Thrust fault geometry of the Range, and

NICHOLAS B. WOODWARD Department of Geological Sciences, University of Tennessee, Knoxville, Tennessee 37996

ABSTRACT the Sevier thrust belt interacted with the Ancestral Teton or Targhee uplift. The large number of thrusts in the Snake River Range and the The Snake River Range is located at the north end of the Absa- truncated folds and thrusts shown by Rubey (1958) and Eardley (1960) roka fault system in the Idaho-Wyoming-Utah thrust belt, and it ad- have led others to suggest that the imbrication occurred from east tc west joins the Teton Mountains. Nine imbricate sheets of the Absaroka at the buttress (break-back sequence) (Kopania, 1983; Wiltschko and system form a shingled array of overlapping thrusts. Most fault no- Eastman, 1983). This is contrary to the over-all hinterland-to-foreland menclature in the range dates from early, discontinuous, reconnais- thrusting sequence described by Armstrong and Oriel (1965), Oriel and sance studies wherein names were applied from the top thrust down whether or not the thrust sheets separated by this method are, or ever were, connected as mechanical blocks. Fault names are revised here to consider the later: J continuity of sheets as the major criteria for defin- ing the block to w hich a name should be applied. The Absaroka thrust and St. John thrust are recognized to be equivalent different parts of a major transfer zone, using the continuity of sheets as the main criteria. The map pattern of the range is dominated by major re-entrants to the west in the traces of the St. John and Absaroka thrusts. The re-entrants in both major faults are caused by folding in their foot walls related to lateral ramping of each thrust upward to the south- east. The folding of the St. John thrust above the Indian Creek Culmination also folds the thrust sheets above it, helping to mark the emplacement of the imbricate sheets as being from top to bottom and west to east. The Absaroka system thrusts formed from 20 to 40 km west of their present position. They are folded by underlying faults of the Jackson or, farther south, the Darby system, indicating that the Ab- saroka system wsis emplaced into its present position by motion on these underlying faults. Discussions concerning mechanical interac- tion between the thrust belt and a foreland buttress (ancestral Teton or Targhee uplift) concentrate on intensities of deformation and changes in geometry adjacent to the proposed buttress. The individual imbricate geometries and regional changes in trend are more related to a changing stratigraphic package and the lateral thrust ramps than to geographic proximity to any proposed buttress.

INTRODUCTION

The Snake Ri ver Range is located at the northern end of the Idaho- Wyoming-Utah thrust belt. It adjoins the Tetons and Jackson Hole, Wyoming, to the northeast (Fig. 1). The main mountains are composed of imbricated thrust sheets of resistant Paleozoic rocks of the Absaroka thrust system, whereas the eastern foothills up to are composed of resistant folded and faulted Mesozoic rocks within the Jackson (Prospect) thrust sheet. The structures are unconformably overlain by Tertiary clastics and volcanoclastici of the Snake River Plain to the north and northwest. The area is interesting because of its excellent exposures of thrust faults and thrust-related structures. Horberg and others (1949) and Rubey (1955) suggested that it is this area where the Idaho-Wyoming thrust belt Figure 1. Generalized map of the Wyoming salient of the Cordil- "ran into" the Teton Mountains "buttress." Although the present Teton leran fold and thrust belt. The largest area in the north is occupied by Range is very young, Love (1983, and references therein) maintained that the Absaroka thrust sheet.

Geological Society o f America Bulletin, v. 97, p. 178-193, 13 figs., February 1986.

178

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/2/178/3434629/i0016-7606-97-2-178.pdf by guest on 27 September 2021 Figure 2. The stratigraphie units exposed in the Snake River Range are from Cambrian to Cretaceous in age (generalized from 700+meters Frontier U.S. Geological Survey quadrangle mapping and the author's measured sections). Isopach trends in Paleozoic rocks crosscut the range from northeast to southwest so that thicknesses are 700 + Aspen approximations.

240 Bear River Armstrong (1966), Wiltschko and Dorr (1983), and to data presented here. This paper focuses on the geometry of the many thrusts of the ï=m 135 Gannett Absaroka system. The relative timing of the Mosquito Pass and Jackson i5cr~ Stump S Preuss thrusts, which occur between the Absaroka fault and the Teton Block 240 Twin Creek (Fig. 3), is discussed only briefly. 120 Nugget 120 Ankareh PREVIOUS WORK 240 Thaynes The most recent mapping in this mountain range was by U.S. Geo- 300 Woodside logical Survey geologists in the 1960s and 1970s. This work includes 160 Dinwoody 1:31,360 maps of the Driggs (Pampeyan and others, 1967) and Gams 60 Phosphoria Mountain quadrangles (Staatz and Albee, 1966) 1:24,000 maps of the Conant Valley (Jobin and Schroeder, 1964a), Thompson Peak (Jobin and 300 Wells Soister, 1964), Palisades Peak (Jobin, 1965), Teton Pass (Schroeder, 150 Amsden 1969), Observation Peak (Albee, 1973), Alpine (Albee and Cullens, (paleokarst) 1975), Ferry Peak (Jobin, 1972), and Pine Creek quadrangles (Schroeder 300 Mission Canyon and others, 1981). Previously, Gardner (1961) mapped part of the area; 160 Lodqepole and, during the 1940s, students from the University of Michigan field 120 Darby station at Camp Davis (see reference list for individuals) mapped and I 20 Bighorn 60 Gallatin named many of the thrusts. The and NEW the 300 Gros Ventre quadrangle were mapped by the writer in 1978 and 1979 to connect the structures in the northern and southern parts of the range. The Michigan 300 meters Flathead Fm theses and USGS reports outline the stratigraphy of the range, which is Dal summarized by Wanless and others (1955) (Fig. 2). During mapping and

Figure 3. Summary map of the nine thrust sheets (Fig. 4A-I) in the Snake River Range. Their recognition is based on two major criteria: (1) continuity of stratigraphic separation along a main fault trace; (2) stratigraphic cut-off line geometry showing that sheets have consistent three-dimensional, discrete fault shapes as mechanical blocks. The positions of structure sections in Figure 5 are also shown.

