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AAPG Bulletin

Abstract


Volume: 36 (1952)

Issue: 8. (August)

First Page: 1574

Last Page: 1612

Title: Structure and Origin of Northern Sangre De Cristo Range, Colorado

Author(s): John W. Gabelman (2)

Abstract:

Reconnaissance of the Lower Paleozoic sediments diagonally crossing the northern Sangre de Cristo Range of southern Colorado disclosed an imbricate zone of east-dipping thrusts associated with parallel, normal, and transverse faults on both sides of the range. Open folds sharpen southeastward from Wellsville, breaking into thrusts causing several repetitions of the section, then dying out without transition through folds. The folds at Orient also break into thrusts to the north and appear to denote the southern end of the thrust zone. Two areas offer evidence for shallow thrusting confined to the Lower Paleozoic. Transverse faults commonly displace thrusts. The steep western slope of the range is considered the scarp of a normal fault.

Four ages of Laramide and Tertiary deformation are recorded. The zone of westward-directed thrusting is part of a broader zone including the Pleasant Valley fault and similar thrusts west of the San Luis Valley. This zone is probably continuous with the early Eocene London-Weston thrust belt of South Park and the Mosquito Range. The northeast-directed folds and thrusts of the Sangre de Cristo Range south of Orient have no counterpart in the northern range. Large normal faults in the Wellsville-Orient thrust zone indicate vertical uplift of a pre-Cambrian area and are correlative with late Eocene uplift on the south. Downfaulting of the San Luis Valley from Oligocene to Recent outlines the present trend of the range and explains its divergence from the zone of thrusting.

Text:

INTRODUCTION

During 1950 a reconnaissance was made of the Lower Paleozoic limestone belt diagonally crossing the northern Sangre de Cristo Range of south-central Colorado from Wellsville in Fremont County to Orient in Saguache County.

Previous workers have shown that the range from Orient south resulted principally from middle Eocene northeast- and east-directed compression in a zone marginal with the Sierra Blanca pre-Cambrian massif.

Until the present time the extreme rugged topography, thick mantles of glacial, alluvial, and vegetative cover, and the lack of known economic mineral deposits have impeded study of the northern part of the Sangre de Cristo Range. However, workers in adjacent regions have projected structures into the region, and have postulated that a continuation of the Mosquito Range-South Park thrust belt, which resulted from west-directed compression, would be found. These projections have been found generally correct.

PHYSIOGRAPHIC FEATURES

The Sangre de Cristo Range trends north-northwest through south-central Colorado from New Mexico to Salida where it terminates against the Arkansas River Valley (Fig. 1). The range is narrow, 10-20 miles wide, and its western slope is much steeper and more precipitous than the eastern slope. It is bordered

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Fig. 1. Index map.

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on the west by the wide, flat San Luis Valley which in turn rises gradually to the San Juan Mountains of southwestern Colorado. East of the range are Huerfano Park and the Wet Mountain Valley. These separate the Sangre de Cristo Range from the Wet or Greenhorn Range which is the easternmost range of southern Colorado and lies en echelon with the Front Range at the north. South Park, which is west of the Front Range and aligned with Huerfano Park and the Wet Mountain Valley, is a broad flat intermountain plain comparable in size with, and approximately at the same elevation as, the San Luis Valley. It is bordered on the south by low volcanic hills and the Arkansas River Canyon, and on the west by the Mosquito Range and the low hills of Coffman and Aspen ridges extending south from that range. West of these lies the broad valley of the upper Arkansas River which is generally aligned with the upper San Luis Valley across Poncha Pass, though the Arkansas is approximately 1,000 feet lower. The Sawatch Range, reported to be the highest mountain range in the United States, borders the upper San Luis Valley and the upper Arkansas Valley on the west. It lies en echelon with the Sangre de Cristo Range and is connected with it by the low Poncha Hills which terminate the San Luis Valley on the north.

REGIONAL GEOLOGY AND PREVIOUS INVESTIGATIONS

The south-central part of Colorado has received considerable attention in the reports dating from the Hayden Survey. However, the Sangre de Cristo Range has been investigated less than most other areas, and that part of the range from Orient north has remained virtually unknown.

The Sangre de Cristo Range was originally described as a large anticline in Pennsylvanian sediments with an exposed pre-Cambrian core by Endlich (1877, pp. 108-39). R. M. Bagg in 1908 (p. 741) noted, opposing Endlich's statement, that a pre-Cambrian core in the fold was not exposed in the vicinity of Rito Alto Peak southeast of Orient.

In 1932 Burbank (pp. 39-41) noted that in Kerber Creek, on the western edge of the San Luis Valley near Villa Grove, northwesterly folds and east-dipping thrust faults of the same age are overridden from the south by later east-west thrusts. Lower Kerber Creek occupies a window-like area in the upper San Luis Valley where Paleozoic sediments appear from beneath the Quaternary fill of the Valley and the Tertiary lavas of the Cochetopa Hills. The east-west Kerber Creek thrusts disappear eastward beneath the gravels of the San Luis Valley, but evidently they turn southeastward (beneath the Valley) and reappear on the west flank of the Sangre de Cristo Range several miles south of Orient. In both areas granite has been thrust north and northeastward over Paleozoic sediments.

Later Burbank and Goddard (1937) traced this zone of thrusting southeastward across the Sangre de Cristo Range into Huerfano Park. The paper resulting from their reconnaissance is a comprehensive treatment of the entire Laramide and Tertiary structural history of the Sangre de Cristo region. They describe the zone of deformation as being marginal with the Sierra Blanca pre-Cambrian

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massif. The rising and expanding of the massif during Eocene time were accomplished by two stages of deformation. The earlier stage produced the south- and southwest-dipping thrusts and folds of the marginal zone. While the thrusts in Kerber Creek, though of low dips, involve the overriding basement and probably steepen in depth, Burbank and Goddard (1937) offer evidence that the thrusts in Huerfano Park are confined to the Lower Pennsylvanian marine shales resting directly on granite and that deformation was confined to the sediments sliding over a fairly stable basement. They state (p. 971):

Thus one may picture the early stages of marginal deformation as having been produced by sheets of granite thrust out from the edges of the Sierra Blanca massif and overriding the marginal zone crumpling and imbricating the sedimentary blanket in front of them.

They estimate the crustal shortening from this deformation to be 9½-11 miles in 8½ miles across Huerfano Park.

The large backbone fold of the Sangre de Cristo Range is shown by Burbank and Goddard (1937, pp. 951-57) to be a diapiric, or injective, anticline in which the core consists of metamorphosed Lower Pennsylvanian marine shales enclosed by a zone of fresh Pennsylvanian marine and continental sediments. The metamorphosed sedimentary core was misinterpreted by Endlich as pre-Cambrian. The origin of such a structure is the upward injection of mobile layers of plastic flow, probably caused, in this case, by intense compression. Zones of shearing mark the boundary between metamorphosed and unmetamorphosed sediments and represent planes along which injection was aided.

The foregoing interpretation implies that that part of the range occupied by the diapiric fold was uplifted by lateral compression during the deformation marginal with the Sierra Blanca massif, which is mostly pre-Huerfano or pre-Bridger (thus lower to middle Eocene). Burbank and Goddard (1937, pp. 971-72) show that the massif itself was raised by differential vertical uplift in post-Bridger time (upper Eocene to lower Miocene), although uplift probably was initiated with or shortly after marginal deformation. The maximum vertical uplift in Blanca Peak was about 3 miles. This uplift is also correlative with the final uplift of the Wet Mountains east of Huerfano Park and the Wet Mountain Valley.

The depression of most of the massif and simultaneous downfaulting of the San Luis Valley occurred from late Oligocene to the end of Tertiary time (Burbank and Goddard, 1937, p. 965), and outlined the present western margin of the Sangre de Cristo Range as a series of normal fault scarps. They attribute the cause of downwarping and faulting to the foundering of large crustal blocks into zones made weak and deficient by the evacuation of large volumes of lava in the San Juan Mountains on the west.

It is significant that most of the Miocene lavas accumulated on a surface which did not rise sharply from the valley, so that most of the depression of the valley had occurred before the late Miocene, and the volcanic pile itself occasions the gentle topographic rise toward the west. Siebenthal (1910, pp. 50-51) dates the earliest land-deposited sedimentary formation in the San Luis Valley as late

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Miocene. This is the Santa Fe formation in which the sediments are interbedded with lava flows. Atwood and Mather (1924, p. 121) described the sloping of the Pliocene Hinsdale basalt from beneath the level of the valley to the summit of the eastern part of the San Juan Mountains. Also, faulting near Fort Garland displaces the Pliocene Los Pinos gravel (Atwood and Mather, 1932, p. 99). These features indicate a continued depression of the San Luis Valley trough and elevation of the eastern San Juan Mountains throughout the late Tertiary and Quaternary.

The structure of the region north and northeast of the Sangre de Cristo Range is characterized predominantly by large folds and related thrust faults which are earlier than the deformations described. Once the principal range areas were formed, they were influenced collectively by vertical uplift in the late Tertiary and Quaternary.

The Front Range and its echelon companion, the Wet Mountains, are the easternmost of a series of large anticlinal folds trending north-northwest across north-central Colorado. Just west of these ranges a series of downfolded and faulted intermountain synclines or "parks" forms a linear trend parallel with the ranges. These are from southwest to northwest: Huerfano Park, Wet Mountain Valley, South Park, Middle Park, and North Park. The Wet Mountains and Huerfano Park have already been shown as influenced in their present form by late Eocene deformations.

West of the line of large intermountain synclines are the echelon Mosquito, Ten Mile, and Gore ranges which have the same trend and are controlled largely by faulting.

West of the Mosquito Range, and separated from it possibly by a syncline only in the northern area, is the Sawatch Range.

The structure of the eastern ranges is relatively simple and representative mostly of the earliest Laramide deformation, but the central Colorado ranges, while originating from the same deformation, are complicated by later structures.

The series of "range-and-valley" folds from the Front Range west to the Sawatch Range originated from westward directed compression which began in Late Cretaceous (Pierre) time with the arching of folds and culminated in lower Eocene time with the overturning and breaking of folds into high-angle east-dipping thrusts, which cut the Denver formation in the eastern part of the region. The faults are commonly found between the ranges and downfolded valleys and occur in several prominent zones.

