Linkages of Laramide Thrusts, NorthernSangre de Cristo Range, Colorado

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Authors Bedford, Janice M.

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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 beensubmitted in partial fulfillment of requirements for the Master of Science degree at the University ofArizona and is deposited in the Antevs Reading Room to made available to borrowers as are copiesof regular theses and dissertations.

Brief quotations from this manuscript are allowable without special permission, provided thataccurate acknowledgment of source is made. Requests for extended quotation from orreproduction of this manuscript in whole or in part may be granted by the head of the department,or the graduate program coordinator, when in their judgment the proposed use of the material is inthe interests of scholarship. In all other instances, however, permission must be obtained from theauthor.

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

APPROVAL BY RESEARCH ADVISORY COMMII"I'EE

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

Maj r Advisor Clement G. Chase)r

Peter J. ney

Df Atipst /5'4t

a vz- a, Ì'7Date

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 Rocky Mountains have been mapped asdiscrete units with little investigation into the linkage between displacements. The ElkRange- Sawatch and Elkhorn thrust systems displaced Precambrian, Paleozoic, andMesozoic rocks toward the southwest. The Sangre de Cristo Range and Wet Mountainsthrust systems displaced rocks toward the northeast. The opposite vergence and oppositelydirected displacements between these systems must be accommodated, both at presentlevels of exposure and at depth. Mapping of the Kerber Creek area west of the northernSangre de Cristo Range by J.M. Bedford helped answer the question of the linkagebetween the opposing Elk Range- Sawatch and Sangre de Cristo Range thrusts.

In the Elk Range- Sawatch system the westward displacement on the thrusts isinterpreted as a minimum of 11 km in the southern Elk Range with displacement decreasingtoward the north. Bryant (1966) interpreted the timing of faulting as Paleocene in age. Inthe Sangre de Cristo Range northeast -verging thrust system the minimum estimatedeastward displacement is 8 km. Burbank and Goddard (1937) interpreted the displacementas Eocene 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 facingdisplacement. Its internal structure most closely resembles that of the more proximalSangre de Cristo deformation. Thrusts in the Kerber Creek area place Precambrian rocksover Laramide(?) and Paleozoic sedimentary rocks. The structures verge toward the north -northeast and represent a minimum of several kilometers of displacement. These Laramidestructures are exposed where overlying Tertiary volcanics are eroded.

In the Northern Sangre de Cristo Range a set of E -W trending faults intersects thegenerally N -S trending Laramide thrusts, possibly representing a partitioning of northeasttransport into N -S and E -W components. The E -W trending deformation can be correlatedacross the San Luis Valley with the thrust faults in the Kerber Creek area. Correlation ofthe Kerber Creek thrusts with Sangre de Cristo faults extends the northeast directedLaramide deformation 22 km northwest of the edge of the Sangre de Cristo Range. Thusthe 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 thrustsystem may reveal how the opposite vergence and minimum 8 -10 km of displacement areaccommodated.

2

STATEMENT OF PROBLEM AND BACKGROUND

Late Mesozoic to Early Cenozoic deformation of the Foreland Fold and Thrust belt

in the western United States can be divided into two structural zones; a western zone of

imbricate thrusting of the passive margin sequence known as the Sevier belt and an eastern

zone of basement -involved thrusts of Laramide age (Figure 1). The basement -involved

thrusts of the Southern Rocky Mountains 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 Central Colorado 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 fault 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- Sawatch 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 Snowmass Mountain to just north of Tomichi dome, a distance of 80

km (Bryant, 1966). The thrusts displace Precambrian, Paleozoic and Mesozoic rocks over

rocks as young as Cretaceous. 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 Late Cretaceous

Mesaverde Formation in the Marble quadrangle (Gaskill and Godwin, 1966) and is

intruded by post -early Eocene bodies which metamorphosed rocks containing Paleocene

plant fossils (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 geochronology 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 Orogeny

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 margin as South America and Africa

collide with the North American continent (Kluth, 1986 and Kluth and Coney, 1981). This

deformation began in the Early Pennsylvanian and ended in the Early Permian. The

deformation extending deep into the North American plate may 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 Front Range -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 Laramide

orogeny (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 Rio Grande Rift, a structural zone between

the Colorado Plateau 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 New Mexico 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 Cambrian Sawatch Sandstone which is found in other parts of

south -central Colorado is missing from the Kerber Creek area. The Ordovician 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 fossil

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). Mississippian 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(?), Paleozoic and 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). An upthrust 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 of Laramide 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 deformation in 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 east in 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 Creek area 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 this interpretation.

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 Quaternary 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 Oligocene 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 the structurally 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 County, Colorado: UnpublishedMaster'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 Bonanzacenter and adjacent parts of the San Juan Volcanic field, South -central Colorado: NMGS 22nd AnnualField Conference, p. 183 -190.

24

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

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

Burbank, W. S., and Goddard, E. N., 1937, Thrusting in Huerfano park, Colorado, and related problems oforogeny 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 CentralRio 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 Memoir152, p. 33 -50.

Coney, P. J., 1987, The regional tectonic setting and possible causes of Cenozoic extension in the NorthAmerican Cordillera in Coward, M. P., Dewey, J. F., and Hancock, P. L., Continental ExtensionalTectonics, 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 Mountainforeland 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: AmericanAssociation 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 Paper475 -C, p. C35 -C38.

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

Godwin, L. H., and Gaskill, D. L., 1964, Post -Paleocene West Elk laccolithic cluster, west -centralColorado 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 GeologicalSurvey Professional Paper 525 -B, p. B9-1312.

Kirk, E., 1930, The Harding sandstone 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 Associatinof 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: UnpublishedMaster'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 duringTertiary 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 Neogene structure ofnorthern Sangre de Cristo Range, south -central Colorado: American Association of PetroleumGeologists Bulletin, v. 68(7), p. 941.

Lindsey, D. A., Reino, F. C., and Soulliere, S. J., 1985, Minturn and Sangre de Cristo formations ofSouthern Colorado: prograding fan -delta and alluvial -fan sequence shed from Ancestral RockyMountains: 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 Cristoformations of Southern Colorado: A prograding fan -delta to alluvial -fan sequence shad form AncestralRocky 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 San Juan Mountains,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 RioGrande depresiion in the Southern Rocky Mountains, in Curtis, B. F., ed., Cenozoic history of theSouthern 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 MountainForeland: American Association of Petroleum Geologists Bulletin, v. 49, no. 7. p. 966 -992.

Reed, J. C., Bickford, M. E., and Tweto, O., 1993, Proterozoic accretionary terranes of Colorado andsouthern Wyoming in Reed, J. C., et ah, eds., Precambrian: Conterminous U. S., The Geology ofNorth 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 deepreflection data and by gravity in Boyd, R. G., ed., Resources of the Wind River Basin, WyomingGeological Association 30th Annual Field Conference Guidebook, p. 227 -234.

Tweto, O, 1975, Laramide orogeny in the Southern Rocky Mountains in Curtis, B. F., ed., Cenozoichistory 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 SurveyBulletin 1422 -D, 22 p.

26

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

Tweto, O., and Sims, P. K., 1963, Precambrian ancestry of the Colorado Mineral Belt: Geological Societyof America Bulletin, v. 74, p. 991 -1014.

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 Creek 1:250,000.

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 deformation across 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°

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Figure 3

Figure 1. Map after Hamilton (1988) showing western United States and "selectedstructural elements ".

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150

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Kerber Creek Area Geologic Map

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