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ROCKY MOUNTAIN FORELAND
BASINS AND UPLIFTS
EDITED BY
JAMES D. LOWELL
ASSOCIATE EDITOR
ROBBIE GRIES
Published by
Rocky Mountain Association of Geologists
Denver, Colorado
1983
© Copyright by the Rocky Mountain Association of Geologists.
ARTHUR LAnS LIlJRARt
COLOl\ADO SCHOOL 01 MINES
GOLDEN. COLORADO 80401
ROCKY MOUNTAIN ASSOCIATION OF GEOLOGISTS -
1983
219
LARAMIDE AND NEOGENE STRUCTURE OF THE
NORTHERN SANGRE DE CRISTO RANGE,
SOUTH-CENTRAL COLORADO
DAVIDA. LI NDSEY1
BRUCE R. JOHNSON1
PAM. ANDRIESSEN2
ABSTRACT
The Sangre de Cristo Range from Blanca Peak northward to the Arkansas River in Colorado is composed mostly
of Precambrian crystalline rocks and upper Paleozoic clastic sedimentary rocks. These rocks have been folded and
faulted by Laramide compressional forces from Late Cretaceous to Eocene time. Laramide structures are large arcu·
ate thrust plates that intersect and overlap one another to form a northwesterly trending belt that extends across the
mountains from Huerfano Park to Valley View Hot Springs. All of the thrust plates within the range are bounded by
west·dipping faults, some of which extend into the basement of Precambrian crystalline rocks. Along the east side of
the range, the Alvarado fault is interpreted tentatively as an east·dipping thrust bringing Precambrian crystalline
rocks west over Paleozoic rocks. Thrust plates of Paleozoic rocks, and possibly those of Precambrian rocks, are in·
ternally folded; the folds tighten and decrease in amplitude toward the leading edge of the plate. Thrust faults are
dominantly high· to medium·angle reverse faults along the leading edge of the thrust plates but flatten at depth. Total
shortening within the range is approximately 8 km at the latitude of Westcliffe and about 14 km farther south near the
latitude of the Great Sand Dunes.
During Neogene time the Sangre de Cristo Range was uplifted and the adjoining San Luis and Wet Mountain
valleys were down·dropped by extensional rift faulting. Rifting followed late Oligocene intrusion of stocks, sills, and
dikes of mafic to felsic igneous rock into the Precambrian and Paleozoic sedimentary rocks of the range. The horst
of the Sangre de Cristo Range probably began to rise in late Oligocene time, rose rapidly in early Miocene time, and
rose again in late Miocene to Quaternary time. Flows of mafic lava were erupted from faults along the southwest
side of the Wet Mountain Valley and in the San Luis Valley. Zones of Laramide thrusting along the west and east
margins of the range were reactivated to form the Sangre de Cristo and Alvarado normal faults, respectively, so that
the floor of the Neogene sedimentary and volcanic fiff of the San Luis Valley is 2,()()()'7,OOO m below the top of the range
and the floor of the Wet Mountain Valley fill is about 2,000 m below the range. As it was uplifted, the entire range may
have been tilted gently eastward. Rifting is still in progress in the San Luis Valley, west of the range, but may have
ceased in the Wet Mountain Valley.
INTRODUCTION
During the summers of 1981 and 1982, most of the Sangre
de Cristo Range from Blanca Peak to the Arkansas River (Fig.
1A) was remapped as part of the U.S. Geological Survey's in·
vestigation of the northern Sangre de Cristo Range. Prior to
that time, one of us (Lindsey) examined the Pennsylvanian and
Permian sedimentary rocks of the area, in part to discover strati·
graphic markers that could be used in delineating the struc·
ture of the area. A zone of imbricate thrust plates, including
thrusts known or previously suspected but never before mapped
fully, was identified in the central part of the region studied.
Many small igneous intrusions were mapped, and together with
structural and geochronologic data, serve to delineate and date
the Neogene uplift of the present range and downdropping
of the adjoining San Luis and Wet Mountain valleys. Much of
the data still remain to be analyzed, so this report is prelimi·
nary and the conclusions are subject to revision. The area
, u.S. Geological Survey, Box 25046. Federal Center, Denver CO 80225.
• Z.W.O., Laboratorium voor Isotopen-Geologie, De Boelelaan 1085, 1081 Am·
sterdam, The Netherlands.
The Netherlands Organization for Advancement 01 Pure Research supported
geochronologic work for this study by providing PAM. Andriessen a NatoSciencE' fellowship for his stay in the United States. We acknowledge the
assistance of S.J. Ashe, C.A. Brannon, A.M. Bruce, A.F. Clark, B.L. DeAngelis,
A.J. Flores, K. Hafner, G. Meyer, A.F. Paschal, A.A. SChaefer and D.M. Walz
in the field. E. Rivera prepared mineral separates; CW. Naeser provided the
Zircon fission track age al'l(! commented on interpretation of fission track ages;
A.G. Tysdal, A.B. Taylor, and O. Tweto commented on the manuscript.
described here is the central part of the mapped area, from the
Great Sand Dunes National Monument on the south to Valley
View Hot Springs on the north.
