles wide. Alternating hard and soft sedimentary formations, make distinctive cuestas and strike valleys where they dip away from the mountain front. The plains extend to the mountain front where faulting has eliminated the foothills (PI. i, Figs. 2, 3). Purpose and scope.—Much unpublished information has been obtained by field geologists since the publication of the revised Geologic Map of Colorado (1935) and Professional Paper 223 of the U. S. Geological Survey (1950). The detailed geologic field study of the eastern flank of the Front Range by the writers had four purposes: (i) to map the Precambrian formations, describe their lithology, age relations, and structure; (2) to map the formations in the foothills belt, present their lithology, stratigraphy, and structure; (3) to determine interrelations of lithology and structure of exposed and buried Precambrian rocks to structures in sedimentary formations of the foothills belt and bordering plains; (4) to provide information concerning Precambrian rocks and structures to aid interpretation of geophysical data in the foothills and adjacent plains. Field work.- -Margaret Fuller Boos examined and mapped parts of the Precambrian terrane of the Front Range in 1921—1922, 1928-1929, and 1932-1934. Local areas were mapped in detail between 1934 and 1943. Earlier work was reviewed in the field and previously unmapped areas were examined in 1950-1951 and 1953-1955. C. Maynard Uoos mapped the sedimentary formations of the eastern foothills of the Front Range between 1930 and 1952, and throughout 1954 and 1955. FIG. i—Index map, showing locations of areas mapped in Figures 4-11. W Y O M I N G ~C O L~O R^A 0 Jl 2606 Fig 1 C. MAYNARD BOOS AND MARGARET FULLER BOOS Fig 2 1 ' ^£i-D_co A D A M S CO I'LATE I FIG. 1. View west from Windy Saddle of Lookout Mountain showing Clear Crtek Cunyon atu] mature uplands north and south. FIG. 2. View south along foothills belt from north end of Red Rocks Park, west of Denver, Colorado. Precambrian of east base of Front Range at right (!'); nearly flat Fountain beds at U ilU<» Springs (!•"); same beds dipping east at Red Rocks (f); Dakota hogback at left (I)); Lyons forniaii.<n (L), and Lykins (LK), and Morrison beds (M). FIG. 3. View north across Colorado Springs ramp (U) and Manitou embaymenl from Cheyenne Mountain to Rampart Range ( K M ) ; plains at (1'J; foothills north of Garden of the Gods (!•'); trace ni Cheyenne Mountain thrust at (T). FIG. 4. Drape fold on north side of Livermore syndine adjacent to highway between Liverrmire and Laramie, Wyoming. Dips increase from near flat on northwest side at (N) to 50°- 70" on south east side over subsurface fault. Steepest dips at (A). Man above (m). Procedures.— Field mapping was guided by aerial photographs of the Forest Service (U. S. Department of Agriculture), scale 1-20,000. Geologic field data were plotted on planimelric maps (U. S. Soil Conservation Service) as far north as Waterlon. Topographic maps of the U. S. Geological Survey, enlarged to i24,000 scale, provided a base for the geology of the northern and central parts of the eastern flank of the Range and the foothills belt. Both writers investigated the contact between the Precambrian rocks and the sedimentary rocks of the foothills belt, and the structures in both areas. The conclusions presented were reached jointly. Text, maps, and cross sections are based on field work. All available publications and the graduate theses in geology at the Colorado School of Mines and the University of Colorado were examined and considered in the preparation of this report. Credit for available information is indicated on Figure 2 and in appropriate places in the text. Authors are not quoted directly. FIG. 2.—References; numbers refer to list at end of article. 2608 C. MAYNARD TECTONICS OF FRONT RANGE, COLORADO BOOS A.\D MARGARET FULLER HOOS TABLE I. CORRELATION CHART OF MAJOR PRECAUBKIAN FORMATIONS, KASTERN SLOPE, FRONT RANGE For reference purposes the major faults, fault blocks, and folds not previously identified in print, and the distinctive structural units are given geographic names (Figs. 3, 4-12). Formations and Their Symbols discontinuity. The Precambrian crystalline rocks of the Front Range consist of high-rank metasedimenlary formations, layered mela-igneous rocks and thick cjuartiiir* intruded by at least six generations of granitic rocks (Boos, M. F., 1954, p. 117). About one-third of the exposed Precambrian rocks are metamorphic and mure than two-thirds are granitic (Fig. 3). The foothills belt consists of sedimentary formations (nearly 3 miles lliiiL) of every geologic age except Silurian and Devonian. I'KECAMBKIAN GEOLOGY Both north and south ends of the Range are occupied by two large bath'olitht. The central part is a complex mosaic of small granite batholiths and irregular plutons enclosed by folded schists, gneisses, quartzites, and other metamorphu formations (Table I; Fig. 3). Stratigraphy and lilhology.-—'f\\e Precambrian crystalline formations of the eastern slope of the Front Range are: Rhyolite porphyry of the Little Fountain Creek and Rock Creek areas 1950Olympus granite, pegmatite, and aplite (Boos, M. F., 1954, p. 120; Boos and Boos, n,j». 10. PMount p. 322; Boos, M. F., 1956, abst.) 9. Silver Plume-type granite, including Log Cabin, Longs Peak, Kenosha, Indian CrerL, lad Cripple Creek varieties and their genetically related pegmatites and aplites (Ball et al, lyoA. p. 45; Lovering el al., 1950, p. 28) 8. Pikes Peak granite, including Mount Rosa and Windy Point varieties and genetically rtUlnJ anorthosite, aplite, and pegmatite (Kinlay, 1916, p. 4; Lovering et al., 1950, p. 28) 7. Boulder Creek granite, gneissoid granite, pegmatite, and aplite (Ball et al., 1908, p. 51; B<*J» and Boos, 1934, p. 320; Boos and Aberdeen, 1940, p. 701; Lovering el al., 1950, p. 25) 6. Quartz diorile and hornblendite (Lovering et al., 1950, p. 25) 5. Gneissoid granite and granite gneiss (Lovering el al., 1950, p. 25) 4. Mt. Morrison migmatite, granite, gneissoid granite, aplite, and pegmatite (Ball el al., 100*. (• 47; Lovering et al., 1950, p. 23; Boos, M. F., 1954, p. n8) 3. Coal Creek quartzite (Adler, 1930; Lovering et al., 1950, p. 23; Pinckney, 1933) 2. Swandyke hornblende gneiss and quartz gneiss (Lovering et al., 1950, p. 18) i. Idaho Springs series of formations (Ball, 1908, p. 38; Boos, M. B., 1954, p. 117) / 3 3 4 5 6 _ GENERAL GEOLOGY Regional setting.—The Front Range is a massive, flat, fault-hounded arili, about five times as long as wide, and eight times as wide as high (Kig. ,}). The body of the Range consists chiefly of 1'recambrian crystalline formations. I |>turned sedimentary formations of Paleozoic, Mesozoic, and Tertiary age constitute the eastern foothills belt. The asymmetrical Denver basin under the pUint east of the Range is deepest very close to the'mountain front (Fogarty, 10.51, I'l 6). The broad tripartite south end of the Range plunges into the ("anon City basin. The Precambrian terrane and the eastern foothills belt merge with the l.aramie Range of Wyoming and its foothills belt without topographic or geolugu 2609 { i 'i 1 Basic dikes (BD) Rhyolite porphyry (RP) Mount Olympus granite (MOG) Silver Plume-type granite (SPG)' Pikes Peak granite (PPG) Windy Point granite (WPG) Mount Rosa granite (MRG) B.iulder Creek granite (UCG) MOG SPG(CCG) PPG — — BCG MMG — IS ISQ Idaho Springs series (IS) Quartzite (ISQ) Marble and time silicates (ISM) ISQ Quartz gneiss (ISQ) Biotite-sillimanite schist, etc. (ISS) ISS RP RP? RP SPG PPG SPG PPG WPG MRG PPG WPG — MMG MMG — ISQ ISQ ISO ISQ _ _ BD BD 7 BD S BD MOG SPG(LCG) SHGf MOG SPG(LPG) MOG SPG — MMG — MMG MMG ISQ ISQ ISQ ISQ ISO ISO IS? }|Q ISQ MOG SPG PPG WPG — MMG —- — — — Locations: (i) Royal Gorge arch-West Wilson Creek; (2) Mt. Pittsburg Red Creek arch; (3) Colorado Springs ramp* Ute Pass area; (4) Perry Park-Rampart Range area; (5) Golden Fault area (6) Boulder-Red Hill area; (7) Green RidgeBuckhorn-Milner Mtn. area; (8) Cache la Poudre-Livermore area. Longs Peak (LPG), and Log Cabin (LCG) variants of • Silver Plume type includes variants: Cripple Creek (CCG), Lon the typical granite of the type locality. t Sherman granite (SHG) in Colorado at south end of Sherman batholith of Wyoming is identified as Pikes Peak granite (PPG) by U.S.G.S. in Prof. Paper 233, p. 28, Fig. g, and PI. i. D. Marble, calcareous rock, skarn, quartzite, bedded hornblende gneiss, and garnet gneiss, all of sedimentary origin (Boos, M. F., I9sr, p. 1533) C. Quartzite, quartz gneiss, amphibolite, and mica schist (Fuller, M. B. (M. F. Boos), 1924, p. 50; Lovering et al., r(J5o, p. 20) B. Biotite schist, quartz Motile gneiss, phyllite, cordierite schist, and staurolite schist (Ball, 1908, p. 39; Boos, M. F., 1954, p. 117; Travis, ig$t>, p. 796) A. Biotite-sillimanite schist and injection gneiss (Ball et al., 1908, p. 37; Boos and Boos, 1934, P- 3°°) The descriptions that follow are planned: (i) to help geologists and geophysicists interpret the character of the various Precambrian formations and structures in the crystalline "basement complex" beneath the foothills and plains east of the Range; (2) to assist in the identification of crystalline rocks encountered in wells drilled for oil and gas; and (3) to provide information about the Precambrian rocks that will aid interpretation of geophysical data. Significant details of color, texture, general appearance, lithology, and mineral content of each Precambrian formation are included. METAUORPHIC ROCKS The Idaho Springs series consists of at least four distinctive, apparently conformable formations of sedimentary origin (Ball, 1908, p. 38). In general, the formations can be correlated from place to place along the Range (Table I; Boos, M. F., 1954, p. 117). Lovering and Goddard (1950, p. i) estimate the thickness of the Idaho Springs at 20,000 feet. Between 25,000 and 30,000 feet of metasedimentary beds, chiefly biotite-sillimanite schist, injection gneiss, and quartzite, were measured by M. F. Boos (i93S —I 943) •'• t'ie Denver Mountain Parks area. More than 10,000 feet of undifferentiated Idaho Springs beds occur in the Buckhorn area. The descriptions of the formations that follow apply chiefly to the eastern flank of the Front Range. 26io C. MAYNARD BOOS AND MARGARET FULLER BOOS Biotite-sillimanite schist, the basal formation,'is dark gray to black. It is locally soft and fissile (PI. 2, Fig. la). The foliation follows the original bedding of the shaly rock from which it was derived by regional metamorphism (I.ovcring, 1929, p. 65). Knots or discs of quartz and sillimanite (PI. 2, Fig. 2b), or of quartz and mica (Thurston, 1955, p. 12; PI. 2, Fig. 2a), crushed red garnets, pencils of quartz or pegmatite, and numerous crumpled stringers and thin sills of pegmatite characterize the formation (PI. 2, Fig. ib). The schist is not hard, dense, and competent, but interlocking platy minerals give it a tough, highly linear texture. Innumerable flattened prismatic rods of black tourmaline darken the schist adjacent to Silver Plume-type granite. Magnetite is an abundant local accessory. In cores or drill cuttings the schist can be identified by felted biotite intergrown with sillimanite, prismatic black tourmaline rods, and knotty, rusty to solid discs of quartz and sillimanite (PI. 2, Fig. 23, b). The formation is a few hundred feet thick in the northern and southern parts of the Range and several thousand feet thick in the central part. The bintile schist named by Ball (1908, p. 38) is chiefly quarlz-biolite gneiss and schist on the eastern flank of the Range. It overlies the biotite-sillimanite schist and injection gneiss and underlies quartzite. The chief rock is light gray to medium gray and fine-grained. Relict bedding planes have filmy concentrations of biotite. The quartz-biotite schist contains less lit-par-lit aplite and pegmatite than the biotite-sillimanite formation. Interlocking quartz grains produce a fairly competent rock. Neither primary muscovite or sillimanite is present. The phyllite members of the formation crop out extensively around Rattlesnake Park and Waltonia on Big Thompson River (Fig. 5). The staurolite and cordierite schists of the Buckhorn-Horsetooth area extend eastward under the foothills belt. Travis (1956, p. 796) identified similar beds in the Precambrian of the southwest end of the Range. The third formation of the Idaho Springs series consists chiefly of quartz gneiss, quartzite, and granitized quarlzite, interlayered with hornblende gneiss of probable sedimentary origin. Quartz gneiss, impure quartziles, and mica schist members are locally interbedded. The gneiss is light gray or nearly black (PI. 2, Figs. 33, 3b). Surfaces weather reddish to brown. Quarlzite members crop out adjacent to the foothills and extend under the foothills belt (Fig. 3). The dark gray to almost black quartzites along Big Thompson Canyon and the lower canyon of Cache la Poudre River (PI. 2, Figs. 4, 5) derive their color from numerous minute biotite flakes and hornblende crystals embedded in the quartz grains. The light gray to white quartzites of Phantom Canyon and West Wilson Creek lack mafic minerals and are interbedded with poorly exposed, soft, glittering white sericite schist (Figs. 3, 13). At least 10,000 feet of Precambrian quartzite is exposed in the Big Thompson and Cache la Poudre areas. M. F. Boos (1954) measured 10,000-12,000 feet of interbedded quartzite and schist in the Phantom Canyon-West Wilson Creek TECTONICS OF FRONT RANGE, COLORADO 2611 PLATE 2 FIG. la. Fissile biotite-sillimanite schist ami mica phyllite minutely crumpled; pegmatite at (P). Denver Mountain Parks area. FIG. ib. Quartz gneiss with visceral and ptygmatically folded pegmatites. FIG. aa. Discs of quartz and mica in phyllite exposed along lower canyon of Cache la Poudre Canyon northwest of Fort Collins. FIG. ah. Knotty biolite-sillimanite schist, Buckhorn area NW. of Fort Collins. FIG. ja. Lower Canyon of Big Thompson River. Walls of dark canyon show distinct beds, striking N. 6o°~7o° W., of quartzite and sills of Mount Olympus granite porphyry (O) dipping SW. about 80°. View is east to foothills. FIG. 3!). Quartzite interbedded with phyllite, Big Thompson River area. FIG. 4. Marble and calcareous quartzite beds exposed west of \Vah Keeney, Denver Mountain Parks. Massive marble below (M) grades up into quartzile (0). 2612 C. MAYNARD BOOS AND MARGARET FULLER BOOS areas (Figs. 12, 13). There may be some duplication in the thickness due to lack of distinctive horizon markers in closed folds. The quartzite and quartz gneiss are hard, dense, tough, color-banded, locally cross-bedded competent rocks. Rectangular joint blocks have razor-sharp edges that splinter irregularly. The rock could be readily identified in cores or drill cuttings by its physical characteristics. The uppermost formation of the Idaho Springs series consists of platy, well bedded quartzite, amphibolile, and at least three calcareous and lime-silicate members (Figs. 3, 7). The quartzites are stratified, grade into calcareous beds, and are .cavernous-weathered adjacent to marble beds (PI. 2, Fig. 4; PI. 3, Fig. 2). The marble outcrops are lenticular to thin-bedded, siliceous, white to dirty gray or greenish, dense to coarsely crystalline, and everywhere associated with quartzites and amphibolites (Boos, M. F., 1951, p. 1533). Rounded detrital quartz grains stand out on weathered surfaces. Pistachio-green epidote grains and blue-green amphibole prisms liberally speckle the surfaces. Coarse-textured skarn underlies the calcite-rich beds. The gneissic quartzite associated with the top calcareous member contains "eyes" of magnetite and biotite surrounded by white halos. Calcareous quartzite and siliceous marble members range in thickness from j to 30 feet. Drag folds and recumbent folds duplicate the outcrops (PI. 3, Fig. j) except at the base of low quartzite cliffs. Green epidote, brown garnet, blue-green hornblende of the cellular calcareous quartzite, green amphibolite, and white to greenish siliceous marble could be recognized readily in cores or cuttings. Swandyke gneiss is chiefly hornblende gneiss and schist interlayered with thin beds of quartz-biotite schist (Lovering, 1950, p. 19). Hornblende gneiss of Swandyke age on the eastern flank of Front Range is dark green to almost black and layered to massive. It crops out stratigraphically above the quartzite and calcareous beds at the top of the Idaho Springs series (Boos, M. F., 1954, p. 118; PI. 3, Fig. i). Relict amygdules indicate a lava-flow origin for some of the gneiss. Dikes of hornblende gneiss intrude the Idaho Springs formations. Swandyke gneiss fills the troughs of synclines and Idaho Springs beds occupy the flanks. Anticlines of Idaho Springs beds are bordered by Swandyke beds (Figs. 7, 8). Lovering (1950) estimates the Swandyke to be 6,000 feet thick in the Montezuma area. The formation is much thinner in the south part of the Front Range than in the type locality. Coal Creek quarlzile, according to Lovering (1950, p. 23), is stratigraphically above the Swandyke formation on the north side of Coal Creek syncline (Fig. 6). Coal Creek quartzites and quartz-pebble conglomerates (Pinckney, 1953), at least 14,000 feet thick also overlie injection gneisses and Mount Morrison migmatite. The cross-faulted and dislocated syncline of Coal Creek quartzite and mica TECTONICS OF FRONT RANGE, COLORADO 2613 schist beds widens and flattens northeast. Quartzite and associated rocks underlie the foothills formations north of Coal Creek. The light color, dense texture, and hackly fracture distinguish the quartzite in cores or drill cuttings. GRANITES At least six generations of granitic formations crop out on the eastern flank of Front Range (Fig. 3). They intrude the Idaho Springs beds and the Swandyke gneiss (Boos and Boos, 1934, p. 306; Lovering, 1929, p. 63), belong to a comagmatic province, and are polygenetic. The youngest is fine-grained red rhyolite porphyry and the oldest is gneissic quartz monzonite. Areal distribution, relative age, genesis, and structural relief are shown in Tables I and II. Mount Morrison formation (quartz monzonite gneiss, Ball, 1908), occurs in plutons and stocks of irregular to lenticular shape west of Denver (Figs. 7, 8; Boos, M. F., 1954, p. 117), adjacent to Log Cabin batholith northwest of Livermore and between the Longs Peak-St. Vrain batholith and Log Cabin batholith in the Cache la Poudre area (Fig. 4), at the south end of Front Range (Figs, n, 12) and enclosed in gneiss and schist west of Roxborough Park (Fig. 8). Typical Mount Morrison granite is pink to tan, delicately gneissic and medium- to fine-grained (PI. 3, Figs. 3, 4). The characteristic fine-banded texture is due to the parallel elongation of closely and evenly spaced films of platy biotite and hornblende needles that alternate with micropegmatitic aggregates of granulated quartz and feldspar. Parts of the granitic gneiss are saturated with ill defined pegmatite. It shows dragged and contorted structure against non-granitic host rocks, and is migmatitic where it interfingers with gneiss and schist. The granitic rocks grade laterally into impure quartzites. Mount Morrison migmatite and gneiss are massive to thin-platy. Main joints parallel the foliation. The granite gneiss and gneissic aplite of Lovering (1950, ]>. 