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Linkages of Laramide Thrusts, Northern Sangre de Cristo Range,

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Authors Bedford, Janice M.

Publisher The University of Arizona.

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Link to Item http://hdl.handle.net/10150/249231 LINKAGES OF LARAMIDE THRUSTS, NORTHERN SANGRE DE CRISTO RANGE, COLORADO

by

Janice M. Bedford

A Prepublication Manuscript Submitted to the Faculty of the

DEPARTMENT OF GEOSCIENCES

In Partial Fulfillment of the Requirements

For the Degree of

MASTER OF SCIENCE

In the Graduate College

THE UNIVERSITY OF ARIZONA

1994 STATEMENT BY THE AUTHOR

This manuscript, prepared for publication in the Geological Society of America Bulletin has been submitted in partial fulfillment of requirements for the Master of Science degree at the University of Arizona and is deposited in the Antevs Reading Room to made available to borrowers as are copies of regular theses and dissertations.

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SIGNED: 444."--cA.-,Aliettia-t-CP

APPROVAL BY RESEARCH ADVISORY COMMII"I'EE

As members of the Advisory/Research Committee, we recommend that this prepublication manuscript be accepted as fulfilling the research requirement for the degree of Master of Science.

Df Atipst /5'4t Maj r Advisor Clement G. Chase) r a vz-a,Ì'7 Date

Peter J. ney 1

LINKAGES OF LARAMIDE THRUSTS, NORTHERN SANGRE DE CRISTO RANGE,

COLORADO

JANICE M. BEDFORD, CLEMENT G. CHASE, AND GEORGE H. DAVIS

UNIVERSITY OF ARIZONA, TUCSON, ARIZONA

ABSTRACT

Laramide thrust belts in the Colorado have been mapped as discrete units with little investigation into the linkage between displacements. The Elk Range- Sawatch and Elkhorn thrust systems displaced , Paleozoic, and Mesozoic rocks toward the southwest. The Sangre de Cristo Range and thrust systems displaced rocks toward the northeast. The opposite vergence and oppositely directed displacements between these systems must be accommodated, both at present levels of exposure and at depth. Mapping of the Kerber Creek area west of the northern Sangre de Cristo Range by J.M. Bedford helped answer the question of the linkage between the opposing Elk Range- Sawatch and Sangre de Cristo Range thrusts. In the Elk Range- Sawatch system the westward displacement on the thrusts is interpreted as a minimum of 11 km in the southern Elk Range with displacement decreasing toward the north. Bryant (1966) interpreted the timing of faulting as in age. In the Sangre de Cristo Range northeast -verging thrust system the minimum estimated eastward displacement is 8 km. Burbank and Goddard (1937) interpreted the displacement as in age. The timing of the faulting in the two areas is not necessarily different. The Kerber Creek area lies between the two regions of oppositely facing displacement. Its internal structure most closely resembles that of the more proximal Sangre de Cristo deformation. Thrusts in the Kerber Creek area place Precambrian rocks over Laramide(?) and Paleozoic sedimentary rocks. The structures verge toward the north - northeast and represent a minimum of several kilometers of displacement. These Laramide structures are exposed where overlying Tertiary volcanics are eroded. In the Northern Sangre de Cristo Range a set of E -W trending faults intersects the generally N -S trending Laramide thrusts, possibly representing a partitioning of northeast transport into N -S and E -W components. The E -W trending deformation can be correlated across the with the thrust faults in the Kerber Creek area. Correlation of the Kerber Creek thrusts with Sangre de Cristo faults extends the northeast directed Laramide deformation 22 km northwest of the edge of the Sangre de Cristo Range. Thus the link between the oppositely verging structures must continue toward the northwest, possibly beneath the Bonanza volcanic field. Further mapping toward the southern extent of the Elk Range- Sawatch thrust system may reveal how the opposite vergence and minimum 8 -10 km of displacement are accommodated. 2

STATEMENT OF PROBLEM AND BACKGROUND

Late Mesozoic to Early Cenozoic deformation of the Foreland Fold and Thrust belt in the western can be divided into two structural zones; a western zone of imbricate thrusting of the passive marginsequence known as the Sevier belt and an eastern zone of basement -involved thrusts of Laramide age (Figure 1). The basement -involved thrusts of the have been a subject of controversy for the last several decades. When the Colorado Rocky Mountains were mapped in the late 1960's and

1970's the basement- involved thrusts of the region were interpreted as discrete structures with little investigation into the linkage between displacements.

Much of the mapping of this region was completed during a period when isolated, vertically uplifted blocks were the dominant model of thrust geometries associated with basement -cored uplifts (Prucha et al., 1954). "Thin- skinned" deformation geometries were not thought to be applicable to deformation involving crystalline basement. The upthrust model, in which reverse faults become vertical with depth, depicts the basement-involved uplifts as isolated structural units. This model implies very little horizontal displacement and requires a separate deformational event to cause significant folding found near the faulting. This model does not require any systematic lateral continuity of regional deformation.

The thrust model, however, requires lateral continuity of regional strain. Thrusts imply greater horizontal transport and link the formation of folds to movement along the thrust faults. Displacement along thrust faults must maintain continuity in plan view through linkage along tear faults or partitioning of the strain onto multiple surfaces or into folds. South contains thrust systems whose linkage is uncertain. This is exemplified on a regional scale by the southwestern verging Elk Range -Sawatch and

Elkhorn systems and the northeasterly verging Sangre de Cristo Range and Wet Mountain systems (Figure 2). 3

COCORP deep seismic data from Wyoming has shown that field data that has been used to support a vertical uplift model may be misleading. In the Wind River Range features once mapped with the previously popular upthrust model are actually low -angle thrusts at depth. Couples and Steams (1978) published an upthrust model of the Wind

River Mountains in which they theoretically modeled the vertical upthrust of the mountains and rejected the possibility of a listric geometry. The same year, Smithson et al.

(1978) published their work on the COCORP seismic data for the Wind River thrust delineating a deep listric fault geometry. Following this work, many of the "thick- skinned" uplifts have been shown to be the result of deep crustal, listric thrust faults.

STRATEGIC LOCATION OF THE KERBER CREEK AREA

Historically the Elk Range- thrust system of Central Colorado has been viewed as part of an upthrust package. These west -verging structures (Figure 3) have been mapped from to just north of , a distance of 80 km (Bryant, 1966). The thrusts displace Precambrian, Paleozoic and Mesozoic rocks over rocks as young as . Some of the most recent mapping of the system was accomplished as part of the University of Minnesota field camp in the Gunnison area (J.H.

Stout and C. G. Chase, oral communication, 1993). Hanging walls contain nappe -like isoclinal recumbent folds of Paleozoic strata; fold cores contain ductilely deformed

Precambrian basement (J.H. Stout, personal communication, 1993). Westward displacement on the thrusts was interpreted by Bryant (1966) as a minimum of 11 km in the southern Elk Range with apparent displacement decreasing toward the north. Bryant

(1966) interpreted the timing of faulting as Paleocene. The fault cuts

Mesaverde Formation in the Marble quadrangle (Gaskill and Godwin, 1966) and is intruded by post -early Eocene bodies which metamorphosed rocks containing Paleocene plant (Godwin and Gaskill, 1964). The unroofing sequence of the Sawatch Range recorded in the basinal sedimentary rocks also suggests a Paleocene or younger age for 4 motion along the Elk Range thrust (Bryant, 1966). This evidence however does not rule out an Eocene age for fault motion.

