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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.
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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
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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-
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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,"
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of Colorado," U. S. Geol. Surrey Cir. 68; Chart jy, Oil and Gas Inv. Ser.
2676
C. MAYNARD BOOS AND MARGARET FULLER BOOS
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of Maxwell Reef, Boulder Count)', Colorado," unpublished thesis, Univ. of Colorado.
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'
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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