25) grade into Mount Morrison migmatite in several places. Aplites and pegmatites of Mount Morrison genesis are gneissic. The pegmatites are not zoned (Boos, M. F., 1954, p. 126). Mount Morrison granitic rock extends east an unknown distance under the foothills belt. Its identity in cores and drill cuttings could be recognized by the fine-grained gneissic texture, intensely granulated quartz and feldspar, and accessory magnetite, and allanite (orthite). Quartz diori/e and hornblendite crop out in Deer Creek and Turkey Creek canyons (Fig. 7; Boos, M. F., 1954, PI. i). The diorite is mottled gray to almost black. The rock is massive, medium to coarse in texture, and pits on weathering due to the break-down of light-colored feldspars. Both diorite and hornblendite probably occur in the basement complex beneath the foothills and plains areas southwest of Denver. The mottled appearance, magnetite, pyrite, titanite, and dark green hornblende content aid identification of the diorite in drill cuttings or cores. 26i4 C. MAYNARD BOOS AND MARGARET PULLER BOOS TECTONICS OF IfRO\T RANGE, COLORADO 2615 & 4* ,.,-:• ' '-.-"'^ .A *''• ^>i&:5H' ''4&« ^my^tttM PLATE 3 l'"ic. i. Massive hornlilende gneiss of the Swandyke formation on Bergen Park road, I)m»o FIG. 5, Dark gray gneissic Boulder Creek granite near margin of batholith. Secretion-t>|* | matite at (S). Fin. 6. Gray gneissic Boulder Creek granite with biotite-llecked pegmatite (2). Boulder Creek granite, aplile, and pegmatite, identified by Hall (1908, p|>. 4951) as "archean q u a r t z monzonite," is the next to oldest of the kindred 1'reiimbrian granites and is not Archean in age (Lovering el al., 1950, I'l. i; lious, M. K.. 1954, p. 119). The granite is consistently light gray to dark gray, f a i n t l y banded to gnciisic and biolite-rich. Strung-out aggregates of black biolite and embayed porphyro- PLATE 4 FIG. i. Hand specimens of Pikes Peak granite from Pikes Peak on left anil specimens of Sherman granite from Virginia Dale on right. Great similarity of widely separated granites. VIG. 2. Trachytoid Silver Plume-type granite from near Silver Plume, Colorado. FIG. 3. Mount Olympus granite. Haphazardly scattered lilack biotite flakes in medium finegrained background. FIG. 4. Brown-stained concentric rings of orbicular appearance (Liesegang rings?) in Mount Olympus granite near shears close to foothills. FIG. 5. Dakota beds turn from N.-S. course (N-N) to S. 30° \V. ami dip near vertical north of Golden adjacent to trace of Golden underthrust (G-G). FIG. 6. Dellvue anticline. North end at N, dip E. on upthrown side, fault line scarp al S-S, downthrown side D. FIG. 7. Reversed dip in Dakota and Denton beds S. of U. S. Hwy. 40 near Golden underthrust. Minor flexure at D, Benton beds at B. View is south. FIG. 8. Low-angle underthrusts and drag folds in Precambrian quartz gneiss (O) and hornblende gneiss (N) near foothills west of Morrison. Flat drag fold (d) activated by thrust (U-IP). Massive bed (H) slightly distorted by thrust (U-U'). blasts of potash feldspar distinguish the outcrops (PI. 4, Fig. 4). The fresh rock is medium- to coarse-grained, massive and tough. It disintegrates to gray to rusty gravel of biotite-tlecked quartz and feldspar crystals. Pegmatites of Boulder Creek genesis contain books of primary biotite (Boos, M. F., 1954, p. 127). The coarse gray granite of Boulder Creek and M o u n t Evans batholiths, Fairy Hill and Deer Creek plutons, and the plutons of Cache la I'oudre area grade into a 2616 TECTONICS C. MAYNARD BOOS AND MARGARET FULLER BOOS border zone of strongly gneissoid gray granite in which the gneissic texture parallels the foliation of the enclosing metamorphic rocks. No other granite of tinFront Range has produced so many aplite dikes and sills (PI. 3, Fig. o). The Boulder Creek batholith (Fig. 6) probably extends east under the foolhills belt several miles. There are few marginal features adjacent to the sedimentary formations. The rock could be identified in drill cuttings or cores b> abundant fresh black biotile, accessory allanite (orlhite), and magnetite. The amount of magnetite might influence magnetic surveys in areas underlain by extensive bodies of Moulder Creek granite. Pikes Peak (Sherman) granite, apllle, anil pegmatite crop out over 80 per cent or about 1,250 square miles of the south third of the Range, and less than 150 square miles of the north end (Table II). There are few, if any, dikes or sills of Pikes Peak granite in the central part of the eastern H a n k of the Range. Pikes Peak and Sherman granites are nearly identical in mineral content, texture, colur, and response to weathering (M. V. Boos, 1935, p. 1038; Covering and (Jo'ddard, 1950, p. 28). Typical Pikes Peak granite is red or gray, coarse-textured, and spoiled haphazardly with dark clusters of inlergrown biotite and hornblende shreds scattered among the quartz and potash feldspar crystals (PI. 4, Fig. i). The unweathered rock is tough and competent. The coarsely gneissic border phase near the south end of the Range contain:, many large, embayed poikilitic feldspars. Dikes of spotted, line-grained Windy Point granite, small plulons of Mount Rosa granite (Finlay, 1910), pods „( anorthosite, and innumerable dikes of aplile and pegmalile genetically related to Pikes Peak granite are present in the roof and the border of Pikes Peak batlioliih. Pegmatites genetically related to the Pikes Peak granite are well zoned. They have a conspicuous coarse-textured core of graphic feldspar and bluish or while quartz. The dikes contain the same accessory minerals as the parent granite (Boos, M. F., 1954, p. 139). The pegmatites are noted for rare-earth minerals (I.overing, 1956, p. 28). Purple fluorite and blood-red hematite stains in feldspar should identify cores and drill cuttings of Pikes Peak granite. The surface of coarse-textured Pikes Peak (Sherman) granite disintegrates on weathering to coarse, angular mineral gravel. The smooth balanced boulders and grotesque "hoodoos" that remain arc partly buried in the grits. Arkosic mineral gravels of Pikes Peak and Sherman origin are a kind of "granite wash." The gravel deposits, accumulated at the base of the sedimentary rock section beneath the foothills and plains, could be reservoir sites for oil or gas. Siher Plume type granite, aplite, and pegmatite occur in five small batholillis, iv merous plutons and many dikes and sills in the eastern flank of Front Range (Fig- 3)- The granites that compose Log Cabin, Longs Peak-St. Vrain, Kenosha, and Cripple Creek batholiths, and Indian Creek plutons are nearly contemporaneous variants of the typical granite of Silver Plume batholith (Boos, M. F., i 954 , OF FRONT RANGE, COLORADO 2617 TABLE II. GRANITES OF FRONT RANGE Name Area (Fig. j) (Square Miles)' Structural Relief (Feel)' Genesis Magmatic, fracture filling RllYOLlTE PORPHYRY Less than i sq. mi. in two areas 200-300 MOUNT OLYMPUS GRANITE, I'oRi'iivHiTic GRANITE. PECIUATITE AND APLITE 15-20 in scattered bodies 6,ooo+, Continental Magmatic, fracture Divide to foothills filling, schlieren SILVER PLUME-TYPE GRANITE AND VARIANTS 1,600-1,700 8,000, l.ongs Peak to foothills C'hielly magmatic Log Cabin batholith *75 4 ,000 Magmatic Longs Peak-St. Vrain, batholith 650 8,000 Aggressive and permissive, magmatic Indian Creek plutons 100 3,000 Magmatic 150-175 5,000 Aggressive sloping Kenosha batholith 250 3,000 Magmatic sloping and assimilation Cripple Creek batholith 200 2 ,OOO Aggressive sloping I ,5OO-I,600 8,000 Magmatic sloping and assimilation !|25° 8,000 Magmalic 125-150 3,000 Magmatic? BOULDER CREEK GRANITE 450-475 6,000 Magmatic and metasomatic Uoulder Creek batholith 140-150 4,000 Magmatic and metasomalic Mount Evans batholith QO-lOO 6,000 Magmatic 1OO-I5O 2,000 Magmalic? 50-60 1 ,000 Magmatic? 250 in scattered areas 4,000 Granitization (palingenesis and metasomatism) Silver Plume batholith PIKES PEAK (SHERMAN) GRANITE Pikes Peak batholith Sherman batholith ia Colorado Tabernash stocks Storks— small MOUNT MORRISON GRANITE Slocks, sills and irregular bodies • Figures are approximate and determined from geologic and topographic maps. p. 129; Levering and G'bddard, 1950, p. 28; Boos and Boos, 1934, P- 320; Boos and Aberdeen, 1940, p. 697). All variants are termed Silver Plume-type in this paper. Typical, fresh Silver Plume-type granite is massive, hard and tough. It is flesh-colored to tan, fine- to medium-grained, and nowhere gneissic. The trachyloid texture and flow linealion are due to the preferred orientation and parallelism S E D I M E N T A R Y FORMATIONS OF FOOTHILLS BELT Sedimentary formations along the eastern flank of the Front Range are of every geologic age except Silurian and possibly Devonian. Nowhere in the foothills of the eastern flank are all of the formations present in a single geologic section. Four major lithologic units are represented (Table I I I ) . The basal unit consists of marine limestone and sandstone formalions only a few hundred feet thick, from Cambrian to Mississippian in age. They lie unconformably on Precambrian rocks and form a narrow inner foothills belt around the southern third of the Range. D. v F 5 1 o 11 •* !3 •£ rJU-IXi X. U. CQ *^. c/J >-4 E a a £ i & c5 6| | ^ 5 gV^a 1-3 • i; £ U ~£ -2 ^3"°^J* v£ 3 « a, /; PQ **, -J-, J £O c ; gu" O il 1- OQ 7' a o j" c c 1 % = C ^ ;W .3 13 -j "S'l Cc ng -5u Q, 3 S§ P* £33 5u: ^.u : 6, B, fill 1 1 1 E c rt *ti c c u- 2 *( 1 Ml c/l & 3 ..HI f I 0 c 0 |j 8 1 a. ~£ * g g OQ !!!= 11 1 <3£'& x « o L £ 1 \ 'c 1 1 M- I? c 1 !••«• ! a gL B 4 1 P § j 3« '-? Q Ij , I lo Ute Pass Sawatch ari O. & •|1E Jijtij 3. 1" 2 « 3 Ute Pass Sawatch "a cq g glesidc mntain of numerous tabular Carlsbad twins of potash feldspar enclosed in finer-grained feldspar and gray quarlz (PI. 4, Fig. 2). Shreds of biotite and grains of m a g n e t i t e give some exposures a dark gray appearance. Silver Plume-type granite intrudes the older granites and the metamorphic formations of the eastern flank of Front Range. The granite produces scanty mineral gravel. Pegmatites genetically related in Silver Plume-type granite are fresh, flesh-colored to gray, and consist of abundant smoky quartz, flesh-colored potash feldspar, silver-colored muscovite, and the accessory minerals of the parent granite (Hoos, M. F., 1935, p. 1042). Silver Plume-type granite of the Longs Peak-St. Vrain and Log Cabin batholiths borders the foothills bell and extends east into the basement complex. The granite in drill cuttings and cores should show sub-parallel twinned potasli feldspar crystals, silver-colored muscovile, euhedral, minute, hyacinth zircons, clear apatite, and the other characteristic accessory minerals. Mount Olympus granite, afUle, and pegmatite are closely associated w i t h Silver Plume-type granite (Hoos, M. F"., 1954, p. 120). Irregular to lensing pluimis, dikes, and sills are satellilic to bodies of Silver Plume-type granite from the Continental Divide to the foothills and from Cripple Creek north to Log Cabin batholith (Fig. 3; Table II; Hoos, M. F., 1956, absl.). M o u n t Olympus granite is gray to white, medium- to fine-grained, and competent. The characteristic "salt-and-pepper" appearance of fresh and weathered surfaces is due to the even but unoriented distribution of small flakes of black biotite and silver-colored muscovite among gray q u a r t z and white feldspar crystals (PI. 4, Fig. 3). Orbicular staining is common (PI. 4, Fig. 4). Its pegmatites are large, white lo flesh-colored zoned bodies that contain accessory black tourmaline, beryl, pitchblende, and other rare-earth minerals. Silver-colored muscovile is an abundant constituent where host rock is mica schist or phyllite (M. F. Hoos, 1954, p. 129). Mount Olympus granile in well cuttings or cores shows distinctive white to gray color, pepper-and-salt texture, fresh black biotite and silvery muscovile. Rliyolile porphyry is the youngest granitic rock of Precambrian age. M. F. Hoos identified sheets of it cutting older rocks north of Rock Creek. McLaughlin (1947, p. 1951) identified it south of Little Fountain Creek where Manitou limestone overlies the reddish, fine-grained rhyolite unconformably (Fig. 10). 2619 TECTOMCS OF I'RO.\T RA\GE, COLORADO BOOS A.YI) MARGARET FULLER BOOS Morrison C. MAYXARD HART OI 2018 U 5 I o H •f* 5 O 2 H •S a^ 3S t_ e^ C | t "o "^ ^ c iSl^cSoo J2 OB. t 1 I I I ; 1 3 := J U, ^ g~3 • t&X k- i 1—1 \ >, H C S8 I '1 « O P gm lll!ili[I I ft 3 S 3 O tJ- 'q 3 •p .3 fc (U2OU3J 8 rt u a in a *d a i5 (S y -^-H IS & P .0 2 q <s -- U.K c .2 Q, a .3 B 1 .a I E ^ o 5 'K ,«.,•d 6 pi •1 ' '• 2622 C. MAVNARD BOOS A.VD MARGARET FULLER BOOS The second lithologic u n i t is composed of terrestrial and littoral arkose, red shale, and sandstone formations of late Paleozoic and early Mesozoic age that attain maximum thickness of about a mile. The unit lies unconformably on the lower marine unit and overlaps it in the northern two-thirds of the foothills bell where it lies on Precambrian. The. third unit consists of marine Cretaceous formations, from Dakota to Laramie, that have a total thickness of nearly 2 miles. They make up the outer part of the foothills belt and fill synclinal basins flanking the Range on east ami west. The fourth lithologic u n i t consists of Tertiary deposits of continental type t h a t lie unconformably, in places, above the folded sedimentary beds of the foothills belt and cover large areas in the adjoining basins. The total thickness is several thousand feet. The lowest, or marine older Paleozoic unit consists of Sawatch sandstone, overlain successively by Ute Pass dolomite, Manitou limestone, Harding sandstone, Fremont limestone, Williams Canyon formation, and Madison (Leadville) limestone. Unconformities occur between most of the formations. Extensive outcrops of Paleozoic formations in Manitou Park (Fig. 10) area connecting link between the formations of the eastern foothills belt and the same formations in central Colorado. Outliers of early Paleozoic rocks east of Phantom Canyon on Pikes Peak arch (Fig. n) and east of Manitou Park on the west side of Rampart Range (Fig. 10) indicate that the south part of the Front Range was covered by early Paleozoic beds. Mississippian fossils in cobbles in the base of the Fountain formation in the northern foothills belt indicate that Mississippian limestone extended onto the north part of the Range. The second, or dominanlly redbed, lithologic unit is in striking contrast to the lowest unit. It contains a thick series of terrestrial and littoral red elastics, shales, siltstone, and evaporites. Varicolored silly shales, lensing sandstones, gypsum, and thin fresh-water limestones make up the top beds. Glen Eyrie shale is the basal formation near Colorado Springs. Elsewhere the Fountain formation is at the base. Ingleside formation, Lyons sandstone, Satanka shale, Forelle-Glennon limestone, Lykins-Chugwater formation, Jelm-Entrada sandstones, and the Morrison formation occur in stratigraphic succession. Jelm, Ingleside, and Satanka formations pinch out toward the south and Lyons sandstone pinches out toward the north. The whole unit is 2,000-6,000 feet thick. Variations are due chiefly to differences in the thickness of the Fountain formation from place to place. The third or marine Cretaceous unit begins with the Dakota group that overlies .Morrison formation disconformably. Above are the lienton group, Niobrara group, Pierre formation, Fox Hills formation, and Laramie formation in ascending conformable sequence. The total thickness of the third unit is about 10,000 feet. The fourth or continental type unit consists of Arapahoe formation at the base TECTONICS OF FRONT RANGE, COLORADO 2623 overlain by Denver, Green Mountain, Dawson, White River, and Arikaree formations in ascending order. The total thickness is more than 3,600 feet. White River beds and Arikaree gravels lie across the truncated edges of foothills beds and onlap the Precambrian in the northern end of the Range. They dip very gently and spread over wide areas in the Denver basin. M A R I N E OLDER 1'ALEOZOIC FORMATIONS Sawalch sandstone of middle Cambrian age is unconformable on Precambrian rocks, chiefly Pikes Peak granite. It is 0-85 feet thick, dark brown, hard, mediumto coarse-grained, evenly bedded, gritty, and glauconitic in places. The thin basal conglomerate contains angular white