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reconnaissance to clarify fault interrelationships, it became clear that thrust traces. Structure contour maps (Figs. 4A-4I) and balanced sections (Figs. nomenclature in the range suffered from a lack of consistency. Thrust 5A-5E) illustrate the shapes of the redefined thrust sheets in the range. names which were originally established in the southern part of the range These figures are derived from the previously mentioned quadrangle maps, were carried northward erroneously, not by direct mapping, but by pre- my mapping of the Mount Baird quadrangle, and local remapping of serving the stacking sequences of thrust names from highest to lowest. A critical areas. Thrust surfaces were renamed on the basis of the new proposed simplified nomenclature is used in Figure 3 and other maps here. mapping that documented the faults separating the major thrust sheets. The contour maps include hanging-wall cutoff positions for the different THRUST FAULT GEOMETRY stratigraphic contacts in each thrust sheet on the basis of available map data. A cutoff line is the intersection of the fault with a stratigraphic The thrust sheets in Figures 3 and 4 were defined on the basis of contact. Where cutoff lines are closely spaced, the thrust cuts through the continuity of the stratigraphic section in each fault block and on the section at a high angle (a ramp); where they are broadly spaced, the thrust preservation of, or regular change in, stratigraphic separation along fault cuts through the section at a low angle (a flat). Where cutoff lines parallel

Figure 4. Structure contour maps of each of the thrust sheets in the range, based on the author's mapping of thrust traces and compilation from U.S. Geological Survey quadrangles in Woodward (1981). Teton Pass is shown in each map where appropriate. A trailing edge branch line as described by Beyer and Elliott (1982) is the line along which a thrust merges with the next overlying thrust; they are the western limits of the respective thrust sheets at depth. A leading edge tip line (C) is the line separating slipped and unslipped material in front of a blind thrust. The contours for the Little Elk and Needle thrusts are on the lowest imbricates in the respective systems. Hanging-wall (HW) cut-off lines are mimed for the appropriate stratigraphic units (see Fig. 2). A. The Baldy thrust sheet. B. The Needle thrust underlies a set of imbricates of Cambrian, Ordovician, and Devonian rocks from the Grey's River north to Elk Creek, where it may correlate with one of the upper faults within the Little Elk thrust system. C. The three main imbricates of the Little Elk thrust system are well exposed in Sheep Mountain, Sheep Creek Peak and Mount Baird and Elkhorn Peak. They are numbered 1-3 from lowest to highest. D. The three-dimensional geometry of the Elk thrust sheet can be defined because of its excellent exposure. The Palisades Creek W indow (PCW) helps to define the cut-off line positions by limiting their positions along Palisades Creek. E. The Ferry Peak thrust sheet is a generally homoclinal, west-dipping panel of rock from the South Fork of Indian Creek to the Grey's River. It probably merges with the underlying St. John thrust a short distance west of its present exposures. F. The Thompson thrust system (or duplex zone) of upper Paleozoic imbricates is best exposed from Neeley Cove south to the North Fork of Indian Creek. Only the basal thrust of the system is contoured. The ex|>osed duplex is folded between Elk Creek and North Fork of Indian Creek by structures beneath the St. John thrust sheet. G. The Black Mountain thrust sheet designation is new and derived from its best exposure. It is not believed to be part of the St. John tlirust sheet as proposed by Staatz and Albee (1966), because it is not continuous with the main body of the St. John farther south. H. The St. Jtohn thrust displacement dies out to the northwest, as can be seen by the convergence of the IP/P cutoff line in the hanging wall and foot wall. To the south, the thrust surface is folded over the Indian Creek culmination. The St. John and Absaroka faults merge for a short distance at J. I. The Absaroka thrust hi is the largest slip at the north end of the range but joins the St. John at J. From J south to the Grey's River, it is the floor thrust to the Absar oka horse (Fig. 3). The two major faults merge finally just south of the Grey's River.

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the regional trend of the thrusts, they define frontal ramps and flats. Cutoff sidered as Maestrichtian in the southern part of this thrust belt (Oriel and lines which are oblique to regional trends define lateral ramps and flats. Armstrong, 1966; Royse and others, 1975). Locally, it is only demonstra- Dahlstrom (1969, 1970), Royse and others (1975), and Boyer and Elliott bly post-Frontier Formation (Cenomanian). The Mosquito Pass thrust (1982) discuss such thrust relationships more thoroughly. within the Jackson sheet (Fig. 3) was correlated by Albee and others The sequence of emplacement of thrust sheets discussed is from west (1967) with the Darby thrust. The Darby thrust was dated as middle to east and overlying to underlying. The Absaroka fault motion is con- Paleocene by Dorr and Gingerich (1980). The Jackson (Prospect) thrust

oooo 6000 -TRAILING EDGE BRANCH LINE FOR FERRY PEAK AND ST.JOHN THRUSTS

Teton Pass

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system is latest Faleocene or early Eocene (Dorr and others, 1977; thrust sheet deeply dissected by erosion, because the fault geometries with Wiltschko and Dorr, 1983) in the Hoback area 50 mi to the southeast. respect to bedding in the sheets change within each one. The Baldy thrust (Fig. 4A) is exposed in the northern part of the ABSAROKA FAULT SYSTEM range, where it is deeply dissected and occurs as several klippen. It is cut off on the west very near, or at, the surface by the Grand Valley normal Baldy-Little Elk-Needle Thrust Sheets fault (Figs. 4A and 5B). Between its northernmost exposure and Painey Creek, it is a flat sheet, placing Cambrian on Mississippian rocks. Bedding The western peaks of the range consist of folded and imbricated in both the hanging wall and foot wall parallel the thrust (a hanging-wall lower Paleozoic rocks of the Baldy, Little Elk, and Needle thrust "sheets," flat on a foot-wall flat). Between Rainey Creek and Palisades Creek, the (Figs. 3 and 4A-4C). Despite the apparent continuity of structures along Baldy thrust surface steepens and cuts bedding at a 30°-40° angle (ha iging- strike indicated by USGS maps, these imbricates are not simply the same wall and foot-wall ramps). The effect is to preserve stratigraphic separation

Figure S. These five balanced and restored sections cross the range from northeast to southwest (Fig. 3). The Absaroka system is narrower in the northern part of the range probably because the Grand Valley Normal Fault (GVNF) crosses the thrust trends. It follows the main Absaroka foot-wall ramp at the southern end of the range (C, D, and E), but may not farther north. Thi; restored sections are Vi the scale of the deformed sections for simplicity of presentation. Where there are breaks in the restored stratigraphy, it is because the GVNF cuts off the original trailing edges of the sheets so that higher thrusts cannot be accu- rately restored. The thrust symbols are as follows. Jackson Thrust System: MMT, Mount Manning thrust; MCT, Mahogany Creek thrust; M, Maho- gany thrust; JT, Jackson thrust. Darby Thrust System: MPT, Mosquito Pass thrust; DT, Darby thrust; Absaroka