The Elkhorn thrust zone on the eastern edge of South Park has been described by Washburne (1910, pp. 307-08) and Stark et al. (1949, pp. 120-23) as a fairly flat east-dipping thrust fault. Its projected trend is continuous with the Williams Range thrust on the north which has been interpreted by Lovering as an underthrust (1932, pp. 650-63; 1935, p. 44). A similar, but steeper, thrust trends northwest through the center of South Park (Washburne, 1910, pp. 307-08; Stark et al., 1949, pp. 123-27). Along the western border of South Park, east of the

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Mosquito Range and Coffman Ridge is a zone containing the Weston and London faults in which overthrusting is also from the east. West of Alma this zone crosses the Mosquito Range and joins with the Mosquito fault. Southward it is somewhat obscured beneath the Oligocene (Stark et al., p. 138) lavas of South Park, but reappears near the Arkansas River and continues south. Just west of this zone is the much debated Mosquito fault, which is steep and whose dip varies from east to west. The fault was recognized as a steep pre-mineral thrust north of Ten Mile Creek, and a post-mineral normal fault southward by Emmons (1886, pp. 284-92; and Emmons, Irving, and Loughlin, 1927, p. 80). Behre (1939, p. 70) considered the dips predominantly eastward and called it a reverse fault. Later, Stark et al. (1949, p. 128) described the diverse attitudes of the fault from Leadville through South Park, and although they state that the only observed dips near South Park are westward, they classify the fault as reverse because of its similarity to other reverse faults at the east. They term the associated thrust zone the "Mosquito-Weston" zone. The interpretation most agreeable to the writer is that of Emmons, Irving, and Loughlin (1927, p. 80), and expressed later by Burbank (1933, p. 286), that the fault has a dual nature, representing early reverse faulting in certain areas and modified later by normal block faulting. South of Leadville the fault is considered entirely normal. The importance of this relationship in correlation with the normal faults of the Sangre de Cristo Range is discu sed later.

Relations in the Arkansas Valley and on the west slope of the Sawatch Range are obscure, but a zone of northwest-trending, east-dipping thrusts of the South Park type is exposed along the west flank of the range (Burbank, 1933, p. 286; Stark, 1934).

Although there are minor differences in age of the early structures (Pierre to post-Denver), and they have been modified by later block faulting of at least two substages, the folds and thrusts in the region resulted from the same orogeny which preceded intrusion of the Early Tertiary stocks and batholiths (Crawford, 1924, pp. 379-80; Burbank, 1933, p. 287), and were earlier than the eastward-directed thrusting in the central Sangre de Cristo Range.

The structure of the northern Sangre de Cristo Range was essentially predicted by Burbank. In describing the London-Weston thrust zone he states (1933, p. 286):

There are several large faults in the region just north and east of Salida that bound the steep east limbs of great synclines and in line with the zone of the London and Weston faults. This zone of faulted folds therefore appears to extend throughout the length of the Gore and Mosquito ranges and southward across the Arkansas River into the northern end of the Sangre de Cristo Range.

In 1937 Burbank and Goddard stated (p. 936):

North of Wet Mountain Valley, the east side of the [Sangre de Cristo] range is bounded by a prominent fault, which probably continues southeastward beneath the alluvial deposits of the valley^hellip

This statement refers to a fault unnamed in the records, and here termed the

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Fig. 2. Geologic and structure map of northern Sangre de Cristo region.

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Pleasant Valley thrust for convenience. It is shown on the State geological map (Burbank et al., 1935) as mapped by Burbank, Butler, and Johnson.

Burbank (1932, p. 38), in discussing the northern San Luis Valley and west slope of the Sangre de Cristo Range, states that evidence indicates the part of the valley north of Mineral Hot Springs does not have a simple synclinal structure as suggested by Siebenthal (1910, pp. 34-37, 50-54). He states further:

In the vicinity of Orient the Ordovician and Carboniferous sedimentary rocks overlie the pre-Cambrian rocks and dip steeply eastward beneath the west slope of the Sangre de Cristo Range.

Charles R. Butler (1949, p. 1960) briefly described the pre-Cambrian core of the northern range and the Lower Paleozoic sediments lapping on its east side. He considers the block of pre-Cambrian isolated in the sediments a horst.

The structural and geologic map of the region (Fig. 2) is a compilation of the writer's observations in the northern Sangre de Cristo Range and south and southwest of Poncha Springs, and the published maps of Burbank in Kerber Creek (1932) and the central Sangre de Cristo Range (1937), J. B. Stone at Orient (1934), J. W. Rold at Wellsville (1950), R. D. Crawford at Maysville (1913), and the State geologic map (1935).

STRATIGRAPHY

In the reconnaissance of the Sangre de Cristo Range, only the Ordovician to Mississippian rocks were studied carefully; pre-Cambrian, Pennsylvanian, and Permian rocks were examined only generally. No detailed stratigraphic sections were measured. The writer, therefore, has drawn liberally from the stratigraphic studies of others in the region (Fig. 3).

PRE-CAMBRIAN ROCKS

The pre-Cambrian crystalline rocks of the region represent an ancient complex of gneissic metasediments invaded by granite of at least two types and ages.

In most of the map area west of the Pleasant Valley fault, the pre-Cambrian rocks are metasedimentary ranging from basic hornblendic gneisses to acidic schists and quartzites. All are injected in varying degrees by stringers and dikes of granite and pegmatite, and in places by small granite plutons. The gneissic granite of the plutons is not similar to either the Pikes Peak or Silver Plume types found farther east. The gneissic banding is normally well developed. High in the pre-Cambrian northern section of the Sangre de Cristo Range this banding is portrayed to such a degree and on such a large scale that the rocks were mistaken for Paleozoic sediments from a distance. The metasediments are folded into a series of distorted isoclinal folds that trend generally north-south. The dips a e steep to vertical.

East of the Pleasant Valley fault the pre-Cambrian is composed entirely of Pikes Peak granite in which are distributed minor patches of undigested metasediments and later Silver Plume granite. The Pikes Peak granite is ordinarily pink and very coarse-grained, and is directly traceable into the Pikes Peak area

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at the east. The Silver Plume granite is gray, not so coarse-grained, and is correlated with the Silver Plume area by virtue of its lithologic similarity (Stark et al., 1949, pp. 21-22).

CAMBRIAN ROCKS

The Cambrian Sawatch quartzite probably once covered the region. In the Monarch district, on the west edge of South Park, and in Kerber Creek, small, thin, isolated patches of quartzite ranging from several inches to 15 feet thick (at Calumet; Osborn and Rainwater, 1934, pp. 21-23) underlie the Manitou dolomite and indicate the quartzite was present, and was removed by pre-Ordovician erosion.

ORDOVICIAN ROCKS

Manitou dolomite:
The Manitou cherty dolomite, Lower Ordovician in age, is the lowest continuous bed that unconformably overlies the pre-Cambrian. It is ordinarily thin-bedded, dark brown to gray, fine-grained crystalline dolomite with lenses and irregular nodules of white or gray chert aligned in zones parallel with the bedding. Chert is most prominent at the base and in a zone near the top, although it is generally prevalent throughout. The lower part is commonly sandy.

Harding quartzite:
The Middle Ordovician Harding quartzite rests conformably on the Manitou, and is remarkably uniform in character and thickness

Fig. 3. Paleozoic formations in northern Sangre de Cristo Range.

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throughout the northern Sangre de Cristo Range. The formation is typically thick-bedded, white, fine to medium-grained quartzite, and is very hard and compact. Shaly partings occur near the top. Oxidation of included iron oxide, probably magnetite, colors weathered surfaces dark red to brown. The Harding is noted for containing fossil fragments of fish plates (Kirk, 1930, pp. 456-65; Johnson, 1945, pp. 21-22), especially near Canon City, and these may be found here and there in the formation throughout the Sangre de Cristo region.

Fremont dolomite:
The Upper Ordovician Fremont dolomite is conformable on the Harding and consists of fairly uniform dark gray to black, crystalline dolomite. It is generally thick-bedded to massive, becoming thinner-bedded toward the top and has blocky jointing. Especially characteristic of this formation are the weathered surfaces which are gray and rough. They are covered with multiple ridges and spines enclosing small, cup-shaped depressions. This feature is found in the Leadville limestone and in places in the Manitou, but is best developed in the Fremont. Various zones are fossiliferous, containing coral fragments and brachiopods. The formation is commonly chert-free, but black or gray chert has been observed in the upper part by Burbank at Kerber Creek (1932, p. 11), by Rold at Wellsville (1950, ge logic section), and by the writer on Coffman Ridge. The upper part also becomes sandy in places and contains beds of lighter-colored calcareous dolomite that resemble the overlying Chaffee formation. Burbank states that the formation becomes more calcareous toward the top (1932, p. 11), but Rold described a gradation just the reverse of this at Wellsville (1950, p. 9).

The thickness of the Fremont varies somewhat, but is markedly uniform considering the unconformity and erosion surface at its top (Fig. 3). Brainerd, Baldwin, and Keyte (1930, p. 84) measured only 178 feet of Fremont in the Wellsville area, compared with Rold's 229 feet. The writer measured 400 feet of dolomitic beds below what are believed to be the Parting sandstones at the south end of Coffman Ridge in Park County, but some of the upper beds may correspond with those included in the Parting by Rold.

Ordovician-Devonian contact:
The unconformity at the top of the Fremont is difficult to find because the beds above and below are conformable and the intervening Silurian erosion surface apparently has very little relief. One begins to doubt the presence of an unconformity; yet no Silurian deposits have been found, and it is obvious that Silurian erosion did occur at the north. The Fremont gradually wedges out between Trout Creek Pass and Weston Pass in the Mosquito Range, and the Harding disappears a short distance farther north. Through north-central Colorado, these formations are intermittent or absent. Since uplift and erosion did occur, it seems logical that the unconformity should exist at the sharpest stratigraphic and lithologic break. Accordingly, because of their similarity to the Fremont, the dolomite bed , even though sandy, immediately below the Parting sandstones and shales have been included in the Fremont by several writers. In Kerber Creek, Burbank (1932, p. 11) considers the contact

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"the base of the prominent sandstone which is persistent throughout the district." Johnson (1934, p. 23) does the same at Trout Creek Pass, and also mentions a basal conglomerate. Osborn and Rainwater (1934, p. 32) also put the contact at the base of the sandstone, thus leaving only 25 feet of Parting at Calumet. In the San Juan region at the southwest the beds of Parting age have lost their dominantly sandy character and consist of shales, sandy limestones, and dolomites. At Ouray these beds have been called the Elbert formation. The Elbert wedges out gradually northeastward beneath the quartzite and sandy beds of the typical Parting type which in turn pinch out southwestward from Leadville. In the Monarch district the Parting contains only one sandy bed and the Fremont-Parting conta t is placed by the writer between the typical black Fremont dolomite and the yellow-weathering argillaceous and dolomitic beds characteristic of the Elbert farther west. If this basis of division is extended into the Arkansas Valley region, the contact should be placed 15-20 feet below the quartzite beds of the Parting. Kirk (1931, p. 231) includes in the Devonian 35 feet of thin-bedded limestone, sandstone, and shale beneath the 21-foot sandstone bed at Wellsville.

DEVONIAN ROCKS--CHAFFEE FORMATION

Parting quartzite member:
The Parting quartzite member of the Chaffee formation is the most varied interval in the Lower Paleozoic section. It contains thin beds of shale, limestone, dolomite, quartzite, and sandstone, and is generally recognizable even from a distance by its over-all pinkish or red color.