Southeast of the Elk Range is the Sangre de Cristo Range, containing a major thrust system with vergence opposite to that of the Elk Range (Figures 2 &3). The Sangre de

Cristo Range thrust system is complex and is composed of a minimum of nine thrust sheets. The thrusts displace Paleozoic and Precambrian rocks. Four sheets contain lower

Paleozoic and Precambrian rocks, three sheets contain mainly upper Paleozoic rocks, one thrust sheet contains upper Paleozoic and Mesozoic rocks, and the uppermost sheet contains solely Precambrian rocks. The thrusts vary in orientation at the surface from vertical to horizontal (Lindsey and Soullierre, 1984). The majority of the faults dip moderately toward the west. Laramide structures in the Sangre de Cristo Range are generally characterized by shallow, brittle deformation (Gabelman, 1952). This east- verging thrust system has been mapped the entire length of the Sangre de Cristo Range.

Minimum estimated eastward displacement is 8 -14 km (Lindsey, et al., 1984).

Displacement was estimated to be of Eocene age by Burbank and Goddard (1937) based on stratigraphie relationships within the Cretaceous to Eocene formations of the east side of the

Sangre de Cristo Range.

The estimated displacement does not take into account the translation of the uppermost thrust sheet, which displaced solely Precambrian basement. Since there are no known piercing points on this thrust, the actual displacement is unknown. If the displacement is similar to that on the other thrusts within the Sangre de Cristo Range at this latitude, the displacement of the top thrust sheet containing Precambrian basement may be a minimum of several kilometers.

To address the question of the linkage of the two oppositely verging systems, J.M.

Bedford mapped the Kerber Creek area west of the northern Sangre de Cristo Range. By investigating the vergence and minimum displacement in the Kerber Creek area we hoped to define the zone of regional strain accommodation. Kerber Creek lies between the two 5 systems but is more proximal to the Sangre de Cristo deformation (Figure 3 ). In Kerber

Creek, the Tertiary Bonanza volcanic sequence has been eroded to expose Laramide( ?) and

Paleozoic sedimentary rocks and Precambrian basement. Thrusts mapped in this area place the Precambrian rocks over Paleozoic sedimentary rocks. Thrusts of similar displacement are exposed in the Sangre de Cristo Range. These faults are correlative to the Kerber Creek structures. Opening of the San Luis Valley separated the Kerber Creek structures somewhat from their correlative Sangre de Cristo counterparts.

SUMMARY OF TECTONIC HISTORY

Multiple Precambrian Intrusions /Deformations

During the Precambrian the western United States was subjected to multiple orogenic events. This is recorded in the metamorphic, magmatic and structural fabrics of the basement. These events for Colorado are summarized by Tweto (1977, 1980). The oldest rocks of Colorado are >1.8 Ga gneissic rocks derived from both metasedimentary and metavolcanic rocks. These gneissic rocks record two periods of folding, with the second occuring approximately 1.7 Ga. Granitic intrusions 1.7, 1.4, and 1.0 Ga have been dated in Colorado. Sedimentary and volcanic sequences, the Uncompahgre Formation and the Uinta Mountain Group, are 1.72 to 1.46 Ga and 1.4 Ga to 950 Ma respectively (Tweto, 1980). Details of the on these rocks were compiled by

Reed et al. (1993). Four major systems of shear zones and faults resulted from

Precambrian deformation. The shear zones formed during the late stages and following the

1.7 Ga intrusive event and following the 1.4 Ga intrusive event. A third episode of folding also accomanied the post 1.4 Ga faulting (Tweto, 1980). These pre- existing structures may have created crustal anisotropy that influenced the development of later structures.

These rocks are here mapped as undifferentiated Precambrian basement. 6

Ancestral Rocky Mountain

As the Antler orogeny ended on the west coast of North America, the Ancestral

Rocky Mountain Orogeny began in the continental interior. The Ancestral Rocky Mountain

Orogeny is thought to be intraplate deformation related to the Ouchita -Marathon Orogeny taking place along the southern North American marginas South America and Africa collide with the North American continent (Kluth, 1986 and Kluth and Coney, 1981). This deformation began in the Early and ended in the Early . The deformation extending deep into the North American platemay be comparable to structures forming north of the modern Indian -Asian collision (Kluth, 1986).

As the late Paleozoic Ancestral Rocky Mountain uplifts and basins developed, pre- Pennsylvanian rocks were eroded from the uplifted areas. The resulting exposure of

Precambrian basement rocks led to the deposition of Pennsylvanian and Permian arkosic sediments into the Central Colorado Trough (Tweto, 1975). Minturn Formation arkosic, micaceous sandstones are believed to have been derived from granitic basement exposed in the cores of Ancestral Rocky Mountain Uncompahgre and -Apishapa uplifts

(de Voto, 1972). Stratigraphic relationships suggest a relatively distal depositional origin with a generally coarsening upward sequence for the Minturn in the Northern Sangre de Cristo area (Lindsey et al., 1983).

Cretaceous Seaway

During the Cretaceous western North America was covered by an interior seaway.

By Cretaceous time the Ancestral Rocky Mountain uplifts had eroded and the basins filled.

Repeated transgressions and regressions deposited 1500 -3000 m of marine and continental rocks over the entire area. As the waning Cretaceous sea was retreating to the northeast, 7

Laramide deformation was approaching from the southwest (Tweto, 1975). Within the broader region, Laramide structures clearly reactivate Precambrian and Ancestral Rocky

Mountain faults.

Laramide Orogeny

Plate reconstructions of the tectonic development of the western margin of North

America reveal a tectonic setting 80 -40 Ma that is similar to the modem Andes of South

America. Oblique convergence of the Farallon plate at 14 cm/yr produced the (Coney, 1978). Reynolds and Coney (1977) also have interpreted changes in the dip of the subducting slab under the North American margin at this time. The area where basement cored "thick- skinned" uplifts are present may have been above a sub -horizontally dipping slab.

The orogen is characterized by basement -cored uplifts bounded on one or more sides by large -scale listric thrusts. Uplifts may have up to 10 -12 km of vertical structural relief. The amount of horizontal displacement on these structures is debated. Uplifts developed between 80 and 40 Ma at a distance of 1000 to 1500 km from their associated subduction margin (Jordan et al., 1983). The structure and development of the system is highly debated. Laramide Rocky Mountain deformation has similarities to the modern

Sierras Pampeanas of Argentina (Jordan et al., 1983).

Post -Laramide Tectonics

Approximately 40 Ma the relative motion between the Pacific plate and the North

American plate changed drastically. Convergence rate dropped from approximately 14 cm/yr NNE to 8 cm/yr ENE (Coney, 1978). This change in rate was accompanied by a change in the subduction slab orientation (Reynolds and Coney, 1977). 8

The change in subducting slab dip from a subhorizontal to steeper "normal" subduction angle was accompanied by a widespread "ignimbrite flareup" along much of the western Cordillera (Coney, 1978). Decrease in convergence rates accompanied by heating of the thickened crust of the western Cordillera resulted in the Mid -Tertiary metamorphic core complex extension (Coney, 1987).