quartz pebbles. Sawatch sandstone crops out around the Colorado Springs embaymenl, in Manitou Park, and as far north as Perry Park (Figs. 9, 10). Ute Pass dolomite is thin, dark red, crystalline, and sandy. The beds are hard, even, and have thin red or green shale and glauconite partings and crop out around Colorado Springs embayment and north to Perry Park. Thickness ranges up to 22 feet. Maher (1950, Chart 39) separated the Ute Pass dolomite from Manitou limestone. Manitou limestone is unconformable on Ule Pass and Sawatch formations (Cross, 1894). The formation is light gray to purplish or pink, cherly, buff-weathering and thin-bedded to massive. It is of Ordovician age and has a thickness of 218 feet in Williams Canyon northwest of Colorado Springs (Fig. 10). Outliers of the limestone lie on Precambrian granite in the south end of the Front Range. Outcrops have been found as far north as Perry Park (Figs. 9-12). The Harding formation crops out in the foothills at the south end of Front Range but thins and disappears, by erosion of the upper beds, south of Colorado Springs. It is unconformable on Manitou limestone, overlaps it, and wedges out by onlap against Grape ('reek arch south of Canon City. The sandstone is yellow, fine-grained, thick-bedded to slabby, and interbedded with red to purple sandy shale. It is slightly calcareous and fossiliferous. The Harding is 140 feet thick at the type locality and is Ordovician in age. Maher (1950) considers the Harding equivalent to a part of the Simpson formation of the Mid-Continent region (Figs, n, 12). Fremont magnesian limestone is disconformable on Harding sandstone and is overlain unconformably by the Williams Canyon formation. It is 283 feet thick northwest of Canon City but thins by erosion of the upper beds around the southern end of the Front Range. It crops out in the foothills belt as far north as Little Fountain Creek (Fig. n) and just fails to cover the top of Grape Creek arch (Fig. 12). Maher (1950) considers the Fremont as the approximate age equivalent of the Viola limestone of the Mid-Continent region. The Williams Canyon formation crops out in the foothills of the Colorado Springs embayment and as far north as Perry Park (Figs. 9, 10). It is unconformable on the Fremont and Manitou formations and unconformable below the •a t :' 2626 C. MAYNARD BOOS AND MARGARET PULLER BOOS Madison. In general, the formation consists of thin white to gray limestone beils with partings of gray calcareous shale and a few thin sandstone beds. In the northern part of Oil Creek graben it is red-stained, sandy, and has red shale partings. The age is doubtfully Devonian or it may be Mississippian. No fossils have been found in it. Its nearest lithologic counterpart is the I'arting quaruite of central Colorado (Ilrainerd el al., 1933, pp. 387-91). The formation is about 50 feel thick northwest of Colorado Springs and 65 feet thick in Manilou Park. It crops out locally around the south margin of Front Range (Figs, n, 12). The sandy phase of the Williams Canyon beds in Oil Creek embayment is the only indication of shoreline fades in the older Paleozoic formations of the southern Front Range. Madison limestone is lower Alississippian in age and up lo 214 feel t h i c k (Hrainerd et al., 1933, p. 391). It is equivalent to the Madison formation of Wyoming and Montana and the Leadville formation of central Colorado. Madison Letls crop out in Perry Park, in (he Colorado Springs area, and in Manitou Park. TERRESTRIAL AND L1TTOKAL CLASTIC FORMATIONS The Glen Eyrie formation crops out at the south end of Rampart Range (Fig 10). It is unconformable on Madison (Leadville) limestone, contains fossils of lower Pennsylvania!! (Des Moines) age, and grades up i n t o the red arkosic beds of the Fountain formation. McLaughlin (1947, p. 1947) measured 362 feet of (lien F,yrie beds. The dark gray to black shales (Finlay, 1916, p. 6) and evenbedded non-arkosic, thin yellow sandstones of the (ilen Eyrie formation o c c u r nowhere else in the foothills of the Front Range. McLaughlin (1947) extends the (ilen Eyrie shale east into the Denver basin. The Fountain formation is present everywhere in the eastern foothills bell of (he Front Range. It consists of red, irregularly bedded, coarse-grained, urkunii sandstone and conglomerate, thin red micaceous shale and siltstone ( M c L a u g h l i n , '947. PP- 1950-60). The coarse Fountain elastics grade into marine Pennsylvanian on the east side of the Denver basin and intertongue w i t h Ingleside limestone:, in the northern foothills. No consistent slratigraphic markers subdivide the formation. The Fountain grades up into the Lyons sandstone south of the terminat i o n of the Ingleside tongues. It is about 1,500 feel thick at (he north end of the Range and 4,500 feet thick at Colorado Springs. In the Canon City basin it is about 2,500 feet thick (Figs, n, 12). The Ingleside formation crops out in the foothills belt from Lyons to the stale line (Figs. 4-6; Butters, 1913, p. 75; Davis, 1947). It consists of interstratilied pink sandstones, sandy limestones, gray, fine-grained massive limestones, and red shales. The Ingleside is more than 600 feet thick at the Colorado-Wyoming line. Its limestone beds are thickest and most numerous at the north and tongue out south. The uppermost limestones extend farthest south. Satanka shale consists of two tongues. The red silly shale of the lower tongue TECTONICS OP FRONT RANGE, COLORADO 2627 is conformable above the Fountain formation and lies between Fountain and Lyons north of Hig Thompson River. The tongue is about 200 feet thick at the state line where the Lyons pinches out. The upper red shale of the Salanka unites with the lower tongue at the Wyoming line (Callier, 1948). The upper tongue in Colorado lies between Lyons sandstone and the "crinkly lime" or (ilennon member of the Lykins (LeRoy, 1946, p. 31) and is, equivalent to the Harriman shale member of the Lykins formation. Lyons sandstone is conformable on the Fountain as far north as Big Thompson River (Fenneman, 1905). There is a transition zone up to 100 feet thick between Fountain and Lyons formations. The formation is nearly 300 feet thick at Lyons, Colorado. North of Owl Canyon (Fig. 5) the Lyons thins to a few feet near the slate line. In general, the Lyons is a fine-grained, well sorted, cross-bedded, cream to pink to red-brown sandstone of beach and bar origin. It grades eastward into marine sand (Thompson, 1949, pp. 52-72). Eldridge (1896) named the Lyons the "creamy sandstone" al Morrison, west of Denver (Fig. 8). Massive vertical slabs of pink Lyons sandstone crop out in the Garden of the Gods near Colorado Springs (Fig. 10). At the soulh end of the Range ihe formation is red, soft, and shaly. The beds form a single stratigraphic unil wilh ihe Lykins on Red Creek arch (Fig. n). Fogarty (1952) correlates ihe Lyons with the Big Blue series on the east side of the Denver basin. The Forelle formation consists of fine-grained, thin-bedded limestone and interbedded red shale. It contains Permian fossils. The Forelle formation lies above the Satanka shale and below the Chugwater and is equivalent to the (ilennon limestone member of the Lykins formation (LeRoy, 1946; Hroin, 1956; and Figs. 4, 5)The Lykins formation crops out throughout the foothills belt. It consists of about 600 feet of red shale and siltstone. Thin limestone members, the "crinkly limes," occur 100 feet above the basal red shale. The calcareous members are replaced by gypsum and anhydrite beds locally. LeRoy (1946) calls the basal red shale Harriman. The "crinkly beds" are Falcon limestone, Bergen shale, and (ilennon limestone in ascending order. The bulk of the Lykins formation, in the Strain shale above the "crinkly beds," is equivalent to the Chugwater of northern Colorado and Wyoming. The Jelm-Entratla formations consist of massive, soft, pink to white crossbedded sandstone. The even-bedded, buff-weathering ledge, overlain by green shale above the Jelm north of Owl Canyon is possibly Sundance (Fig. 5). Heaton (1939) correlated lenses of sandstone in the foothills between Morrison and Lykins formations with the Entrada formation. In Figures 4-12, the sandstones between Lykins and Morrison formations are mapped as Jelm-Entrada. Sundance(?), Jelm, and Entrada sands at the north end of the Range are 225 feet thick at Boxelder. Jelm and Sundancef?) beds pinch out toward the south. The lenses of Entrada that occur at intervals south in the foothills have maximum thickness of about 60 feet. 26 34 C. MAYNARD BOOS AtVD MARGARET FULLER BOOS TERTIARY CONTINENTAL DEPOSITS The Denver formation is disconformable on (he Arapahoe. Its dark basal conglomerates, grayvvacke, and gray, Ian, and buff sandstones are 434 feet thick on Green Mountain (Reichert, 1956, p. 108). The basaltic lavas t h a t cap North and South Table mountains and extend i n t o Green Mountain are about two-thirds of the way up in the Denver formation. K. \V. Brown (1943) puts the CretaceousTertiary boundary in the lower part of the Denver formation on fossil evidence. Green Mountain conglomerate is unconformable on the Denver formation ami occurs only on Green Mountain, west of Denver (Fig. 7). The lower member is 50-150 feet thick and consists of boulders up to 6 inches in diameter t h a t are predominantly basalt and andesite and less amounts of Precambrian cobbles. The upper member, 275-375 ^eel thick, is predominantly of Precambrian boulders up to 6 feet in diameter. Amounts of volcanic rocks decrease upward. One of the shale members contains plant fossils of Paleocene age. The formation is considered by Reichert (1956) to be a very coarse (luvialile deposit equivalent to the lower Dawson of Dane and Pierce (1936). Dawson conglomerate consists of coarse, light-colored arkosic sandstone and conglomerate, poorly cemented, and having torrential cross-bedding. It has a thickness of 2,000 feet or more near Castle Rock. It is of Eocene age and covers large areas in the Denver basin. Dawson formation is dragged up by the Rampart upthrust and dips steeply away from the mountains south of Monument (Fig. 9). The White River formation of Oligocene age lies unconformably over the truncated formations of the foothills belt at the north end of Front Range (Fig. 4). Brule clays of the lower member are white, pink, greenish, ashy, calcareous, and about 150 feet thick. They are overlain by the coarse, gritty, thick-bedded, lleshcolored sands of the Chadron member, about 100 feet thick. The Brule and Chadron members make up the "Chalk Cliffs" scarp. Arikaree gravels of Afiocene age cap the Chalk Cliffs. The gravels are part of aborder widespread sheet t h a t overlaps the foothills onto the low edge of the mountain (Fig. 4). STRUCTURE Regional tectonic pattern.—The complex, faulted arch of the Front Range trends north-south to N. 20° VV. in the wide anticlinal mountain belt of central Colorado (Oriel, 1954, p. 42; Warner, 1956). The Range is nearly five times as long as wide and is trilobate at the south. Cross folds, horsts, grabens, and thrustfault belts are imposed on the general anticlinal pattern. The Precambrian body of the Range is bordered by the narrow eastern foothills belt of faulted, tilted, and locally folded sedimentary formations (Goddard and hovering, 1937). The Range occupies a positive Precambrian area rejuvenated by several orogenies (McCoy, 1953). The east flank consists of folded and dislocated schist, gneiss, and quartzite buttressed by granite intrusions ranging from dikes and sills to batholiths. The competency of the Precambrian formations under erogenic TECTONICS OF FRONT RANGE, COLORADO 2635 stress, and their ability to transmit force was partly controlled by rocks that range from fissile schist to massive granite. The development of granitic bodies within the disturbed groundwork of foliated rocks determines the tectonic pattern of the Range as a whole and the crystalline basement complex under the foothills belt. The lithologic range of sedimentary rocks from plastic shale to tough sandstone and arkose, influenced the tectonic pattern in the foothills belt. Relatively rigid sedimentary formations (Lyons, Dakota, Timpas) are disjointed close to fault planes. Thrust faults have low dips in soft plastic shale (Pierre, Benton), dip more steeply in relatively rigid sandstone, and are near vertical in the crystalline rocks of the Precambrian. Tectonic elements of the I'recambrian (Fig. 3) are: (i) positive, competent granite batholiths, stocks, and groups of plutons that buttress the Range from end to end; (2) layered or foliated meta-igneous and metasedimentary formations, host to the igneous bodies, all complexly folded, faulted, dislocated, and occupying the less competent areas between the granites; (3) faulted, sheared, and structurally weak belts of Precambrian genesis partly occupied by pegmatite and aplite; (4) faults and folds of Laramide and later genesis and reactivated, structurally weak belts of Precambrian genesis; (5) five rectangular, northwesttrending, fault-bounded segments or lineaments that cross the Range and the eastern foothills belt from northwest to southeast; and (6) thrust-fault belts in the east margin of the Precambrian and the eastern foothills belt. i. The major bodies of granite occur: (A) along the general north-south Precambrian axis (Fig. 3), (B) adjacent to the foothills on both east and west flanks, and (C) in north and south ends of the Range. The granites occupy, in outcrop and probably at depth, 70-75 per cent of the exposed Precambrian terrane. The shape, attitude, and internal structure of granite bodies of Mount Morrison genesis conform to the primary patterns of the metasedimenlary formations from which they were derived by metasomatism or palingenesis, or both (Boos, M. F., 1954, p. 119). Sante Fe Mountain, Mount Morrison, and Falcon Mountain stocks grade into impure quartzite and interconnect by sheets of granitized quartzite. The bodies of gneissic Mount Morrison granitic rock are tougher, stronger, and more massive than the mica schist, gneiss, and amphibolite around them. Small, funnel-form batholiths and stocks of Boulder Creek granite reinforce the schist and gneiss near the base of the eastern flank (Fig. 3). The marginal gneissic granite of stocks and batholiths contains innumerable aplite and pegmatite dikes. Pikes Peak batholith in the south third of the Range and Sherman batholith at the north end are major crystalline bulwarks, extend across the Range, and continue under the eastern foothills (Fig. 3). Small batholiths, stocks, and lensing plulons of Silver Plume-type granite &&£<X* %$SxX>< 2638 C. MAVXAKD BOOS A.\D MARGARET FULLER BOOS and Mount Olympus granite border the north and south margins of Pikes IVA batholith and the south margin of Sherman batholith (Boos and Aberdeen. 1940, p. 703). The compound Longs Peak-St. Vrain batholith occupies the northcentral part of the Range (Boos and Boos, 1934, Fig. 3). The Log Cabin balholiili is separated from Sherman batholith by folded gneiss and schist. 2. In general, the metamorphic formations dip away from the major graniiii bodies and are accommodated to their outlines. Folds are tight and closely spaml between batholiths and stocks. The N. 60° W. to east-west folds in the castcentral part of the Range extend under the foothills belt. The anticlines are deeply truncated, but the synclines indicate the size, shape, and extent of the primary Precambrian folds. The east-west to northeast structural grain of the Precambrian in the From Range indicated in some geologic reports (Cross, 1894; Ball, 1908; George, loot;, Van Hise and Leith, 1909; Thurston, 1955) is not general for the east llanL Early geologic investigations were chiefly near trails, roads, and railroads along the major streams such as Clear Creek, Golden Gate, Big Thompson, and Cache la Poudre rivers. The water courses are entrenched along local east-west ti. northeast folds, fractures, and foliation of gneiss, schist, and quartzite between major granite bodies on the north and south. The geologic reports fail to note the areal limits of northeast structures. A major exception is Coal C'reek syiulinr that extends northeast-southwest across the general grain of the Precambrian from Central City to the foothills and probably northeast beneath the sedimentary formations (Lovering el al., 1950, PI. i; Moench, 1954. Several minor northeast Precambrian trends affect the rocks adjacent to llie foothills belt (Figs. 5, 7, 8). The shift from well developed northwest to northeast folds and fractures takes place along a north-south Precambrian structural axis. This axis is within a Laramide hinge area, where the formations in the foolhills belt and the crystalline basement bend steeply down into the Denver basin. The north-south fold centered by Precambrian rock at Deer Creek southwest of Denver, a minor Precambrian high in the Fountain formation near the mouth of Turkey Creek Canyon, and east-west folds on either side of Clear Creek Canyon are in the hinge. The five quartzite-rich areas of Cache la Poudre and Big Thompson areas, Bear and Turkey creeks west of Denver, and Phantom Canyon and Wilson Creek in the south end of the Range, are strong tectonic elements reflected in the adjoining foothills terrane. The faults, shears, and complex minor folds in the disturbed Precambrian rocks of the Boulder-Jamestown area, mapped by Lovering, Goddard, and others, have counterparts, witli local variations, in the east flank of Front Range from Wyoming to Oil Creek graben (Figs. 4-12). 