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(€ against M) with much less slip. South of Palisades Creek, the Baldy Elk-Ferry Peak Thrust Sheets thrust dips steeply west within west-dipping Cambrian rocks and loses all stratigraphic separation. The footwall of most of the Baldy, Little Elk, and Needle thrusts Three flat sheets of Cambrian and Ordovician rocks are stacked comprises the trailing edge of lower thrust sheets (Fig. 3). To the north, the under Needle Peak, south of Elk Creek. As mapped by Moore and others Elk sheet (the Elk thrust is called the "Gopher Canyon thrust" by Staatz (1984), the Needle sheet is a steeply west-dipping imbricated sheet of and Albee, 1966) underlies the Baldy sheet between Pine Creek and Cambrian through Devonian rocks extending for 20 km along the west Palisades Creek (Fig. 5B). It underlies the Little Elk thrust from Palisades edge of the range, from Elk Creek south to the Grey's River. The contour Creek to Elk Creek (Fig. 8), and it also underlies the Needle thrust from maps (Figs. 4B, 4C) show, however, that the individual Needle imbricates Elk Creek to Blowout Canyon. Farther south, the Ferry Peak sheet under- south of Elk Creek cannot simply be traced northwestward. The imbri- lies the Needle thrust from Indian Creek to Grey's River (Fig. 5E). Unlike cated Needle thrust sheet is another major sheet like, and in a similar the Baldy-Little Elk-Needle sheets which substitute for one another along structural position to, the Baldy sheet farther north. There are no lateral strike by a shingle-like overlapping, the Elk and Ferry Peak sheets are structures in the area indicative of major changes of shortening along strike equivalent in position but are never in contact with each other. The Needle between Baldy Mountain and Needle Peak. The Little Elk blind thrust sheet directly overlies the St. John thrust sheet between the Elk and Ferry system (Fig. 4C) serves as a transfer zone for displacement between the Peak sheets. Baldy sheet on the north and the Needle sheet on the south. The Elk sheet is the most widely exposed in the range and includes a The Little Elk system is a group of two-to-four blind thrusts. The stratigraphic section of Cambrian through Pennsylvanian rocks. There is faults in Cambrian through Devonian units die upward into a fold of also a window through the sheet in Palisades Creek Canyon which allows Mississippian Lodgepole and Mission Canyon Limestones. The thrust calculation of its displacement and limits the cutoff line positions of the complex extends ~ 10 km along strike and is exposed on the sides of Sheep Darby, Lodgepole, and Mission Canyon Formations. The exposed width Mountain, Sheep Creek Peak, and Mount Baird (Figs. 6,7). Little Elk and of the sheet decreases to the north as the cutoff-line spacing decreases, Sheep Creeks erode windows through the whole set of slices and into their (compare Figs. 5B, 5C), indicating that the northern exposures are near that footwall. end of the imbricate sheet.

Figure 5. (Continued).

Thrust System: AT, Absaroka thrust; SJT, St. John thrust; BMT, Black Mountain thrust; BT, Baldy thrust; ET, Elk thrust; TT, Thompson thrust; LET, Little Elk thrust; NT, Needle thrust; BCT, Blind Canyon thrust; FT, Ferry thrust; EMT, Elk Mountain Thrust. The stratigraphic horizons shown are synoptic: LP, lower Paleozoic, includ- ing the Cambrian, Ordovician, and Devonian; UP, upper Paleozoic, in- cluding Mississippian, Pennsylvanian, and Permian. J, T, and K designations are from Figure 2.

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Figure 6. View north across Little Elk Creek at the hanging-wall i ramp- foot-wall flat, thrust pattern of the Little Elk thrust. It puts Cambrian Gal- latin and Gros Ventre Formations on top of the Mississippian Mission Can- yon Formation at the west end of the canyon and Ordovician, Devonian, and Mississippian rocks on top of Mis- sion Canyon Formation shown in this photo in the main part of the Ciinyon. To the east, the Little Elk thrust dies out as a blind thrust in the upper Mis- sion Canyon.

Figure 7. View north from the an- al the Little Elk thrust system across Elk Creek Canyon. The three ithrusts form a blind imbricate complex which probably never penetrated through the massive Mission Canyon Limestone. The blind thrust in front is the Little Elk thrust, and higher imbricates may be equivalent to the Baldy thrust (at the north end) or the Needle thrust (at the south end). The size of individual im- bricates in the Little Elk and Needle systems strongly suggests shingle-like overlaps of small sheets which cannot be traced uniquely along strike. Strati- graphic symbols are from Figur e 2.

Where the lilk sheet narrows in outcrop width, the Black Mountain The Ferry Peak thrust sheet (Figs. 3,4E, 5E) intervenes betw een the thrust sheet (Fig. 4G) (erroneously called the "St. John sheet" by Staatz St. John trailing edge and the Needle imbricates between Indian Creek and Albee, 1966) widens and progressively gains stratigraphic separation. and Grey's River. It includes a section from Cambrian through Mississip- This is another transfer zone between two overlapping thrust sheets. Short- pian rocks, and occupies the same structural position as the Elk sheet. No ening across the pair is relatively constant because there are no lateral cutoff lines are preserved in the hanging wall, and so the original shape of faults to indicate that any part has moved farther than another part. the north end of the sheet is uncertain. Isolated slices or klippe of the sheet The southern end of the Elk thrust sheet (Figs. 4D, 9) is located are not recognizable farther north. between Elk Creek and Blowout Canyon. The Needle thrust sheet of Cambrian and Ordovician rocks overlies Devonian rocks in the southern Thompson Thrust System end of the sheet at the termination. The Elk sheet thins from northwest to southeast. At Elk Creek, it is composed of Cambrian through Mississip- The main foot wall of the Elk and Ferry Peak thrusts consists of pian rocks. The southernmost exposure of the Elk sheet is a thin septum of Mississippian rocks on the trailing edges of the Black Mountain and the St. Devonian rocks between the Needle thrust and the main foot wall of John thrust sheets. Trailing edges are intensely imbricated in the center of Mississippian rocks. Formation cutoff lines in the Elk hanging wall change the range. This imbricated zone is called the "Thompson thrust system" trend sharply south of Elk Creek (Fig. 4D), defining a lateral or oblique because its lowest fault was so named in the Thompson Peak quadrangle ramp. (Jobin and Soister, 1964) (Figs. 3, 4F, 5D). It involves Mission (Canyon,

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Figure 8. View northward of Mount Baird ridge across Elk Creek near southern termination of the Elk thrust underlain by several horses of Cambrian and Ordovician rocks.

Figure 9. The imbrication of the Mississippian Mission Canyon Forma- tion paleokarst on the trailing edge of the St. John thrust sheet is best ex- posed on the north wall of Indian Creek, seen here looking northwest from the air.

Amsden, and Wells Formation rocks. The western faults of the imbricate upper part of the Mission Canyon Limestone. The best exposed part of the zone merge upward with the Elk thrust along Elk Creek making it a duplex, on the canyon wall of Indian Creek (Fig. 9), is composed entirely "duplex" (Fig. 5C). This duplex (Boyer and Elliott, 1982) is also preserved of horses of the paleokarst (Woodward, 1981). The intense imbrication of in three nested klippen on Powder Peak east of Elk Creek, and on the the Mission Canyon paleokarst propagated upward and eastward into the north fork of Indian Creek (Garden thrust complex of Woodward, 1981; Amsden Formation and then died out into folding in the massive Wells Fig. 9). Formation and the overlying Triassic rocks. Northwestward from Neeley Cove (Fig. 4F), the Thompson thrust is a single fault, placing Pennsylvanian Wells Formation rocks against the Black Mountain Thrust Sheet Triassic Woodside and Dinwoody Formations. The Wells Formation oc- curs in an eastwardly asymmetric anticline, which is cored by duplicated In the Driggs quadrangle, the thrust sheet beneath the Elk thrust was Amsden Formation. The massive upper Wells sandstones and dolomite called the "St. John sheet" by Staatz and Albee (1966), who followed the cap the duplex. In the headwaters of Rainey Creek, the Thompson thrust structural stacking sequence established earlier at the south end of the (identified by its association with the same anticline) merges with the Elk range. Maps of the thrust sheets along the range, however, show major thrust in an area of multiple slices of Wells Formation. The Thompson problems with this correlation. If Staatz and Albee's interpretation is cor- thrust zone is thus areally restricted and includes rocks of only a thin rect, the St. John thrust sheet, which transported a section of Cambrian to stratigraphic interval. It originated in a well-developed paleokarst in the Mississippian rocks at Black Mountain, is represented only by the over-