Overlying the dark gray dolomite of the Fremont, is a series of thin, interbedded, yellow to brown and pink, argillaceous, and in places sandy dolomites. This series is 14 feet thick at Monarch (measured by writer) and 20 feet thick at Wellsville (Rold, 1950, geologic section). Above this is a fairly persistent purple to red or green shale bed, 3 feet thick at Monarch, 2 feet thick at Wellsville (Rold, 1950, p. 44), and about 10 feet thick at Calcite. Overlying the shale is a series of thinly interbedded quartzites, shales, and limestones above which occurs a fairly pure white quartzite bed 10-25 feet thick.

The lower argillaceous series is likened to the Elbert formation of the San Juan Mountains by Rold (1950, p. 9), who calls this lower portion of the Parting the Elbert facies. However, a correlation with the Lower Devonian Fairview shale of the Gold Brick district in Gunnison County (Worcester and Crawford, 1916, p. 55) is evident, and the name Fairview facies seems more applicable because it is geographically nearer the region discussed.

In the southern part of the region, the Parting is not so easily divisible into a lower argillaceous and calcareous facies and an upper quartzite facies. Here the beds are more heterogeneously mixed, although the same rock types and red, yellow, and green colors are present and there are fairly abrupt lateral lithologic changes. The formation seems most representative of the Fairview facies.

Dyer dolomite member:
The contact between Parting and Dyer is gradational through a thin series of quartzite and calcareous rocks. Just as the Parting

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is distinguishable by its over-all reddish color, the Dyer is recognizable from a distance by the yellow-weathering color of the light gray limestones and dolomites. Except for chemical composition, the rock is remarkably uniform for long distances. From Wellsville to Orient and at Kerber Creek it is lithographic, light gray, and massive toward the bottom, and thin-bedded to platy at the top. At Wellsville all but the top 20 feet is dolomite (Rold, 1950, p. 44). The lower part contains considerable chert and the upper part is a series of interbedded gray and yellow limestones. To the south the member is all limestone with some siliceous impurities, but with very little chert. In the Monarch district on the northwest, the Dyer is generally limestone though dolomitic in places. Througho t most of central Colorado the Dyer does not exceed 100 feet in thickness. In the Mosquito Range it varies from 75 to 95 feet (Johnson, 1934, pp. 24-25). The areas of abnormal thickness, Orient, Bushnell Ridge, and Monarch, are also areas of rather tight folding, and it is possible that these thicknesses are in part caused by severe deformation.

MISSISSIPPIAN ROCKS

Leadville limestone:
An unconformity separates the Dyer from the Leadville limestone, although it is commonly difficult to find in the field. However, south and west of South Park the contact can be placed by means of a color change, and throughout most of central Colorado it is marked by the presence of one or more thin quartzite beds.

The Devonian limestones are light gray or brownish gray and weather to a yellow or "buckskin" color, but the Mississippian limestones vary from gray to black and weather light gray or blue. North of Trout Creek Pass the Dyer changes color and becomes the same gray and blue as the Leadville.

The quartzite at the Dyer-Leadville contact has been described in the Mosquito and northern Sawatch ranges. It has been named the Gilman sandstone member from the type locality near Gilman in Eagle County where it is 15 feet thick (Lovering and Tweto, unpublished report, p. 24). In south-central Colorado it is poorly developed.

At Trout Creek Pass there are two thin quartzite beds separated by a few feet of gray limestone. At Wellsville (Rold, 1950, p. 43) the Gilman is described as a limestone formational breccia 8 feet thick in which fragments of black limestone lie in a quartz sandstone matrix. At Orient the Gilman equivalent consists of a lower 5-foot white quartzite and an upper one-foot white quartzite separated by several feet of intra-formational conglomerate. Burbank mentions no basal quartzites in Kerber Creek, but states (1932, p. 13) that the basal 10-15 feet of Mississippian is interbedded shale and shaly limestone in beds ¼-3 inches thick. Evidently a northwest-southeast facies change in the Gilman member begins between Orient and Kerber Creek and ends near Monarch where there is little, i any, lithologic evidence of a depositional break between the Devonian and Mississippian. At Monarch the division is arbitrary and is considered by the writer at the top of the yellow-weathering dolomitic limestones, with a 40-foot

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bed of black dolomite as the lowest bed in the Leadville. A black dolomite bed in Kerber Creek of possible correlation with the one at Monarch, occurs 80 feet above where Burbank placed the Leadville contact. The gray limestone overlying the black dolomite at Monarch contains the intra-formational conglomerate considered characteristic of the base of the Leadville limestone at Wellsville and Orient.

The Leadville limestone consists of a thick series of massive, gray to black, fine-grained to dense limestones that weather light gray or blue. The lower part is ordinarily chert-free, but the upper part which is in some places thinner-bedded contains black and gray irregular chert nodules. In the Mosquito Range, Leadville area, and most of the northern Sawatch Range the Leadville is normally dolomitic, but in south-central Colorado it is limestone. A distinctive feature of the Leadville is its cavernous character. Openings from the size of small vugs to solution caves hundreds of feet long have been developed along the bedding and, in less degree, along the joints.

The thickness is variable because of the prominent erosional unconformity at the top of the formation.

Pennsylvanian overlap south of Orient:
The southernmost appearance of the Lower Paleozoic sediments in the Sangre de Cristo Range is near Crestone, 16 miles south of Orient, indicating the proximity of the southern boundary of the central Colorado Paleozoic depositional basin. At Crestone (Burbank and Goddard, 1937, p. 939) the Harding quartzite has already overlapped the Manitou and rests directly on the pre-Cambrian with less than its normal thickness. The overlying formations through the Leadville limestone are present, though thin. The exposures at Crestone are complicated by shear faulting along the east edge of the uplifted Sierra Blanca granite massif; therefore, it can not be determined accurately whether the thinness is caused by faulting or proximity to an ancient shore line. However, in all exposures southward in t e Sangre de Cristo Range and Wet Mountain Valley, Pennsylvanian sediments rest directly on the pre-Cambrian; and the boundary of the early central Colorado basin is presumed to be not far south of Crestone, although its position shifted from period to period (Burbank, 1933, p. 278). A Lower Pennsylvanian marine section correlative throughout central Colorado rests on both the Lower Paleozoic beds and the pre-Cambrian throughout overlapped areas.

PENNSYLVANIAN AND PERMIAN ROCKS

The unconformity at the top of the Leadville is widespread and represents the beginning of uplift of the ancestral Rockies. Following slight erosion the region was submerged during deposition of the Pennsylvanian marine sediments. At the beginning of Permian time high mountain ranges were built in portions of Colorado, the erosion of which supplied tremendous volumes of Permian clastics to continental depositional basins in which they may be observed in great thicknesses. The ancient San Luis Range (Melton, 1925, pp. 84-89) was uplifted during this time in the position of the present San Luis Valley, and its destruction furnished

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the great thickness of sediments that make up the greater portion of the Sangre de Cristo Range.

The Pennsylvanian marine section may be divided roughly into two parts: a lower shaly part and an upper redbed interval. The lower part is several hundred feet thick and consists of alternating black shales, limestones, and gritty sandstones, with many combinations of these three types. The shales and limestones predominate. The shaly section is persistent throughout the Pennsylvanian areas of Colorado, and in central Colorado has been named the Belden shale (Brill, 1942, p. 1385). In Trout Creek it is black shale with many thin interbedded limestones and is 300 feet thick (Stark et al., 1949, pp. 37-38). At Calumet (Osborn and Rainwater, 1934, pp. 38-39) and south into the Wellsville area, the black shale decreases and is replaced in importance by gray limestone and red gritty sandst ne beds. In the Sangre de Cristo Range, black shale beds are scarce, and the interval contains mostly red sandstones and micaceous shales. At Kerber Creek, Burbank (1932, p. 13) measured 200 feet of black shales and sandstones in about equal amounts which he calls the Kerber formation, equivalent to the Belden. Thus, there is a transition from sandy to shaly predominance from the Sangre de Cristo Range across the San Luis Valley and west. At Monarch the formation has been called the Garfield formation and comprises 2,600 feet of black shale with a few limestones, sandstones, and conglomerates. In the San Juan Mountains it is known as the Hermosa formation.

Overlying the black Belden shales in the Sangre de Cristo Range is a thick interval of red marine shales, limestones, thin sandstones, and conglomerates which Melton (1925, pp. 810-12) and Johnson (1929, p. 6) include with the overlying Sangre de Cristo conglomerates, but which actually are independent of that formation. The interval is predominantly shaly, becoming sandy and conglomeratic near the top. There is a distinct tectonic break separating these beds from the Sangre de Cristo formation in most places, and Burbank (1937, p. 940) discusses evidence showing the lower beds to be Pennsylvanian and the Sangre de Cristo formation to be Permian.

The marine character of the Pennsylvanian redbeds apparently is widespread. The "lower member," 1,200 feet thick, of the Maroon formation described by Burbank at Kerber Creek (1932, p. 14) meets this description. Overlying the "lower member" is a middle conglomeratic member which may represent the division between Pennsylvanian and Permian. On the west edge of South Park, Stark et al. (1949, pp. 39-40) describe a middle shale and limestone member, 1,500 feet thick, of the Weber(?) formation, which corresponds with the Pennsylvanian marine beds of the Sangre de Cristo Range. The Weber(?) may be correlative with the Battle Mountain formation of Brill (1942, p. 1379) or the Minturn formation of Tweto (1949, p. 190), and either of these names would be more acceptable. The middle shale and limestone member underlies the Coffman conglomerate member, the uppermost of the Weber(?) beds, which in turn is overlain by the Maroon formation, considered mostly Permian in age.

The break at the top of the Pennsylvanian marine beds represents an uplift

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which probably initiated Permian time. Subsequently, the continental redbeds of the Sangre de Cristo formation were deposited. The San Luis Range at the west supplied coarse conglomeratic debris on a torrential scale to the deepening basin in the position of the present Sangre de Cristo Range, allowing great thicknesses to be deposited. Melton (1925, pp. 807-15) estimates the formation to be 13,000 feet thick near Crestone, and 4,300 feet thick near Pleasant Valley and the Arkansas River. The formation is probably correlative with the Maroon of the Mosquito Range and has some counterpart in the Kangaroo of the Sawatch Range (Crawford, 1913, p. 70), but its extreme conglomeratic character is found only in the Sangre de Cristo Range.

TERTIARY IGNEOUS ROCKS

Igneous rocks of Tertiary age are relatively rare in the northern Sangre de Cristo region, although in most adjoining areas they are abundant and compose a major part of the exposed surface.

Here and there intrusive sills and dikes of gray intermediate porphyry are the only intrusive bodies known near the Lower Paleozoic limestone belt. Two dikes several feet thick occupy faults associated with the thrust zone at Calcite. They are well exposed in the limestone quarries and have had some baking effect on the confining limestone, but no silication or other alteration was noted. Stone (1934, p. 318) described small sills of monzonite porphyry at the Orient iron mine. These occur principally at the Dyer-Leadville contact, and also in the Manitou, Harding, and Fremont formations. Butler (1949, p. 1960) described several felsite porphyry sills in the Ordovician and Devonian beds on Bushnell Ridge, north of Hayden Pass.