Between 30 and 20 Ma the East Pacific Rise made contact with the western margin of North America. Basin and Range extension is correlated to the destruction of the East

Pacific Rise as the western North American transform system developed (Coney, 1987).

Approximately 26 Ma extension began in the , a structural zone between the and the continental interior. The change in plate configuration following the end of the Laramide Orogeny resulted in the Rio Grande extension. Rio

Grande extension resulted in the collapse of Laramide uplifts in and Colorado

(Chapin and Cather, in press).

STRATIGRAPHY OF THE KERBER CREEK AREA

Stratigraphic studies have been carried out in the Kerber Creek area and northern

Sangre de Cristo Range by Burbank (1932), Gabelman (1952), de Voto (1972), Knepper

(1974), and Lindsey et al. (1 983). Their stratigraphic nomenclature (Figure 4) and measured sections were used as a guide in the mapping of the Kerber Creek area.

Precambrian Basement

The Precambrian basement is composed of coarse -grained 1.7 Ga granites with large plagioclase laths, quartz, biotite, and sparse hornblende. Zones of gneissic texture are interspersed within the granite. The gneiss, biotite -rich with quartzose layers is part of the >1.8 Ga gneissic complex of Colorado and southern Wyoming (Tweto, 1980). Where 9 exposed in the cores of the folds of the Kerber Creek area, the gneiss is complexly deformed. These rocks are mapped in this study as undifferentiated Precambrian basement.

Ordovician Manitou Formation

The Manitou Formation unconformably overlies Precambrian basement in the

Kerber Creek area. The Sawatch Sandstone which is found in other parts of south -central Colorado is missing from the Kerber Creek area. The Manitiou

Formation is a thin- bedded to massive gray limestone /dolomite. It weathers a gray -brown color with smooth outcrops. White to grey chert layers and nodules are present throughout the formation (Gabelman, 1952). Burbank (1932) estimated the formation thickness to be

30 -70 m.

Ordovician Harding Formation

The Harding Formation is a quartzite /sandstone with diagnostic fish plate fragments (Kirk, 1930) found along the top horizon. The Harding weathers to a maroon color and forms blocky talus slopes (Gabelman, 1952). Burbank (1932) estimated formation thickness to be approximately 20 -30 m. The contact with the Manitou is gradational.

Ordovician Fremont Formation

The Fremont Formation is predominantly a limestone with thin bedding toward the top and thick to massive bedding toward the bottom. It is locally dolomitic. The Fremont weathers to a blue -gray color with a rough, hackley texture. Black to grey chert layers are 10 abundant in the upper part of the Fremont. This formation contains rare coral and brachiopod fragments (Gableman 1 952). Burbank (1 932) estimated formation thickness varying from 70 to 100 m.

Devonian Chaffee Group

The Chaffee Group is divided into the Dyer Dolomite and Parting Sandstone members. The contact with the Fremont Formation is a low relief unconformity. The Dyer

Dolomite is a fine -grained grey dolomite, thin-bedded toward the top and massive toward the bottom, and tends to weather to a yellow -brown color. The Parting Sandstone is a thin bedded pink to red sandstone with characteristic cross laminae and interbedded limestones

(Gabelman, 1952). Knepper (1974) estimated formation thicknesses as 0 -20 m and 0 -40 m for the Parting and Dyer members, respectively. In the Kerber Creek area the individual members are sometimes difficult to distinguish, and are mapped simply as the Chaffee Group.

Mississippian Leadville Formation

The Leadville Formation is massive gray limestone and dolomite. The upper part of the formation contains both white and black chert layers and nodules. The Leadville weathers a light grey to blue color with a relatively smooth texture. Intraformational breccias are present throughout the Leadville (Knepper, 1974). The contact with the Chaffee Group is unconformable (Gabelman, 1952). limestones regionally have a terra -rossa development recording a large -scale karsting event following

Mississippian deposition. Formation thickness was estimated by Knepper (1974) as 70-

120 m. 11

Pennsyvanian Kerber Creek Formation

The Kerber Creek Formation contains clean quartz sands with cross bedding and graded bedding. The quartz sandstones are interbedded with carbonaceous shales and thin limestone layers. Abundant marine fossils in the limestone layers date this formation at

Morrowan and Atokan. The depositional setting of the Kerber Creek formation is marine and deltaic plains. This formation is correlated with the Belden formation further to the north (de Voto, 1972). Formation thickness was estimated by Burbank (1932) at 70 m.

Pennsylvanian Minturn Formation

The Mintum Formation is primarily red to green to grey, micaceous arkosic, sandstone, shale and siltstone. The coarser beds contain cross -bedding with graded bedding. The Minturn displays abrupt changes in facies and formation thicknesses over short distances. Plant impressions and marine fossils in interbedded carbonates as well as stratigraphie relationships reveal that this formation is DesMoinesian and Atokan in age (de Voto, 1972). Lindsey et al. (1983) interpreted this formation to be deltaic and fan deposits shed from the Uncompagre and Front Range -Apishapa Ancestral Rocky Mountain uplifts.

Minturn outcrops tend to be low hills. The contact with the Kerber Creek Formation is gradational over several meters (Bridwell, 1968).

Laramide "Wagner Breccia"

The "Wagner Breccia" in the Kerber Creek area is a previously unmapped unit that contains sub -angular blocks of granite ranging in size from 1 cm - 5 m. The average clast size is approximately 3/4 meter. The breccia is clast supported, with a feldspar -rich matrix.

Between some of the largest granite clasts are rounded cobbles resembling the Minturn 12 formation ( "dirty" micaceous arkosic sandstone, greenish in color). The formation is found in roadcuts near the intersection of Kerber Creek and Little Kerber Creek.

Outcrops of granite breccia are interspersed with areas containing float of "Minturn formation ". The nature of the contacts between these two types of rocks is unknown. One possibility is a talus slope or debris flow deposit with coarse granitic breccia mixed with finer micaceous sandstones. Another possibility is a fault contact between the breccia and the Minturn formation. The interspersed patches of Precambrian and Mintum float support the talus slope /debris flow hypothesis.

Tertiary volcanic rocks unconformably overlie the breccia. The lack of volcanic rock fragments in the breccia puts a lower limit on its age. The breccia appears to contain

Precambrian granite boulders and Pennsylvanian Minturn formation. Since the Minturn formation is interpreted as distal fan deposits from the Ancestral Rocky Mountain orogen, the incorporation of rounded cobbles of lithified Minturn in the breccia implies a later origin. The large granite boulders suggest a talus slope, perhaps created during Laramide deformation.

A Laramide origin for the "Wagner Breccia" would make this the only Laramide rock exposed in the Kerber Creek area. To create a talus slope deposit, a steep slope is required. Laramide thrusting and folding would have created such slopes in this area. This breccia may have formed at the toe of the Kerber thrust which places Precambrian granite over Paleozoic rocks ranging from Ordovician Manitou to Pennsylvanian -Permian Mintum.

The Kerber Thrust is currently exposed 2 km to the south of the breccia outcrops (Figure

6).