The Sangre de Cristo Range first was believed to be a simple
anticline (Endlich, 1874) modified only slightly by faulting (8agg,
1008); the faulted nature of the range was recognized only later
(Johnson, 1929). The zone of folding and faulting herein described was proposed by Burbank and Goddard (1937, p. 949951) who projected an extensive zone of folds and thrusts
through the Sangre de Cristo Range between those known
near Kerber Creek northwest of the range and those in Huer·
fano Park to the southeast (Fig. 1B). Burbank and Goddard
(1937) believed that this zone was marginal to the Precambrian
massifs of Blanca Peak and the Culebra Range, and was formed
by uplift and overriding of the Precambrian massifs onto adja·
cent sedimentary rocks of Paleozoic and younger age. Evidence
for marginal thrusting was described from the Kerber Creek
area and Huerfano Park;. ine region between was interpreted
as a belt of intensely folded strata with subordinate thrusted
structure. Thrusting and folding in Huerfano Park has been
dated as Late Cretaceous to Eocene, based on 1· the presence
of unconformities separating strongly folded rocks of Late Cretaceous age from gently folded rocks of Paleocene and young·
er age, and 2· the presence of red detritus derived from Paleozoic
rocks in strata of Eocene age (Burbank and Goddard, 1937,
p.959; Tweto, 1975, p.31).
Much of the northern Sangre de Cristo Range was mapped
during the 1950's and 1960's by a large number of workers
(Clement, 1952; Litsey, 1958; Burford, 1960; Koch, 1963; Karig,
RtX:I<Y MOUNTAIN AS5CX:IA TION OF GEOt.OOISTS
,I
I
"i.
220
DAVID A. LINDSEY, BRUCE R. JOHNSON, AND PAM. ANDRIESSEN
\
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EXPLANATION
Figure 1.
Index maps showing eA) northern Sangre de Cristo Range
and nearby geographic features in souttrcentral Colorado,
and (B) geologic features of same area (modified from
Tweto, 1979a~ Both laramide and Neogene displacement
shown on Alvarado fault.
1964a; Volckmann, 1965; Munger, 1965; Nolting, 1970). DeVoto
and Peel (1972) and Scott and Taylor (1974) compiled and extended some of this mapping. The maps show some of the
thrusted structure of the area and confirm Burbank and Goddard's (1937) projection of a zone of folding and thrusting
through the central part of the range. Large thrust sheets also
were mapped north of Blanca Peak (Burford, 1960). Much of
the thrusting in the central part of the range was attributed
to gravity sliding off adjacent uplifts (Karig, 1964a, p. 114-119;
1964b). The Spread Eagle Peak thrust was mapped and westward thrusting of the Precambrian Wet Mountain block onto
the Paleozoic rocks of the Sangre de Cristo Range was proposed (Munger, 1965).
We interpret the Laramide folding and thrusting of the northem Sangre de Cristo Range to be the result of regional compression of the Rocky Mountain foreland, but we do not exclude
the possible role of block uplifts in generating compressional
structures along their flanks. The mechanism of Laramide deformation has remained controversial to this day: folding and
thrusting by compression, vertical uplift, strike-slip movement,
and combinations of these all have been proposed. Compression was favored early in the century (Chamberlin, 1919); recent
evidence supports compression as the cause of Rocky Mountain foreland deformation (Smithson and others, 1979; Gries,
1981; Brown, 1982), and compression by a shear couple has
been demonstrated by laboratory models of deformation (Sales,
1968). Vertical uplift has been advocated by many workers in
recent years (Prucha and others, 1965; Matthews, 1976). Burbank and Goddard (1937, p. 963-965, 971) appear to have attrib-
I
T,
""v
Q.Lat.,nry.w:! Upper Ten...,. sedtmeOtl
Tert..,..,
IgOeOUI
rocks
~
TM:
I..o¥wer T...,......
:~~ ~.
Pll6eozOK' ..omern:~ rods
: ... :
Pr.carnbrwr.
Memzotc sediment.,., toea,
cryst_"" rock.
Contact
~
Ma,ot thrust t8U1t
-L
MaJO' normal teull
uted the thrusting of the Sangre de Cristo region to lateral compression generated by uplift of adjacent Precambrian terranes,
and Tweto (1975, p. 37) advocated Laramide uplift of late Paleozoic highlands as a mechanism for deformation of sedimentary
rocks depoSited in basins adjacent to the highlands. Recently, some Laramide structures of Eocene age in southern Colorado and northern New Mexico have been interpreted as strike
Slip in origin (Chapin and Cather, 1981).
The opening of the Rio Grande rift in Colorado in Neogene
time interrupted the regional pattern of Laramide uplifts (Tweto,
1979b). The rift is a complex system of north-south horsts and
grabens that extend from north-central Colorado to Mexico.
The Sangre de Cristo Range, with its Laramide structure essentially intact, stands as a horst as much as 1,600 m above the
San Luis Valley to the west and the Wet Mountain Valley to
the east.