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Figure 10. The frontal folds of the Elk, Thompson, St. John, and Absa- roka thrust sheets are exposed by ero- sion in Palisades Creek Valley. This view at the north wall of that canyon shows an eastward overturned, anti- cline of Wells Formation (IPw) thrust over a syncline of Triassic rocks (Tw, Woodside Formation; "Ed, Dinwoody Formation; "Ea, Ankareh Formation; "Et, Thaynes Formation) in the St. John thrust sheet. According to Jobin and Soister (1964), a minor thrust oc- curs in the core of this syncline.

turned limb of thi; syncline (Fig. 10) in Palisades Creek Valley. Farther St. John Thrust Sheet south, the St. John sheet again transports a section from Ordovician to Triassic. It is more likely that the major thrust sheet at Black Mountain If the Black Mountain and St. John thrusts differ, where does the St. (Figs. 3, 4G) is another shingle in the imbricate pile which terminates John thrust extend northwestward from Palisades Creek? At Neeley Cove southward in the fault slices of Pennsylvanian Wells Formation between in the Palisades Peak quadrangle (Jobin, 1965), the St. John thrust juxta- Rainey Creek and Palisades Creek (Fig. 11). poses a syncline of Pennsylvanian, Permian, and Triassic rocks onto the The Black Mountain sheet is a gently west-dipping, homoclinal pack- western limb of an anticline of Triassic and Jurassic rocks in the Absaroka age which thins southeastward at the base of the sheet by the fault cutting sheet. This syncline is the same as the syncline in Figure 10, at Palisades up-section gently southward. The exposed outcrop width and probable slip Creek. The simplest explanation indicates that the St. John thrust should decreases as that of the Elk sheet increases. Shortening across the two again underlie the east limb of the fold. This fault was called the " Dead- remains roughly constant. man Creek fault" by Jobin and Soister (Thompson Peak quadrangle, 1964). The same fault was called the "Poison Creek fault" by Pampeyan and others (1967) and Staatz and Albee (1966). North of Neeley Cove, the width of the St. John sheet above the St. John-Deadman Creek-Poison Creek thrust narrows progressively toward Pine Creek. Stratigraphio sepa- ration also decreases, so that the fault places homoclinal panels of Tliaynes on Thaynes Formation (compare Figs. 5B and 5C). The writer would call the continuous fault under the east limb of the syncline the "St. Jchn" in all locations and conclude that, like most others in the range, the Si;. John thrust dies out along strike. Some of the confusion is based on mapping of Jobin and Soister's Deadman Creek thrust with two branches. The west branch puts Pennsyl- vanian rocks on Cretaceous Ephraim strata, whereas the east branch puts Ephraim rocks against Jurassic Twin Creek Limestone. The western branch is the St. John thrust with the same stratigraphic separation it has farther south. The eastern branch, however, is an extensional fault parallel to the St. John's trace. Listric normal faults are common in the entire thrust belt (Royse and others, 1975), and this is a local example. Southeastward from Neeley Cove, definition of the geometry of the St. John sheet is vital to understanding the over-all geometry of the range. On the south side of Elk Creek, the St. John and the Absaroka thrusts merge into a single thrust. About 2 km farther southeast, the two faults Figure 11. Tracing folds is as important as tracing fault surfaces diverge again at Fall Creek (Figs. 4H, 41). The St. John thrust trace then in properly naming thrusts and thrust sheets. The Absaroka sheet for swings westward around the Indian Creek drainage, to near the junction of most of its length has an eastward asymmetric anticline in its hanging North and South Forks of Indian Creek, where it swings southeastward wall. The St. John sheet has a chevron syncline of Triassic rocks. The again to the Grey's River. This westward re-entrant in the fault trace is Black Mountain thrust sheet has a hanging-wall ramp anticline which called the "Indian Creek Culmination" in the following description. dies out southward. In the same area, the Thompson thrust's hanging- The St. John thrust sheet occupies the width of the range between Elk wall ramp anticline plunges north and dies out west of the Black Creek and Indian Creek (Figs. 3, 5D). It includes a section of Ordovician Mountain sheet. The Elk sheet is marked by a hanging-wall anticline Bighorn Formation through Triassic Thaynes Formation. The fault is ex- over the ramp in. the Devonian Darby and Mississippian Lodgepole posed in two windows (one in South Fork Elk Creek and one in South Formations. Fork Fall Creek) and along the north wall of the North Fork Indian Creek

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Figure 12. A. Generalized geo- logic map of the fold in the St. John thrust called the "Indian Creek Culmi- nation" (LM, Lower Mesozoics, Trias- sic and Jurassic; UP, Upper Paleo- zoics, Permian, Pennsylvanian, and Mississippian; LP, Lower Paleozoics, Devonian, Ordovician, and Cambrian). A lateral ramp is marked by strati- graphic cut-off lines trending along or oblique to the thrust transport direc- tion. In this case, the St. John cuts up- section in any northeast-southwest transport direction cross section and is folded by structures within the culmi- nation; therefore, it was emplaced be- fore the last deformation within the culmination. Unfolding the upper Paleozoic-lower Mesozoic contact line, however, creates a north-northeast- to south-southwest-trending cut-off line for the adjacent formations, a lateral ramp. This map shows the deformed foot-wall lateral ramp. B. The hang- ing-wall lateral ramp, which moved off the foot-wall lateral ramp, is preserved in the Fall Creek drainage 9 km north- east of the foot-wall structures. Pre- thrusting (with the future fault shown) and post-thrusting sections along AB are shown. C. The restored hanging- wall and foot-wall lateral ramps should fit back together when each is un- folded. This schematically shows the shape of the St. John fault surface prior to thrust motion.