3. Faults and shears of Precambrian genesis are nearly vertical or dip steeply east, trend northwest to north-northwest, and dislocate both granites and folded metamorphics. Some are upthrown on the northeast, others have strike-slip dis- TECTONICS OF FRONT RANGE, COLORADO 2639 placement, and a few are partly fdled with pegmatite. Belts of Precambrian genesis are less than \ mile wide but are tens of miles long. The slightly younger northeast-trending faults and shears are complementary to the major northwest ones. Lovering and Goddard (1950, PI. i) show two major fracture systems that extend northeast almost across the Range. 4. Faults of Laramide or younger origin greatly disturb the Precambrian terrane west of the Denver basin, the formations in the foothills belt, and the basement complex. Some of the northwest-trending faults established during the Precambrian were locally reactivated by Laramide or younger orogenies. 5. The major structural segments of the Range consist of live en icheluti, fault-bounded lineaments (Fig. 3). Main structural features are: (a) prominent, nearly continuous fault and shear zones along the northeast and southwest boundaries; (b) long dimensions are northwest-southeast; (c) each sub-rectangular segment contains one or more major granite batholiths or groups of stocks; (d) the boundary fractures cross structurally weak areas of folded and sheared metamorphic beds and the eastern foothills formations; (e) the faulted and sheared zones are in part of Precambrian genesis, but were rejuvenated and extended through the foothills formations by Laramide diastrophism; (f) the Precambrian structures in the segments extend beneath the foothills formations; and (g) the segments cross the entire Range. No distinction is yet possible between the features primarily of Precambrian genesis and those due solely to dislocations of Laramide origin. The Kenosha-Cripple Creek segment (Fig. 3, I) is bounded by traces of Parkdale-Mdntyre, Elkhorn, and Williams River thrusts, Cooper Mountain fault east of Oil Creek graben, and the faults along the west side of Pikes Peak and Mount Evans batholiths. Internal structural components are Cripple Creek and Kenosha batholiths, Oil Creek graben, Royal Gorge arch, Wilson Creek graben, Webster and Twelvemile Park basins, and Grape Creek arch. The segment terminates in the Williams River Mountains al the northwest. The Pikes Peak-Mount Evans segment is a great horst (Fig. 3, II). The internal structural features are Mount Pittsburg-Red Creek arch, Cripple Creek and Pikes Peak arches, Cheyenne Mountain fault block, Mount Evans batholith, and part of Silver Plume batholith. The lineament terminates in the Vasquez Mountains. The Cheyenne Mountain-Ute Pass fault on its east margin is offset at the northwest by the Ralston fault. The quartzite, gneiss, and schist belt intruded by Mount Pittsburg stock of Silver Plume-type granite at the southeast margin of the Range is parallel with the adjacent foothills formations (Fig. n). The Idaho Springs-Rampart segment (Fig. 3, III) extends from the Colorado Springs ramp on the southeast (Fig. 9) to the Tabernash stock on the northwest. The Rampart Range horst of Pikes Peak granite, Rampart fault, Perry Park platform, Manitou Park graben, Indian Creek plutons, Mount Morrison, Falcon, and Santa Fe Mountain stocks, and extensively folded and faulted areas of metasedimentary beds make up the segment. The Floyd Hill breccia reef is the north- 2640 C. MAYNARD BOOS AND MARGARET FULLER BOOS east boundary and the Golden thrust belt terminates the segment on the east. The Longs Peak-Boulder segment (Fig. 3, IV) extends from the Golden llirust fault belt on the southeast and east, to Grand Valley on the northwest and J l u i k horn-Redstone Creek faults on the northeast. The Boulder Creek and Longs Peak-St. Vrain batholiths are the major granite bodies. Northwest-trending en echelon faulted folds displace the eastern foothills formations and the Precambrian (Figs. 5, 6). Several breccia reefs dislocate the southwest part of the segment. The Log Cabin-Sherman segment (Fig. 3, V) crosses the Range north-norlhwest from Milner Mountain, north of Big Thompson River, to the south end of the Laramie basin. Major tectonic components are the blunt south end of the Sherman batholith, Log Cabin batholith, and small, massive plutons of Moulder Creek granite south of Poudre Canyon quartzites. The Virginia Dale-Round Butte platforms, Livermore graben, and Ingleside fault unit are foothills structures of the segment (Figs. 4, 5). 6. The east margin of the Range is a discontinuous thrust fault belt 3-5 miles wide (Fig. 3). Underthrusts are more extensive than overthrusts. Upthrusts dip steeply west or are nearly vertical. Wedges of Precambrian crystalline rock between thrusts make a shatter belt 2-3 miles wide adjacent to the foothills. The Golden thrust-fault belt (Fig. 3), from Coal Creek on the north to Jarre Canyon on the south, consists of underthrusls in the foothills formations bordered by metamorphic rocks and small faulted plutons of Mount Morrison, Boulder Creek, and Mount Olympus granites. Wedges of crystalline rock between west-dipping polished fractures overlie vertical Fountain beds west of Denver and in Cherry Gulch south of Mount Vernon Canyon. The fractures in the Precambrian north of Golden Gate and west of Denver are replicas of thrusts in the Golden fault zone of the foothills belt (Gabelman, 1948). Imbricate thrust slices of Pikes Peak granite top Benton shale south of Jarre Canyon (Fig. 9) and tear faults extend west into the Rampart Range. Fractures in the Rampart fault zone are nearly vertical. North-south faults, parallel with the Rampart fault (Fig. 10), are upthrown successively at the west to the summit of the Rampart Range. The Ute Pass-Cheyenne Mountain fault belt consists of steep upthrusts. The fractures dislocate Pikes Peak granite and the siliceous ferruginous gneiss south of Manitou. The sole of Cheyenne Mounlain-Ute Pass fault overlies foothills beds from Little Fountain Creek to Woodland Park (Fig. 10). The upthrusts steepen mountainward but diminish in number and throw. Great curving faults dislocate the crystalline rocks around the south end of the Front Range from Little Fountain Creek to Twelvemile Park (Figs, n, n). The traces are sub-parallel with the folds in metamorphic beds, the strike of the foothills Precambrian contact, and the foothills belt. Tectonic elements of the eastern foothills belt are modifications of the simple monocline. The strata between the upper limit of the foothills belt and the bot- TECTON1CS OF FRONT RANGE, COLORADO 2641 torn of the Denver basin descend east from about 8,000 feet above sea-level to 5,000-8,000 feet below. Dips of 2o°-35° E. are uniform and uninterrupted on Milner and Horsetooth mountains (Fig. 5) and Mount Pittsburg fault block (Fig. n). Where the monocline is steep it is sharply bent at top and bottom. Tensional displacements show in the upper bend (or hinge) and compressive features in the lower one. This is the mobile belt in which much of the Laramide uplift took place. It extends west into the bordering Precambrian. The tectonic elements are: (i) minor anticlinal folds; (2) en echelon, tilted, southeast-plunging fault blocks; (3) north-south to northwest faults; (4) northeast faults and associated drape folds; (5) wedge-shaped grabens that indent the mountain front; (6) extensive north-south to northwest thrust faults; and (7) broad shelf-like "platforms" or ramps. The elements combine or repeat to produce the complex structural pattern of the foothills belt. 1. There are two or three lines of north-south folds in the foothills belt. Firstline folds are high, steep, and level-crested. Open folds are cut off by transverse faults at one end and plunge steeply at the other. Closed folds are nearly symmetrical and the axes plunge gently at both ends (Fig. 3). Carter Lake (Figs. 5, 6) is a first-line open fold and Wellington anticline is a second-line closed fold. The Clarks Lake structure may be a third-line fold. 2. Southeast-plunging fault blocks that offset the foothills belt southwest from Bellvue to Lyons (Figs. 3, 5, 6) have mountainous dimensions. The faults on their west margins are en echelon and trend northwest. Maximum throws are thousands of feet and decrease basinward. The sedimentary formations of the east slopes form asymmetric plunging folds at the southeast. Similar small structures occur as far south as Deer Creek (Fig. 8). 3. The entire sedimentary rock section, more than 11,000 feet thick, is faulted out at the east base of the Rampart Range against the Precambrian rocks of a mountain front almost 1,000 feet high (Fig. 9). The Fountain formation east of Ute Pass fault in Manitou Park graben is hundreds of feet below the granite of the Rampart Range. 4. Northeast-trending normal faults and drape folds form the basinward edge of Round Butte and Loveland platforms from Boulder north to the Wyoming stale line (Figs. 3, 4, 7). Minor anticlines are between and parallel with the northeast faults. 5. Fault-bounded, V-shaped grabens containing nearly horizontal sedimentary formations indent the east and south margin of the Front Range at Livermore (Fig. 4), Manitou Park (Fig. 9), Oil Creek (Fig. 12), and southwest of Canon City (Fig. 12). 6. Thrust faults cut through or override the sedimentary formations along more than a third of the foothills belt and are evidence of strong horizontal compression of both east and west margins of the Range. In general, vertical exceeds horizontal displacement. Precambrian rocks overlie sedimentary formations of the foothills beds at Cheyenne Mountain and west of Denver. In the Golden 2642 C. MAYNARD BOOS AM) MARGARET FULLER BOOS fault-belt area the sedimentary rocks of the Denver basin and the underlying basement complex moved down and west beneath the Precambrian rocks of the eastern flank and the sedimentary formations of the eastern foothills. Multiple thrusts were penetrated by the drill at Soda Lakes and at Deer Creek (Fig. 8). Thrust faults have low dips in soft shales, and steeper dips in harder formations. The outcrops of the foothills formations are narrow belts due to thrust faulls at Boulder and at Golden. The outcrops of the beds on the active, plainsward side of the thrusts curve west parallel w i t h the trace and are close to the Precambrian foothills contact (Pigs. 6, 7). The fault traces are parallel w i t h the foothills beds, curve northwest into the Precambrian rocks, or end in transverse (east-west) tear faults. Where the f a u l t trace has sharp curvature, multiple faults have developed. 7. The dip of the foothills monocline flattens, locally, into broad, step-like, gently tilted shelves or platforms (Pig. 13, sees. AA', GG'), faulted along ai least one margin and intermediate in elevation between the foothills bell and (he Denver basin. The Precambrian basement rock is also step-faulted. Sofl sedimentary formations, curved down over faulted and tilted Precambrian blocks, make drape folds. The Perry Park platform is I h i n l y covered by sedimentary formations (Pig. o; Fig. 13, sec. G(J'). Round l i u l t e (Fig. 4; Fig. 13, sec. AA') and Lovelaml platforms (Fig. 6) are thickly covered. The Virginia Dale platform (Fig. 4; Pig. 13, sec. AA') retains only a thin patch of sedimentary rock cover. The Colorado Springs ram]) is a similar but more steeply l i l t e d p l a t f o r m . Sedimentary rocks are eioded from the upper part, but a bevelled wedge of Paleozoic formations covers I he Precambrian on the lower end of the ramp. TECTONIC UNITS The foothills belt and adjacent Precambrian areas consist of fifteen leclonii units: (i) Virginia Dale-Sand Creek-Round B u t t e , (2) Livermore, (3) Ingleside, (4) Cache la Poudre, (5) Big Thompson-ew eclielon faults, (6) Moulder, ( 7 ) Denver Mountain Parks-Golden f a u l t , (8) Perry Park-Rampart Range, (<;) Colorado Springs-Manitou Park, (10) Cheyenne M o u n t a i n , (n) M o u n t Pill burg-Red Creek, (12) Pikes Peak, (13) Oil Creek, (14) Royal Gorge-Wilson Creek, and (15) Twelvemile Park-Webster Park. The Virginia Dale-Sand Creek-Round Jiulle unit (T. 11-12 N., K. 69 71 \V.; Fig. 4) consists of two platforms separated by the Sand Creek folded belt. The base of the sedimentary rock section in the Sand Creek folds is about 3,000 feet below Virginia Dale platform, and Round H u t t e platform is 2,000-3,000 feet lower. The Virginia Dale platform consists of Sherman and Silver Plume-type granites and gabhro dikes. Deadman fault zone in folded schist and gneiss divides it (Fig. 4). The uplhrown (west) block of Sherman and Log Cabin granites is deeply trenched along faults. The platform east of Deadman is a broad, low, truncated arch of Sherman granite cut by northwesti-trending, nearly vertical TECTONICS CF FRONT RANGE, COLORADO 2<>43 faults offset by northeast to east-west faults. The fractures extend into the basement complex under the foothills. Gneiss, schist, and small plutons of Mount Olympus granite north of Halligan fault make up the south part of the platform. The beds of Fountain formation in the shallow northwest-trending topographic depression on the west side of the arch are nearly parallel with the Precambrian surface of the platform, which tilts southwest at a low angle. The beds curve down against Halligan fault. Fountain to Chugwater beds on the east Hank of the arch dip east io°-2o° into the Sand Creek folded belt. The axes of Sand Creek folds are generally north-south (Fig. 4). The Hanks are steeper on the west, like other folds in the foothills belt. Sand Creek structures include a small northeast-faulted syncline en echelon with the sharp, northeast-trending north end of the Sand Creek anticline. The north-south axes of Sand Creek anticline and Table M o u n t a i n syncline are separated by a transverse zone of south dips. Boxelder anticline and Table M o u n t a i n syncline are parts of one fold (Coke, 1934). The high, steeply folded anticline 2 miles southwest of Table M o u n t a i n has a level crest, plunges steeply at the north end and Hares at the south. Kast of Sand Creek Valley, a north-plunging anticlinal horst (Fig. 4; Fig. 13, sec. AA') is separated from Sand C'reek folds by normally east-dipping Dakota and Morrison beds. The anomalous structures of the folded belt are: (i) anticlines and synclines (not saddles) along a common north-south trend, (2) anticlines that blunt out at one end by abrupt Hat toning of dips, and (3) anticlinal and synclinal axes w i t h disconnected trends. Minor northeast faults and the northeast end of Sand Creek anticline are en echelon and parallel with other structures in the region. No simple compressive forces could produce the discordantly arranged folds and faults of the Sand Creek beh and not disturb the uniformly dipping formations at the east and west (Fig. 4). Drape folds on the south and east suggest an origin for the irregular arrangement of the structures of the Sand Creek belt. The irregularity of Sand Creek folds resulted from tilting and crowding of relatively rigid Precambrian fault blocks below the incompetent red shale, siltstone, and evaporile during east-west compression. Formations from Jelm to Niobrara dip io°-2o° K. from Sand Creek fold belt to Round liutte platform where Pierre shale cover is nearly flat. Dips are 4°-7° W. on Round B u t t e Dome in Sees, i and 12, T. n N., R. 69 W. (Fig. 4). Hygiene sandstone and shale, draped over a buried fault at the east edge of Round Butte platform (Figs. 4 and 13, sec. A A'), dip 7o°-85° SK. The drape fold extends southwest into a fault between nearly vertical Niobrara and Dakota beds. The Livermore graben unit (T. 9-10 N., R. 70-71 W.; Fig. 4).—The southwest branch of the Livermore embayment is a fault.