GEOLOGY
The Neogene horst of the Sangre de Cristo Range (Figs. 1B,
2, and 3) is separated from the Neogene grabens of the San
Luis and Wet Mountain valleys by two large normal faults: the
Sangre de Cristo fault on the west and the Alvarado fault on
the east. Both faults are interpreted here to follow zones of reactivated thrust faults of Laramide age. The horst itself contains numerous thrust plates of Precambrian crystalline rocks
and Paleozoic sedimentary rocks. The plates can be divided
into 1- those that consist mostly of Precambrian rocks and a
thin cover of lower Paleozoic rocks, which define the margins
of the adjacent San Luis and Wet Mountain highlands of LaraF¥:JCKY MOUNTAlN ASSOCIA TION OF GEQ.OOISTS
"
LARAMIDE AND NEOGENE STRUCTURE mide time, and 2- those that consist of upper Paleozoic sedimentary rocks and a veneer of Mesozoic sedimentary rocks
that were compressed between the two uplifts (Fig. 2). An autochthonous terrane of upper Paleozoic sedimentary rocks lies
between the thrusted terranes.
Rocks in thrust sheets derived from the Laramide highlands
are best exposed along the west side of the range, but they occupy almost the whole of the range from Medano Pass southward. Along the west side of the range, the Precambrian rocks
consist of gneiss belonging to a 1,700-1,800 my age group,
granodiorite of a 1,700 my age group, and quartz monzonite
of a 1,400 my age group (Fig. 2). Gneiss also is exposed at
scattered localities east of the Alvarado fault, along the east
side of the range. About 100 m of lower Paleozoic sedimentary
rocks rest unconformably on Precambrian rocks along the west
side of the range: the contact dips steeply and is faulted in
places, but commonly the main horizon of faulting is about 30100 m above the base of Paleozoic rocks. In the northern part
of the study area, the Ordovician Manitou Limestone overlies
the Precambrian, but southward, near the village of Crestone,
the Manitou is missing and the Ordovician Harding Sandstone
(locally a quartzite) onlaps Precambrian quartz monzonite.
Above the Harding, the Ordovician Fremont Dolomite, the Mississippian(?) to Devonian Chaffee Group (sandstone, shale, and
dolomite) and the Mississippian Leadville Limestone have been
tentatively identified at scattered localities along the west side
of the range (Koch, 1963: Clement, 1952). Some of the Paleozoic rocks have been metamorphosed (Karig, 1964b). The basal
part of the Pennsylvanian and Permian Crestone Conglomerate
phase of Melton (1925) or Crestone Conglomerate Member of
Bolyard (1959) unconformably overlies Precambrian rocks locally
along the eastern margins of some of the thrust plates.
Thrust plates in the interior of the range are composed mostly
of parts of a 3,700-meter-thick section of sedimentary rocks of
late Paleozoic age (Fig. 2). The upper Paleozoic strata were deposited by coalescing alluvial fans and fan deltas along the
northeast flank of the late Paleozoic San Luis highland; the
strata contain abrupt facies changes and onlap the highland to
the southwest. The Pennsylvanian Minturn Formation consists of 2,000 m of sandstone and lesser conglomerate, shale
and limestone, and is faulted at the base. The Minturn is absent
owing to nondeposition in most of the southwestern part of
the area. The Pennsylvanian and Permian Sangre de Cristo
Formation, composed of about 1,700 m of red sandstone and
conglomerate, is faulted and eroded at the top. The Sangre de
Cristo Formation was mapped undivided in the autochthon
and the eastern thrust plates, but it has been divided into a
lower sandstone member and an upper (Crestone Conglomerate phase of Melton, 1925; Crestone Conglomerate Member of
Bolyard, 1959) member in the western half of the range; to the
southwest, the entire Sangre de Cristo Formation is represented
by the Crestone Conglomerate Member. The conglomerate
overlies and interfingers with sandstone of the lower member
in the northern part of the study area, but rests unconformably
on progressively older strata to the southwest, where it finally
rests unconformably on Precambrian crystalline rocks of the
late Paleozoic San Luis highland. A thin (100 m) section of
Jurassic Entrada Sandstone and Morrison Formation overlies
upper Paleozoic strata at Loco Hill southeast of the range (Fig.
2). The upper Paleozoic sedimentary rocks comprise the fill of
the late Paleozoic central Colorado trough that, together with a
veneer of Mesozoic rocks, was compressed between the San
Luis and Wet Mountain highlands during the Laramide orogeny.
Scattered intrusive and extrusive igneous rocks in the study
area postdate Laramide structural features. Igneous rocks in-
SANGRE DE CRISTO RANGE
221
clude the Oligocene Rito Alto stock, composed of tonalite and
granite (Scott and Taylor, 1974), the Oligocene and Miocene
volcanic rocks at Goat Creek (Scott and Taylor, 1975), and flows
of olivine basalt of Miocene(?) age in the foothills east of
Medano Pass (Burford, 1960) (Fig. 2). Numerous dikes and sills
of Oligocene and Miocene age, not shown on Figure 2, have
been mapped throughout the range. The oldest Tertiary intrusive rocks of the range, as well as many igneous intrusions elsewhere in the region, probably were emplaced near the beginning
of Rio Grande rifting (Tweto, 1979b).