Tertiary Volcanics and Intrusives

Small intermediate to felsic porphyritic bodies intrude the Paleozoic sedimentary rocks in the Kerber Creek area. Volcanic rocks, Rawley andesite and Bonanza tuff, from 13 the Bonanza caldera lie unconformably over Laramide(?), Paleozoicand Precambrian rocks. The Tertiary volcanic rocks of this region have been dated by potassium -argon to between 35 and 30 Ma (Lipman et al., 1970).

STRUCTURE OF THE KERBER CREEK AREA

Structural Overview

The initial mapping done by Burbank (1932) showed low angle thrust geometries.

Such a thrust plate model implies more shortening than an upthrust model and requires regional strain continuity. Bridwell (1968) and Knepper (1974) mapped the Kerber Creek area using an upthrust interpretation (Figure 5 & 6). Anupthrust model implies very little shortening and requires no regional strain continuity.

(1952), Bridwell (1968), and Knepper (1974) interpreted the Laramide structures of this area to be the result of two different episodes ofLaramide deformation; an early episode of folding followed by a later episode of reverse faulting. However, both types of features can be explained using a single phase of deformationin which fault propagation folds and fault bend folds affect the hanging wall rocks above thrusts faults.

Remapping of the area originally covered by Bridwell (1968) showed exposures of

Paleozoic and Precambrian rocks folded and faulted with generally E -W trending faults but

with fold axes trending both NW -SE and E -W (Figure 6). This deformation is very similar

to brittle N -S/E -W directed faults and folds 10.5 km to the eastin the Sangre de Cristo

Range. Laramide deformation in the Kerber Creek area is partially obscured by the rocks

from the Bonanza caldera. The caldera was defined geophysically by Karig in 1965, and

the associated volcanic rocks originally mapped by Burbank (1932). Mayhew (1969)

mapped most of the normal faults associated with the east side of the caldera. Knepper 14

(1974) summarized these rather well. Displacement along these faults is relatively small.

The faults form a radial pattern about the town of Bonanza. A series of tuffs (Bonanza tuff) and andesite flows (Rawley andesite) were extruded from the caldera approximately

35 -30 Ma (Lipman et al., 1970). Where these volcanic rocks are eroded the Laramide structures of the Kerber Creek Area are exposed.

General Description of the Map Area

The Kerber Creek area contains four major faults: the Kerber thrust, the Noland thrust, the Clayton thrust, and the Villa Grove thrust. Smaller faults such as the Soda

Spring fault and Cody Spring fault cut the folds of the area. There are four major and three minor folds: the Western syncline, the Central anticline, the Noland syncline, the Clayton

Cone anticline, and the folds associated with the Clayton and Villa Grove thrusts. The geography and geology of Kerber Creek are summarized in Figure 6.

The sequence of faults from southwest to northeast is Kerber, Noland, Clayton, and Villa Grove. The western syncline and central anticline are north of the Kerber thrust.

The Noland syncline is south of the plunging nose of the Central anticline in the hanging wall of the Noland thrust and the footwall of the Kerber thrust. The Clayton Cone fold in the eastern portion of the map area is associated with the Villa Grove fault.

Determination of Fault Dips

Outcrop exposure in the Kerber Creek area is relatively poor. The actual fault planes are not exposed. The majority of the contacts were mapped based on float.

However, locations usually were constrained to within 5 -10 meters. Based on these locations, the three point approach was used to determine strike and dip of the faults.

These dips are represented in the cross section A -A" and on the Kerber Creekarea map 15

(Figure 6 & 7). The dips range from 8 °N on a portion of the Noland thrust to 44 °S on a portion of the Villa Grove thrust.

As a test, similar three -point calculations were performed on Bridwell's (1968) map. His cross sections show steeply dipping faults but his mapped topography -fault intersections indicate shallowly dipping faults. Three -point solutions support the shallow dipping interpretation.

Faults

The Kerber thrust is the southernmost of the faults exposed in the Kerber Creek area. It places Precambrian basement over Pennsylvanian Mintum Formation along its western trace and over Precambrian basement and overlying Paleozoic rocks (Manitou to

Leadville) along its eastern trace. Where the Precambrian is thrust over Precambrian rocks south of Noland syncline in Noland Gulch, the contact becomes hard to follow. The fault orientation was determined with the three point method. To the west where contact locations are approximate, the fault dips steeply toward the north and strikes generally

N86 °W. The central portion of the thrust dips shallowly (7 ° -25 °) toward the south and strikes generally N80 °W. The eastern portion of the thrust dips shallowly, approximately

8° toward the south and strikes N47 °W. As the fault disappears into granite -on- granite contacts, the relationships between topography and contacts result in conflicting dips which suggests a near -vertical dip for this portion of the fault. These orientations are represented on the cross section and on the Kerber Creek area map (Figures 6 & 7).

The Noland thrust is structurally below the Kerber thrust (Figure 7). The Noland thrust carries Precambrian rocks over Ordovician and Pennsylvanian rocks. A hill of deformed Ordovician Harding and Manitou(?) is at the northernmost exposure of the Noland thrust. These rocks are caught between two fault splays (Figure 7). The Noland syncline was formed as part of the hanging wall which rode over the inferred flat -ramp 16 geometry of the Noland thrust. Surface data are the only control on thisinterpretation.

From three -point determinations, at the surface the central and eastern portion of the

Noland thrust dips shallowly (10 ° -31 °) toward the south and strikes N69 °W. The western portion dips shallowly to the north 2 °-8° and strikes N84 °W to N45 °W. The orientation of this part of the fault may have been affected by motion along the nearby Kerber thrust.

Between the Noland and Kerber thrusts in the Noland Syncline there are two small faults present. These minor faults displace the axis of the syncline (Figure 6). They are oriented generally N -S and appear to have a near vertical dip. These "tear faults" in the syncline may act as displacement direction indicators.

The Clayton thrust is located in the area between the Noland thrust and Clayton

Cone. This thrust places Pennsylvanian Mintum over itself along its eastern length. Two folds developed in the Minturn suggest flat -ramp geometry at depth. This is represented on cross - section A -A" (Figure 7). Toward the west this fault places a depositional sequence of Ordovician and Precambrian rocks over the Paleozoic (Manitou to Minturn) and

Precambrian sequence. Three -point determination of the orientation of the thrust indicates a dip ranging from 9° to 22° south and an average strike of N73 °W. The eastern portion has a strike of N75 °W but dips shallowly (2 °) toward the north. Locations are approximate where this orientation was derived. The stratigraphic separation on this fault is less than the full thickness of the Mintum Formation, estimated at 70 m by Burbank (1932), along the eastern portion of the fault and less than the thickness of the Ordovician units along its western length. North of Kerber Creek, this fault cuts the Central anticline and places

Manitou over Fremont with a maximum of 200 m of stratigraphic offset. Further to the west this fault disappears into a Precambrian on Precambrian contact.

The Villa Grove fault is the northernmost of the Kerber Creek thrusts. It cuts the rocks in a series of low hills south of Clayton Cone. This fault places the Paleozoic sequence from the Kerber Creek Formation to the Fremont Formation over the Precambrian core of an anticline. The fault trace cuts off the Paleozoic along the western limb of the 17

Clayton Cone anticline. The fault is characterized by a zone of silicification. Three -point determination of the Villa Grove fault's orientation shows a strike and dip of N81 °W, 44 °S.