-bounded graben that contrasts structurally with the shallow northwest branch on Virginia Dale platform. The graben is 7 miles long, up to 4 miles wide, tapers at the southwest, and opens into Livermore syncline at the east. Precambrian rock between the Halligan fault and the boundary fault of the 2646 C. MAYNARD BOOS AND MARGARET FULLER BOOS graben includes the east part of the Log Cabin batholith, small bodies of Mount Olympus granite, and a bell of folded gneiss and schist striking east and west in northeast (Tig. 4). The structurally weak area north and northwest of the graben is strike-faulted, parallel with the boundary f a u l t and dislocated by cross f a u l t s that converge under the graben. The fractures are inherited from I'recambrian time and tendency to failure persisted as late as Laramide orogeny. Fountain and Ingleside beds in the graben dip gently east, steepen i n t o Livermore syncline and are turned up along the boundary faults. The syndine lias a steep south limb and a gently dipping north limb where three northeast-trending drape folds cross it (PI. i, Fig. 4). The drape folds have steep southeast dips and make sharp Y-shaped outcrops across the synclinal axis. The beds between I he drape folds dip gently southeast and curve roundly across the axis. The step-like structures cross Livermore syncline diagonally, indicating that the drape folds preceded the development of the syncline. They are controlled by east to northeast structures in the basement complex. South of the graben, in the angle between the south boundary fault and the north-south belt of lower foothills beds, the Precambrian is buttressed by Pikes Peak (Sherman) and Silver Plume-type granites (Fig. 4), that extend east under the foothills monocline. The f l a t t e n i n g of the sedimentary formations there and the north-plunging nose of Chugwaler beds are over a structural high in the basement complex. The Ingleside unil is characterized by u n d e r l h r u s t faulls (T. 9 N., R. 69 \V.; Fig. 4) that emerge along the lop and east face of the Dakota cuesta south of Owl Canyon and about a mile east of Ingleside. The broad lop of the Dakota hogback has a disconnected chain of seven short, boat-shaped centroclines of upper Dakola sandstone exposed for about 5 miles between two branches of Ingleside fault (Robertson, 1950; Broin, 1952). Dakota, Benton, and Niobrara beds curve west in a broad arc parallel with t h e fault trace, but Lyons and Ingleside cuestas are straight north-south. Dips are steepest in the west part of the arc. The westward displacement of the foothills formations and the sleep dips close to the Ingleside f a u l t trace are due to underthrust from the east bounded by tear f a u l t s at the north and south. Ingleside faults are the northernmost underthrust belt of the eastern foothills. The Cache la I'audre unit (T. 8-9 N., R. 69-71 W.; Figs. 4, 5) extends from Rist Canyon across Cache la Poudre River to the granite area south of Livermore and from Bellvue north to Ingleside fault (Baker, 1948; Broin, 1952; Thompson, 1949). The Precambrian between Cache la Poudre River and Rist Canyon consists of quartzite, mica schist, spotted mica schist (PI. 2, Fig. 2a), and phyllite intruded by irregular plulons of Boulder Creek granite. The beds are tightly folded, f a u l t e d , locally overturned, and cut by three sets of faults. The metamorphic beds dip steeply away fro M the fault-bounded gr mite bodies or are tightly folded between them. Earliest faults trend north-south to N. 30° W. and contain peg- TECTONICS OF FRONT RANGE, COLORADO 2647 matite. Other faults, parallel with fold axes, terminate against northwest Laramide faults which invade the foothills belt. Many branch and feather out northwest in fissile Precambrian beds and extend east and southeast under the foothills (Fig. 5). A triangular wedge of foothills beds indents the Precambrian northwest of liellvue along a faull that curves north i n t o N o r t h Fork fracture system. Fountain beds turn up against the upthrown Precambrian block northeast of the f a u l t . Bellvue anticline (Sees. 19, 30, T. 8 N., R. 69 \V.) is cut by a north-south faull. The west part of the dome is beneath the alluvium of Cache la Poudre River except a small outcrop of Lyons sandstone on the west flank that is nearly vertical. Ingleside beds make a great dip-slope arch over the east flank (PI. 4, Fig. 6). The syncline north of Bellvue anticline is sub-oval, warped, and twisted, is cut by the north end of the Bellvue fault (Figs. 4, 5; Thompson, 1938; Hunter, 1956), and is crumpled and broken against the northwest-trending fault north of Cache la Poudre River. The liig Thompson-en echelon fault unil (T. 3-7 N., R. 70-71 W.; Figs. 5, 6, 13, sees. BB', CC') extends from Risl Canyon south to Lyons. Sub-parallel northwest-trending faulted folds, downlhrown on the southwest, successively offset the formations of the foothills bell and the adjacent Precambrian. The narrow V-shaped re-enlrants of foothills beds extend 1-5 miles northwest into the Precambrian areas. The maximum throw of each fault is several thousand feet. The accumulated lateral displacement of the mountain front is 7 miles southwest. The u n i t consists of four structural sub-units: (i) Horsetoolh-Carter RidgeMilner M o u n t a i n , (2) (Jreen Ridge-Big Thompson, (3) Rattlesnake Park-Blue Mountain, and (4) Little Thompson-Rabbit Mountain. i. The Horsetooth-Carter Ridge-Milner Mountain sub-unit extends from the east-west faults along Rist Canyon to the Buckhorn fault zone (Fig. 5). Horsetoolh and Redstone faults in the foothills belt extend northwest through the crystalline rocks and branch. The faulted horsts of Boulder Creek granite and folded metamorphic beds adjacent to the foothills extend northwest to I'oudre River. Minor faults parallel with Buckhorn and Redstone faults go southeast under the foothills but do not disturb the sedimentary beds. The east-tilted block of Precambrian rock in Milner Mountain is 6 miles long and i j miles wide. Its Precambrian crest is 1,500 feet above the Fountain beds in Buckhorn Valley. Foothill beds on the east dip 2o°-35° into the Denver basin. Precambrian structures are depressed and offset from the great fault block of Horsetooth M o u n t a i n along Horselooth fault. The northeast-tilted wedge of Carter Ridge consists of tightly folded phyllite, dark quartzite, staurolite schist, and sills of Mount Olympus granite. Foothills partly cover its east slope. The Precambrian is structurally lower than in Horsetooth-Milner Mountain fault blocks, but the phyllites on top of Carter Ridge are about t,ooo feet above Dakota beds in Buckhorn Valley. Total vertical displacement is at least 3,500 feet. 2648 C. MAYNARD BOOS AND MARGARET FULLER BOOS Sedimentary beds on the east side of Horsetooth and Milner Mountain dip 20°-30° E. into the Denver basin. The Fountain and Lyons formations are offset at Long Gulch (Sec. 35, T. 8. N., R. 70 W.) and near Stout (Fig. 5) by Spring Canyon and Redstone faults, respectively, that do not displace the upper formations of the foothills belt (Callier, 1948). The south-plunging structure of Milner Mountain continues 4 miles soulh in the narrow, tightly folded Big Thompson anticline. The fold is faulted down on the west (Cutter, 1949; Culligan, 1948) and plunges steeply at its soulh end. A broad curve in Hygiene sandstone outcrops (Sees. 33-34, T. 5 N., R. 69 \V.) indicates its south extent. Fountain to Lykins beds in the narrow fault trough west of Horselooth Kidgc, in upper Redstone Creek Valley, dip I5°-20° NE. The beds between Horselooth and Redstone Creek fault turn up against Horsetooth fault. Between Milni-r Mountain and Carter Ridge the trough is broadly synclinal. 2. The Green Ridge-Big Thompson sub-unil is not as deeply indented liv faull-wedges of foolhills beds as ihe sub-unit on the north. The foothills-l'rccambrian contact at Big Thompson River is 4-5 miles southwest of the same contact on the east side of Milner Mountain. Mica schist, phyllite, and sheets of Mount Olympus granite in Green Ridge are upthrown northeast of Green Ridge fault. Similar northwest-trending fractures cut the Precambrian between Green Ridge fault and Big Thompson River. The competent, N. 6o°-jo° \V.-trending dark quartzites along Big Thompson Canyon contain three groups of porphyritic Mount Olympus granite sills (I'l. i, Fig. 33). The sill-bearing quartzites crop out from 2 miles north of the river to i mile south of it and extend east under the foothills belt (Fuller, M. B. (M. F. Boos), 1924; 1956, abst.). South of Big Thompson River, the siliceous beds arc interlayered with phyllite, intruded by small stocks of Mount Olympus granite, and are offset by Dixon Gulch fault and north-south tension fractures. The faults terminate west of or within the foothills belt. The sedimentary rock section northeast of Green Ridge contains 4,000 feet of Fountain to Pierre beds that dip 2o°-25° NK. to Buckhorn fault (Baker, 1948). The sharp synclinal fold adjacent to the fault is due to drag. The south-plunging nose of Fountain and Lyons beds at the end of Green Ridge continues into Carter Lake anticline. The fault at the north end of the Carter Lake fold may be a continuation of Green Ridge fault. Ingleside sandstone on the narrow, deeply truncated, north-south Carter Lake anticline (Rue, 1949) dips 65° W. and 25°-35° K. The southwest flank of the structure is cut by a fault aligned with Dixon Gulch fault. Minor faults east of the Carter Lake fold trend northeast toward the low Deininger or Dry Lake fold (Fig. 5). 3. The Rattlesnake-Blue Mountain sub-unit extends 10 miles south from Saddle Notch fault to the Boulder County line (Fig. 6). The northwest-trending, cross-faulted Blue Mountain block consists of hornblende gneiss, phyllite, mica TECTONICS OF FRONT RANGE, COLORADO 2649 schist, and east-dipping sheets of granite that continue under the Carter Lake area. Little Thompson fault emerges along the west base of Blue Mountain. Rattlesnake Park graben of Fountain and Lyons beds is northeast of the intersection of Little Thompson fault with a prominent northeast fracture. The granite southwest of Little Thompson fault contains numerous inclusions of gneiss and schist and extends east under the foothills of the Dry Creek area. Three generations of faults dislocate the Precambrian: (i) east-west to northeast tension faults, (2) Dry Creek and Dixon Gulch breccia reefs and other northnorthwest fractures and (3) late north-south tension fractures, uplhrown chietly on the west. The Precambrian-Fountain contact is offset a mile southwest where Dixon Gulch and Saddle Notch faults go under the foothills formations at the north end of Carter Reservoir syncline. West of Carter Lake, beds on the east side of the Blue Mountain fault block strike north-south for 7 miles and dip i5°-2o° E. as far as Little Thompson fault. Dakota beds dip 65° W. and 25° E. in the Little Thompson anticlinal horst (Fig. 6). The Bertlioud oil field, 3 miles east of Little Thompson anticline, produces a small amount of oil and gas from a low southplunging anticline. Subsurface closure is against a northeast fault (Lavington, I954). 4. Sedimentary beds west of Little Thompson fault dip 2o°-3o° NE. A mile south, downthrown Fountain beds on the southwest side of a north-south fault extend over Precambrian granites about 3 miles. Dowe Pass anticline, between the faults, extends southeast into the northwest end of the Rabbit Mountain structure (Quam, 1932; Ali, 1950). Rabbit Mountain anticline extends south from the Blue Mountain fault block. The fold is broad, asymmetric, plunges south, and is steeper on the east or basinward side t h a n on the west. The granite of Longs Peak-St. Vrain batholith appears to extend east under the foothills beds and may be present in the basement complex as far east as Rabbit Mountain anticline. The steep east flank of the structure suggests a concealed underthrust from the east, north of St. Vrain River. Lyons anticline (Ali, 1950; Fig. 6; Fig. 13, sec. CC') has steep dip slopes of Lyons sandstone, an axial valley of Fountain beds, and plunges south. A shallow syncline separates it from Rabbit Mountain anticline on the east. The Boulder unit (T. 1-3 N., R. 69-71 W., Figs. 6, 7, 13, sec. DD') extends soulh across foothills and adjacent Precambrian formations for 20 miles. An eastwest fault, south of Lyons, dislocates sedimentary formations and granite (Fig. 6). Coarse-grained, massive Silver Plume and Mount Olympus granites (Boos and Boos, 1934, p. 318; Lovering and Goddard, 1950, PI. II) in the north end of the unit are separated from Boulder Creek batholith by northeast-folded metasediments. Coarse, massive, gray Boulder Creek granite extends east to and below the foothills formations (Lovering and Goddard, 1950, PI. II, N. \; p. 16). The northwest faults, west of Lyons, are parallel with those at the south end of the Big Thompson-eH ichelon fault unit (Fig. 5). 2650 TECTONICS OF FRONT RANGE, COLORADO C. MAYNARD BOOS AND MARGARET FULLER BOOS Maxwell, Hoosier, Livingston, Rogers, and oilier breccia reefs (Lovcring and (joddard, 1950, p. 237, PI. II, PI. 23; Krill, 1948; MacCornack, 1944; Van Valk enburgh, 1953) in the steep northeast-dipping Laramide fault system trend N. 25°-5o° W. across the east Hank of the Front Range and end in the foothills hell. The reefs are brecciated, silicified, iron-stained zones a few feet to j mile wide Poorman and Copeland reefs strike nearly east-west across the northwest rrcf> Northwest-trending reefs of Precambrian genesis are partly occupied by pi^matile. Those that cross the foothills belt are of Laramide age or are older (I'rccambrian?) fractures reactivated. The faults that die out below the upper bed.-. «i the foothills belt are due to weak trends in the Precambrian. The asymmetrical, steeply folded, cross-faulted Coal Creek syiuline of I'ncambrian quartzite, mica schist, phyllite, and thick conglomerate beds (Adlt-r. 1930; Pinckney, 1953; Fig. 7; Fig. 13, sec. I)])') extends from Ralston shear zdiir northeast across the south end of the Moulder unit as far west as Central Ciiy (Moench el al., 1954)- Coal Creek thrust fault extends from Rogers breccia rcrf zone around the east end of the syncline, where Coal Creek quartzite overlies { mile of foothills formations. Drew Hill, Rogers, and Livingston breccia reefs divide the syncline into four segments, each faulted down toward the west (Jranitized quartzite and miginatite of Mount Morrison age border and probably underlie the structure (Fig. 7). Red Hill arch, south of Lyons, is steepest at the o u t e r foothills bell and c u r \ t s up over the Precambrian to a nearly Hat top. The formations swing southwest and dip steeply southeast at its south end. The half arch overlies Precambrian gneiss, schist, and granite near ihe southeast end of Longs Peak-St. Vraiu halholith. Fountain beds cover the hinge line west of the steepest part of the foothills monocline. The Tertiary porphyry sill in the Fountain beds of the Red Hill area and other minor intrusions at the northeast end of the Central Mineral Hch (Loveringand Goddard, 195°, PI- II, pp. 22, 25) cut foothills formations as youn^ as Niobrara south to Boulder and may be present in the Denver basin northeast of Ralston fault and Coal Creek syncline. The narrow, sharply folded and faulted anticline and syncline in Niobrara beds at Sixmile Creek plunge south. Small dikes of Tertiary? igneous rock intrude sedimentary formations of the Sixmile fold. The faulted, south-plunging Haystack anticline 2 miles east is partly outlined by Hygiene sandstone (Fig. 6). Concealed Tertiary intrusions may have silicified the quartzitic Dakota and Lyons sandstones in it. Steeply dipping foothills beds and faults of large throw occur where Boulder Creek batholilh extends east under the margin of the Denver basin. The downthrown inlier of east-dipping Fountain beds 4 miles northwest of Moulder is separated from the main foothills belt by granite. A strike thrust fault of small displacement 2 miles north of Boulder cuts vertical to overturned Dakota beds. The active block is on the west (Hunter, 1956, map). A down-faulted block west of Boulder is between Poorman dike and Hear j , 2651 Canyon fault. Only t h i n slices of some of the foothills formations crop out in the fault zone on the east face of Flagstaff Mountain, and much of the foothills section is missing. Poorman fault is a tear at the north end of the block. Outcrops of beds south of Poorman fault are east of corresponding beds at the north. The lowest (Fountain) is displaced farthest east. At the south end of the block, where the fault curves east in Bear Canyon, formations in the downthrown block are displaced west of those in the foothills on the south. The downthrown block appears to have rotated clockwise and down, the south corner underthrust west, the north corner lilted east (Woodbury, 1942). Traces of Ihe Maxwell and Hoosier breccia reefs extend into the foothills belt south of Boulder. The massive, east-dipping Fountain beds are duplicated by faulling on (Jreen Ridge. Hoosier reef curves east and ends as a hinge faull in ihe lienlon formation. Steeply dipping Fountain beds wesl of Hoosier faull sland higher on South Boulder Peak than the Precambrian on ihe west. A thin wedge of Fountain on ihe wesl or downthrown side of a branch fault, one mile south of Hoosier fault, lies unconformably on Coal Creek quartzile. Outcrops of the Milliken member of the Fox Hills sandstone and ihe coalbearing members of ihe lower Laramie formalion northeast of Marshall are repealed by curving northeast faults (Fig. 7). The faults and the folds between them extend about 20 miles northeasl in line with the projected axis of the Coal Creek syncline. Most of the faults are downthrown on the northwest (Johnson, 1935)The Loveland platform, a broad structural bench like Round Butle platform, extends from the Marshall fault belt on the southeast as far north as Milner Mountain (Figs. 3, 6, 7). The southeast-plunging noses of the offsel faull blocks that indent its northwest margin are major structures in the basement complex. The fault blocks exerl some control on the folds in sedimentary formations on the platform. The vertical Fountain outlier west of Livingston reef near Plainview is separated from ihe foothills belt by a tilled fault block of Precambrian quartzile that ends in a southeast-plunging faulted nose north of Coal Creek (Fig. 7). The Coal Creek thrust cuts foothills formations from Fountain lo Pierre. Fountain and Lyons beds in the ridge north of Ralston Butle (Fig. 13, EE') are sliced into narrow strips by branches of the Coal ("reek thrust. Overturned Dakota sandstone is above the Irace of ihe main faull in ihe easl face of ihe ridge. Underground exploralion for fire clay in ihe Dakota beds crossed the ihrusl inlo highly dislurbed black shale (Prommel, 1957, oral communication). The Denver Mountain I'arks-Golden fault unit (T. 2-8 S., R. 68-71 \V.; Figs. 7, 8; Fig. 13, sees. EE', FF') extends from Ralston fault and Coal Creek lo Jarre Canyon. Faulled and folded metamorphic rocks between Boulder Creek batholith and Pikes Peak batholilh surround small plulons of Boulder ('reek granile, Silver Plume-lype granile, and Mount Morrison granilic rock. At the north, the south- 2652 C. MAYNARD BOOS AND MARGARET FULLER BOOS east lobe of Boulder Creek batholith is bordered by closely folded phyllite, giu-i». schist, quartzites, and tourmaline-rich pegmatites. Drew Hill breccia reef c ri)>x-» the Precambrian northwest-southeast and offsets slightly the foothills-I'm a in brian foothills contact (Fig. 7). It extends far northwest of the area shouit in Figure 7. Several faults along Ralston Creek drainage converge into the l l r u m m Ranch "trunkline" fault that is uplhrown on the northeast. The fractures ml the Precambrian of an important uranium-bearing area. Gabelman (1948), Chapman (1948), Bozbag (1943), and Lovejoy (195:) tarefully mapped and described the closely folded, east-wesl-trending beds of phyllite, mica schist, injection gneiss, quartzite, and hornblende gneiss south uf