Only two stock-like bodies of intrusive porphyry are known in the area discussed. A stock north of Wellsville was partly mapped but not described by Rold (1950, geol. map). The other stock, occupying the crest of Nipple Mountain in Pennsylvanian sediments east of Orient, is probably the northernmost of a line of stocks extending southeast along the crest of the range.

The only extrusive flow rocks known are two small bodies capping ridges near Howard. The flows are considered remnants of the once, more widespread, Tertiary lavas that blocked off the south end of South Park and flowed into the Arkansas Valley. The probable Oligocene age of these volcanics has been established (Stark et al., 1949, p. 138), yet the flow east of Howard inclined steeply toward the river occupies a surface that so closely approximates the present topography that a Quaternary origin may be implied. This flow may have been fed from the Pleasant Valley fault after the present topography had been established.

STRUCTURE

GENERAL DESCRIPTION

The Sangre de Cristo Range north of Orient is separated into two parts by Hayden Pass. The range north of the pass is narrow and developed mostly on

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pre-Cambrian crystalline rocks. South of the pass it is developed on Pennsylvanian rocks. The topographic northern termination of the range is at Salida. A structural termination, if one exists, is not apparent.

The band of upturned Lower Paleozoic sediments flanking the east side of the range trends directly southeastward into the region from similar attitudes on the east flank of the Mosquito Range through the small folds at Coffman and Aspen ridges east of Nathrop. The structures recorded in the sedimentary belt offer no evidence for a geological northern termination of the range. The structures in the sedimentary belt are indicative only of a large anticline, broken on its eastern flank, which apparently extends from the southern Mosquito Range on the north, through Aspen Ridge and into the Sangre de Cristo Range on the south. The topographic northern termination of the Sangre de Cristo Range and the divergence between its crest and the axis of the anticline indicate that the range and an icline are probably not closely related. The band of Lower Paleozoic sediments enters the area at Wellsville, passes through Calcite at the base of the range, and rises continually higher on the range flank until it crosses to the west side through Hayden Pass.

The geologic dividing line of Hayden Pass is well expressed topographically, for it is the lowest point in the range crest, about 11,000 feet in elevation, and does not rise above timber line. When seen from a distance, the divided parts of the range are outstanding as separate units.

The west slope of the range is much steeper than the east slope and forms a linear eroded scarp (FOOTNOTE 3) throughout its length bounding the San Luis Valley.

South of the pass the band of carbonate and quartzite sediments, maintaining the same south-southeast trend and in places repeated by thrusting, gradually descends on the west slope because of the divergence of the sedimentary strike and the range crest. The sediments maintain the same steep easterly dips which on the west slope are into the range. The limestone section falls to the bottom of the slope, and at Orient, disappears beneath the Quaternary fill of the San Luis Valley. The range south of the pass is developed almost entirely on Pennsylvanian and Permian sediments which dip eastward varying amounts, forming a large homocline. The range becomes much wider south of the pass, and its central highest parts are supported by occasional small stocks of more resistant intrusive rock that form its backbone. The increased width is not evident on the west slope which still forms a linear scarp, but is developed on the east slope which swings southeast in the transition from Pleasant Valley to the Wet Mountain Valley. Baldy Peak which lies considerably east of the range crest is the northern extremity of this increased width.

South of Orient the east-dipping homocline gradually arches into the large diapiric fold described by Burbank (1937, pp. 951-53). The significance of the Orient district as a geologic dividing line is that the east-dipping structures extending

FOOTNOTE 3. Scarp is used here in the sense defined by Billings (1946, pp. 161-65) that even though the original fault surface has been destroyed, the dissected and flatter slope may still be termed a scarp.

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southeast from the Mosquito Range and resulting from westward-directed thrusting are here replaced by west-dipping structures caused by northeast- and eastward-directed thrusting.

Pleasant Valley lies east of the northern segment of the range, and is the fairly broad valley of the Arkansas River between the short canyon at the northern terminus of the range near Wellsville and the Arkansas River Canyon which extends from Coaldale to Canon City. The valley coincides with the axis of a large syncline which is bounded on the east by a high-angle thrust. Both the thrust and syncline are parallel with the faulted Lower Paleozoic outcrop band flanking the range, and resulted from the same Laramide deformation.

FOLDING

All of the larger folds in the areas investigated, with the possible exception of those at Orient, are contemporaneous with the Laramide "range-and-valley" folding of the region on the northeast, and were influenced by the westward-directed thrusting which followed the overturning and breaking of folds.

Pleasant Valley syncline:
The large asymmetric syncline that underlies Pleasant Valley is a deeply eroded intermountain park depression (sections FF^prime and GG^prime, Fig. 7). It is the major syncline between the northern end of the Wet Mountain anticline, and the Sangre de Cristo anticline whose axis was west of the present range. The sedimentary area in Red Gulch, north of Cotopaxi, is a small faulted syncline on the west flank of the Wet Mountain anticline.

The Pleasant Valley syncline extends northwest diagonally crossing Badger Creek and is obliterated by the Whitehorn stock. It reappears north of the stock and is probably correlative with the small syncline in Bassam Park, east of Nathrop, and dies out south of Trout Creek Pass. The Bassam Park syncline lies between the small Coffman Ridge anticline and the upturned sediments on the east flank of the Sawatch arch. Southward the Pleasant Valley syncline disappears beneath the fill at the northwest end of the Wet Mountain Valley. However, evidence presented by the accompanying fault indicates that this syncline may be the true structural continuation of the Wet Mountain Valley.

The syncline is developed principally on Pennsylvanian and Permian redbeds. The east-dipping axial plane is on the eastern edge of the fold about a mile west of the Pleasant Valley fault. The east flank steepens rapidly from the axis until the Permian beds are nearly vertical or overturned and are cut off sharply by the fault. The west flank occupies most of the fold. The beds gradually steepen in dip, rising on the flank of the range until they are made vertical by thrusting, thus exposing the Lower Paleozoic limestones. Before thrusting this flank of the fold continued farther west, and for some distance south of Hayden Pass, the entire range width is considered part of the west flank of the syncline.

Wellsville folds:
The folds mapped by Rold (1950) near Wellsville are small, though severe, and probably are related more to the substage of deformation associated with the northern Sangre de Cristo thrust zone than to the earlier

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"range-and-valley" folding which produced the Pleasant Valley syncline. At the north end of the Wellsville-Orient line of thrusting the Wellsville folds seem to replace the thrusts as major features. Thrusts are still present at Wellsville, and Rold presents evidence to show the folds are partially due to drag; but the displacement on these thrusts is much less than it is farther south. Evidence north of the small intrusive body north of Wellsville is lacking, but the folds and numerous oblique faults imply that this particular thrust zone may be dying out through folds. Farther north it may be replaced by another thrust zone.

Rold shows two main folds which trend northwest parallel to the fault zone, the eastern Lofgren anticline and the western Wellsville syncline. Both folds are broken by thrusts. Smaller related folds complicate the structure somewhat, but need not be considered here.

Drag folds:
Most of the thrust faults have associated drag. The configuration of some pre-existing folds broken by faults was modified by movement, but with few exceptions the folds in the central part of the thrust belt owe their existence to fault movement and are of the same age as the faults.

Rold's Rogers anticline (1950, p. 24) is a shallow asymmetric fold formed in the upper plate of his Rogers fault by the slight turning down of the sediments into the thrust from their normal monoclinal attitude on the west flank of the Pleasant Valley syncline.

A sharp asymmetric syncline, in the underplate of a steep thrust, extends from Cherry Creek to a position high on the range above the Hayden Creek drainage basin. The axial plane dips steeply east presumably parallel to the thrust surface. The beds on the east flank steepen to vertical and overturned attitudes; those on the west slope gently upward toward the range crest at about 30° forming the continued western flank of the Pleasant Valley syncline. Southeastward an axis seems aligned with a thrust plane which passes across the east shoulder of Galena Peak. The fold is believed to have originated principally from movement on the adjacent thrust. There may have been a corresponding anticline in the upper thrust plate, but traces of it in the sediments have eroded away.

The shallow anticline east of and perpendicular to the normal fault in Hayden Creek has not been throughly investigated, but is probably a slight wrinkle in the upper plate of the thrust to the west. Folds of this east-west trend are rare in the region.

Folds near Orient:
J. B. Stone (1934) mapped two adjacent tight folds at Orient, a north-plunging syncline joined on the east by a north-plunging anticline. He notes the absence of important faults in the district and does not describe either folding or faulting north of Black Canyon. Since the thrust zone extends from Calcite to the vicinity of Black Canyon without prominent folding and because the middle Eocene marginal thrust zone of Burbank passes just south of Orient where it makes a major directional change, the question arises as to which deformation system the Orient folds belong.

The Wellsville-Orient thrust zone appears to continue southeast from Lime

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Canyon as far as the bottom of Black Canyon with no major change in the type of structure except that the thrust displacements are decreasing. The sediments maintain the same indipping attitude. Black Canyon is then a sharp dividing line south of which a new type of structure suddenly appears with tight folds, as described by Stone, replacing the thrusts. The fold axes trend generally parallel with the thrusts on the north. From Stone's map, which does not include Black Canyon, the folds, if projected, should appear north of the canyon, but they do not. Also the Mississippian-Pennsylvanian contact is apparently much higher on the north side of the creek than on the south. These are the principal reasons for showing the Black Canyon fault, not recognized by Stone, and actual outcrops w ich have not been observed.

The folds at Wellsville are parallel with the regional structural trend, and plunge south; and, although they do not replace the thrusts entirely, the folds become the main features through gradation. Although the change from faults to folds at Orient is abrupt, the same parallelism of both structural types exists, and the folds plunge north, opposite from the early folds of the "range-and-valley" compression. A possible interpretation is that the thrust zone dies out at Wellsville and Orient with a transition through folds that plunge toward the zone center. The folds at Orient would then be overridden by the thrusts trending southeast from Kerber Creek.

Two folds additional to those mapped by Stone are indicated between Orient and the range crest. Stone states that although the dip near Orient is into the mountains, it can not continue far, because the crest of the range is anticlinal. Since the east flank of Stone's anticline again places the beds in an indipping position, a syncline is needed between the anticline at Orient and the anticlinal range crest. Although the area above Orient was not visited, a syncline is believed evident on aerial photographs, and its position is recorded on the structure map (Fig. 2). This fold also probably disappears beneath the Kerber Creek thrust extension. Burbank (1937, Pl. I) shows the axis of the northward extension of the diapiric anticline near the range crest to extend along the crest past O ient before it swings northwest and dies out (Fig. 2). Since the portion of the range above the Lower Paleozoic limestone band south of Hayden Pass was not examined, the features of this fold termination and its relation to the proposed Black Canyon fault remain unknown. However, since the fold does not involve the limestone band, it must die out in the Pennsylvanian.