The San Luis and Wet Mountain valleys are rift valleys that
had begun to sink by late Oligocene time (Lipman and Mehnert,
1975). The northern part of the San Luis Valley contains about
500 m of Miocene and younger sediments and volcanic rocks
(Davis and Stoughton, 1979); farther south, in the vicinity of the
Great Sand Dunes, as much as 6,000 m of sediments and volcanic rocks fill the basin (Davis and Keller, 1978). The Wet Moun·
tain Valley contains as much as 400 m of Miocene and younger
sediments (Scott and Taylor, 1975, p. 12).
LARAMIDE STRUCTURE
As many as eight thrust plates have been identified in the
central part of the northern Sangre de Cristo Range (Fig. 4).
Four plates, the Wild Cherry, Crestone, Sand Creek, and Deadman Creek, are composed entirely of Precambrian and lower
Paleozoic rocks. Interpretation of the Deadman Creek fault as
a thrust is tentative, being based mainly on its similarity and
proximity to the Crestone thrust; other thrusts in Precambrian
rocks may have escaped detection because of the absence of
mappable rock units. Three thrust plates, the Spread Eagle
Peak, Marble Mountain, and Huckleberry Mountain, are composed mostly of upper Paleozoic rocks. The Loco Hill plate
consists of upper Paleozoic and MesozoiC rocks; it passes
northward into an autochthonous terrane of upper Paleozoic
rocks. Each thrust plate is bounded on the east by a large
arcuate fault ranging in dip from vertical to about 30° west and
interpreted as a thrust (Figs. 5A and B). The faults interpreted
as thrusts are concave up and concave southwest in map view.
In detail, the thrusts commonly branch upward and laterally,
showing a bifurcating pattern in map view. The attitude of individual fault branches ranges from vertical to horizontal. The
main thrusts truncate sedimentary contacts, folds (Figs. 5B and
C), and even whole thrust plates in map view; the latter relationship is interpreted to represent the overriding of one thrust plate
byanother.
Most of the thrust plates have been folded and some have
been faulted. Folding is evident in plates composed of sedimentary strata, but may have affected Precambrian rocks as
well. The western parts of three thrust plates, the Spread Eagle
Peak, Marble Mountain, and Huckleberry Mountain, are composed of Sangre de Cristo Formation folded into large synclines (Fig. 50 - the Gibson Peak syncline in the Spread Eagle
Peak plate); the synclines in all three plates may have been
connected prior to thrusting because their axial zones are in
the Crestone Conglomerate Member. The Spread Eagle Peak
thrust plate contains three major east-facing folds that tighten
and decrease in amplitude eastward toward the leading edge
of the thrust. The folds are truncated by the thrust. The Marble Mountain thrust divides northward into branches that show
left-lateral separation of the base of the Sangre de Cristo Formation. Such horizontal separation of sedimentary contacts
reflects tearing of the Marble Mountain thrust plate along its
north flank. The axial traces of folds in the autochthon have
been overridden by the Spread Eagle Peak thrust plate.
ROCKY MOUNTAIN ASSOCIA TION OF GEOLOGISTS
DAVID A. LINDSEY, BRUCE R. JOHNSON, AND P.A.M. ANDRIESSEN
222
I'
EXPLANATION
QuATERNARV AND TERTIARY
B
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s-dlmen1l
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VolcanIC roeks
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intrUSIve locks
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V06cM'IIC rodtl
JURASSIC
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PERMIAN ANO P£NNS'fl"A.NIAN SANGRE DE CRISTO FM
~o c.. ~_
c'
~
Congoomoo'"
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LUIS
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Figure 2.
________
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________
10 ICtLOMETERS
~o
Simplified geologic map of part of the northern Sangre de Cristo Range, from the Great Sand Dunes to Valley View Hot Springs.
Place names used In text GC, Goat Creek; LH, Loco Hili; M, Marble Mountain; MP, Medano Pass; V, Valley View Hot Springs;
VL, Venable Lakes.
ROCI<Y MOUNTAIN ASSOCIATION OF GEOtOOISTS
LARAMIDE AND NEOGENE STRUCTURE -
SANGRE DE CRISTO RANGE
223
A
A'
METERS ABOVE SEA LEVEL
4200
SPREAD EAGLE
SYNCLlNE\
SAN LUIS
VALLEY
3800
ALVARADO
FAULT
3400
3000
OT,
26001-__------------~
2200
Xgo
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S'
4200+
.sPREAD EAGLE SYNCLINE
3800
3400
SANGRE DE CRISTO
FAULT
3000
2600
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/~
3400 : SANGRE DE CRISTO'
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Figure 3.
Cross sections through the northern part of the Sangre de Cristo Range. Line of sections shown on figure 2; symbols same
as figure 2.
In general, strike-slip movement may be present locally on the
flanks of thrust plates, but does not seem to have been a major
style of faulting within the part of the range investigated. Three
lithofacies in conglomerates of the Sangre de Cristo Formation
can be distinguished by distinctive clasts of red syenite, granodiorite of the 1,700 my age group, and gneiss of the 1,700-1,800
my age group. The conglomerates, deposited by northeast·
flowing paleostreams, have been telescoped by thrusting but
have not been offset laterally across the Spread Eagle Peak
and Sand Creek thrusts. Facies changes in conglomerate east
of the Sand Creek thrust are aligned closely with differences in
Precambrian source rocks west of the fault.