Belcher Hill and the fault patterns that affect them. Crawford Gulch, Drew Hill, and Guy Gulch breccia reefs cross the area northwest-southeast. Broadly curving east-west faults, north and south of Clear Creek, antedate the reef-type s t r u i tures. The folded Precambrian of the Golden Gate Canyon and Clear Creek areas extends east under the foothills in the upper plate of Golden underthrust. At Mount Vernon Canyon (Fig. 7), the Precambrian-Fountain contact is faulted and slightly offset by east-west fractures that branch and disappear n o r t h w e s t . The folded metamorphic beds between Golden Gate Canyon and Bear Creek are interlayered with sheets of Mount Morrison granitic rock that merge at the southeast in Mount Morrison and Falcon Mountain (Fig. 8). Between Clear Creek and Bear Creek, and significantly west of the dee|«:>t part of the Denver basin, the Idaho Springs, Swandyke, and Mount Morrison beds that make up Apex anticline, Cody Park syncline, Shingle Gulch anticline, Genesee Mountain syncline, McCoy syncline, and other folds, strike east and southeast nearly at right angles to the foothills trend (Boos, M. I'., 1954; Fig. 8). Sawmill Gulch shatter zone, 3 miles west of the foothills, is north-south. It dislocates the crystalline rocks from Mount Vernon Canyon to South Turkey Creek. The mile-wide fault zone along Bear Creek consists of innumerable subparallel steeply dipping fractures that shatter quartzite, injection gneiss, and Mount Morrison granitic rock. The faults extend east under the foothills. The Bear Creek fractures may be tear faults that articulate at depth with the Golden underlhrust belt. The extensive gneiss, schist, and thin-bedded quartzite areas between Hear Creek and the north end of Pikes Peak batholilh (Fig. 8) are reinforced by the Indian Creek plutons (Boos and Aberdeen, 1940), Mount Morrison granitic material, and small stocks of Boulder Creek granite (Boos, M. F., 1954). The granites also bulwark the metamorphic formations of the Bear Gulch-South Platte Canyon area. The closely folded area of schist and gneiss south of Deer Creek, north of Pikes Peak batholith, and west of Roxborough Park is relatively weak and incompetent. The prominent northwest trends in it, within a mile of the foothills, turn northeast from the hinge and continue northeast beneath the foothills belt. Floyd Hill breccia reef, Phillipsburg reef, and minor parallel faults have little TEC TOXICS OF FRONT RANGE, COLORADO 2653 effect upon the foothills belt. Northwest-trending faults in the Bear Gulch area curve east and disappear beneath the formations of the Waterton area. South of Mount Vernon Canyon, adjacent to the foothills belt, the crystalline rocks in anticlinal horsts are comparable, in a small way, with Rampart Range (Fig. 8). Minor dislocations along north-northwest-trending faults offset the north-south to N. 25° \V. Precambrian-foothills contact west of Ralston Butte and northwest of Waterton. The Precambrian ridge in the core of Deer Creek anticline is 4 miles long and probably continuous, below the Fountain, with the fault wedge of crystalline rocks west of Ken Caryl Ranch (Fig. 8). The Precambrian structural grain is east-west to northeast and is a continuation of the trends in the hinge between the Denver basin and east flank of the Front Range. The foothills belt of the Denver Mountain Parks-Golden fault unit begins, at the north, in the Ralston Butte fault block that dips steeply east, plunges south, and ends in the narrow carinate Beartooth anticline (Hunter, 1947). Rogers breccia reef cuts the west side of Ralston Butte and ends at Ralston Reservoir. Fountain beds on the western, downthrown side of the reef are several hundred feet lower than on the crest. The north extent of the Golden thrust fault zone is difficult to trace. Its course, north of Golden, is clear as far as Ralston dike (Fig. 7). Basic igneous intrusions mask the trace in the Ralston dike area. It does not show in soft Pierre shales or below the gravels of pediments south of Coal Creek. The writers believe that the north end of the Golden underthrust belt extends to Coal Creek thrust. Both faults are low-angle, west-dipping underthrusts of large displacement. The trace of Coal Creek thrust, extended 2 miles south-southeast, meets the north end of Ralston dike and the projected Golden thrust. The "middle sandy zone" (Hygiene) of Pierre shale is parallel with the projected trace of Coal Creek thrust in the active (footwall) block, as the Niobrara is in the Ingleside thrust block (Fig. 4). Butler (1950) found that Pierre thickness appeared to decrease from 8,000 feet to 5,000 feet in the i\ miles north of Ralston Reservoir. He attributed the decrease to downthrow along a continuation of the Golden fault. West-dipping Laramie beds southeast of Ralston dike are offset J mile east of the trend of Laramie outcrops between Marshall and Leyden Ridge (Fig. 7). The beds are overturned, silicified and near a fault trace. The offset is an east-trending lobe of the Golden fault (Stewart, 1955). The Golden thrust cuts southwest across the foothills belt south of Ralston dike. The south tip of the Dakota hogback is bent sharply west by lateral drag (PI. 4, Fig. 5). North of Clear Creek (Fig. 7) the fault zone contains narrow, subparallel warped strips of Dakota, Morrison, and Lyons beds. South of Clear Creek a sliver of Niobrara is wedged between Fountain and upper Pierre beds. The fault trace west of Golden is close to the Precambrian-foothills contact. Nearly 11,000 feet of section is faulted down and under the Precambrian and only a few hundred feet of Fountain beds are exposed in the upper plate of the 2656 C. MAYNARD BOOS AND MARGARET FULLER BOOS thrust. Near-vertical to overturned Laramie and Fox Hills beds are parallel with the curving trace of the underthrust from Ralston dike to Morrison, hut the Fountain-Precambrian contact is nearly straight south-southeast (Figs. 7, 8). The west twist of the Beartooth fold axis sou.th of Ralston Butte, the westward drag of Dakota outcrop north of Golden (PI. .4, Fig. 5), and the west curve of Laramie outcrops indicate at least 5,000 feet of horizontal offset due to underthrusting. Vertical is twice horizontal displacement. The active block of Golden underthrust (Stommel, 1951; Stewart, 1955) on the plainsward side, moved down and west toward the Precambrian area because of east to west compressive forces active in Laramide time. South of Clear Creek, the Golden fault zone consists of sub-parallel westdipping thrusts in both Precambrian and foothills areas. North and south of Mount Vernon Canyon (Fig. 7) a thrust separates Precambrian gneiss, quaruite, and granite from upper Fountain beds. The thrust begins at Cherry Gulch, north of Red Rocks Park, at a tear fault, and extends about 3 miles north. West of Morrison, several small, nearly horizontal shears disturb Precambrian siliceous beds along Bear Creek Canyon (PI. 4, Fig. 8). Broken Precambrian rock overlies a narrow vertical block of basal Fountain beds near ihe south end of Red Rocks Park. The fractures in the Golden thrust zone affect Precambrian crystalline rocks 1-3 miles west of the foothills, between Mount Vernon Canyon and Deer Creek (Figs. 7, 8). Numerous low-angle shears were encountered in drilling Precambrian at the oil seep in Halfmile Gulch, 3,000 feet west of the foothills contact and north o( Golden Gate Canyon. The drill did not succeed, at total depth of 1,854 feet, in reaching sedimentary formations below the Golden thrust zone. Southeast <if Morrison, S. D. Johnson's Pallaoro well No. i (Fig. 8) drilled through four major fault zones. The stratigraphic section is inverted between the upper fault, 5,700 feet below the surface and the lowest one at a depth of about 8,100 feet. The formations below are in normal stratigraphic order. The reversed section indicates a sheared recumbent fold or nappe. The Golden fault trace is about a mile east of the Pallaoro well (Fig. 8). Dip of the fault plane computed from the upper fault in Pallaoro well to its emergence is about 50° SW. Multiple thrusts were also encountered in deep tests drilled on Deer Creek anticline. The sole of the Golden thrust is warped. It is nearly flat in soft shale. Seismic studies and shallow core drilling south of Golden found east inclination of the sole for a short distance (Stommel, 1951). The fault plane dips more steeply west in harder rocks. Nearly horizontal overturned Benton shale beds (PI. 4, Fig. 7) exposed where U. S. Highway 40 crosses the Dakota hogback west of Denver show extreme drag and overturn above the fault (Stewart, 1955). The trace of the Golden fault zone can not be located definitely within the wide expanse of Pierre shale southeast of Morrison. The upper plate probably overrides Laramie and Fox Hills beds east of Ken Caryl Ranch. Laramie and Fox Hills beds dip more steeply at the outer margin of the foothills belt than the formations stratigraphically below. Laramie dips are 50° 70° TECTONICS OF FRONT RANGE, COLORADO 2657 1C. as far south as Jarre Canyon (Fig. 8). Mining in Laramie coal has shown that at depths of a few hundred feet the dips decrease abruptly in the active (lower) plate of the underthrust. The structural profile of the foothills belt in the upper plate is convex east between Morrison and Deer Creek (PI. I, Fig. 2). Niobrara formations are vertical south of Deer Creek, a few hundred feet above a thrust in the Golden fault belt. Dakota beds at Deer Creek bend east from 25° dip at the top of the hogback to 55° at its eastern base. Northwest-trending faults offset basal Fountain beds and the Precambrian west of Ken Caryl Ranch and northwest of Waterton. The fault block of Precambrian rock that goes under Fountain beds reappears in the midst of Fountain beds north of Deer Creek and in Deer Creek Valley. Fountain beds dip away from the partly buried ridge to make Deer Creek anticline and a syncline between it and the mountainward Precambrian contact. The tilted block of Precambrian northwest of Waterton plunges south and the Precambrian does not reappear at the surface in South Platte Valley. Foothills formations of the Roxborough Park area trend S. 25° E. for 5 miles, then curve S. 40° E. for 2 miles. Dips are consistently 5o°-7o° E. from South Platte River to the south end of Roxborough Park (Fig. 8). North of Jarre Canyon, Pierre shale exposures narrow from i^ miles to less than J mile in the same manner that the exposures decrease north of Ralston dike (Kinnaman, 1954). The underthrust that crosses Niobrara, Benton, Dakota, and Morrison formations diagonally southeast of Roxborough Park curves southwest into the Precambrian at Jarre Canyon in the same way that Coal Creek underthrust terminates foothill outcrops at the north end of the Golden underlhrust zone. East of Jarre Canyon thrust, the formations parallel with its trace curve west toward the mountains as in Golden thrust south of Clear ('reek. A scrap of Niobrara limestone in a bend of the fault trace north of Jarre Canyon duplicates a structure in the Golden thrust belt south of Clear Creek. Pikes Peak granite overlies Niobrara beds that dip 30° W. at the mouth of Jarre Canyon. The evi1 dence suggests that the fault complex is the south end of the Golden thrust belt, although its trace can not be followed continuously in the soft Pierre shale. South of Jarre Canyon Precambrian crystalline rock overlies a narrow strip of Lyons formation faulted against Dawson arkose. In the same fault zone, imbricate slices of Pikes Peak granite lop Benton shale north of Jackson Creek. The Perry Park-Rampart Range unit (T. 11-13 s-. R - 67-68 W.; Figs. 8-10; Fig. 13, sec. GG') extends from the north end of the Pikes Peak batholith and Jarre Canyon along the Rampart Range and the foothills to Colorado Springs ramp and west to Devils Head fault. Rampart Range is a massive anticlinal horst of Pikes Peak granite faulted up on the east and west, and 4-8 miles wide. The sleep, nearly unbroken "rampart" of its eastern face from Perry Park to (ilen Eyrie gives the Range its name. Windy Point granite and inclusions of metamorphic rock in the top of the horst suggest local proximity to the roof of Pikes Peak batholith. The granite is at least 2,500 feet thick and may underlie the sedimentary formations east of the Range. 2660 C. MAYNARD BOOS AND MARGARET FULLER BOOS Northeast tear faults articulate with the upthrust zone at the east base of the Range. West of Perry Park, tear faults extend northeast into the crystalline complex beneath Perry Park. Long, curving tension faults cross the Range and the Precambrian in Manitou Park graben (Fig. 8). The numerous faults in Pikes Peak granite were identified in situ by slickensides along aligned sheared and broken zones. The discolorations and ironstained fractures grade into long, gouge-filled sireaks. Faults that cross pegmatile and aplite dikes and linear inclusions of metamorphic rock have measurable offsets. Many faults feather out or branch and contain pegmatites and quartz veins. In many places the signs of f a u l t i n g in granitic rocks are aligned with topographic trends such as sags, gaps, cols, and long straight valleys. Main fault systems show clearly on aerial photographs. The strong northeast to east-west faults that cross Rampart Range between Woodland Park and Manitou make the hinge between the level-crested Rampari Range, the southeast-tilted Colorado Springs ramp, and Manitou Park graben (Fig- 9). Perry Park and Round 1) title platforms are similar. Sedimentary formations strike north and northwest, dip gently northeast on the fault-bounded Perry Park platform, and cover an area nearly 8 miles long and 4 miles wide. Foothills beds dip steeply away from the Precambrian at the west onto the platform. Laramie and Fox Hills formations dip steeply northeast from the platform inio the Denver basin. An upthrust, north-dipping block between the granite of Rampart Range and the main Perry Park shelf consists of Pikes Peak granite overlain by thin older Paleozoic formations that dip north at low angles (Kohh, 1949; Aslani, 1950). The full sedimentary rock section is faulted out for n miles south of Perry Park. Dawson beds are upturned to vertical against Pikes Peak granite in the Rampart fault zone in places, but do not show lateral thrust or overturn. However, at Cathedral Rocks (Fig. 10) overturned foothills formations crop oui below the Rampart faultline scarp in three small, fault-bordered wedges between the Precambrian on the west and overlapping Dawson beds. A similar group of faulted wedges dips steeply away from the Precambrian b miles north of Colorado Springs. Curved branches from Rampari fault zone that trend southeast divide the foothills beds into three east-dipping blocks (Fig. 10). The Laramie and Foxhills formations of the area swing southeast away from the foothills belt toward the southern end of the Denver basin. The Colorado Springs ramp-Manilou Park graben unil (T. 9-13 S., R. 67 69 W.; Figs. 9, 10) constitutes a structurally depressed area of Precambrian anil sedimentary rocks 35 miles long and 4-8 miles wide. The thick section of Fountain and older Paleozoic beds in the unit indicates that the area has been structurally low for a long time. Colorado Springs ramp is a blunt, wedge-shaped, fault-bounded, inclined block of Precambrian rocks partly covered by sedimentary formations that dip i5°-3o° SE. from the south end of Rampart Range. The ramp is 6 miles wide, TECTONICS OF FRONT RANGE, COLORADO 2661 8 miles long, and is bounded on the southwest by the Ute Pass-Cheyenne Mountain fault zone. Tightly folded metasedimentary beds, down-faulted in Pikes Peak granite, extend southeast under the sedimentary beds of the ramp. The sedimentary rocks are bevelled and deeply trenched along Queens Canyon, Williams Canyon, and Fountain Creek gorge west of Manilou (Fowler, 1952). Minor folds and faults affect the foothills formations adjacent to Ute Pass fault (Morgan, 1951). The steep, faulted foothills belt of the Rampart fault zone extends through the Garden of the Gods to Fountain Creek along the east margin of the ramp. The southwest-trending foothills formations at the lower end of the ramp south of Fountain Creek bend steeply down into the Denver basin. Colorado Springs ramp is separated structurally from Manitou Park graben by northeast-trending horsls of Pikes Peak granite (Fig. 10). Manitou Park graben is 30 miles long and 4 miles wide. It lies between Ute Pass fault on the west and Devils Head fault zone on the east. Sawatch to Fountain beds that dip west at low angles partly cover the Pikes Peak granite of the south half of the graben. Fountain beds adjacent to Ute Pass fault are vertical but are not overturned or overriden from the west. The north half of the graben is entirely in Precambrian. The west-dipping Paleozoic formations in Manitou Park are linked to the sedimentary beds of Colorado Springs ramp by scattered outliers on the hinge north of Green Mountain Falls. The Cheyenne Mountain unil (T. 14, 15 S., R. 67 W.; Figs. 10, n) extends from the Engelman fault zone west of Manitou through Crystal Park and Cheyenne Mountain to the Hlue Mountain fault zone and Little Fountain Creek. Precambrian rocks crop out from the Ute Pass-Cheyenne Mountain fault zone on the east to St. Peters faults. The uplhrust Cheyenne Mountain block consists of Pikes Peak granite, irregular bodies of Windy Point granite, and plutons of Silver Plume-type granite. Several grabens of folded mica schist and injection gneiss on the southwest Hank are surrounded by Pikes Peak granite. Narrow faulted slices of ferruginous and siliceous gneiss, schist, and quartzite are caught in Cheyenne Mountain fault zone between Fountain beds of Colorado Springs ramp and shattered Pikes Peak granite above. Pikes Peak granite is overthrust