The diapiric fold was placed with the middle Eocene deformation marginal to the Sangre de Cristo thrust zone by Burbank (1937, p. 954). Although there is a variation in trend between the diapiric fold and the smaller folds at Orient, they could well belong to the same deformation. The disappearance of the Orient folds beneath the Kerber thrust is no impediment, because the diapiric fold is known to have pre-dated thrusting. However, the former theory placing the Orient folds with the lower Eocene thrusting is favored because of their parallelism with the Kerber Creek folds, and because the diapiric fold curves northwest paralleling the Kerber thrust zone while the Orient folds do not.

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Kerber Creek folds:
The series of asymmetric folds in lower Kerber Creek and the importance of their relation to the Kerber thrust zone have been described by Burbank (1932, p. 39); only a brief mention of their part in a regional pattern is made here. The folds come into evidence about 2 miles west of Villa Grove and consist of two anticlines and two synclines, all of which disappear beneath the Kerber thrust plates (Fig. 2). The folds trend northwest generally parallel with the Wellsville-Orient thrust zone, and the Pleasant Valley syncline and fault and are broken in places by steep east-dipping thrusts (section GG^prime). They plunge south. It is evident that these folds were not caused by drag on thrusts, and their correlation in age with the Pleasant Valley syncline is concluded. Burbank (1932, p. 39; 1937, p. 936) has already pointed out the correlation of the Kerber folds and related northwest thrusts with the lower Eocene Sawatch structure on the northwest.

Sangre de Cristo anticline:
The projected connection between the folded sediments of Kerber Creek and the steep east-dipping beds on the faulted west flank of the Pleasant Valley syncline indicate either a large anticline with its axis near the east side of the present San Luis Valley, or a great thrust fault which broke a monocline extending from the Pleasant Valley syncline to Kerber Creek, and lifted the sediments in the northern Sangre de Cristo Range thousands of feet above those in Kerber Creek. A fault is believed to be present on the west side of the range, but it is much later than the folding which is considered responsible for the dip of the beds, and is believed to have less displacement than necessary to drop the Lower Paleozoic beds from their elevation projected from the Sangre de Cristo Range thousa ds of feet above the valley floor to their present position in Kerber Creek. Furthermore, near the latitude of Orient, sediments are found at nearly the same elevation on both sides of the San Luis Valley, the dips of which indicate an intervening anticline. The steep east dip of the sediments north of Orient along the edge of the valley could hardly have resulted from deformation in the range, because they are so low on its western margin.

The anticline (section GG^prime) is here denoted the Sangre de Cristo anticline, and probably covered most of the northeast San Luis Valley and northern Sangre de Cristo Range: its eastern flank being marked by the trend of the band of Lower Paleozoic sediments. On the north, the only evidence for the anticline is this upturned, east-dipping band. On the south, the sedimentary band converges with the Kerber Creek sedimentary area illustrating the south plunge of the fold.

East-dipping sediments on the east flank of Burbank's easternmost anticline are exposed in Clayton Cone, west of Villa Grove. This dip is the same as that of the sediments which reappear north of Orient on the west flank of the Pleasant Valley syncline. With the southern end of the Sangre de Cristo anticline in the intervening valley area, the sediments must reverse to a west dip east of Clayton Cone, and there must be a small syncline between the Sangre de Cristo anticline and the easternmost anticline of Kerber Creek. This relation extends the area of

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small folds eastward a short distance beneath the valley gravels. The small syncline is not shown in section GG^prime, because its exact position is not known and the south plunge of the folds creates the possibility that the sediments illustrating it may not be present in the bedrock in the plane of the section.

Poncha synclinal area:
Small patches of Lower Paleozoic sediments were noted by the writer north of the volcanic area of the Cochetopa Hills and in line with the projected northwest trend of the Kerber Creek folds. The principal occurrences are in Poncha Creek, Gray's Creek, and near Maysville on the South Arkansas River. The beds in most of these areas dip east or lie nearly horizontal. As near as can be determined, the aggregate attitudes indicate more folds comparable in size with those at Kerber Creek. The folds are complicated by high-angle faults, interpreted as thrusts from the east, which bound the patches on one side. It is not known how far northwest the folds extend between Maysville and the Mount Princeton batholith, since this interval is covered by Arkansas Valley gravels. However, they were prob bly continuous with the Kerber Creek folds, and formed an east-west series extending from the Kerber Creek-Maysville axis west to the Monarch district where other similar folds are found.

The sedimentary area of small folds is bounded on the west by the pre-Cambrian core of the large central anticline of the southern Sawatch Range, and on the east by the crystalline core of the Sangre de Cristo anticline. The boundary with the Sangre de Cristo anticline is believed complicated by thrust faulting which is discussed later. When projected northwest, the folds trend into the pre-Cambrian and intrusive rocks in the undisputed core of the Sawatch Range. However, the areas near Maysville and Poncha Creek are influenced by diverse faulting which could have dropped some of the sedimentary patches below their normal position, and the south plunge of the folds could have caused the pre-Cambrian-sedimentary contact to rise northwestward until it was above the Sawatch Range.

The resulting configuration of the large sedimentary area, of which only small patches are left, was probably that of a structural southeast-plunging embayment, here called the Poncha synclinal area, between the southern Sawatch and Sangre de Cristo anticlines which sloped off a large high region at the north.

Configuration of early Eocene Sawatch arch:
The Classification of the proposed Poncha synclinorium and the Sangre de Cristo anticline as contemporaneous with the Laramide "range-and-valley" folding permits some conclusions to be drawn about the Sawatch arch which is of the same age.

The band of Lower Paleozoic sediments flanking the Sangre de Cristo anticline crosses the northern Sangre de Cristo Range diagonally and continues northwest along the edge of South Park, interrupted by the small Bassam Park syncline and Coffman Ridge anticline, and extends into the Mosquito Range with the same east-dipping attitude. There is very little evidence to show that this upturned band is not the true east flank of the original Sawatch arch.

Vanderwilt (1948, p. 77) states that the synclinal zone between the Sawatch

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and Gore ranges extends southeast through the Leadville district as far as Salida, an opinion shared by many earlier workers. This syncline would separate the Sawatch anticline from the anticline west of South Park, and could also join with the Poncha synclinorium. However, the south plunge of the Poncha synclinorium would carry any sedimentary evidence of the syncline thousands of feet above the present surface throughout most of the length of the upper Arkansas Valley. The Poncha synclinorium is believed to die out northward, but even the presence of such a feature so far above the exposed sedimentary flanks of the two anticlines would mean the syncline was a minor fold on the greater Sawatch arch which extended from South Park to Taylor Park and Tomichi Creek on the west side of th Sawatch Range.

The separation of the Sawatch-Gore trough from the South Park-Middle Park synclinal trough by anticlinal ranges remains doubtful, and it may be principally the result of faulting. The east-dipping sediments in the Sawatch-Ten Mile trough, and probably the Sawatch-Gore trough which is continuous with the former, are monoclinal and reverse to a west dip only near the Gore and Mosquito faults. The trough strongly resembles the broken west flank of the South Park-Middle Park synclinal trend which was separated from the Park synclines by thrusts from the east; thus initiating the Mosquito, Ten Mile, and Gore ranges. Similarly, the southern end of the trough in the Leadville district is not definitely synclinal.

South of Leadville, even though the Arkansas Valley is wide and extensively filled with gravel, it narrows in places, as near Granite, and the pre-Cambrian may be traced continuously across it except for a narrow strip of fill along the river. Furthermore, the sediments rise sharply when projected westward from Coffman Ridge, 2,000 feet above the Arkansas River.

Thus from Granite to Salida, the Sawatch arch was probably one broad fold between South Park and Taylor Park. But at its southern end it divided into two projections, the Sangre de Cristo and southern Sawatch anticlines, separated by the Poncha synclinorium.

FAULTING

Thrust faults:
The thrust faults of the northern Sangre de Cristo Range trend generally north-northwest and dip steeply northeast. They are early Eocene and represent the late intense stages of "range-and-valley" folding. The entire deformation was caused possibly by movement of the ancient Front Range highland westward toward the old central Colorado basin.

The faults may be segregated into three groups: the Pleasant Valley fault between the Wet Mountain anticline extension and the Pleasant Valley syncline; the Wellsville-Orient group between the Pleasant Valley syncline and the deformed Sangre de Cristo anticline; and the northwest faults in Kerber Creek on the flanks of small folds west of the Sangre de Cristo anticline.

By projection, the London-Weston fault zone of the Mosquito Range is easily

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correlated with a broad zone containing both the Pleasant Valley fault and the Wellsville-Orient group. More accurate correlation must depend on mapping between Wellsville and Turret.

Pleasant Valley thrust:
The Pleasant Valley thrust is an east-dipping, high-angle thrust along which the basement Pikes Peak granite has been brought up against Permian sediments on the west. The sediments have been dragged into a vertical position by fault movement. The amount of thrust displacement is not known.

Toward the northwest, the fault gradually rises on the east side of Pleasant Valley, crosses Badger Creek diagonally, and dies out near the southwest edge of South Park.

The projected trend of the Pleasant Valley fault would pass close along the edge of South Park toward Coffman Ridge, and although it apparently dies out, may be renewed in a north-south-trending zone of thrusts on the west edge of the Park near Herring Creek. This northward extension of the fault would approach closest to the London and Weston faults.

Southward the fault disappears beneath the gravels of the Wet Mountain Valley with no indication of decreased displacement. Burbank and Goddard (1937, p. 936) describe the possible southeast continuation of the fault:

North of Wet Mountain Valley, the east side of the [Sangre de Cristo] range is bounded by a prominent fault, which probably continues southeastward beneath the alluvial deposits of the valley and joins with a similar steep fault which separates Huerfano Park from the west side of the southern Wet, or Greenhorn, Mountains.

They show the fault as vertical east of Huerfano Park (1937, p. 950). Burbank's conclusion is supported by the continuous exposure of pre-Cambrian rocks east of the fault between Pleasant Valley and the southern Wet Mountains. West of the fault, Pennsylvanian and Permian rocks are exposed along the east flank of the Sangre de Cristo, their continuity being broken only by coverings of alluvium or post-Permian sediments. The fault, then, is structurally the east boundary of the Wet Mountain Valley which must swing northwest where it is partly dissected by tributary canyons of the lower Arkansas Canyon, and is structurally continuous with Pleasant Valley finally terminating at the southwest edge of South Park.

The Laramide position of the Wet Mountains is clearly en echelon with the Front Range as demonstrated by the Canon City embayment; and the axis of the Wet Mountain uplift, when projected northwest, trends into South Park (Fig. 1). It is likely that the fold plunges northwest beneath the Tertiary lavas and sediments of the park and dies out.