The largest, and one of the most intact, thrust plates of sedi·
mentary rocks is bounded on the east by the arcuate Spread
Eagle Peak fault (Figs. 2, 4, and 5B); it serves as a model for the
thrust plates. The thrust fault itself has been traced for almost
35 km and partly circumscribes a thrust plate more than 10 km
wide. Numerous stratigraphic markers in upper Paleozoic
strata provide good control on the folded internal structure. Bed·
ding-plane faults reported previously (Karig, 1964a, p, 65-68) were
not substantiated; mapping of marker beds in the lower part of
the Sangre de Cristo and upper part of the Minturn formations
revealed a normal stratigraphic succession,
Detailed cross sections of the Wild Cherry Creek thrust, the
northern part of the Spread Eagle Peak thrust, the folded autochthon, and the Alvarado fault were prepared to further analyze
the structure of the range (Fig. 3). Guidelines used in preparing
the sections emphasize fundamental aspects of the structure
of the thrust plates. 1· Large open folds such as the Gibson
Peak syncline (Fig. 5D) are essentially concentriC, except where
sharply overturned, but the smaller, tighter folds near the lead·
ing edge of the thrust plates must have been deformed by de·
collement or faulting. There is not enough room to accommo-
SANGRE DE CRISTO HORST
WET MOUNTAIN VAllEY GRABEN
J('
SAN lUIS VAllEY GRABEN
n'
"
10
30
I
KILOMETERS
Figure 4.
Map showing Laramide and Neogene tectonic features
of the Sangre de Cristo Range from the Great Sand Dunes
to Valley View Hot Sprtngs. Area shown same as figure 2;
LH. Loco Hill; M. Marble Mountain; V. Valley View Hot
Springs; VL. Venable Lakes.
ROCKY MOUNTAIN ASSOCIATION OF GEOLOGISTS
DAVID A. LINDSEY, BRUCE R. JOHNSON, AND PAM. ANDRIESSEN
224
"--
..
FrXKY MOUNTAlN ASSOCIA T10N OF GEOLOGISTS
LARAMIDE AND NEOGENE STRUCTURE - SANGRE DE CRISTO RANGE
date tightly folded strata in the deep axial portions of anticlines
and the shallow axial portions of synclines. Thus anticlines
must pass down into faults or decollements, and synclines must
pass up into the same structures (Dahlstrom, 1969a). The occurrence of marker beds from west to east in the range is not
sufficient to construct truly balanced cross sections (Dahlstrom,
1969b) but the law of conservation of volume has been observed as much as possible, and is accommodated on the sec·
tions by showing tectonic thickening of beds and faulting in
the axial zones of tight folds. 2- Structures exposed at the sur·
face have been extrapolated into the subsurface by observing
the same structures at lower topographic and structural levels
along strike; this method is essentially the same as that of
down·structure viewing (Mackin, 1950). Some structures can
be followed along strike from an open fold to a tight fold to a
fault in the upper axial zone of a syncline or the lower axial
zone of an anticline. The phenomenon is best observed in folds
of the autochthonous sedimentary rocks southeast of the
Spread Eagle Peak thrust, and also along the northern axial
trace of the Cotton Lake anticline, within the Spread Eagle Peak
thrust plate (Fig. 2). These structures demonstrate conservation
of volume in the axial zones of folds. The principle of downstructure viewing was applied also to predict the subsurface dip
of faults. The southem part of the Wild Cherry Creek fault, seen
to dip steeply west at the surface along the line of section CoG'
(Fig. 3), flattens to about 30° west dip 1·2 km south and 1,000 m
below the line of section. 3- The final consideration in projecting thrust faults to depth is the most likely location of decollement horizons. Reverse faults that flatten locally into thrusts
have been mapped over most of the study area near the contact
between lower Paleozoic carbonate rocks and shales and upper
Paleozoic sandstones and shales; this horizon contains abundant thin-bedded shaley rocks that could serve as a convenient
~ollement. The leading edges of the Spread Eagle Peak and
Marble Mountain thrusts underlie lower Minturn Formation, although the former fault cuts up-section along the flanks of the
thrust plate. Nearly all the thrust faults on the west side of the
range coincide with thin-bedded lower P~leozoic sedimentary
rocks. In projecting faults to depth, the decollement horizon at
the base of the Minturn Formation was used when geometric
considerations permit. Nevertheless, thrusts on the west side
of the range cut Precambrian rocks, indicating that many and
perhaps all the thrusts extend into the basement.
The cross sections (Fig. 3) clearly reveal two major west-dipping thrusts, the Wild Cherry Creek and the Spread Eagle Peak
faults. The Wild Cherry Creek thrust dips as low as 30° west
beneath Precambrian gneiss and granodiorite, placing the Precambrian rocks more than 1,000 m over the uptumed west limb
of the Gibson Peak syncline. Both the Wild Cherry Creek and
Spread Eagle Peak faults branch upward into faults that vary
from nearly vertical to low angle. The Cotton Lake anticline
(Fig. 2) in the eastern part of the Spread Eagle Peak thrust
plate (section C-C~ passes down-structure northward into a tight
overturned and faulted anticline (section A-A? as it approaches
the north flank of the thrust plate. Accurate reconstruction of
Agure5.
