Thrust fault geometry of the Snake River Range, Idaho and Wyoming 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 Teton Pass 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 Mount Baird and NEW the Palisades Dam 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. 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 180 N. B. WOODWARD 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.