Wellsville-Orient thrust zone:
The Wellsville-Orient thrust zone, following the outcropping band of Lower Paleozoic sediments, projects northwest toward a thrust which repeats the band south of Turret. Farther north this projection appears to join with the southward extension of the dying-out Mosquito fault zone. This junction is structurally incompatible since one zone is characterized by thrust faults, the other by normal faults. Furthermore, the normal fault zone

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is dying out southward and might be replaced by thrust faults. These relations are reminiscent of the Mosquito fault north of Climax in the Ten Mile Valley where thrust movement replaces normal movement toward the north, and the possibility of the late Mosquito fault having acted along a previously formed thrust zone is recalled. Later statements will introduce the possibility of late normal faulting continuing southward on a zone west of the projected Mosquito zone, into the Arkansas Valley.

The Wellsville-Orient zone was traced from Taylor Gulch, north of Howard Creek, to the vicinity of Black Canyon. Since the basement and Permo-Pennsylvanian areas were not investigated thoroughly, the presence, distribution, and relations of faults in those areas remain unknown. However, faulting of the Pennsylvanian was observed in West Creek and is known to exist at Wellsville and on Rito Alto Peak in Custer County. Therefore, the width of the thrust zone may be greater than that shown in Figure 2.

Three thrust faults are shown north of the Arkansas River at Wellsville which trend more northwest than other members of the zone, and extend into the Pennsylvanian. The displacement of these faults is small, ranging from 175 to 600 feet (Rold, 1950, pp. 27-28). No explanation can be offered for the variant trend of these faults and the related folds unless rotation during possibly later diagonal faulting moved them out of parallelism with pre-existing structures. Mapping on the north may disclose some structural element, such as the small stock, to which the fault strikes could be related. It is noted that on the state geologic map a slight southwestward projecting bulge in the Pleasant Valley fault occurs nearly opposite the Wellsville thrusts, and it is conceivable that a local cha ge to a more southwest direction of compression may have caused the variation.

The next thrusts on the south are the Norberg and Rogers faults (Rold, 1950, pp. 26-31) which originate northwest of Taylor Gulch. Rold describes the Norberg fault as vertical with a predominantly horizontal component of movement along which the northeast side moved southeast. He derives this direction of displacement from the respective position of vertical sediments on either side of the fault. This fault was traced from Taylor Gulch southward into Howard Creek, and throughout this distance it is a high-angle thrust following the top of the Leadville limestone in the lower plate. The Dyer limestone has been thrust over the Leadville throughout most of the fault length (section AA^prime). The fault relations shown on Rold's map permit interpretation of the fault as a thrust, "vertica in this area," and cross-cutting the beds in the lower plate preparatory to dying out in the pre-Cambrian on the northwest. The vertical attitude of the fault and related beds can not persist in depth, and vertical, updip movement could place the beds in their present position as effectively as horizontal movement which is not compatible with high-angle thrusting on the same fault a short distance away. In most exposures, the thrust and related beds in both plates have generally the same strike and dip, indicating a large thrust displacement. The fault apparently dies out not far south of Howard Creek.

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The Rogers fault (Rold, 1950, p. 28) is nearly parallel with, and northeast of, the Norberg fault. It originates in Spring Gulch, south of Wellsville, and cross-cuts the Lower Paleozoic sediments southeasterly before swinging parallel with the strike of the sediments at Taylor Gulch. From Taylor Gulch south, it follows the top of the Leadville limestone in the lower plate, which is the upper plate of the Norberg fault. The sediments from Fremont dolomite through Leadville limestone and into the Pennsylvanian are exposed in the upper plate of the Rogers fault; thus a third of a series of imbricated thrust plates is made lapping on the Sangre de Cristo mountain front (section AA^prime). The dip of the beds is steeper in this plate than in the other two, and in many places approaches the vertical. The fault continues southeast at the top of the Leadville, is displaced by a cross fault in West Creek, and is finally lost in the thick forest of Cherry Creek where it evidently dies out since it does not reappear at the south. Between Howard and West creeks, the Harding quartzite has been thrust up over the Leadville limestone.

For a brief interval between Cherry and Stout creeks, no thrusts are in evidence, but a sharp asymmetric drag syncline with an east-dipping axial plane begins low on the mountain front north of Stout Creek and rises sharply southeast to timber line. South of Stout Creek there is a thrust on the east flank of the syncline along which the pre-Cambrian has been thrust over Ordovician beds. For most of its course, this thrust follows gullies nearly perpendicular to the main drainage direction and is not well exposed. However, two small thrusts (Fig. 4) exposed in the south wall of Stout Creek are thought to express the same

Click to view image in GIF format. Fig. 4A. [Grey Scale] Easternmost of two small thrusts in Stout Creek, looking southeast. Displacement decreases up-section. Manitou is broken, Harding is folded, and Fremont is undisturbed.

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relations as the larger thrust. Both of these dip steeply northeast and have thrust the pre-Cambrian contact 20-50 feet up on the east side. In both places, the Manitou is displaced and the faults die out in the Manitou and Harding by flattening into bedding-plane thrusts and slightly arching the Harding and lower Fremont. The small thrusts originate in depth and die out toward the surface, while the large thrusts steepen. The larger thrust continues southeast, crossing the range crest south of Hayden Pass where it cross-cuts into the Pennsylvanian and dies out.

North of Galena Peak a thrust originates in the Lower Paleozoic sediments approximately in line with the axis of the drag syncline. The syncline evidently breaks into a thrust which becomes nearly parallel with and follows the Mississippian-Pennsylvanian contact across the east shoulder of Galena Peak, through

Click to view image in GIF format. Fig. 4B. [Grey Scale] Westernmost of two small thrusts in Stout Creek, looking southeast. Granite contact is thrust westward, Manitou-Harding contact is slightly broken, and Harding is essentially undisturbed. Fault flattens into bedding-plane thrust at Manitou-Harding contact.

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Hayden Pass and into Hayden Pass Creek on the west side of the range. For a short distance below the pass, it is in the Lower Pennsylvanian marine shales. This fault does not involve the basement rocks, but in Hayden Pass Creek it is joined by another thrust which originates in the basement on the west side of the range on the north. The second fault cross-cuts the Lower Paleozoic sediments to the top of the Leadville limestone where it joins the former thrust. The combined fault extends southeast along the Mississippian contact into the Silver Creek drainage where its features were not studied. However, it probably dies out, since traces of it were not found in Steel Canyon.

North of Steel Canyon a thrust begins in the Lower Pennsylvanian sediments and gradually cross-cuts to the top of the Leadville limestone in the lower plate. It then follows this contact southeast across Steel and Lime canyons into Black Canyon where relations are obscure, and it may be dying out. Throughout most of the fault length, the Dyer limestone has been thrust over the Leadville and the fault plane apparently is nearly parallel with the beds.

A smaller thrust branches from this thrust just north of Steel Canyon and extends to and possibly beyond the Black Canyon fault. It is confined entirely to the Mississippian, greatly increasing the thickness of the Leadville, except for an area in Lime Canyon where the Pennsylvanian is involved in the lower plate. Also branching from the main thrust is a high-angle normal fault which cross-cuts the Lower Paleozoic formations in Steel Canyon, then swings parallel with the sediments at the granite contact. It cuts out the Manitou dolomite completely in Lime Canyon and partly farther south. Assuming that the Pleasant Valley fault swings sharply southeast beneath the Wet Mountain Valley gravels and emerges on the east side of the valley, an explanation is offered for the dying-out of the one at Orient. Orient is on a line nearly perpendicular to the trend of the fault where the fault swings sharply southeast. The increased width of the Pleasant Valley syncline continuation would allow more room for relief of compression caused by the fault, and the thrust zone on the syncline's west side would no longer be necessary.

Shallow thrusting and age of Wellsville-Orient zone:
General consideration of the thrust faults portrays a linear zone of thrusting with evidence of its termination on both ends at Wellsville and Orient, although there may be other zones on the north. Both ends of the zone are characterized by several overlapping "bedding-plane" thrusts confined to the Lower Paleozoic, but the center of the zone contains fewer thrusts which cut sharply across the sediments along the dip and involve the basement rocks.

The similarity between the ends of the zone is remarkable. In each case, there are two thrusts and one longitudinal normal fault. The thrusts seem to occur at the top of the Mississippian in the lower plate, and where observed all of them are nearly parallel with the sediments, except at the ends of the faults where there are cross-cutting relations. In this respect, they approach bedding-plane faults. In no case is the basement involved. The result in cross section is a series

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of imbricated thrust plates of Lower Paleozoic beds which were piled on each other by movement from the east.

The fact that no basement rocks were found in any of the upper thrust plates leads to the conclusion that on the ends of the zone the thrusts are shallow and did not enter the crystalline rocks. It might be argued that there is insufficient range of observation to draw such a conclusion, and that the basement might be

Fig. 5. Sections AA^prime, BB^prime, and CC^prime.

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involved at no great depth. However, the same relations were noted throughout a vertical range of observation of more than 1,500 feet and horizontally for about 5 miles on either end of the zone. In this entire range, the thrusts were found without exception at the top of the Leadville limestone. North of Calcite, the beds in the top plate are steeper than those in the next lower plate, but throughout a vertical topographic relief of more than 1,000 feet, the same formations are

Fig. 6. Sections DD^prime and EE^prime.

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found on each side of the fault. It is therefore concluded that the fault planes do not cut across the formations abruptly in dip, although there may be certain parts where cross-cutting is more pronounced than in others. However, the faults must cut across all the repeated horizons in order to have repetition. In reconstructing sections across these imbricated areas (sections AA^prime and EE^prime) it was found that many thousands of feet of thrust movement are needed to allow imbrication of the same series of beds into their present positions without involving basement rocks and maintaining the low angle of cross-cutting. Movement of this magnitude is necessary before any of the faults are free to enter the basement complex, and in such a distance it is believed that the trough of t e Pleasant Valley syncline would be approached or encountered so that the fault planes would have inclinations approaching the horizontal.

Deformation would therefore be shallow. The faults can hardly extend through the Pleasant Valley fault, or sediments certainly would appear on its east side, since the faults are either in the sediments or not far below them if they do enter the granite. They may extend to the Pleasant Valley thrust underneath the syncline, but this is not likely.

Similar thrust faults may occur in the Pennsylvanian marine beds, but these have not been studied, and no places are known where Lower Paleozoic or pre-Cambrian rocks are thrust over the Pennsylvanian. If such faults are present, they are probably shallow and confined to the Pennsylvanian marine section just as the others are confined to the Lower Paleozoic limestones and dolomites.

The theory of shallow thrusting is not new in the Sangre de Cristo region. Burbank and Goddard (1937, p. 962) found evidence that the eastward-directed thrusting in Huerfano Park and the area on the west was shallow and confined essentially to the Pennsylvanian marine shale sediments which rest directly on the granite. They concluded that deformation was confined to these sediments by eastward-directed compressional forces, sliding them along the unaffected basement, and imbricating the whole series.