225
this fold is facilitated by the mapping of a distinctive turbidite
member in the Minturn Formation. A major fault splits north
off the Spread Eagle Peak fault (section A-A 1; although the fault
is a thrust near the line of section, it steepens northward into
a high-angle reverse fault having less than 2,000 m of vertical
separation.
The stratigraphic sections of Minturn Formation on opposite
sides of the Spread Eagle Peak fault differ in important aspects;
turbidites are abundant in the middle of the Minturn Formation
within the Spread Eagle Peak thrust plate, but have not been
found northeast of the fault; limestone beds near the top of the
Minturn Formation are thin west of the fault, but dominate a
zone 60 m thick at the top of the Minturn east of the fault. Some
of the marker units observed to end at the Spread Eagle Peak
fault have been traced many kilometers in their respective tectonic blocks. Abrupt termination of such laterally perSistent
units at the Spread Eagle Peak fault is indicative of major displacement.
Gently folded autochthonous sedimentary rocks of the
Sangre de Cristo Formation (Fig. 3, section A-A 1 pass southward into tightly folded and faulted strata (section G-G1 where
the leading edge of the Spread Eagle Peak thrust plate impinges on the autochthon (Fig. 4). Southward, the autochthon
passes into openly folded Sangre de Cristo Formation; these
folds tighten southward into thrust faults at Loco Hill (Fig. 5C),
east of the encroaching thrust plates of the Marble Mountain
and Huckleberry Mountain faults (Figs. 2 and 4). Folds, highangle reverse faults, and thrusts comprise a continuum of struc·
tures that pass into one another along strike, depending on the
degree of compression.
The Alvarado fault is interpreted tentatively as an east-dipping
thrust bringing Precambrian gneiss west over the Sangre de
Cristo Formation (Fig. 4), as proposed by Munger (1965). The
fault is well exposed in rugged terrain only at the latitude of the
cross sections, where its dip is nearly vertical. A three-kilometerlong branch of the Alvarado fault, exposed in the Sangre de
Cristo Formation west of the main fault, clearly dips about 40°
east and joins the main fault (Figs. 2 and 3, section G-G~. The
Sangre de Cristo Formation is tightly folded and overturned to
the west beneath the branch of the Alvarado fault; from there,
the west-facing Spread Eagle syncline extends north more than
10 km along the west side of the Alvarado fault. These westfacing compressional features are interpreted as evidence that
the Alvarado fault, in part or in its entirety, may have been a
Laramide thrust that dipped east beneath the Wet Mountain
Valley. The fault also is exposed for a short distance north of
Loco Hill, where its sinuous trace in gentle terrane is compatible
with a low dip angle. Most of the remainder of the Alvarado
fault is concealed; it is not known to be straight and simple as
shown on the map (Fig. 2), but cannot be shown otherwise O~'I­
ing to lack of data. In any case, the presence of scattered outcrops of Precambrian gneiss along much of the east side of
the fault indicates that the Wet Mountain Valley was part of an
upthrown block in Laramide time.
Southwest-northeast crustal shortening of sedimentary rocks
Photographs of thrust faults and associated folds In the northern Sangre de Cristo Range: (A) Marble Mountain thrust; view
looking· south at Marble Mountain; thrust brings west-dlpplng Minturn Formation (Pennsylvanian) over gentiy dipping Sangre de
Cristo (Pennsylvanian and Permian) Formation. (B) Spread Eagle Peak thrust; view south across Venable Lakes; anticline In
Minturn Formation between segments of the fault; near·vertlcal Sangre de Cristo Formation east of fault. (C) Aerial view of Loco
Hili thrust, showing truncation of syncline In Jurassic Morrison and Entrada formations In foreground; view north from above
Loco HilI. (0) Gibson Peak syncline In Sangre de Cristo Formation, looking northwest across the North Fork of Crestone Creek.
Symbols In photographs: MMT, Marble Mountain thrust; SEPT, Spread Eagle Peak thrust; F, branch of Spread Eagle Peak thrust;
Jm, Morrison Formation; Je, Entrada Sandstone; PPsu, Sangre de Cristo Formation, undivided; Pm, Minturn Formation, undivided;
Pml, Minturn Formation, lower part.
ROCKY MOUNTAIN ASSOCIATION OF GEOLOO/STS
226
iI
I
:
,
i
I
II,
,
II .
DAVID A. LINDSEY, BRUCE R. JOHNSON, AND PAM. ANDRIESSEN
in the range is conservatively estimated at about 8 km (or 40 per·
cent of the estimated original width of 20 km) along the line of
the cross sections. Southward from the line of sections, the
exposed width of folded and thrusted sedimentary rocks de·
creases from about 12 km to as little as 4 km east of the Sand
Creek thrust. Crustal shortening on the order of 14 km (or 70
percent of original width) appears likely, without taking into ac·
count probable shortening by horizontal displacement of thrust
plates composed mainly of Precambrian rocks. This estimate
is close to that of 15-17 km estimated by Burbank and God·
dard (1937, p. 963) for Paleozoic and Mesozoic rocks in Huerfano
Park.