onto lower 1'ierre shale and scraps of silicified and sheared Dakota and Niobrara beds appear from place to place in the thrust zone at the east base of Cheyenne Mountain (Roy, 1940, abst.). Finlay (1916) states that displacement southeast along the thrust is probably 4 miles. The horizontal displacement may not greatly exceed the thickness of the stratigraphic section overridden, which is about 7,000 feet. The sheared zone of Cheyenne Mountain thrust averages \ mile wide. Three sets of faults (Fig. n) cut the Precambrian south of Cheyenne Mountain: (i) north-south to northwest upthrusts in granite parallel with the main Cheyenne thrust; (2) the many-branched, north-south-trending St. Peter fault and (3) Engelman and other northeast tear faults nearly at right angles to Cheyenne Mountain upthrust. At Limekiln Creek (Fig. 10) overturned foothills beds from Lyons to Niobrara 2664 C. MAYNARD BOOS AND MARGARET FULLER BOOS crop out between thick talus deposits. South of the creek east-dipping Km IT Fountain beds crop out east of the thrust. The Cheyenne Mountain and Ute Pass fault belts are continuous. The nearly vertical faults that border Manitou Park on the west change to low-angle t h r u s i s at the base of Cheyenne Mountain and terminate at Ulue Mountain tear f a u l t . The foothills formations reappear in a wide belt from beneath overthrust granite south of Little Fountain Creek. The Muitnl Pitlsburg-Red Creek unit (T. 16-19 S., K. 66-67 VV.; Fig. n | i » the southeast end of Front Range. The Precambrian part is bounded by I lie Ulue M o u n t a i n and M o u n t a i n Dale fault zones and the eastern foothills licit. The foothills belt includes the wide outcrops of sedimentary formations south of Little Fountain Creek and all of Red Creek anticline (Finlay, 1916; Glockzin and Roy, 1945). The Blue Mountain slock of massive Pikes Peak granile, northeast-trending folded metamorphic rocks, and Mount Pittsburg stock of Silver Plume-type granite are the main tectonic features. The oldest faults trend nearly north-south parallel with Mountain Dale Ranch fractures. Other faults curve southwest along the strikes of fold axes and tabular bodies of Silver Plume granite. Many f a u l t s that strike 6o0-yo° W. extend below the foothills beds. Emerald Valley and Hluc Mountain thrust faults are continuations of prominent Laramide thrusis in ihc Cheyenne Mountain fault zone. The Blue Mountain stock reinforces the Precambrian area bounded by Emerald Valley and lilue Mountain thrusts at the north, and Mountain Dale Ranch and St. Peters faults on the east and west (Fig. n). A belt of faulted and tightly folded quartzite, gneiss, and schist trends north-south lo southwest between lilue Mountain stock and the foothills. Folds, axes, and major f a u l t s are parallel with the trend of the Precambrian-foothills contact, the strike of the foothills beds, and the south margin of Pikes I'eak batholith. The M o u n t Pittsburg fault block, uplhrust along M o u n t a i n Dale K a i u l i fault, is inclined southeast (Fig. n). Coarse-textured Silver Plume-type graniuadjacent to the foothills suggests that the stock is also present under the sedimentary beds in the north part of Red Creek arch. Probably the Precambrian rocks under the foothills beds are as complexly folded and faulted as the crystalline rocks exposed on the north. The trends of folds, thrusts, tear faults, and their convergence in the concealed Precambrian basement could be inferred by projecting the exposed structures for a reasonable distance beneath the sedimentary rock cover. Red Creek anticline, the east prong of the three-lobed south end of Front Range, extends south-southeast nearly 30 miles, and separates the Canon City and Denver basins. Niobrara outcrops on the broad low flanks are steepest on the west. The shallow White B u t t e syncline east of the arch and the small Wildhorse anticline are cut by northeast normal faults of small throw. TECTONICS OF FRONT RANGE, COLORADO 2665 Thin early Paleozoic beds on the east side of Mount Pittsburg fault block dip 15°-25° E. Outcrops trend parallel with the metamorphic beds in the adjacent Precambrian, and do not loop south around the broad nose of Red Creek anticline. The crest of Red Creek arch flattens where Fountain beds overlap early Paleozoics (Fig. n). The curving thrusts of Mountain Dale fault zone are uplhrust on the east. Sedimentary formations on the steep west side of Red Creek arch may be affected by buried faults as far south as Teller Reservoir (Fig. n). The Pikes Peak unit (T. 15-17 S., R. 68-69 W.; Figs, n, 12) contains the broad double arch between Mountain Dale and Cooper Mountain faults and the wide foothills bell on the north side of the Canon City basin from Red Creek arch lo Oil Creek graben. The unil contains ihe bell of Precambrian metasedimentary formalions in Ihe Beaver Creek and Phanlom Canyon areas, Pikes Peak batholith south of Pikes Peak, and the Eightmile Park area. Dislinclive sub-units are (i) Pikes Peak arch, (2) Cripple Creek arch, and (3) the Red CreekEighlmile Park bell. The Pikes Peak arch (Figs. 3, n) between Pikes Peak and the Adelaide fault zone (Lavinglon and Thompson, 1948) consists of massive, coarse-textured Pikes Peak granite and small plutons of Windy Point granite. It is faulted at the south against gneiss, schist, quartzile, and Mount Morrison granitic rock. The folded and strike-faulted belt of metamorphic rocks between Skagway fault and the foothills crosses Pikes Peak arch norlh of Eightmile Park and extends south beneath the sedimentary formations. From the Red Creek arch to Oil Creek graben the pattern of the crystalline basement complex probably controls the alignment of sedimentary formations in the north flank of the Canon City basin. Pikes Peak arch and Cripple Creek arch adjoin along Beaver Creek Valley, where the northeast tension faults on the west side of Pikes Peak arch intersect the northwest-trending faults on ihe east flank of Cripple ("reek arch (Fig. 12). C'ripple Creek arch branches southwest from Pikes Peak arch near Cripple Creek and curves southwest toward Grape ("reek Ridge on the west side of Ihe Canon Cily basin (Fig. 12). The Cooper Mountain fault zone divides il from Oil Creek graben. An axial section of the arch is exposed along Eightmile Creek from Pikes Peak granite near Victor across Mount Morrison granitic rock near Adelaide lo ihe folded and faulled belt of Phantom Canyon quartzite and the schist and gneiss of the Eightmile Park area. The belt of metamorphic and granitic rock adjacent to the foothills that begins at Emerald Valley fault south of Cheyenne Mountain (Fig. n) is downfaulted across Pikes Peak and Cripple Creek arches and probably extends under Oil Creek Valley into the Wilson Creek graben. Mica schist and gneiss beds normally at the base of the Idaho Springs series lie over Phantom Canyon quartzite at the south end of Phantom Canyon. The normal sequence is inverled by closely spaced uplhrusts that trend easl-west lo N. 70° E. The Adelaide and Skagway faulls zones also conlain thrusts from the south. The Red Creek under- ^#^ w fcSU^Tn lj> ^./M£. ;5 ^ ^- • Wztf-'.-' I / //» / V -«\** ^T | / / / 5 ' t > ^ i i \ ' V / ! , III \*S\ \\' \ ' 2668 C. MAYXARD BOOS A.\D MARGARET FULLER BOOS thrust described by Glockzin and Roy (1945) is a continuation of Adelaide fault zone. Manitou, Harding, and Fremont outliers on the low, inconspicuous scarps along the south margin of the Range are unconformable on the crystalline rocks of Pikes Peak unit. The pre-Fountain unconformity channels the older Paleozoic formations so deeply that Fountain beds, at one place or another, lie on all three. The steeply dipping beds in the narrow syncline west of Mountain Dale Ranch fault are crumpled against the Mount Pittsburg fault block. The broad and very shallow syncline of nearly flat Dakota sandstone on Table Mountain is south of the Red Creek underthrusl. Dakota and Niobrara beds dip south a little more steeply than the gradient of Heaver Creek and make a broad V across its valley. Several northwest tension faults cut the Dakota and Niobrara ouicrops west of .Beaver Creek. Each fault is downthrown 10-50 feet into the Canon City basin. Northwest of Eightmile Park, the foothills beds are repeated in three places by faults nearly parallel with the strike of the beds. Downthrow is several hundred feet on the north. Manitou and Harding formations extend high on Cripple Creek arch west of P h a n t o m Creek, and there are outliers of sedimentary formations on the Pikes Peak arch east of Adelaide.. The Oil Creek graben unit (T. 16-17 S., R. 70-71 \V.; Fig. 12) is a V-shaped re-entrant on the north side of the Canon City basin, west of Cripple Creek arch, t h a t heads about 4 miles southwest of Cripple Creek and extends 14 miles south, where it is 10 miles wide. The narrow north end of the graben is in Precambrian t h a t has a thin cover of Manitou, Harding, and Fremont beds that dip south at low angles. The graben drops from Cooper Mountain fault in narrow steps between northsouth faults. Nearly flat Paleozoic beds that cap some of the steps dip steeply west adjacent to the faults. A small re-entrant of complexly folded and faulted beds at Millsap Creek extends under a thrust fault in the east margin. The sedimentary formations south of Felch Creek dip steeply west off Cripple Creek arch. The graben ends at a semicircle of Niobrara outcrops around the north lobe of the Canon City basin (Fig. 12). Parallel crescentic ridges of jumbled Dakota sandstone south of Felch Creek, t h a t alternate with swales of disturbed Morrison beds, rest on undisturbed lower Morrison formation in Oil Creek Valley. The landslide is 2 miles wide, 200 feet or more below Dakota sandstone scarps on either side, and 100 feet or moie above Oil Creek. The older Paleozoic beds at the north end of the graben bend southeast in a drape fold over Twin M o u n t a i n fault in the west margin of the graben (Fig. 12). Within the graben, a downfaulted strip of Dakota and underlying formations in the topographic ridge between Garden Park and Shaw's Park, a narrow southeast-plunging syncline of Lykins and Morrison beds and the short east-plunging nose adjacent to Twin M o u n t a i n fault trend southeast. The Royal Gorge-\\"dson Creek unit (T. 17-19 S., R. 70-71 \V.; Fig. 12) begins TECTONICS OF FROST RA\GE, COLORADO 2669 i the south enc: °f Cripple Creek balholilh and extends across Wilson Creek and loyal Gorge Jf Arkansas River to Grape Creek Ridge. The Precambrian area onUins f o i r sub-units: (i) Rice Mountain horst, (2) Wilson Creek graben, ( ) l-'.oyal ^orge arch, and (4) Grape Creek ridge. PJU'I blocks in the Rice Mountain area consist of Boulder Creek and Silver Plum«" l yP e g ran il e - The east-west broadly curving faults probably offset the ]> re( .mbrian basement in the north end of Oil Creek graben. A'ilson Creek graben, between Rice Mountain and Royal Gorge arch has l'.ee parts: (i) an east-west, south-dipping belt of Fountain and Manitou beds nconformable on Pikes Peak granite and granilized quartzite, (2) a northeastmi ling, closely folded wedge of strike-faulted, black to gray, while-weathering quartiite beds interlayered with soft, silvery mica schist and phyllite, and (3) a belt of northeast-foliated Mount Morrison granitic rocks and granilized quartzite crossed by numerous dikes of Pikes Peak granite, north of Blue Mountain fault. Royal Gorge arch is a north-south, asymmetric block of Precambrian rock 6 miles wide and 10 miles long between Oil Creek graben and Webster ParkTwelvemile Park synclines (Fig. 12). The bordering foothills beds dip steeply east into the Canon City basin and gently west. From the plutons of Boulder Creek granite south of the Twelvemile fault zone, coarse-textured Pikes Peak granite extends west under the shallow basins of the Parks. Mount Morrison granitic rock, Boulder Creek gray granite, tightly folded northeast-trending mica schist, and innumerable pegmatites make up the east flank. A Morrison-Dakota outlier is perched on Pikes Peak granite north of Royal Gorge. Four groups of faults cross the Precambrian and offset the foothills formations on both flanks: (i) northwest-trending faults, (2) Twin Mountain fault, (3) eastwest (jorge faults, and (4) north-south tension fractures. Northwest faults of the east flank are partly filled w i t h Precambrian pegmatites. They were reactivated in Laramide time and displace older Paleozoic formations and Precambrian rocks. The wedges between are minor replicas of the major en echelon fault blocks of the Loveland area. Twin Mountain fault is the west boundary of Oil Creek graben along part of its course and extends across the arch and the north end of Webster Park. Gorge faults cut Royal Gorge arch east-west, offset Webster Park syncline, and extend into the Precambrian west of Mclntyre thrust (Fig. 12). The faults of the Gorge zone are sub-parallel with Grape Creek Ridge and with Wet Mountain thrust. The foothills belt narrows where dips steepen into the Canon City basin near Arkansas River. Harding sandstone and Fremont limestone thin southward and end by onlap against Grape Creek Ridge (Oborne, 1938; Acharya, 1949). The Fremont thins due to pre-Fountain erosion of the upper beds. Morrison beds rest on Precambrian at the crest of Grape Creek Ridge. A mile-wide re-entrant of sedimentary rocks extends west from the Canon City basin about 2 miles between Grape Creek Ridge and Wet Mountain fault. Dips are generally south and southeast from '•(/ ~ 2672 C. MAYXARD BOOS A.\D MARGARET FULLER BOOS Grape Creek Kidge but the foothills formations are overturned and overriden by Wet Mountain t h r u s t fault on the south margin of the re-entrant. The Webster I'ark-Tu'eh'emilc Park unit (T. 17-19 S., R. 71 W.; Fig. 12; Fig. 13, sec. KK'.I consists of two shallow, north-northwest faulted syndines of sedimentary formations on the west (lank of Royal Gorge arch and east of the Parkdale and Mclnlyre Ihrust-fault traces. Twelvemile Park (Sinha, 1951) is a complex, faulted structural depression about 5 miles long and 2 miles wide I hat contains about 1,500 feet of sedimentary rocks of Manitou to Niobrara age. .Much of the central part is covered by terrace gravels. A wide lobe of folded gneiss, schist, and pegmatilic rock of the Parkdale t h r u s t plate overlies the sedimentary formations in the south part of the syncline. The north part of the syncline is cm of! by the Twelvemile fault zone, and Twin M o u n t a i n fault terminates the trough at the south. A small sharp-crested anticline plunges northwest from tineast Hank. Webster Park is a complexly folded and faulted north-south syncline 6 miles long and 3 miles wide between Royal (Jorge arch and Mclntyre thrust. Its 1,800 feet of sedimentary formations (Sinha, 1951), from Morrison to Pierre in age, lie unconformably on Pikes Peak granite and dip gently west. The sharp folds in tinwest side of the syncline are distorted against Mclntyre thrust. The f a u l t north of (irape Creek Ridge offsets and l i f t s the south end of the syncline. S U M M A R Y AND CONCLUSIONS The eastern (lank and foothills of the Front Range rise steeply from the deepest part of tlie asymmetric Denver basin. They contain a narrow border of sedimentary formations 2-4 miles wide and a zone of adjacent Precamhrian rocks of about the same w i d t h . This is a "mobile belt" (T. C. Hiesland,personal communication) in which more t h a n half of the uplift of the east llank of the Front Range took place during the Laramide orogeny. It affords an unusual opportunity to study Precambrian tectonics and their relation t<i the Laramide and post-Laramide tectonics of the eastern foothills and tinDenver basin. The earliest tectonic pattern is Precambrian and has a dominant n o r t h w e s t trend (Lovering, 1932, p. 660), manifest in the intense folding and f a u l t i n g of tinmetamorphic formations that are the oldest crystalline rocks of the Range. The initial pattern was interrupted and disturbed by at least live generations of granitic rocks that bulwark the Range at both ends and reinforce the central area. The dominant northwest trend controlled the orientation of the live major segments of the eastern flank and foothills. Northeast-trending structures of the Central Mineral belt have Precambrian ancestry and were rejuvenated in Tertiary time. Northeast-trending local structures in the Precambrian of the mobile belt were a factor in determining the location of Laramide folding. There is no record of Paleozoic tectonics t h a t can be differentiated from those of the Precamhrian in the eastern flank of the Front Range. Unconformities bt- TKCTOXICS OF FRO\T RAXGE, COLORADO 2673 tween the formations of the foothills belt carry a record of broad regional uplifts and minor warpings. Typical structures of Laramide origin along the eastern flank and foothills belt are repeated in various combinations in the fifteen structural units along the eastern margin. They vary in complexity from a simple east-dipping monocline to north-south folds parallel with the mountain front, normal faults of great throw, broad step-like "platforms" bordered by drape folds, tilted fault-bordered en echelon fault blocks, extensive thrust faults, some of them multiple and overlapping, wedge-shaped grabens t h a t indent the m o u n t a i n front, and northeasttrending faults and folds that may be of post-Laramide age. The many structural types represent the varied reactions w i t h i n a t h i c k section of sedimentary rocks resting on a complex base of strongly deformed metamorphic and igneous rocks in response to compressive forces and differential uplift of Laramide mountain building. The record of east-to-west underthrusting shows t h a t the west part of the Denver basin was a part of the mobile belt and moved relatively down and under the border of the range. Thrust faults are present along a third or more of the eastern foothills belt and adjoining Precambrian. Underthrusts were developed in the lower part of the mobile belt and overthrusts in the upper "hinge zone" where upward relief of stress was easier. Most of the thrusts appear to have developed from shears w i t h o u t the formation of large recumbent folds, with the exception of the Soda Lake thrusts. The thrusts appear to steepen westward and w i t h depth. Where they underlie the mobile belt the mountains have been wedged up and overlie the margin of the Denver basin but it is not certain whether the wedge structure extends west beyond the mobile belt. Vertical displacement exceeds horizontal along the thrusts, except for local areas in soft formations In general, the thrust surfaces are steeper in hard rock than in soft. Forces of sufficient magnitude to produce the t h r u s t f a u l t s and to displace cubic miles of rock for distances of a mile or more were necessarily carried in large part by the relatively rigid Precambrian foundation of the mobile belt and adjoining areas. The major batholiths of Pikes Peak and Sherman granite at either end of the Range were anchors against stress. The deepest part of the Denver basin and the largest underthrusl fault, the Golden thrust, lie opposite the central part of the Range which contains the greatest proportion of folded and faulted schist and quartzile. The belt of metasedimentary rock that borders the south end of Pikes Peak balholith is a relatively weak structural zone that lies between the blunt southern termination of the Front Range and the Canon City basin. Detailed descriptions of the structure of the eastern flank and foothills of the Front Range are presented by the writers as a contribution to the study of the structure of the Front Range and the bordering basins. It is hoped that the information given here may stimulate further research into the interrelationships of Precambrian and basin structure in other Rocky Mountain areas. 