Burbank's theory was applicable to the entire south-central Sangre de Cristo region. However, it can not be applied to the Wellsville-Orient thrust zone without considerable modification, because only the ends of the zone offer evidence for shallow deformation. The central third of the zone contains two fairly high-angle thrusts that involve the basement and have deeper origins. The amount of thrusting is probably greater here than in the end shallow areas, but it is much less than that on the Pleasant Valley fault. The zone may then be considered a composite concave surface along which increased compression, perhaps induced by the tightness of the Pleasant Valley syncline, and drag on the Pleasant Valley fault were relieved. Compression probably was greatest in Hayden and Hamilton cr eks where thrust movement is greatest and has a deep origin. The shallow deformation is thus local and indicates that movement is decreasing away from the center of the zone and that the zone is supplementary to the Pleasant Valley deformation. In this sense, the zone is somewhat later than the Pleasant Valley

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fault, although it belongs to the same period of early to middle Eocene deformation.

Kerber Creek faults:
The northwest thrust faults in Kerber Creek are closely related to the folds of that area (Burbank, 1932, p. 39) and aid in illustrating the age relationship between the early Eocene compression and middle Eocene compression which has already been described. The faults break the east flanks of folds and have thrust granite over Paleozoic sediments. They are thus of the same type, trend, and age as the Pleasant Valley fault, though somewhat smaller.

The northward extension of the folds to the South Arkansas River has been discussed, and it is believed that the thrust relationships extend as far, as indicated by the faults that cut off the small patches of sediments. The thrust relations are thought to extend into the Monarch district where deformation is particularly severe. Evidence is weak for the zone to extend north of Maysville (Fig. 2), and it is not intended to portray that it is proved continuous with the thrust through Clayton Cone. The fault east of Round Mountain, exposing a window of granite through the Miocene lavas is much later than the projected thrust, since it cuts the lavas and the thrust does not, although it may represent a weak zone which existed since Eocene time. The long zone through Maysville represents he approximate eastern boundary of the patches of Paleozoic sediments, and is generally in the position of a high-angle fault shown on the Tectonic

Fig. 7. Sections FF^prime and GG^prime.

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Map of the United States (Amer. Assoc. Petrol. Geol., 1944). Also, the thrust through Clayton Cone seems to be the strongest in the Kerber Creek area, dying out southward and hinging on a point near Clayton Cone (Burbank, 1932, p. 39), and it should extend some distance north.

Regional effects of early Eocene deformation:
The early Eocene compressional deformation affected the entire area shown in Figure 2 north of the east-west Kerber thrust zone, as well as regions on the northeast and northwest. Sections FF^prime and GG^prime show the effects of this deformation. The amount of crustal shortening in the early Sangre de Cristo-San Luis Valley areas was measured on Section GG^prime, which does not show the possible syncline between the Sangre de Cristo anticline and Burbank's eastern anticline (1932, p. 39). The pre-Cambrian-Paleozoic contact was used as the key horizon. It is estimated that approximately 42 miles of original pre-Cambrian surface have been compressed into 24 miles, a crustal shortening of 18 miles or 42 per cent. These figures are, of course, based on theoretical assumption, and indicate nly the approximate extent of compression.

Cross faults:
Throughout the length of the Wellsville-Orient thrust zone high-angle or vertical faults were noted here and there, cutting the thrusts nearly perpendicularly. They do not appear to be abundant, but many might have escaped notice.

In the Wellsville area the cross faults were accurately mapped by Rold, and apparently are more numerous there than elsewhere along the zone. They trend about N. 70° W., a direction peculiar only to the cross faults of this area, and are described (Rold, 1950, pp. 29-34) as having dominantly horizontal movement which aided in movement of the north part of the Wellsville area eastward past the southern part. This movement was later than the period of compressional folding, and is explained by Rold (1950, p. 38) as a shearing between the westward-directed thrusting of the Mosquito Range and the later eastward-directed thrusting of the south-central Sangre de Cristo Range. However, such conditions should cause the north areas to move westward past the south areas, instead of the rev rse of this which is the case at Wellsville.

Several observations lead to other possible interpretations. The cross faults alone characterize the central part of the Wellsville area with folds predominating on the north and thrusts on the south. Little evidence has been presented to show that cross faults actually are later than the thrusts. Also, it has been shown that the areas for many miles both north and south of Wellsville were being compressed in the same direction, toward the west, even though differentially. If it is considered that the south part of the Wellsville area moved west past the north part, instead of vice versa, the same regional direction of movement is maintained in the discussion, and the cross faults might be considered tear faults in the thrust zone through which movement gains magnitude toward the sout .

If the cross faulting is later than the early Eocene deformation, it is more difficult to determine its cause. There are three logical possibilities: the faults are

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shears diagonal to the compressive direction, they resulted from the axial arcuation of large folds, or they are related to the northern termination of the Sangre de Cristo Range and are late Eocene or Oligocene in age. If projected northwest, the faults would pass near the topographic termination of the range.

In the remainder of the zone the cross faults are fewer and trend either perpendicular to the zone, about N. 65° E., or more rarely about N. 35° E., diagonal to the zone. In only a few instances is the fault strike accurately determinable. Usually the presence of faults was detected by variant relations on either side and the trend was drawn generally between outcrops, many times in gully bottoms. It has been established that the faults in West Creek and Steel Canyon displace the thrust faults and the later normal faults. This indicates the cross faults may be one of the latest deformations in the region.

Normal faulting:
Few normal faults of consequence are known in the northern Sangre de Cristo Range. They fall into two classifications depending on their average strike: one group trending N. 20° W. parallel with the band of Lower Paleozoic sediments, the other N. 40° W. parallel with the topographic crest of the range. Both groups are later than all the compressional thrust faults in the region and are the result of differential vertical uplift. The N. 20° W. group are correlated with the late Eocene or Oligocene uplift of the Sierra Blanca massif on the south (Burbank and Goddard, 1937, pp. 969-71). The N. 40° W. faults are believed related to the Miocene downfaulting of the San Luis Valley.

Three normal faults parallel with the sediments were observed, and others in the pre-Cambrian or Pennsylvanian rocks might have escaped notice. Two of these are in the areas of imbricated thrust plates, one on each end of the thrust zone. In both cases the faults occur at the pre-Cambrian-Paleozoic contact and cause the Manitou dolomite to be thin or absent. The sediments are vertical or nearly so, and the fault planes approach being parallel with the bedding.

The third fault is one of the largest in the thrust belt. It extends from north of Cherry Creek to south of Hayden Pass, and possibly farther on both ends since the fault was lost beneath gravels on the north and not observed on the south. For convenience, it is termed the Hayden Creek fault. Along most of its length Pennsylvanian redbeds have been dropped so as to abutt against the pre-Cambrian in the upper plate of the central thrust of the Wellsville-Orient zone (section CC^prime). For a short interval in Hayden Creek the Lower Paleozoic limestones are exposed east of the fault where erosion has cut deepest. On both ends of the normal fault the thrust joins with it so that the normal fault is longer than the thrust and Lower Paleozoic sediments are found on its west side. The dip o the normal fault is much greater than that of the thrust, and the thrust relations terminate so abruptly against it that the normal fault probably displaces the thrust and is therefore later. A throw of 3,000-5,000 feet is indicated.

A significant feature is that the normal faults have taken advantage of previously existing zones of weakness both in strike and dip, even though an entirely different type of stress was involved. This could mean that the two types of

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deformation, thrusting and normal faulting, were not far removed in time, or that the later uplift was acting along the same trend as the former. Because the faults are of such high angle and there is no visible associated folding, it is concluded that vertical movement rather than compression was responsible. There is no evidence for crustal collapse in the region other than the small dome of Miocene lavas at Bonanza (Burbank, 1932, p. 43), so it is probable that the northern part of the range was uplifted differentially past the area on the east.

Burbank and Goddard have described the differential vertical uplift of the Sangre de Cristo Range farther south (1937, pp. 964, 972-74). In that region the Sierra Blanca massif was uplifted past the zone of marginal east-directed compression on the east side of the massif during the late Eocene or early Oligocene. In the northern region the east boundary of the massif cuts along the west edge of the range past the diapiric anticline, then south of Orient swings west across the San Luis Valley into the Kerber Creek area where it is represented by the Kerber thrust zone. The larger part of the massif existed in the San Luis Valley. The Sangre de Cristo Range from Orient north, therefore, is not connected with the Sierra Blanca massif.

Nevertheless, the normal faults of the northern part of the range are between the thrusts and late downfaulting of the San Luis Valley in age, and are apparently the result of vertical uplift. These factors agree closely with those characterizing the uplift of pre-Cambrian massifs on the south. The normal faults are not considered related to the faults that outlined the eastern edge of the San Luis Valley because of their markedly divergent trend. It is likely that the pre-Cambrian block comprising the northern part of the range is part of another massif whose center was at the northwest, and which was uplifted at nearly the same time as the Sierra Blanca massif just as the Culebra massif of the southern Sangre de Cristo Range is contemporaneous with the Sierra Blanca massif.

The late Oligocene to Recent downfaulting of the San Luis Valley and outlining of the range's present western face have been demonstrated only from Orient south where the exposed edge of the Sierra Blanca massif has been cut off sharply. However, some evidence, even though meager, indicates that the fault zone may extend to the head of the valley and possibly into the Arkansas Valley. The most prominent supporting physiographic feature is the unbroken linear trend of the sharp break at the valley's eastern edge which extends from Crestone north to the head of the valley and crosses the Kerber thrust zone as though it were not there (Fig. 8). The Wellsville-Orient thrust zone is divergent from the range crest and is obviously not responsible for the range's present form. However, the c est is parallel with the linear edge of the valley and the projected fault zone which is the most probable cause for the trend of the northern range. The northward extension of this fault zone is not a new contribution. J. B. Stone (personal communication) noted that the triangular facets projecting from the west side of the range were suggestive of a fault scarp. These features can hardly be attributed to glacial action, because there is no evidence to show that

End_Page 1607------------------------------

glaciers existed either on the west slope of the northern range or in the San Luis Valley. Also Burbank and Goddard (1937, Pl. I) show the normal fault zone to extend north of Orient, but not to the head of the valley. Reconnaissance mapping in the pre-Cambrian area southeast of Poncha Springs has disclosed a group of northwest-trending faults which are of a comparatively recent age since they contain epithermal fluorspar deposits and hot springs, and may be related in some ways to the normal fault zone. Many of the faults are breccia zones more than 100 feet wide. The dip on only one fault is known, that being northeast. If the zone extends beyond Poncha Springs, it loses its excellent topographic expression in the pre-Cambrian hills at the head of the San Luis Valley where it should be exposed. However, it may be discontinuous in that area.