The late Paleozoic central Colorado trough, now restricted
to only a few kilometers width in the Sangre de Cristo Range,
was deformed intensely and was telescoped by southwest·
northeast compression during the Laramide orogeny. Although
vertical uplift of the adjacent San Luis and Wet Mountain high·
lands (Tweto, 1975, p. 37) may have played a role in deformation,
the thrust plates are not interpreted as gravity slides from adja·
cent highlands (Karig, 1964a, p. 114-119) because they are not
intercalated with syntectoniC sediments. Coarse clastic sedi·
ments of Late Cretaceous to Eocene age, shed from adjacent
Laramide highlands, occur in Huerfano Park, but these sedi·
ments are not known to contain landslide depOSits (Burbank
and Goddard, 1937, p. 946-947). Lateral compression is con·
sidered to have been the dominant mechanism of Laramide
tectonism in the Sangre de Cristo Range. Compressive forces
brought the San Luis and Wet Mountain blocks closer to one
another, fracturing and thrusting the margins of the highlands
and squeezing the intervening sedimentary fill of the late Paleozoic central Colorado trough into a complex of folds and thrusts.
NEOGENE RIFTING
The San Luis and Wet Mountain valleys were downdropped
relative to the Sangre de Cristo Range when the Rio Grande rift
formed, mostly in Neogene time (Tweto, 1979b). The major
range-bounding faults are the Sangre de Cristo on the west side
and the Alvarado on the east (Fig. 6). Although many branches
of the Sangre de Cristo fault have been mapped in Quaternary
depoSits, some are concealed, so that the location of some
segments of the main fault remain in doubt (Fig. 2). A major
branch of the fault extends northwest across the San Luis Val·
ley near Valley View Hot Springs (Scott, 1970; Knepper, 1976).
The fault cuts depoSits of Quaternary age at many places along
the east side of the San Luis Valley. The aggregrate movement
of the Sangre de Cristo fault has resulted in an estimated 2,000
m of vertical separation between the base of the Neogene fill
in the valley and the top of the range at the latitude of Crestone
and Valley View Hot Springs (Davis and Stoughton, 1979); south·
ward along the west side of the range, more than 7,000 m of
vertical separation may be present (Davis and Keller, 1978). The
trace of the Alvarado fault along the eastern side of the Sangre
de Cristo Range is poorly exposed and generally concealed be·
neath Quaternary glacial deposits along much of its extent.
Parts of both the Sangre de Cristo and Alvarado faults follow
the general trend of Laramide thrusts (Fig. 4). Segments of
the Sangre de Cristo fault intersect or join the Wild Cherry, Cres·
tone, Sand Creek, and Deadman Creek thrusts; cross sections
(Fig. 3) show that the Sangre de Cristo fault joins the Wild
Cherry thrust in the subsurface. The Alvarado fault is inter·
preted tentatively as the leading edge of a west-directed Lara·
mide thrust that brought Precambrian rocks over upper Paleozoic rocks (Figs. 2, 3, and 4), but the fault, or perhaps concealed
branches of the fault, was responsible for down-dropping of the
Wet Mountain Valley in Neogene time. Thus, major Neogene
,"'...
,"'"'"
EXPlANAnON
I Dike
Of
sill
o A Locaflty of flS8l()n·neck dirt.
31"
7
:IT
E)t
)
"
/
..
,A,\
'\
,~,Cll
I
.... Tronooct of Mg 7
WIT MOUNT ""N VAl.l.EY
,-<,
GRABEN
SANGRE DE CRISTO HORST
.
J,
\
,h
#"
"
~--~----~,
KilOMETERS
Figure 6,
?'t;'-'
)
)~
MP~
'\,
Map showing igneous rocks and Neogene tectonic fea·
tures of the Sangre de Cristo Range from the Great Sand
Ounes to Valley View Hot Springs. Map shows locations
of fission·track ages as follows: (A) felsite dike intruding
Spread Eagle Peak fault, 26.5:!: 1.1 my on zircon; (6) Per·
mian and Pennsylvanian Sangre de Cristo Formation,
3290 m elev., 15.0:!: 3.9 my on apatite; (C) Pennsylvanian
Minturn Formation, 3,440 m elev.. 18.9:!: 4.7 my on apatite;
(0) Minturn Formation, 3,930 m elev., 23.7:!: 9.5 my on
apatite; and (E) syenite clast from Sangre de Cristo For·
mation, 3,920 m elev., 17.3:!: 4.0 my on apatite. Transect
of figure 7 shown also; area shown same as figure 2;
GC, Goat Creek; MP, Medano Pass; V, Valley View Hot
Springs.
rift faults probably formed in part by reactivation of Laramide
thrusts; the rifts acted as normal faults in Neogene time, in part
reversing the direction of Laramide movement. Both thrusts in
compressional tectonic settings and normal faults in exten·
sional settings flatten at depth even though the faults cut Precambrian basement, so that thrusts are favorable strUcfures
for reactivation during rifting. Parallelism of Neogene rift faults
with reactivated Precambrian faults in southern Colorado sug·
gests a very early ancestry for some of the Neogene faults
(Tweto, 1979b).