The fault trends toward the San Luis Valley and is covered by alluvium. To the east along the fault the zone of silicification in the Paleozoic rocks become wider. As it disappears under the alluvium its thickness is approximately 130 m. A small hill toward the east of the Clayton Cone area contains silicified Leadville and Kerber formations. This outcrop is interpreted as the continuation of the Villa Grove fault as it cuts the eastern limb of the Clayton Cone anticline.

The Clayton thrust cuts off the north -northwest trending Cody Spring fault. The

Cody Spring fault in its northern extent places Precambrian basement over Mississippian

Leadville Formation. Three -point determination of the strike and dip reveals a N15°W, 20E orientation for the northern part of the fault. This fault continues across the Kerber Creek

Valley along the eastern flank of Wagner Hill. Vegetation and talus from the steep slopes above make accurate location of this portion of the fault difficult. Approximate three -point determination on the southern portion of the Cody Spring fault results in a strike and dip of

N54 °W, 16 °NE or N46 °W, 33 °NE. These may not be true representations due to ambiguity in the contact location.

The western flank of the Central anticline is also faulted. The fault places Paleozoic rocks (Manitou to Kerber Creek) over Precambrian. The topography- contact relationships show a steeply dipping north- northeast striking fault.

Along the southwestern flank of the Central anticline a small fault of minimal stratigraphic displacement (< 70 m) places Ordovician Fremont over Ordovician Manitou.

Displacement decreases toward the southwest until no obvious stratigraphie separation is noticeable 1/2 mile away. These hangingwall rocks are also bounded on the west by an inferred fault. There is no definitive evidence that this contact is a fault but the abrupt change in stratigraphy across the valley and the presence of other small faults in the stratigraphy east of the 18 inferred fault. The Paleozoic rocks can be traced to the west to Soda Springs Gulch. To the west the "Wagner Breccia" is exposed. The "Wagner Breccia" can be traced toward the east to Soda Springs Gulch. The contact between the two sections is overlain by

Quaternary alluvium. Both units are also unconformably overlain by the Tertiary Bonanza tuff. Thrust slices repeat sequences of rocks in the Kerber Creek area. These were not mapped by Bridwell in 1968. Examples include thrust slices of Kerber Creek sandstones south of Kerber Creek and repetition of the Ordovician Harding Quartzite in the Soda

Spring area of the west limb of the Central anticline (Figure 6).

West of the Central anticline, south of Kerber Creek, thrusts repeat two slices of

Kerber Creek formation. The presence of carbonaceous shale layers with clean quartz sandstones in the middle of the arkosic Minturn Sandstone defines two imbricate faults.

Another possible interpretation of these cleaner sandstones with carbonaceous shales is an alternation of the depositional environment to Kerber Creek marine, deltaic plain conditions then returning to terrigenous Minturn fan deposits. This cycle would have to occur twice.

However, the slope above these two anomalous "Kerber" outcrops is covered by brecciated

Minturn float. The breccia may be indicative of thrusts causing the repeat of section.

Folds

The Western syncline is located along the western portion of the Kerber Creek map area. It is mainly buried under Quaternary alluvium, Tertiary volcanics and the Laramide

"Wagner Breccia". The western limb is exposed in a hill between Little Kerber Creek and

Columbia Gulch. The limb is composed of Paleozoic rocks (Manitou to Leadville). The eastern limb of the fold is exposed in the Soda Spring Gulch area. Stereographic determination reveals the fold axis to be 7 °, S41 °E and the approximate axial plane to be

N40 °W, 80 °E. 19

The Central anticline is east of the Western syncline. The western limb of the anticline contains Paleozoic rocks (Manitou to Kerber) in fault contact with Precambrian rocks and Paleozoic rocks (Manitou to Minturn) in unconformable contact with

Precambrian rocks. The eastern limb contains Paleozoic rocks (Manitou to Minturn) deposited unconformably on Precambrian rocks. The Chaffee group defines the top and plunging nose of the Central anticline. The core of the gently plunging anticline is

Precambrian basement. The Central anticline is defined stereographically by a fold axis

19 °, S33 °E and an axial plane striking N31 °W and dipping 83 °NE. The Clayton thrust cuts the eastern limb of the fold in Cody Gulch.

Between the Kerber thrust and the Noland thrust, the Noland syncline is defined by a series of Paleozoic rocks (Manitiou to Leadville) deposited on Precambrian rocks. The fold is divided into three sections by north -trending vertical faults. The largest portion, the western third, is defined by a fold axis oriented 4 °, S61 °E and an approximate axial plane oriented N60 °W, 67 °NE. The central portion contains Paleozoic rocks (Manitou to

Fremont) in a fold defined by a fold axis 6 °, S86 °W and an axial plane oriented S86 °W,

88 °N. The eastern portion of the syncline contains Paleozoic rocks (Manitou to Fremont) and is defined by a fold axis oriented 1°, N87 °W and a axial plane oriented N87 °W, 85 N.

The geometry of the fold may reflect the geometry of the underlying Noland thrusts. The shape of syncline can be explained by a flat -ramp geometry of the fault (Figure 7).

The eastern portion of the Kerber Creek area, the Clayton Cone area, contains the

Clayton anticline. The anticline contains Paleozoic rocks (Manitou to Kerber) with a

Precambrian core. The fold is defined by an approximate fold axis 10 °, S46 °E and an axial plane N43 °W, 72 °NE. The Villa Grove fault separates the western limb and nose of

Clayton anticline from the fold's crystalline core.

Minor folds found in the limestones suggest a top to the north displacement. These structures however are not necessarily solely from Laramide deformation. 20

Striations

Fault striations were found in only three places in the Kerber Creek Area. Two of the outcrops were north of the Kerber thrust. Striations in Mintum Sandstone are 12 °,

N22 °E; 28 °, S62 °E; and 26 °, S56 °E,.The third outcrop was south of the Villa Grove fault at Clayton Cone. The striations in Kerber Creek sandstone are 4 °, S26 °W; 7 °, N16 °W;

22 °, S25 °E; 18 °, S 12 °W; 28 °, S 18 °W; and 3 °, S22 °W. Figure 8 shows stereoplots of these striations. The distribution may imply a slight left lateral component of motion along the faults. The scarcity of data points, however, makes this only a possible interpretation.

Relationship Between the Faults and the Folds

Previous workers required a separate event for folding and faulting. However, flat- ramp geometries along the faults in the Kerber Creek area can be used to explain the folds of the area. Figure 7 shows one possible flat ramp geometry. Fold geometry and surface dip are the only constraints placed on the placement and shape for the fault.

The flat -ramp geometry suggested in Figure 7 would put a limit on the amount of displacement on that fault. The syncline shapes requires displacement less than one flat length. If the ramp /flat geometry is somewhat periodic then displacement would then need to be slightly less than a multiple of the period of flat/ramp length. Pairing that displacement magnitude with the slip direction suggested by the few striations we have and the tear fault geometry on the Noland Syncline leads to the conclusion that displacement on the Kerber Thrust is a minimum of one km toward the north. Displacement on the other thrusts in the Kerber Creek area would be smaller than the Kerber Thrust displacement.