Canyon. The fault rides within the Bighorn Formation (a hanging-wall flat) for 2.5 km along North Indian Creek and along most of its trace south to the Grey's River. The westward re-entrant in the St. John's trace forms a half-window around folding and blind thrusting between Indian Creek and the Snake River (Fig. 12A). Structure contours on the St. John surface (Fig. 4H) (Woodward, unpub. data) show that it overlies an elliptical culmination in its foot wall, the long axis of which trends northwest-southeast parallel to SE the general structural strike. The cutoff-line geometry for the St. John NW hanging wall farther northeast gives an indication of the origin of this N. Side Indian Ck S. Side ln£jan Ck foot-wall deformation. Ji

The Pennsylvanian/Permian cutoff line in the hanging wall (in the Jn T(«tw South Fork Fall Creek drainage) trends north-northeast-south-southwest, nearly parallel to the thrust's transport direction. It defines a hanging-wall PH> lateral ramp through at least the Wells, Phosphoria, Ankareh, and Thaynes r or Formations. The foot-wall cutoff lines for the St. John should also show a M similar lateral ramp. The location of the foot-wall lateral ramp is precisely -a _/ -a. . the highly deformed area between the north and south forks of Indian •e A ... Creek. ^ Basal Decol lernen t. When folding and imbrication is removed, the Mississippian through V " \ • \ y \ t — — V y ^ ' Triassic foot-wall cutoff lines also trend nearly parallel to the transport

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direction. The lateral ramp preserved in the hanging wall, therefore, is only the fault dip is steeper than the dip of the hanging-wall beds. The foot-wall part of a larger one in which the St. John thrust surface cuts up to the south faults reflect imbrication of the foot-wall lateral ramp that is equivalent to from a flat in the Mississippian to a higher flat in the Jurassic [the fault the hanging-wall lateral ramp seen on the map (Woodward, 1981). once overlay all of the Triassic rocks presently preserved on Middle Ridge Northwest of Elk Creek (Figs. 5A, 5B, 5C), the St. John thrust (Fig. 4H), including the Ankareh Formation]. diverges westward from the trace of the Absaroka thrust. An eastward, The folding on Middle Ridge in the upper Paleozoic rocks is most asymmetric anticline occurs at the leading edge of the Absaroka horse. The intense at the lateral ramp and dies off to the south. North of Indian Creek, Absaroka sheet from Elk Creek northward nearly to Pine Creek is com- the St. John thrust surface dips 20° northwest, and from Indian Creek posed of this frontal hanging-wall ramp anticline. The Deadman Creek southeast to the Grey's River, it dips 45° southwest. This indicates that the extension fault and the St. John thrust (as used here) occurs on its west St. John fault was; folded after its emplacement by the deformation of the limb over that distance. The northwestward loss of the frontal anticline on foot wall at the lateral ramp under Middle Ridge. An alternative hypothe- the Absaroka thrust coincides with the loss of the syncline which was sis, that the pre-existing Middle Ridge folds were truncated (both limbs cut mentioned previously in regard to the St. John thrust sheet. North of Pine across later) by the St. John thrust, is rejected because the equivalent part Creek, the Absaroka fault underlies a homoclinal, west-dipping sheet that of the hanging wall is preserved 9 km (Fig. 12B) to the northeast, and no includes the St. John thrust as a minor fault within it, placing Thaynss over truncated folds are seen there. Thaynes. The St. John thrust underlying a sheet composed of the Cambrian The St. John-Afasaroka Thrust System through Triassic Thaynes section is the most important one at the Grey's River. This fault underlies the same body of rock that is called the "Absa- The St. John and Absaroka fault surfaces coincide between Elk roka thrust sheet" farther south. At Indian Creek, the St. John thrust has Creek and Fall Creek and then diverge to the southeast (Fig. 3). The St. 9-10 km of slip based on the offset hanging-wall and foot-wall lateral John thrust swings westward around the Indian Creek culmination and ramps. The Absaroka thrust has about 15 km of slip based on the offset of then southeastward. The Absaroka swings in an eastward curve from Fall the frontal hanging-wall ramp from its foot-wall ramp which controlled Creek, where its trace strikes northwest-southeast to the Grey's River the Grand Valley fault location (Royse and others, 1975). Where they where it strikes no rtheast-southwest. The two major thrusts join again just merge, the combined fault has the sum of the two slips. The slip (a.nd the south of the Grey's River (Fig. 3). As shown in Figures 5C and 5D, this importance of the St. John thrust) decreases to the northwest as the Absa- indicates that the Absaroka imbricate sheet at the south end of the range is roka gains slip. Where the St. John dies in the Thaynes Formation at Pine a very large horse (a block enclosed entirely by fault surfaces). It is con- Creek, the Absaroka again becomes the major frontal thrust of the s ystem. fined both above and beneath by thrusts which presumably diverged from Overall, the Snake River Range exposes a major transfer zone between a single fault to the west (beneath Grand Valley) and which merged two faults which carry the Absaroka name at the north and south ends of eastward again into one fault. Farther south, the main frontal thrust of the the area, where they put Paleozoic over Cretaceous rocks, but which are Absaroka system places Cambrian rocks over Cretaceous rocks and is separately named as the "Absaroka" and the "St. John" (with imbricates) called the "Absaroka fault" (Rubey, 1973; see discussion of thrust nomen- in the transfer zone. clature in Royse and others, 1975). Between Fall Creek and the Grey's River, however, the St. John sheet contains the lower Paleozoic rocks, and RELATIVE THRUST TIMING the Absaroka juxtaposes upper Paleozoic rocks onto Cretaceous strata. The Absaroka horse is imbricated internally by the Blind Canyon The Absaroka horse carries a stratigraphic section of Mississippian thrust (BCT, Fig. 3) and its minor associated thrusts. They duplicate upper Lodgepole Formation to Cretaceous Bear River Formation (Fig. 41). The Paleozoic and Triassic rocks in both branches of Indian Creek. The overly- hanging-wall cutoff lines from the Snake River northward are closely ing St. John thrust is strongly folded by this lower deformation at the spaced and trend northwest-southeast, showing that the fault cuts up- eastern side of the Indian Creek culmination (Figs. 3,4H, 5E). Locally, the section steeply eastward. The map is an oblique view across this frontal Blind Canyon thrust penetrates into the St. John hanging wall and places hanging-wall ramp. A ramp anticline from Phosphoria through Gannett foot-wall Nugget Formation against hanging-wall Amsden Formation Group is well exjwsed. The horse is internally imbricated by the Blind across a vertical fault. This indicates that the St. John thrust sheet was Canyon thrusts and is also cut by northwest-southeast-trending extension emplaced prior to the Blind Canyon thrusting within the Absaroka horse. faults (Fig. 5E). The St. John also predates the folding associated with the Indian Creek From the Snake River to south of the Grey's River, where the Absar- culmination, because it is folded by it. The St. John thrust was emplaced oka and St. John merge, the shape of the horse changes. The Amsden- before the Absaroka frontal thrust formed and isolated the Absaroka Wells cutoff line is the only one preserved, but it trends northeast-south- horse. As there appears to be over-all displacement balance between the west, nearly parallel to the transport direction (Fig. 13A and B). At the Absaroka and St. John thrusts, folding of the St. John by motion on the Snake River, the Absaroka sheet includes a section of Lodgepole to Absaroka indicates that the deformation was partly concurrent and pro- Thaynes. Along the Grey's River, the Absaroka fault has cut up-section in gressive, rather than episodic. The St. John fault began motion at Indian its hanging wall to the Wells Formation. The southern end of the Absa- Creek synchronously with the "Absaroka" thrust at Pine Creek. This roka horse marks another major lateral hanging-wall ramp, which cuts northern branch of the Absaroka propagated southward to the east of the through the upper Paleozoic and lower Mesozoic section. propagating St. John fault. The two faults isolated the Absaroka sheet as a The Stewart Peak Culmination (Fig. 3) (Lageson, 1984) lies south of horse in their area of significant overlap. The main fault to the north the Absaroka hone and beneath the combined Absaroka-St. John thrust. became a foot-wall imbricate of the St. John farther south. Its structure is important in order to understand the Absaroka contour map Structures above the St. John thrust also are folded by the Indian (Fig. 41). The culmination has several thrusts of upper Paleozoic through Creek culmination. The Ferry Peak and Needle thrust sheets both lie on Cretaceous rocks in its center (Lageson, 1984; Schroeder and others, top of the steeply west-dipping part of the St. John sheet on the west side 1981). The steep Absaroka contours reflect folding of the thrust surface of the culmination. The fault surfaces are folded around the western corner and the hanging-wall lateral ramp by the foot-wall imbrication, because of the culmination (Fig. 4C) and predate the St. John.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/2/178/3434629/i0016-7606-97-2-178.pdf by guest on 27 September 2021 Figure 13A. Generalized geologic map of the north margin of the Stewart Peak Culmination (geology from Lage- son, 1984, and Jobin, 1972). Many thrusts in the hanging wall of the Ab- saroka and St. John thrust sheets merge with these major faults south- ward.