Rifting in the region of the northern Sangre de Cristo Range
began in late Oligocene time. A limit on the inception of rifting
is fixed by the 29-my-ald Gribbles Park Tuff, which occurs in
the fill of two paleovalfeys that crossed the site of the northern
part of the range in late Oligocene time (Taylor, 1975; Scott and
Taylor, 1975, p. 10); one of the paleovalleys is at Goat Creek
(Fig. 2). Rifting and uplift of the range disrupted these drain·
ages sometime after deposition of the Gribbles Park Tuff. Rift·
ing probably had begun when the oldest volcanic rocks filling
the rift of the San Luis Valley were emplaced 26-27 my ago (lipman and Mehnert, 1975).
Many of the igneous intrusions of southern Colorado were
emplaced during the initial stages of rifting (Tweto, 1979b). The
Oligocene Rito Alto stock (Marvin and others, 1974; Scott and
ROCKY MOUNTAlN ASSOCIA TION OF GEOLOO/STS
LARAMIDE AND NEOGENE STRUCTURE Taylor, 1974) and many dikes and sills were intruded at the site
of the Sangre de Cristo Range (Fig. 6). Small dikes and sills
have been mapped throughout much of the range; they typically
strike north, northwest, and northeast and follow jOints, faults,
and bedding planes. Locally, dikes of felsic rocks have in·
truded the Laramide Crestone and Spread Eagle Peak thrusts;
the unsheared character of the dikes indicates that, at least
locally, these thrust faults have not moved since intrusion. One
dike, intruding the Spread Eagle Peak thrust,has a minimum age
of 26.5± 1.1 my. (C.w. Naeser, written communication, 1982).
East of Medano Pass (Fig. 2), flows of Miocene(?) olivine basalt
overlie and locally dip into a possible branch of the Alvarado
fault (Burford, 1960, p. 107).
Emplacement of igneous rocks was followed by uplift and
erosion of the Sangre de Cristo Range in early Miocene time.
Detrital apatite from Paleozoic sedimentary rocks, dated by the
fission·track method, ranges in age from 15.0 ± 3.9 my to 23.7 ±
9.5 my at sample localities that vary in elevation from 3290 m
to 3930 m along a transect of the range (Fig. 6). The young
dates of detrital apatite from sedimentary rocks of Permian and
Pennsylvanian age were caused by thermal resetting, as shown
by the wide distribution of reset apatite across the range and
the tendency for ages to correlate with elevation among closely
spaced localities (Fig. 7). Fission tracks in apatite are annealed
at about 120° C within a period of 1 my (Naeser, 1981). The
apatite dates indicate cooling of the host rocks below the tem·
perature sufficient to reset apatite 1~23 my ago, probably when
the range was uplifted and eroded rapidly.
Sediment eroded from the Sangre de Cristo Range in Neogene time accumulated in the San Luis and Wet Mountain val·
leys. The northern part of the San Luis Valley contains about
500 m of Neogene sediments (Davis and Stoughton, 1979), and
the Wet Mountain Valley contains as much as 400 m (Scott and
Taylor, 1975, p.12). Large alluvial fans along the west side of
the range record Quaternary uplift and erosion; extensive dis·
sected pediments in the southern part of the Wet Mountain Val·
ley (Fig. 8) record uplift and erosion of the adjacent range in
late Neogene or Quaternary time.
The Sangre de Cristo Range may have tilted gently east duro
ing Neogene uplift. Fission·track ages of apatite from upper
Paleozoic rocks at the westernmost localities along a transect
through the range (D and E of Fig. 6) are consistent with east·
ward tilting (Fig. 7). The two localities are nearly the same elevation, but apatite from the west em part of the range (locality E)
is about 7 my younger than from the central part of the range
(locality D), indicating that the western part of the range has
been uplifted and eroded more, or has been uplifted more recently, than the central part. Extensive pediment surfaces in
the southern part of the Wet Mountain Valley appear to have
been tilted east, as shown by eastward incision of north·flowing
streams into the pediments (Fig. 8). Tilting also may be reflect·
METERS ABOVE
.~
SEA LEVEL
E 17.3"0
my
0 23 7".5
my
8
1!'.O~3.J my
3
2
o1~______5~1______~110 KILOMETERS
Agure7.
Plot of fission track ages of detrital apatite from Pennlan
and Pennsylvanian sedimentary rocks on an elevation
profile of a transect of the Sangre de Cristo Range. Tranaect and sample localities shown In figure 6.
SANGRE DE CRISTO RANGE
Figure 8.
227
Photograph of late Cenozoic pediments In the southern
part of the Wet Mountain Valley, showing eastward-Incised
drainage pattern; aerial view looking south; Wet Moun·
taln Valley in foreground.
ed by the asymmetric distribution of young fault movement
along the margins of the Sangre de Cristo Range. Faulting ap·
pears to have ceased on the east side of the range, where the
trace of the Alvarado fault is concealed beneath Quaternary deposits; but faulting continues on the west side of the range,
where numerous branches of the Sangre de Cristo fault cut
deposits of Quaternary age (Scott, 1970; Knepper, 1976).
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