LINKAGE OF SANGRE DE CRISTO RANGE TO KERBER CREEK 21

The Kerber Creek area was separated from the Sangre de Cristo Range by the opening of the San Luis Rift Valley, the northern extension of the Rio Grande Rift. The

Rio Grande Rift began opening approximately 26 Ma (Lipman and Mehnert, 1975). The amount of extension across the San Luis Valley is estimated at approximately 1 km at 37° N at the New Mexico - Colorado state boundary (Cordell, 1982). Extension at 38.25 ° N, the latitude of Kerber Creek would be slightly smaller. This is based on a Euler pole at 41 °N with 1.32° clockwise rotation. Hamilton (1988) estimates an Euler pole in central Colorado with a 3° clockwise rotation. In either case the extension is not significant enough across the San Luis Valley to obscure the correlation of structures between the Kerber Creek area and the Sangre de Cristo Range (Figure 9).

In the Northern Sangre de Cristo Range a set of E -W trending faults interrupts the generally N -S trending Laramide thrusts, possibly representing a partitioning of the NE- directed deformation from dominantly N -S faults to both N -S and E -W faults. The E -W trending, south -dipping faults may be correlated with the reverse faults in the Kerber Creek area across the to Recent San Luis rift valley (Lipman and Mehnart, 1975).

Based on fault orientation and the contents of the hanging wall and foot wall, each fault is compared to those across the valley.

Bridwell (1968) and Knepper (1974) correlated the Kerber structures with the

Sangre de Cristo faults. The Sangre de Cristo Range was remapped in 1987 by Lindsey and Soulliere. The Sangre de Cristo faults Bridwell and Knepper correlated to the Kerber

Creek structures were mapped as north dipping features by Lindsey and Soulliere (1987).

Since the Kerber Creek area faults are south -dipping structures, this correlation is suspect.

Correlation between south -dipping faults in both areas would be a more plausible solution.

Even without correlating individual features, it is evident that the deformation continues in a north -westerly direction from the Sangre de Cristo Range, across the San Luis Valley, through the Kerber Creek region and under the Bonanza volcanic field (Figure 9). 22

For example, the Kerber thrust, the structurally highest thrust in the Kerber Creek area, carries solely Precambrian basement. This correlates with thestructurally highest thrust in the Sangre de Cristo stack which also carries solely Precambrian basement.

The north trending faults of the Sangre de Cristo Range disappear into Precambrian on Precambrian contacts. They are last mapped by Lindsey and Soulliere (1987) turning toward the west across the San Luis valley. These faults may also have correlative faults on the west side of the valley north of the Kerber Creek area where Precambrian basement is exposed beneath the Tertiary volcanics. The granite on granite relationships, however, makes these faults difficult to locate.

DISCUSSION

Lindsey and Suilliere (1987) estimated a minimum of 8 -14 km displacement in the

Sangre de Cristo Range based on thrust sheets below those that carry only Precambrian rocks. Since they have no footwall or hangingwall cutoffs to correlate, displacement of the latter is not easily determined. Thrusts which carry Precambrian rocks over Paleozoic rocks are correlated with those present in the Kerber Creek area. The thrusts in the

Paleozoic rocks continue to the north in the Sangre de Cristo Range and disappear into

Precambrian on Precambrian contacts. The displacement on the major Precambrian over

Paleozoic fault slices is transferred into the Kerber Creek area along the Kerber, Noland,

Clayton and Villa Grove thrusts. The actual amount of displacement represented in this area is indeterminate. Lindsey (oral communication, 1993) estimates that there are at least several kilometers of displacement represented by the Precambrian thrust slices in the northern Sangre de Cristo Range. If so, the displacement along those thrusts in the Kerber

Creek area is also a minimum of several kilometers. Striations found near the Kerber and

Villa Grove thrusts indicate motion in a north -south direction with a possible component of left lateral motion. Tear faults in the Noland Syncline support a north -south displacement along the faults north and south of the fold (Figure 6). 23

We did not complete the link between the northeast -verging deformation in the

Sangre de Cristo and the Elk Range -Sawatch southwest -verging deformation. The Sangre de Cristo deformation turns toward more north verging structures in the Kerber Creek area but there it is covered by Tertiary volcanics of the Bonanza caldera. The link between the two thrust systems must continue to the northwest of the Kerber Creek area.

It is possible that detailed mapping of where the Elk Range- Sawatch system disappears toward the south under the Bonanza volcanics may result in a better understanding of the link between the two systems. The opposite vergence and minimum of 8 kilometers of displacement in some way must be accommodated in this area.

The presence of an accommodation zone between the two thrust systems may have had an effect on the position of the Bonanza caldera and Northern San Juan volcanic field.

Crustal weakness due to the accommodation of the minimum 8 -10 km of left lateral displacement may have created a corridor for the Oligocene intrusions. Magma may have taken advantage of the preexisting structures, thereby obscuring any evidence of Laramide deformation. If this is the case the complete trace of the Laramide accommodation zone may never be found.

ACKNOWLEDGMENTS:

Thanks go to Peter Coney and George Gehrels for their assistance and patience during the preparation of this manuscript. We would also like to thank the Wagner family for their hospitality and for granting access to most of the land included in this project.

REFERENCES CITED

Bridwell, R. J., 1968, Geology of the Kerber Creek area, Saguache , Colorado: Unpublished Master's Thesis, Colorado School of Mines, Golden, Colorado, 82 p.

Bruns, D. L, Epis, R. C., Weimer, R. J.,and Steven, T. A., 19 ? ?, Stratigraphie relations between Bonanza center and adjacent parts of the San Juan Volcanic field, South -central Colorado: NMGS 22nd Annual Field Conference, p. 183 -190. 24

Bryant, B., 1966, Possible Window in the Elk Range thrust sheet near Aspen, Colorado: United States Geological Survey Professioal Paper 550 -D, p. Dl -D8.

Burbank, W. S., 1932, Geology and ore deposits of the Bonanza mining district, Colorado: United States Geological Survey Professioal Paper 169, 165 p.

Burbank, W. S., and Goddard, E. N., 1937, Thrusting in Huerfano park, Colorado, and related problems of orogeny in the Sangre de Cristo Mountains: Bulletin of the Geological Society of America, v. 48, p. 931 -976.

Chapin, C. E., and Cather, S. M., in press, Tectonic setting of the axial basins of the Northern and Central Rio Grande Rift in Keller, G. R., and Cather, S. M., eds., Basins of the Rio Grande Rift: Structure, stratigraphy, and tectoic setting: Geological Society of America Special Publication 291, in press.

Coney, P. J., 1978, Mesozoic- Cenozoic Cordilleran plate tectonics: Geological Society of America Memoir 152, p. 33 -50.

Coney, P. J., 1987, The regional tectonic setting and possible causes of Cenozoic extension in the North American Cordillera in Coward, M. P., Dewey, J. F., and Hancock, P. L., Continental Extensional Tectonics, Geological Society Special Publication No. 28, p. 177 -186.,

Cordell, L., 1982, Extension in the Rio Grande Rift: Journal of Geophysical Research, v. 87, no. B10, p. 8561 -8569.

Couples, G., and Stearns, D. W., 1978, Analytical solutions applied to structures of the Rocky Mountain foreland on local and regional scales: Geological Society of America Memoir 151, p. 313 -335.