A

Figure 13B. There is a south- facing hanging-wall lateral ramp in the Absaroka horse marking its southern end beneath the St. John thrust. The cutoff line (Amsden/Wells contact) trends obliquely to the transport direc- tion. This is the poorly preserved hanging-wall part of the lateral ramp. The foot-wall part of the ramp at the Mesozoic level underlies the area to the south and southwest (geology from SEW of the Ferry Peak quad, by Jobin, 1972).

B

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The Thompson thrust zone is clearly folded concordantly with the St. The Relationship of the Absaroka Thrust System John thrust (Fig!». 4F, 5D). Erosion of the north-plunging, antiformal to the Basement Uplifts Thompson zone left, as remnants, the stacked klippen on Powder Peak and the duplex along the western and northern exposed edges of the St. The view that the Tetons acted as a buttress was supported by Hor- John sheet. berg and others (1949) with three lines of evidence: (1) the "crowding and The amplitu de of folding of the St. John and Thompson thrusts convergence of fold axes, accompanied by intensified thrust faulting," decreases northward (upward) so that the Elk thrust is only broadly (p. 257); (2) the change in trend of structures regionally; and (3) the warped. The Baldy thrust sheet lies concordantly on top of the homoclinal interaction of the Jackson thrust with the southwest limb of the Taylor part of the Elk sheet and thus should predate the Elk sheet's emplacement. Mountain anticline west of Teton Pass. They concluded that "the foreland The Black Mountain thrust sheet underlies the Elk sheet and replaces structures developed first and acted as a buttress against which the geosyn- it northward. The: north end of the Elk sheet dips southward off the back cline was crowded" (p. 213). Several lines of evidence lead to the opposite of the Black Mountain sheet and was gently warped in that area by its conclusions with respect to the Absaroka thrust and the structure; of the emplacement. The Elk thrust predates the Black Mountain thrust. Snake River Range. First, the majority of thrusts in the range are part of the Absaroka SUMMARY OF THE ABSAROKA THRUST SYSTEM system which developed in a west-to-east progression. Mapping since the late 1940s demonstrates that the fold axis and thrust orientations are The shapes of the nine thrust sheets of the Absaroka system in the unusual in their excellent exposure but not in their internal geometry. Snake River Range can be used to demonstrate their interactions as a Changes in thrust orientation, if a buttress were present, also should be shingled array of imbricate thrusts of one major fault system. Throughout associated with changes in thrust geometries where the thrusts approach the range, structurally higher sheets are folded by the emplacement of the proposed buttress. The location of Teton Pass is shown in Figure 3 and lower sheets (Jones, 1971). Locally, small scale faults that were associated on each large structure contour map in Figure 4. Although thrusts are with lower thrust;: cut higher ones. The sheets were emplaced in piggyback steeper in that area, the maps show that the thrust system is less imbricated fashion, the oldest sheets on the west and the youngest on the east. There there, not more so. are no truncations of early, lower structures by later, higher ones. Locally, Second, the relative timing of deformation between the Teton Uplift thrusts of the Absaroka system formed from hinterland to foreland, just as and thrusting as proposed by Horberg and others (1949) also is mostly in the regional profession of thrusting did. The resulting order of thrust error. The Absaroka system formed in the Late Cretaceous period-proba- emplacement was Baldy-Little Elk-Needle thrusts, Black Mountain-Elk- bly 20-40 km west-southwest of its present position. The Ancestral Ferry Peak thrusts, and finally the St. John-Absaroka system. Targhee Uplift was proposed as an earlier buttress by Love (1983 and The faults within the Stewart Peak culmination folded the Absaroka other references therein) for an area to the northwest of the present Tetons; surface and thus ¡ire also later foot-wall imbricates. this area is now covered by the Snake River Plain. The north end of the range would have formed nearest this older buttress. There is no change in JACKSON AND DARBY THRUST SYSTEMS thrust geometry to indicate this, nor is there a change of orientation in that part of the range to indicate its presence. Two extra imbricates; of the Two large thrusts occur between the Absaroka fault and Teton Pass Jackson system (Mount Manning and Mahogany Creek thrusts) at the (Fig. 3), the Jaclcson (Prospect) thrust and the Mosquito Pass thrust of northwest end of the range (Figs. 5A, 5B) also show usual thrust geome- Jobin (1965). The Mosquito Pass thrust was called the "continuation of tries (Staatz and Albee, 1966). the Darby thrust" by Albee and others (1967), even though the Darby Present research on the structures of the Absaroka thrust system in loses almost all of its stratigraphic separation into a box fold at Munger the range does not resolve Horberg and others' third suggestion regarding Mountain. It is questionable whether the name "Darby" should be applied the precise structural geometries of faulting at Teton Pass. Schroeder to the minor Mosquito Pass fault, which can be connected with the Darby (1969) and Zeller (1983) disagree about the structures which are present only tenuously across several miles of poorly exposed Cretaceous rocks. and S. Oriel (1984, personal commun.) suggested that both may l)e over- Farther south, this Darby is demonstrably younger than the Absaroka fault simplifying a more complex situation. In any case, the Jackson-I'rospect (Woodward, 1981), and there is nothing in the map geometry (Jobin, thrust system is later than most of the Snake River Range structures, and 1965; Schroeder, 1969; Pampeyan and others, 1967) to suggest otherwise some temporal overlap with foreland faulting cannot be ruled out to the north. In fact, near Teton Pass, where the axial planes of the (Wiltschko and Dorr, 1983). Mosquito Pass folds are vertical or overturned to the west, the overlying Absaroka thrust contours and fold-axial planes are also steep or over- CONCLUSIONS turned. The Moiiquito Pass folds and thrust formed after the Absaroka thrust, and all were overturned westward together as a result of the uplift The style of the Absaroka thrust system in the Snake River Range, of the Tetons. including transfer zones between imbricate sheets replacing one another in The Jackson (Prospect) thrust postdates the Darby to the south, a shingled array, is not well documented at this scale elsewhere in the although the pre«rise relationships are debatable (Royse and others, 1975; Idaho-Wyoming thrust belt. On a larger scale, the Crawford and. Meade Blackstone, 1979; Dorr and Gingerich, 1980). Despite Dixon's (1982) sheets and the Paris and Putnam sheets occupy equivalent structural posi- protest, normal cross-section balancing techniques do not require the tions along strike from one another (Fig. 1). The exposures of the thrusts in Darby to postdate the Jackson thrust. There is nothing in the map geome- the Snake River Range are the result of (1) the present Teton Uplift try to suggest that the Jackson thrust is truncated by the Mosquito Pass affecting adjacent areas and (2) the Absaroka "horse" as a major, thick, fault in a "break-back" style on the eastern edge of the Snake River Range. additional, foot-wall fault slice under the higher sheets. The Indian Creek