A suggestion of the continuation of normal faulting even farther north is offered by the linear relationship and proximity of the San Luis Valley to the upper Arkansas Valley. If the original Sawatch arch extended completely across the Arkansas Valley, then a downfaulted weak zone must have existed to allow the early Arkansas River to follow its present course rather than the flank of the uplift farther east. Also, it is significant that normal movement on the Mosquito fault was traced south to the east side of the Arkansas Valley before it died out or was lost. If the Arkansas Valley is controlled by normal faulting, the relation of this action to the foundering of the San Juan region due to the weight of superimposed lava, as postulated by Burbank and others, becomes less valid beca se of the increased distance from the volcanic area. Smaller volcanic areas in the region could hardly have caused much foundering. A late "basin-and-range" type faulting seems more applicable. In places where the Arkansas Valley is of

Click to view image in GIF format. Fig. 8. [Grey Scale] Steep west slope of Sangre de Cristo Range from north of Hayden Pass to south of Orient. Baldy Peak on left, Electric Peak on middle right. Linear scarp is believed to be upthrown side of normal fault zone.

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considerable width it seems as though two normal faults with a dropped block between are needed to explain the physiographic features of the valley which is 1,500-2,000 feet below the level of South Park on the east. It is only one logical step farther to project the graben structure northward up the Arkansas Valley into the Leadville district where Tweto (1948, Pl. 7) has postulated a graben structure for the valley. Nearly the same physiographic features characterize the interval of projection.

LARAMIDE AND CENOZOIC HISTORY

During Late Cretaceous time uplift initiating the Laramide revolution raised the central Colorado region from the sea and erosion began stripping off the cover of Cretaceous sediments. Vertical uplift may have preceded lateral compression which acted principally toward the west and produced a series of northwest trending asymmetric folds. The major anticlinal mountain ranges were the Front Range, early Wet Mountains, and the Sawatch Range. The larger of the downfolded intermountain synclines became major valleys which accumulated sediments of Denver age. At this time the Pleasant Valley syncline, Sangre de Cristo anticline, and the folds of Kerber Creek and the Poncha synclinorium were formed in the northern Sangre de Cristo region.

Increased compression caused overturning and the rupturing of fold flanks, and extensive thrusting toward the west followed, accomplishing great crustal shortening. The Pleasant Valley fault, the Wellsville-Orient thrust zone, and the northwest faults of Kerber Creek were formed at this time.

In South Park the thrusts of this system cut the Denver formation and permit dating of the end of this first orogeny. The Denver formation has been classed as Late Cretaceous to early Eocene by the United States Geological Survey. The Arapahoe-Denver sediments are composed principally of pre-Cambrian and volcanic debris eroded from the Front Range as it rose. A strong unconformity separates the Arapahoe-Denver group from the underlying Cretaceous and is supposed to represent the early Laramide uplift. However, this classification has been contested. Lovering (1929, p. 91) considers this break between the Laramie and Arapahoe-Denver sediments to be less significant and places the Cretaceous-Eocene contact at the top of the Denver formation. Brown (1943, pp. 77-85), on the basis of good paleontologic evidence, found the Cretaceous-Tertiary boundary 290 feet above the base of the Denver formation and 500 feet above the base of the equivalent lower Dawson arkose. He therefore redefined the Laramie formation to include all disputed beds below this horizon, and considers the Denver of Paleocene age. LeRoy (1946, p. 101) considered the boundary of Brown too indistinct for mapping purposes and proposed the retention of the Arapahoe-Denver formation above the originally defined Laramie, but accepts the age classifications of Brown. Lavington (H. W. Oborne, 1948, pp. 55-56) has found that the Dawson arkose, east of the Front Range, overlaps the underlying Arapahoe-Denver (LeRoy, 1946) and older formations, and considers this the

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major break. He classes the Denver as latest Cretaceous to Paleocene and the Dawson as Eocene.

The previously discussed relations in Kerber Creek show that the earliest structures are post-dated by the southwest- and west-dipping thrusts which cross the Sangre de Cristo Range and displace the lower Eocene Poison Canyon and Cuchara formations in Huerfano Park, but only slightly displace the Huerfano formation of Bridger or middle to upper Eocene age. Thus the second orogeny is interpreted as in its final stages in Huerfano time so that the major deformation was probably middle Eocene in age.

The earliest Laramide orogeny, therefore, must have occurred between the middle Eocene and Upper Cretaceous (Denver time). The classification of Brown and Lavington is acceptable in this case. Thus the first orogenic phase may be considered lower Eocene.

The west-directed and later east-directed deformations have been described by Burbank (1933, p. 287) as possibly originating from the movement of the old Front Range Highland and the San Luis-Uncompahgre landmass toward the central Colorado basin.

The middle Eocene east-directed orogeny was followed by a subphase consisting of vertical uplift. At the south and east the Sangre de Cristo pre-Cambrian massifs and the Wet Mountains were uplifted nearly to their present heights. In the northern Sangre de Cristo the uplift of a massif, whose boundaries are undetermined except on the southeast, is suggested by the normal faults of the Wellsville-Orient zone. This phase of orogeny may have extended to other areas throughout central Colorado and probably initiated the entrenching of meandering streams whose courses were determined by previous Laramide deformations. Uplift probably continued to the end of the Eocene and possibly into the Oligocene.

The next major event in the Tertiary history of the region was the downfaulting of the San Luis Valley which occurred before and during the extrusion of Miocene lavas and created a basin in which accumulated lavas and volcanic-derived sediments. This action is illustrated by the normal faults on the east side of the valley, which dropped the greater part of the Sierra Blanca massif after it had been raised. "Basin-and-range" block faulting of this age is postulated for the northern part of the valley because of the scarp-like configuration and trend of the west slope of the Sangre de Cristo Range. It may have extended northward to create the downdropped outlines of the Arkansas Valley, normal movement on the Mosquito fault, and the normal block faulting of the Leadville district. The basin-and-range" block faulting has been described as late Oligocene to Recent in the San Juan region (Cross and Larsen, 1935, pp. 110-14), but evidence in the northern Sangre de Cristo region indicates that orogenic forces were of the broad regional vertical uplift type just before Pleistocene glaciation, and that the "basin-and-range" deformation was confined to a shorter period in the north. The San Luis downfaulting is probably more contemporaneous

End_Page 1610------------------------------

with the middle Miocene, post-Antero block faulting of South Park (Stark et al., 1949, p. 140). However, late uplift of the Sangre de Cristo Range may have been contemporaneous with downfaulting of the valley. The entrenching of streams, believed to have begun during the late Eocene uplift, was continued possibly until the Pleistocene at the northern end of the range. A flat gravel-strewn surface exists on the northeast side of the range about 1,500 feet above the present level of the Arkansas River. This surface was probably created after the late Eocene uplift since it truncates one of the normal faults believed to be of that age. The flat character of this surface and its gravel cover indicate a long quiescent period during which downfaulting was probably taking place. Uplift was s bsequently renewed and probably continued throughout the Pliocene while the Arkansas was being entrenched nearly to its present level.

Pleistocene alpine glaciers existed only on the east side of the northern Sangre de Cristo Range. These deeply incised the range and cut the lower parts of the larger valleys below the level of the beveled surface previously described.

CONCLUSIONS

Sufficient evidence has been accumulated to show that the majority of structural features in the Sangre de Cristo Range from Orient north to its termination are of a different age and were derived from a different pattern of applied forces from the structures south of Orient. The structures in the north are of the early Eocene west-directed compressional deformation, while those in the south are of the middle to late Eocene east-directed compressional deformation. The present range was uplifted vertically, then received its present shape and trend by the down-dropping of the San Luis Valley on the west.

References:

ATWOOD, W. W., AND MATHER, K. F., "Physiographic History of the San Luis Valley of Colorado and New Mexico" (abstract), Bull. Geol. Soc. Amer., Vol. 35, No. 1 (March 30, 1924), pp. 121-23.

ATWOOD, W. W., "Physiography and Quaternary Geology of the San Juan Mountains, Colorado," U. S. Geol. Survey Prof. Paper 166 (1932). 176 pp., 34 pls.

BAGG, R. M., "Some Copper Deposits in the Sangre de Cristo Range, Colorado," Econ. Geol., Vol. 3 (1908), pp. 739-49.

BEHRE, C. H., JR., "Preliminary Geological Report on the West Slope of the Mosquito Range in the Vicinity of Leadville, Colorado," Proc. Colorado Sci. Soc., Vol. 14, No. 2 (1939), pp. 49-79.

BILLINGS, MARLAND P., Structural Geology, pp. 161-66. Prentice-Hall, Inc., New York (1946).

BRAINERD, ARTHUR E., BALDWIN, HARRY L., JR., AND KEYTE, IVY A., "Stratigraphic Sections in the Southern Rocky Mountains of Colorado," Kansas Geol. Soc. Guidebook, 4th Annual Field Conf. (September, 1930), pp. 74-96.

BRILL, KENNETH O., "Late Paleozoic Stratigraphy of the Gore Area, Colorado," Bull. Amer. Assoc. Petrol. Geol., Vol. 26, No. 8 (August, 1942), pp. 1375-97.

BROWN, ROLAND W., "Cretaceous-Tertiary Boundary in the Denver Basin, Colorado," Bull. Geol. Soc. America, Vol. 54 (January, 1943), pp. 65-86.

BURBANK, W. S., "Geology and Ore Deposits of the Bonanza Mining District, Colorado," U. S. Geol. Survey Prof. Paper 169 (1932). 166 pp.

BURBANK, W. S., "Relation of Paleozoic and Mesozoic Sedimentation to Cretaceous-Tertiary Igneous Activity and the Development of Tectonic Features in Colorado," Ore Deposits of the Western States (Lindgren volume), pp. 277-301, Amer. Inst. Min. Met. Eng. (1933).

BURBANK, W. S., AND GODDARD, E. N., "Thrusting in Huerfano Park, Colorado, and Related Problems of Orogeny in the Sangre de Cristo Mountains," Bull. Geol. Soc. America, Vol. 48, No. 7 (1937), pp. 931-76.

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BURBANK, W. S., ET AL., compilers, "Geologic Map of Colorado," U. S. Geol. Survey (1935), in cooperation with Colorado State Geol. Survey Board and Colorado Metal Min. Fund, edited by G. W. Stose.

BUTLER, C. R., "Geology of the Northern Part of the Sangre de Cristo Mountains, Colorado" (abstract), ibid., Vol. 60, No. 12, Pt. 2 (December, 1949), pp. 1959-60.

CRAWFORD, R. D., "Geology and Ore Deposits of the Monarch and Tomichi Districts, Colorado," Colorado Geol. Survey Bull. 4 (1913). 317 pp.

CRAWFORD, R. D., "A Contribution to the Igneous Geology of Central Colorado," Amer. Jour. Sci., 5th Ser., Vol. 7, No. 41 (1924), pp. 365-88.

CROSS, CHARLES WHITMAN, AND LARSEN, ESPER SIGNIUS, "A Brief Review of the Geology of the San Juan Region of Southwestern Colorado," U. S. Geol. Survey Bull. 843 (1935). 138 pp.

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Acknowledgments:

(2) Geologist, Colorado Fuel and Iron Corporation. The writer is deeply grateful to the Colorado Fuel and Iron Corporation, and particularly to G. H. Rupp, manager of the mining department, for permission to publish this paper; also to D. A. Carter, chief geologist, for constructive criticism of the manuscript.

Copyright 1997 American Association of Petroleum Geologists

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