DeVoto, R. H., 1972, Pennsylvanian and Permian stratigraphy and tectonism in Central Colorado: Colorado School of Mines Quarterly, v. 67(4), p. 139 -185.

Gabelman, J. W., 1952, Structure and origin of Northern Sangre de Cristo Range, Colorado: American Association of Petroleum Geologists Bulletin, v. 36(8), p. 1574 -1612.

Gaskill, D. L., and Godwin, L. H., 1963, Redefinition and correlation of the Ohio Creek Formation (Paleocene) in westcentral Colorado: Art. 69 in United States Geological Survey Professional Paper 475 -C, p. C35 -C38.

Gaskill, D. L., and Godwin, L. H., 1966, Geologic map of the Mrble quadrangle, Gunnison and Pitkin Counties, Colorado: United States Geological Survey Map GQ -512.

Godwin, L. H., and Gaskill, D. L., 1964, Post -Paleocene West Elk laccolithic cluster, west -central Colorado in Geol. Survey Research 1964: United States Geological Survey Professioal Paper 501 -C, p. C66 -C68.

Hamilton, W. B., 1988, Laramide crustal shortening, Geological Society of America Memoir 171, p. 27- 39.

Jordan, T. E., Isacks, B. L., Allmendinger, R. W., Brewer, J. A., Ramos, V. A., and Ando, C. J., 1983, Andean tectonics related to geometry of subducted Nazca plate: Geological Society of America Bulletin, v. 94, p. 341 -361.

Karig, D. E., 1965, Geophysical evidence of a caldera at Bonanza, Colorado: United States Geological Survey Professional Paper 525 -B, p. B9-1312.

Kirk, E., 1930, The of Colorado: American Journal of Science, v. 20, p. 456 -465. 25

Kluth, C. F., 1986, Plate tectonics of the Ancestral Rocky Mountains in Peterson, J. A., ed., Paleotectonics and sedimentation in the Rocky Mountain region, United States, American Associatin of Petroleum Geologists Memoir No. 41, p. 353 -369.

Kluth, C. F., and Coney, P. J., 1981, Plate tectonics of the Ancestral Rocky Mountains: Geology, v. 9, p. 10 -15.

Knepper, D. H., Jr., 1974, Tectonic analysis of the Rio Grande Rift zone, central Colorado: Unpublished Master's Thesis, Colorado School of Mines, Golden, Colorado, 205 p.?

Lindsey, D. A., and Soulierre, S. J., 1987,

Lindsey, D. A., Andriessen, P. A. M., and Wardlaw, B. R., 1986, Heating, cooling and uplift during Tertiary time, northern Sangre de Cristo Range, Colorado: Geological Society of America Bulletin, v. 97, p. 1133 -1143.

Lindsey, D. A., Johnson, B. R., and Andriessen, P. A. M., 1984, Laramide and structure of northern Sangre de Cristo Range, south -central Colorado: American Association of Petroleum Geologists Bulletin, v. 68(7), p. 941.

Lindsey, D. A., Reino, F. C., and Soulliere, S. J., 1985, Minturn and Sangre de Cristo formations of Southern Colorado: prograding fan -delta and alluvial -fan sequence shed from Ancestral Rocky Mountains: American Association of Petroleum Geologists Bulletin, v. 69(5), p. 854.

Lindsey, D. A., Reino, F. C., Ashe, S. J., and Flores, R. J., 1983, Minturn and Sangre de Cristo formations of Southern Colorado: A prograding fan -delta to alluvial -fan sequence shad form Ancestral Rocky Mountains: American Association of Petroleum Geologists Bulletin, v. 67(8), p. 1348.

Lipman, P. W., Steven, T. A., and Mehnert, H., 1970, Volcanic history of the , Colorado, as indicated by potassuim -argon dating: Geological Society of America Bulletin, v. 81, p. 2329 -2352.

Lipman, P. W., and Mehnert, H. W., 1975, Late Cenozoic basaltic volcanism and development of the Rio Grande depresiion in the Southern Rocky Mountains, in Curtis, B. F., ed., Cenozoic history of the Southern Rocky Mountains, Geological Society of America Memoir 144, p. 119 -154.

Mayhew, J. D., 1969, Geology of the eastern part of the Bonanza volcanic field, Saguache County, Colorado: Unpublished thesis, Colorado School of Mines M. S., Thesis T -1226, 94 p.

Prucha, J. J., et al., 1965, Basement -controlled deformation in Wyoming province of Rocky Mountain Foreland: American Association of Petroleum Geologists Bulletin, v. 49, no. 7. p. 966 -992.

Reed, J. C., Bickford, M. E., and Tweto, O., 1993, accretionary terranes of Colorado and southern Wyoming in Reed, J. C., et ah, eds., Precambrian: Conterminous U. S., The Geology of North America, v. C -2, p. 211 -228.

Reynolds, S. J., and Coney, P. J., 1977, Cordilleran Benioff zones: Nature, v. 270, p. 403 -407.

Smithson, S. B., et al., 1978, Question of the Wind River thrust, Wyoming, resolved by COCORP deep reflection data and by gravity in Boyd, R. G., ed., Resources of the Wind River Basin, Wyoming Geological Association 30th Annual Field Conference Guidebook, p. 227 -234.

Tweto, O, 1975, Laramide orogeny in the Southern Rocky Mountains in Curtis, B. F., ed., Cenozoic history of the Southern Rocky Mountains, Geological Society of American Memoir 144, p. 1 -44.

Tweto, O., 1977, Nomenclature of Precambrian rocks in Colorado: United States Geological Survey Bulletin 1422 -D, 22 p. 26

Tweto, O., 1980, Precambrian geology of Colorado in Kent, H. C., and Porter, K. W, eds., Colorado Geology, Rocky Mountain Association of Geologists, p. 37 -46.

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Figures:

1. Figure after Hamilton (1988) showing the extent of Laramide deformation, the edge of imbricate thrusting and the major basement involved thrusts.

2. Regional map with Elk Range /Sawatch thrusts and Sangre de Cristo thrusts showing

Kerber Creek1:250,000.

3. Inset to figure 1. Cartoon showing the relative displacements along the thrust systems of south -central Colorado. Fine line is approximate original position, bold line is approximate post- Laramide position. Between the opposite vergences an accomodation zone must be present. The location of such a zone is in question.

4. Stratigraphy of the Kerber Creek area. Formations are drawn to scale with the exception of the Mintum formation whose thickness varies up to over 1000 m.

5. Cross Sections a) after Burbank (1932) thrust model b) after Bridwell (1968) upthrust model. The thrust model requires lateral continuity of strain and greater displacement along the faults. Both of these cross -sections were drawn based on separate folding and faulting events.

Determination of dip cross -section across Kerber thrust

6. Geography and geology of the Kerber Creek area

7. Cross Section A -N' and flat -ramp geometry model. This model explains synclines in the hangingwalls of the thrust faults but places a limit on the amount of displacement on the faults to approximately 1 flat length.