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Dahlstrom, C.D.A., 1969, Balanced cross sections: Canadian Journal of Earth Sciences, v. 6, p. 743-757. and Stewart Peak lateral ramps contribute to folding and exposing of 1970, Structural geology in the eastern margin of the Canadian : Bulletin of Canadian Petroleum higher thrust structures. Geology, v. 18, p. 332-406. Dixon, J. S., 1982, Regional structural synthesis, Wyoming salient of western overthrust belt: American Association of There are no mapped truncated faults or folds between the Absaroka Petroleum Geologists Bulletin, v. 66, p. 1560-1580. Dorr, J. A., Jr., and Gingerich, P. D., 1980, Early Cenozoic mammalian paleontology, geologic structure and tectonic thrust and the Jackson thrust, nor are there any in the Absaroka system history in the overthrust belt near Labarge, western Wyoming: Contributions to Geology, University of Wyoming, north of the Snake River. Thrusts adjacent to the Teton Uplift have been v. 18, p. 101-115 Dorr, J. A., Jr., Spearing, D. R., and Steidtman, J. R, 1977, Deformation and deposition between a foreland uplift and an locally steepened. The simplest explanation for these geometries is that impinging thrust belt; Hoback Basin, Wyoming: Geological Society of America Special Paper 177, 82 p. Eardley, A. J., 1950, Snake River region of western Wyoming: Wyoming Geological Association, 5th Annual Field each thrust within the Absaroka system formed from hinterland to fore- Conference, Guidebook, Southwest Wyoming, p. 88. land. The entire system was subsequently carried into its present position 1956, Geology of southwest Jackson quadrangle: Wyoming Geological Association, 11th Annual Field Confer- ence, Guidebook, p. 179. by motion on the Jackson and Mosquito Pass thrusts. Finally, the entire 1960, Phases of orogeny in the fold belt of western Wyoming and southeastern Idaho, in Overthrust Belt of southwestern Wyoming and adjacent areas: Wyoming Geological Association, 15th Annual Field Conference, stack was tilted by the present Teton Uplift. There is no evidence in the Guidebook, p. 37-40. Snake River Range to indicate an active mechanical interaction between 1962, Structural geology of North America (2nd edition); New York, Harper Bros. Elliott, D. W., 1977, Some aspects of the geometry and mechanics of thrust belts: Canadian Society of Petroleum Geology, thrusting and basement uplifts, nor any "break-back" thrusts or east-to- 8tb Annual Seminar, Lecture Notes, v. 1 and 2.