2674 C. MAYNARD BOOS AM) MARGARET PULLER BOOS SELECTED REFERENCES (I)'ACHARYA, ASEIUTOSH, 1949, "Geology of (he Grape Creek Area, Fremont County, Colorado," unpublished thesis No. (161, Colorado School of Mines. -ADLER, JOSEPH, 1930, "Geologic Relations of ihe Coal Creek Quartzite in Colorado," unpublished thesis, Univ. of Chicago. (2) ALI, HAMZAII, 1950, "Geology of the Carter Lake Region, from Lyons to North of Hygiene," unpublished thesis No. 676, Colorado School of Mines. (3) ANDERSON, T. P., 1949, "Geology of the Turkey Creek-Strains Gulch Area, JelTerson County, Colorado," unpublished thesis No. 6 j j , ibid. (4) ASLANI, MoKAD-MALEK, I9SO, "The Geology of Southern Perry Park, Douglas County, Colorado," unpublished thesis No. 6X6, ibid. BAKER, A. A., DANE, C. H., AND REESIDE, J. B., JR., 1936, "Correlation of Geologic Formations between East-central Colorado, Central Wyoming and Southern Montana," U. S. Geol. Survey I'm/. Paper i/i .;. (5) BAKER, U. K., 1948, "Geology of the Foothills of the Front Range, between Masonville and Bellvue, Colorado," unpublished thesis, Univ. of Colorado. BALL, S. H., 1908, "Economic Geology of the Georgetown Quadrangle," V. S. Geol. Suney Prof. Paper 63. BASS, N. \V., 1956, oral communication. (6) Jloos, C. MAYNARU, 1956, unpublished maps and manuscript. -T I • .1 • > I-I-- "VO /.} (9) Boos, MARGARET FULLER, AND Iloos, C. MAYNARD, 1934, "Granites of the Front Range: the Log Cabin Batholith" (abst.), Proc. Geol. Soc. America, /yjj, p. 69. Boos, MARGARET FULLER, 1935, "Some Heavy Minerals of Front Range Granites," Jour. Geol., Vol. 43, pp. 1033-40. — — , 1951, "Distribution of Impure Marbles ant! Lime-Silicate Members of the Idaho Springs Formation West of Denver, Colorado" (abst.), Bull. Geol. Soc. America, Vol. 6;, p. 1533(10) -- — , 1954, "Genesis of Precambrian Granitic Pegmatites in the Denver Mountain Parks Area, Colorado," ibid., Vol. 65, pp. 115-41. (n) -- - — , 1955, Field Conference Guidebook, Rocky Mountain Assoc. Geol., PI. — - --- , 1956, "Distribution and Tectonics of the Mount Olympus Granite, Front Range, Colorado" (abst.), Bull. Geol. Soc. America, Vol. 67. —-— , 1957, "Distribution and Petrogenesis of Mount Morrison Granitic Hocks, Front Range, Colorado" (abst.), ibid., Vol. 68. (12) BOZBAG, H., 1943, "Precambrian Geology of an Area near the Mouth of Golden Gate Canyon, Jefferson County, Colorado," unpublished thesis No. 605, Colorado School of Mines. (13) Itoos, MARGARET FULLER, 1956, unpublished maps and manuscript. BRAINERU, A. E., BALDWIN, H. I,., JR., AND KEYTE, I. A., 1933, "Pre-Pennsylvanian Strati^ raphy of Front Range in Colorado," Bull. Amer. Assoc. Petrol. Geol., Vol. 17, pp. 375-06. (14) BROIN, T. I,,, 1952, "Geology of the Owl Canyon-Bellvue Area, Larimer County, Colorado," unpublished thesis, Univ. of Colorado. -- — , 1956, oral communication. | BROWN, R. \V., 1943, "Cretaceous Tertiary Boundary in the Denver Basin, Colorado," Hull Geol. Soc. America, Vol. 54, pp. 65-86. BURBANK, W. S., LOVERING, T. S., GODDARD, K. N., AND EcKEL, E. B., 1935, 6Y<>/<'£/£ .1/J^ of Colorado, U. S. Geol. Survey. (15) BUTLER, C. K., 1950, "Structure of the Post-Cambrian Formations in ihe Vicinity of Coal Creek, Colorado," unpublished thesis, Univ. of Colorado. BUTTERS, R, M., 1913, "Permian and Permo-Carboniferous of the Eastern Foothills of the Rocky Mountains in Colorado," Colorado Geol. Survey Bull. 5. CALLIER.D., 1948, "Upper Paleozoic Stratigraphy of the Masonville-Lyons Area, Colorado, "ibid (16) CHAPMAN, J. J., 1948, "Geology of the Eastern Clear Creek-Golden Gate Canyon Area," unpublished thesis No. 647, Colorado School of Mines. (17) CULLIGAN, L. B., 1948, "Geology of the Loveland Fold Area, Loveland, Colorado," nnf>ub lished thesis, Univ. of Colorado. DANK, C. H., AND PIERCE, W. G., 1936, "Dawson and Lara mi e Formations in Southeastern Part of Denver Basin, Colorado," Bull. Amer. Assoc. Petrol. Geol., Vol. 20, pp. 1308-28. * Numbers refer to Figure 2, showing locations of subject matter of indicated publications. TKCTOMCS OP PKO.\T RA.\GE, COLORADO 2675 DAVIS, KALPII, 1947, "Origin, Age and Correlation of the Ingleside Formation of North-Central Colorado," unpublished thesis, Univ. of Colorado. F.I.DRIUGE, G. II., 1896, in F.MUONS, S. F., CROSS, WHITMAN, AND ELIJRIDGK, G. II., "Geology of the Denver Basin in Colorado," U. S. Geol. Surrey Man. 27. FENNEMAN, N. M., 1905, "Geology of the Boulder District, Colorado," ibid., Bull. 265. (21) FINLAV, G. I., 1916, "Colorado Springs, Colorado," (/. S. Geol. Suney Geol. Alias folio 2113. (22) KMMONS, S. F., CKOSS, C. \V., AND ELURIDGE, G. H., 1896, "Geology of the Denver Ilasin," V. S. Geol. Survey M on. 27. (23) FOGARTY, C. F., 1952, "Subsurface Geology of the Denver Basin," unpublished thesis A'c. 755, Colorado School of Mines. (,23a)FOWLER, W. A., 1052, "Geology of the Manitou Park Area, Douglas and Teller Counties, Colorado," unpublished thesis, Univ. of Colorado. (24) FREDERICKSON, E. A., DE LAY, J. M., AND SAYLOR, \V. W., 1956, "Ralston Formation of Canon City Kmbayment, Colorado," Bull. Amer. Assoc. Petrol. Geol., Vol. 40, pp. 2120-48. (25) FULLER, MARGARET B. (M. F. Boos), 1924, "General Features of Precambrian Structure along Big Thompson River in Colorado," Jour. Geol., Vol. 32, pp. 49-63. (26) GABELMAN, J. W., 1948, "Geology of the Golden Gate-Van liihljer Creek Area, Jefferson County, Colorado," unpublished thesis No. <5jtf, Colorado School of Mines. GEORGE, R. I)., 1909, "Main Tungsten Area of Boulder County, Colorado," Colorado Geol. Surrey 1st Rept. GILBERT, G. K., 1896, "The Underground Water of the Arkansas Valley in Eastern Colorado," U. S. Geol. Surrey ijth Ann. Kepi., I't. Ilf, pp. 551-601. (27) GLOCKZIN, A. K., AND ROY, C. J., 1945, "Structure of the Red Creek Area, Fremont County, Colorado," Bull. Geol. Soc. America, Vol. 57, pp. 819-28. GODDARD, E. N., AND LOVERING, T. S., 1937, "Laramide Fault Pattern of the Front Range," Colorado School of Mines Magazine, Vol. 27, No. 7. HEATON, Ross L., 1933, "Ancestral Rockies and Mesozoic and Late Paleozoic Stratigraphy of Rocky Mountain Region," Bull. Amer. Assoc. Petrol. Geol., Vol. 17, pp. 100-68. , 1939, "Contribution to Jurassic Stratigraphy of Rocky Mountain Region," ibid., Vol. >3. PP1I53-77HORNER, W. P., 10.54, "The Fox Hills Laramie Contact in the Denver Basin," unpublished thesis, Univ. of Colorado. HUNTER, ZENA M., 1947, "Geologic Patterns in the Foothills of the Front Range, BoulderLyons Area, Colorado," ibid. (28) , 1956, "Geology of the Foothills of the Front Range in Northern Colorado," Kocky Mountain Assoc. Geol. (29) JOHNSON, J. F., 1935, "Geology of the Marshall District, Boulder County, Colorado," unpublished thesis, Univ. of Colorado. (29a)KiNNAMAN, R. L., 1954, "Geology of the Foothills West of Sedalia, Douglas County, Colorado," unpublished thesis, Univ. of Colorado. (30) KRILL, K. E., 1948, "Geology of Parts of the Maxwell and Hoosier Breccia Reefs, Boulder County, Colorado," ibid. LAVINGTON, C. S., AND THOMPSON, W. ()., 1948, "Guide to the Geology of Central Colorado," Quar. Colorado School Mines, p. 28. LAVINGTON, C. S., 1954, "Berthoud Field," Oil and Gas Fields of Colorado, Rocky Mountain Assoc. Geol. , 1956, "Structure of the Eastern Flank of the Front Range, Colorado," field Conference Guidebook, ibid. (31) I.EROY, L. VV., 1946, "Stratigraphy of the Golden Morrison Area, JelTerson County, Colorado," Quar. Colorado School Mines, Vol. 41, No. 2. LOVERING, T. S., 1929, "Geologic History of the Front Range, Colorado," Proc. Colorado Sci. Soc., Vol. 12, No. 4, pp. 59-111. , 1932, "Field Evidence to Distinguish Overlhrusling from Underthrusting," Jour. Geol., Vol. 40, pp. 651-63. , 1934, "Preliminary Map Showing Relations of Ore Deposits to Structure in Boulder County, Colorado," I'roc. Colorado Sci. Sue., Vol. 13, pp. 77-88. (32) , AND GOUDARD, E. N., 1950, "Geology and Ore Deposits of the Front Range, Colorado," U. S. Geol. Suney Prof. Paper 223. , AND JOHNSON, J. H., 1933, "Meaning of Unconformities in Stratigraphy of Central Colorado," Bull. Amer. Assoc. Petrol, Geol., Vol. 17, pp. 353-74. (32a)LovEj()Y, E. F. P., 1951, "Geology of the Lower Clear Creek-Mi. Zion Area, Jefferson County, Colorado," unpublished thesis No. 718, Colorado School of Mines. McCoY, A. W., Ill, 1953, "Tectonic History of Denver Basin," Bull. Amer. Assoc. Petrol. Geol., Vol. 37, pp. 1873-93. (33) MCLAUGHLIN, K. P., 1947, "Pennsylvania!! Stratigraphy of the Colorado Springs Quadrangle," Colorado, ibid.. Vol. 31, pp. 1936-81. MAIIER, J. C., 1950, "Detailed Sections of Pre-Pennsylvanian Rocks along the Front Range of Colorado," U. S. Geol. Surrey Cir. 68; Chart jy, Oil and Gas Inv. Ser. 2676 C. MAYNARD BOOS AND MARGARET FULLER BOOS MATIIKWS, li. D., 1900, "The Granitic Rocks of the I'ikes Peak Quadrangle," Jour. Geol., Vol. 8, pp. 214-40. MOKNCII, R. H., HARRISON, J. K., AND SIMS, 1'. K., 1954, "Precambrian Structures in the Vicinity of Idaho Springs, Front Range, Colorado" (ahst.), Bull. Geol. Sue. America, Vol. 65, pp. 1383-84. MOODY, J. I)., 1947, "Upper Montana Group, Golden Area, Jefferson County, Colorado," Bull. Amer. Assoc. Petrol. Geol., Vol. 31, pp. 1454-71. (34) MORGAN, G. B., 1951, "Geology of the Williams Canyon Area North of Manitou Springs, Kl I'aso County, Colorado," unpublished thesis No. 6yo, Colorado School of Mines. (35) MACCORNACK, RICHARD, 1944, "Geology and Structure along a Portion of the Northern Kml of Maxwell Reef, Boulder Count)', Colorado," unpublished thesis, Univ. of Colorado. OBOKNE, H. W., 1938, "Grape Creek Arch," ijlli Annual Field Conference Guidebook, Kansas Geol. Soc., pp. 12-13. ORIKL, STEPHEN, 1934, Field Conference Guidebook, Rock)' Mountain Assoc. Geol., p. 42. (36) PINCKNKY, I). M., 1953, "Structure in the Coal Creek Series, Coal (.'reek Canyon, Colorado," unpublished thesis, Univ. of Colorado. . I'ROMMEL, H. VV. C., 1957, oral communication. (37) QUAM, L. O., 1932, "Geology of Rabbit Mountain Area, Colorado," unpublished thesis, I'niv. of Colorado. (38) KEICIIERT, S. O., 1954, "Geology *J>f the Golden-Green Mountain Area, Jefferson Count), Colorado," Quar. Colorado School Mines, Vol. 49, pp. 6-31, 33-37. , 1956, "Post-Laramie Stratigraphic Correlations in the Denver Basin, Colorado," i Bull. Geol. Soc. America, Vol. 67, pp. 107-12. ' (39) ROBERTSON, L. B., 1950, "Geology of the Ingleside Area Northwest of Kort Collins, Colorado," unpublished thesis No. 68;, Colorado School of Mines. (40) ROBB, G. L., 1949, "Geology of Northern I'erry 1'ark, Douglas County, Colorado," unpub tished thesis No. 661, ibid. ROY, C. J., 1940, "Cheyenne Mountain Overlhrust" (abst.), Bull. Geol. Soc. America, Vol. 51, p. 1942. (41) RUE, K. E., 1949, "Geology of the Carter Lake Region Northwest of Berthoud, Colorado," unpublished thesis No. 66j, Colorado School of Mines. SHIELDS, R. L., 1948, "The Garden Park-Canon City Area," The Compass, Vol. 25, pp. 01-04. (42) SINIIA, B. N., 1951, "Geology of the Region around I'arkdale, Fremont County, Colorado," unpublished thesis No. 7oS, ibid. STEVENS, E. M., 1938, Guidebook, islh Ann. Field Conference, p. 44. STEWART/W. A., 1955, "Structure of the Foothills Area West of Denver, Colorado," FirlJ Conference Guidebook, Rock)' Mountain Assoc. Geol., pp. 25-30. STOMMEL, H. E., 1951, "Seismic Investigations of the Golden-Denver Area," unpublished thesis, Colorado School of Mines. THOMPSON, W. O., 1938, maps in /j//; Ann. Field Conference Guidebook, Kansas Geol. Soc. , 1949, "Lyons Sandstone of Colorado Front Range," Bull. Amer. Assoc. Petrol. 6V.'/.. Vol. 33, pp. 52-72. THURSTON, W. R., 1955, "Pegmatites of the Crystal Mountain District, Larimer C o u n i \ , Colorado," V. S. Geol. Survey Bull, ion, p. 5. (43) TOLLEFSON, O. 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M., 1953, "Refractory Clay Deposits of South-Central Colorado," U. S. Geul. Survey Bull. cjyj. (45) WAUISCIIMIDT, VV. A., 1939, "The Table Mountain Lavas and Associated Igneous Rocks near Golden, Colorado," Quar. Colorado School Mines, Vol. 34, No. 3. WAKNER, L. A., 1956, "Tectonics of the Front Range," Oil and Gas Fields of Colorado, Rock) Mountain Assoc. Geol. (46) WISE, R. A., 1952, "Eastern Front Range and Foothills Geology of Central Jefferson County, North-Central Colorado," unpublished thesis No. 741, Colorado School of Mines. WOODBURY, II. ()., 1942, "Structure of the Boulder Arch," unpublished thesis, Univ. of Colorado. BULLETIN OF THE AMERICAN ASSOCIATION OF PETROLEUM GEOLOGISTS VOL. 41. NO. 12 (DECEMBER, 1957). PP. 2677-2694. 7 FIGS. REFLECTION OF POSSIBLE DEEP STRUCTURES 1!V T R A V E R S E GROUP FACIES CHANGES IN WESTERN M I C H I G A N 1 RICHARD L. JODKY' Billings, Montana ABSTRACT Lilhologic studies of the Traverse group in the Michigan basin have indicated that a large area of western Michigan was isolated from the main sea during Traverse deposition. A West Michigan barrier and resulting West Michigan lagoon can be traced over a large area. The West Michigan barrier shows relation to a pre-Devonian pattern of folding, and this deep tectonic pattern is coincident with the regional gravity of the area. In the West Michigan area it appears possible to trace deep folding by lithologic studies of shallow beds where these folds are no longer apparent. The use of lithologic, fades, and sedimentation studies of shallow "beds combined with regional gravity studies can furnish clues to the tectonic fabric of deeper beds where no direct information is available. •v INTRODUCTION It is notable that members of the oil industry in the Michigan basin do not generally identify in the subsurface the various formations comprising the Traverse group. Oil production from formations in the Traverse group is merely identified as being from the "Traverse" or sometimes from the ''Alpena,." The latter designation is patently a miscorrelation which is finding wide usage. Many writers have subdivided (he Traverse group in the surface section. Possibly the most detailed subdivision of the group was made in Alpena County, in northeastern Michigan, by Warthin and Cooper (1943). Kelly and Smith (1947) described more recent work in an adjacent area, and Cohee (1947) extended these surface subdivisions into the subsurface. Cohee then extended Warthin and Cooper's subdivisions into the extreme northwestern and southwestern parts of lower Michigan, but did not extend them into western Michigan where major Traverse group production is developed. To the writer's knowledge no work has been published to show which specific formations in the Traverse group in western Michigan are prolific producers of oil. To extend these subdivisions into western Michigan and trace specific producing zones, a cross section was constructed by the writer beginning at the American Drilling Company's Packer No. i, Sec. 14, T. 20 N., R. 4 E., as described by Cohee, and continuing to Cook's Wright No. 4, Sec. g, T. 3 N., R. 13 W., in Allegan County. Another section was made joining the first section at Teater's Grewe No. i, Sec. 14, T. 14 N., R. 5 W., and continuing west to McCallum's Mollett No. i, Sec. 3, T. 13 N., R. 15 W., in Oceana County. The location of these sections is shown in Figure t. These sections demonstrate that the members of the Traverse group as 1 Manuscript received, April 22, 1957. Research geologist, Sun Oil Company. The writer thanks Dolores K. Colburg, research technician, Sun Oil Company, for redrafting the illustrations for publication. Corrections of the manuscript and suggestions for the improvement of the paper were made hy George D. Lindberg, Margaret Applegate Kitchen, and Robert J. Cordell, Sun Oil Company, and William A. Kelly, Michigan Stale University. This paper was initialed as a result of advanced graduate work at Michigan State University, East Lansing, Michigan. 2