8. Striations in the Kerber Creek Area 27

9. Continuity of deformationacross the San Luis Valley. Notice match between the deformation in the Sangre de Cristo Range and the Kerber Creek Area. The continuation of the deformation continue to the northwest under the Tertiary Bonanza caldera. 115° 110° 105°

I CANA_ "r". o East Limit of Imbricate Thrusting I Ne ',. - o . MONTANA H Precambrian Basement Involved . _ I Ooc Thrusts / ../- \ ra-41 45° , Ó 45° Major Laramide Folds % / li)clá Limit of Major Cenozoic Normal Faults

40° 40°

Figure 3

35° 35°

a 1 I , 1 0 kilometers 500

Figure 1. Map after Hamilton (1988) showing western United States and"selected structural elements ". D Quaternary

Ik':.t{:; j Terti a ry

Mesozoic

• Paleozoic

• Precambrian

Elk Range-.Sawatch Sangre de Thrust System CristoThrust System

1 kilometers I I I 0 10 20 Williams Range Elkhorn Thrusts Minimum 6 km SW Bryant & Nasser, 1980

Elk Range- Sawatch Thrusts Minimum 11 km SW Bryant, 1966

Wet Mountain N Thrusts Gunni son Minimum 9.5 km NE Jacob & Albertus, 1985

Salida

Sangre de Cristo- Kerber Creek Thrusts Minimum 8 km NE Lindsey, 1984

,..TAMM AI amusa o kilometers 100

Figure 3. Inset to figure 1. Cartoon showing relative displacements along the thrust systems of south -central Colorado. Fine line isapproximate original position, bold line is approximate Laramide position.Arrows are not to scale. Kerber Creek Stratigraphy

Quaternary Alluvium ,. ,. ,. ", ,. ,. A ,. ,. ,. A.A ", A"" A A A ,. A ,. ,. A ,. ,. ,. ,. A ,. ,. ,. ,. ,. A ,. Ie ,. ,. Ie A ,. ,. ,. A A ,. A J ,. ,. ,. ,. ,. ,. ,. ,. ,. A ,. A ,. A It. ,. ,. II. ,. ,. ,. ,. A ,. , ,. ,. ,. ,. ,. A A A A A A A A A A ,. ,. ,. A ,. ,. ,. ,. ,. ,. A ,. A A A A A A A A Mid-Tertiary Bonanza Tuff ,. ,. ,. ,. ,. ,. ,. ,. A A ,. ,. ,.,.,.,.,.,.,.,.,.,. A A A A A A ,. A A .' .'" A A A A A A A"''' ", ...... i • ::: ...::: ::: :::. ..41 Laramide "Wagner Breccia" ~~ ~~~ .:' ~"\~~"\~~"\~"\'~"\~ ;~ ~~~ ~-;,~,",!' L":.'~ ~?l?Ri~?i~~ ~{~ ~~~ ~~~~~'~~ ~~ jr!i~'!.:~r!;~r!l~ ~i~ ~~~ '~, r~ 'Jo' Jo iJo' 'Jo,i~ ~~~L ~~~~~'~?i,~ -~ ~i,~ ~?i JI ~~. not drawn to scale I~ Middle Pennsylvanian Minturn Formation \'!o"\~~~;~~~I' ~: -:. -:..~~~~~~~ I~ ~~~'!o~'!o;~~\"\.~~~~~ ~.~~~~~ ~~~ ~~,;; '~~.~l~ ~~li:Y'!.~}~~~~~ 1~I·i ~~l ~~~'~~~ ~~~.~ i~:"j ~~ L":."- " ~J:i.?ij.·:~ l'i..J:~?l~ ;,;,: ,,;; ~ .. ~~, ~~~;~}~ I~~~~~" '~, .'!.'~' •• ~i:.',~~~ ~~ ~i:.'!o~ ~~ Lower to Middle Pennsylvanian Kerber Creek ~~I~',':' '-;; ~~i ~":.i~":., L":., L":.i ~":.I ~":., L":. ~ ~ ~-.; "I "I ~-?-v. ~~~~ Lower to Middle Mississippian Leadville ~).~~~ ~ ~ ~>y--~ ."< ."< "< "< "< Upper Devonian Chaffee Formation

Upper Ordovician Fremont

Middle Ordovician Harding 150 Lower Ordovician Manitou ,,'",~ ,,>" , ,'.' , .', .'.', , .. ,'" ,',,' ,.,- .,- ,,' ", ", ", ", ", ", ", ", "", ,.,- , ,,' ", ", ", ", ", ", ", ", "", '. ,- , ", ", ", ", ", ", ", ", ", ", ", Precambrian Granite and Gneiss o " ' ", ", ", ", ", ", ", ", ",.,- " ". , ", ", ", ", ", ", ", ", ", ", , " .""" ." ."" ." ." '" ." '" '" a) A BurbankKerber (1932) Fault Noland Fault A' SynclineClayton Villa Grove Fault Clayton Cone Anticline 9000' PC fi Pz PC .! Pz - . . ,- ' . - - - , ' A" 7000' o wl' ..... Pz ,', - -.- ,,.-' _ - -. Pz , '.- ,,,-' /// PC Pz 5000' - -- PC 1 ( ///i./ .-___-'" % // i, b) Bridwell (1968) SynclineNoland A' SynclineClayton Clayton Cone Anticline Kerber Fault. Noland Fault - / Villa Grove Fault... % .... 555-5 . /`1 9000' Clayton Fault -.-- \ \ . ., A 7000'5000' .. ,r afterFigure Burbank 5. Different (1932) interpretations b) upthrust of the Kerber Creek Area fault /fold geometry after Bridwell (1968). PC = Precambrian, Pz geometry. a) Thrust geometry = Paleozoic.. Kerber Creek Area Geologic Map

.: .::::...... :. :::::... . :-.: ~~~ : :::::::: : ~::~

...... ~~dy Spr ng Fault~ :::: ~ : .- :::::::: : ::::: : :: : :: : :::::::: :

o

~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~I Quaternary Mississippian .""I """ """"" ". Alluvium Leadville Formation

Tertiary Devonian Volcanics Chaffee Group

Mesozoic Ordovician Fremont, Harding, Wagner Breccia and Manitou Formations

Pennsylvanian-Permian Precambrian Minturn and Kerber Formations Undifferentiated

Figure 6. Geology and Geography of the Kerber Creek area. Kerber Fault SynclineNoland A' Clayton Cone Anticline 9000' - A PC Nolandr.. Fault ''...... 0 1 Clayton Fault Villa Grove Fault /.P A" 7000' PC ... ?? mall Pz r worm garb PC 5000' _- r , / - / _ ?? `? ,"? ..."1 ? .+ / / / A 9000' - ? I-I ? A" 7000'5000' . / o Figure 7. Geologic cross section and model of thrust geometries in the Kerber Creek i . vv._ Area. PC = Precambrian, Pz = Paleozoic. «S· Q) ~ «S ~ Q) Q) "- ()

~ Q) .0 ~ Q) ~ Q) .J::.... \t-o UJ c: .Q "i:ca en =:. :J ...... «S U) / ...... ")1...... k. ....···....·.h>.... c ...... " ...... II u. .-0 ex:> ... Q) .-cu ~ ~ ::J ... C 0) en iL. Kerber Creek and the Sangre de Cristo Thrust System

D Quaternary \ \ • Tertiary \ Y'" Paleozoic \ \ Precambrian

\--A \ \ -.-... \ \

kilometers I . " , o 10 20 \ \