Enyert, R. L.t 1947, Geology of the Calamity Point area, Snake River Range, Idaho [M.S. thesis]: Ann Arbor, Michigan, west imbrications. University of Michigan. Espach, R. H., Jr., 1957, Geology of the Mahogany Ridge area. Big Hole Mountains, Teton County, Idaho [M.S. thesis]-. Why does the Absaroka system change orientation in the vicinity of Laramie, Wyoming, University of Wyoming. the Snake River (Fig. 1)? An equally good question is, why should it not? Gardner, L. S., 1961, Preliminary geologic map of the Irwin quadrangle, Caribou and Bonneville Counties, Idaho, and Lincoln and Teton Counties, Wyoming: U.S. Geological Survey Open-File Report. The Meade thrust curves sharply in southeast Idaho. Thrust systems are Grubbs, K. L., and Van der Voo, R., 1976, Structural deformation of the Idaho-Wyoming overthrust belt (U.S.A.), as made up of anastomosing shingles, as noted by Price (1973), and as determined by Triassic paleomagnetism: Tectonophysics, v. 33, p. 321-336. Hinds, G. W„ and Andrau, W. E., I960, Geology of a portion of the northern Snake River Range, Bonneville County, different major imbricates become important along strike, significant Idaho: Wyoming Geological Association, 15th Annual Field Conference, Guidebook, p. 57. Horberg, C. L., Nelson, V. E., and Church, V., 1949, Structural trends in central western Wyoming: Geological Society of changes in orientation should result. America Bulletin, v. 6, p. 193-215. Jobin, D. A., 1965, Preliminary geologic map of the Palisades Peak quadrangle, Bonneville County, Idaho, and Teton County, Wyoming: U.S. Geological Survey, Open-file Map. ACKNOWLEDGMENTS 1972, Geologic map of the Ferry Peak quadrangle, Lincoln County, Wyoming: U.S. Geological Survey Geologic Quadrangle Map GQ-1027. Jobin, D. A., and Schroeder, M. L., 1964a, Geology of the Conant Valley quadrangle, Bonneville County, Idaho: U.S. Geological Survey, Mineral Investigations Field Studies Map MF-277. This research was completed as part of a doctoral dissertation at The 1964b. Geology of the Irwin quadrangle, Bonneville County, Idaho: U.S. Geological Survey Mineral Johns Hopkins University, under the supervision of David Elliott. Discus- Investigations Field Studies Map MF-287. Jobin, D. A., and Soister, P. E., 1964, Geologic map of the Thompson Peak quadrangle, Bonneville County, Idaho: U .S. sions of the geology of this area and the region with friends at Johns Geological Survey Mineral Investigations Field Studies Map MF-284. Jones, P. B., 1971, Folded faults and sequence of thrusting in Alberta foothills; American Association of Petroleum Hopkins; Amoco Production Company, Denver; Arco Oil and Gas Com- Geologists Bulletin, v. 55, p. 292-306. pany, Denver; and the U.S. Geological Survey have corrected many errors Keenmon, K. A., 1948, Geology of the Red Creek area, Snake River Range, Wyoming [M.S. thesis]: Ann Arbor, Michigan, University of Michigan. and clarified many of the questions about the range. William Brock, Bob Kopania, A., 1983, Deformation consequences of the impingement of the foreland and northern thrust belt, eastern Idaho and western Wyoming: Geological Society of American Abstracts with Programs, v. 5, p. 296. Wells, Gene Richards, Steve Boyer, and especially Steve Oriel and Dave Lageson, D. R., 1984, Structural geology of the Stewart Peak Culmination Idaho-Wyoming Thrust Belt: American Moore helped immensely. I would also like to thank Rick Allmendinger, Association of Petroleum Geologists Bulletin, v. 68, p. 401-416. Love, J. D., 1983, A possible gap in the western thrust belt in Idaho and Wyoming: Geologic studies of the Cordilleran Frank Royse, Steve Oriel, and Steve Boyer for their critiques of earlier Thrust Belt, Rocky Mountain Association of Geologists, v. 1, p. 247-261. Moore, D., Woodward, N.B., and Oriel, S. S., 1984, Geologic map of *.heMt. Baird quadrangle: U.S. Geological Survey versions of this manuscript. Open-File Report 84-776. Oriel, S. S., and Armstrong, F. C., 1966, Times of thrusting in Idaho-Wyoming Thrust belt: American Association of This work was supported by grants from Amoco Production Com- Petroleum Geologists Bulletin, v. 50, p. 2614-2621. pany, Arco Oil and Gas Company, and TGA Incorporated, all of Denver, Pampeyan, E. H., Schroeder, M. L., Schell, E. M„ and Cressman, E. R., 1967, Geologic map of the Driggs quadrangle, Bonneville and Teton Counties, Idaho, and Teton County, Wyoming: U.S. Geological Survey Mineral Investiga- Colorado. A Geological Society of America student grant and an Ameri- tions Field Studies, Map MF-300, scale 1.31,360. Price, R. A., 1973, Large-scale gravitational flow of supracrustal rocks, southern Canadian Rockies, in De Jong, K. A., can Association of Petroleum Geologists Research-in-Aid grant provided and Scholten, R, eds., Gravity and tectonics: New York, John Wiley and Sons, p. 491-502. additional support. Publication was supported by the Discretionary Fund Royse, F., Jr., 1957, Geology of the Pine Creek Pass area, Big Hole Mountains, Teton and Bonneville Counties, Idaho [M.A. thesis]: Laramie, Wyoming, University of Wyoming. of the Department of Geological Sciences of the University of Tennessee. Royse, F., Jr., Warner, M. A., and Reese, D. L., 1975, Thrust belt structural geometry and related stratigraphic problems, Wyoming-Idaho-northern Utah, in Rocky Mountain Association of Geologists Symposium on Deep Drilling Frontiers in central Rocky Mountains, p. 44-54. Rubey, W. W., 1955, Early structural history of the overthrust belt of western Wyoming and adjacent states: Wyoming BIBLIOGRAPHY Geological Association, 10th annual Held conference, guidebook, p. 125-126. 1958, Geology of the Bedford quadrangle, Wyoming: U.S. Geological Survey Geologic Quadrangle Map GQ-109. Albee, H. F., 1964, Preliminary geologic map of (he Gams Mountain N.E. quadrangle, Teton County, Idaho: U.S. 1973, Geologic map of the Afton quadrangle and part of the Big Piney quadrangle, Lincoln and Sublette Counties, Geological Survey Mineral Investigations Field Studies Map MF-274. Wyoming: U.S. Geological Survey Miscellaneous Geologic Investigations Map 1-686. 1973, Geologic map of Observation Peak quadrangle, Teton and Lincoln Counties, Wyoming: U.S. Geological Schroeder, M. L., 1969, Geologic map of the Teton Pass quadrangle, Teton County, Wyoming: U.S. Geological Survey Survey Geologic Quadrangle Map GA-1081. Geologic Quadrangle Map GQ-793. Albee, H. F., and Cullens, H. L., 197S, Geologic map of the Alpine quadrangle, Bonneville County, Idaho and Lincoln 1972, Geologic map of the quadrangle, Teton County, Wyoming: U.S. Geological Survey County, Wyoming: U.S. Geological Survey Geologic Quadrangle Map GQ-1259, scale. Geologic Quadrangle Map GQ-793. Albee, H. F., Jobin, D. A., and Schroeder, M. L., 1967, Northwesterly extension of the Darby thrust in the Snake River Schroeder, M. L., Albee, H. F., and Lunceford, R. A., 1981, Geologic map of the Pine Creek quadrangle, Lincoln and Range, Wyoming and Idaho: U.S. Geological Survey Professional Paper S7S-D, p. D1-D3. Teton Counties* Wyoming: U.S. Geological Survey Map GA-1549. Albee, H. F., Lingtey, W. S., Jr., and Love, J. D., 1977, Geology of the Snake River Range and adjacent areas: Wyoming Staatz, M. H., and Albee, H. F., 1966, Geology of the Garns Mountain quadrangle, Bonneville, Madison, and Teton Geological Association, 29th Annual Field Conference, Guidebook, p. 769-783. Counties, Idaho: U.S. Geological Survey Bulletin 1205, p. 122. Armstrong, F. C., and Oriel, S. S., 1965, Tectonic development of Idaho-Wyoming thrust belt: American Association of Wanless, H. R., Belknap, R. L., and Foster, H., 1955, Paleozoic and Mesozoic rocks of Gros Ventre, Teton, Hoback and Petroleum Geologists Bulletin, v. 49, p. 1847-1866. Snake River Ranges, Wyoming: Geological Society of America Memoir 63, 90 p. Bally, A. W., Gordy, P. L., and Stewart, G. A., 1966, Structure, seismic data and orogenic evolution of southern Canadian Wiltschko, D. V., and Dorr, J. A., Jr., 1983, Timing of deformation in Overthrust belt and foreland of Idaho, Wyoming, Rocky Mountains: Bulletin of Canadian Petroleum Geology, v. 14, p. 337-381. and Utah: American Association of Petroleum Geologists Bulletin, v. 67, p. 1304-1322. Bastanchury, R. F., 1947, Geology of the Bradley Mountain area, Lincoln County, Wyoming [M.A. thesis]: Ann Arbor, Wiltschko, D., and Eastman, D., 1983, Role of basement warps and faults in localizing thrust fault ramps: Geological Michigan, University of Michigan. Society of America Memoir 158, p. 177-190. Beutner, E., 1977, Causes and consequences of curvature in the Sevier orogenic belt, Utah and Montana: Wyoming Woodward, N. B., 1981, Structural geometry of the Snake River Range, Idaho and Wyoming [Ph.D. dissert.]: Baltimore, Geological Association, Rocky Mountain Thrust Belt Geology and Resources, 29th Annual Field Conference, Maryland, The Johns Hopkins University, 261 p. Guidebook, p. 353-366. 1983, A balanced view of the northern Idaho-Wyoming thrust belt: Geological Society of America Abstracts with Blackstone, D. L, Jr., 1979, Geometry of the Pros pea-Darby and La Barge faults at their junction with the La Barge Programs, v. 15, p. 318. Platform, Lincoln and Sublette Counties, Wyoming: Wyoming Geological Survey Report of Investigations 18, 34 p. Boeckerman, R. F., 1950, Geology of the southwest quarter of the Jackson quadrangle, Wyoming [Ph.D. thesis]: Ann Arbor, Michigan, University of Michigan. MANUSCRIPT RECEIVED BY THE SOCIETY SEPTEMBER 21,1983 Boyer, S. E., and Elliott, D., 1982, Thrust systems: American Association of Petroleum Geologists Bulletin, v. 66, REVISED MANUSCRIPT RECEIVED JULY 10,1985 p. 1196-1230. MANUSCRIPT ACCEPTED JULY 12, 1985

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