JAMES R. STEIDTMANN Department of Geology, University of Wyoming, Laramie, Wyoming 82070 Origin of the Pass Peak Formation and Equivalent Early Eocene Strata, Central Western Wyoming ABSTRACT plex, and evidence from previous investigations is inconclusive with regard to timing. As a The Pass Peak Formation is a Tertiary result, reconstructions of the Tertiary geologic basin-flank deposit exposed throughout much history of this area are contradictory. A thor- of the Hoback Basin; a depression physio- ough investigation of the nature and distribu- graphically distinct from, but structurally con- tion of orogenic sediments and the relations of tinuous with, the northern Green River Basin. the various sequences to one another is neces- Two lithofacies of the Pass Peak Formation are sary if the Tertiary history is to be recon- identified. A northern quartzite conglomerate- structed. sandstone facies, possibly as thick as 3200 ft, The early Tertiary Pass Peak Formation is intertongues southward with a sandstone-silt- orogenic in origin, having been deposited in stone facies 1500 ft thick. This in turn extends response to local or regional uplift, or both. southward and intertongues with the Wasatch Determination of the source and distribution Formation. Arkosic sandstone and conglomer- of the Pass Peak sediments and an understand- ate intertongues with the Pass Peak on the east. ing of the sedimentary processes involved in Trends in grain size and sandstone composi- their deposition are fundamental to the inter- tion, and directional structures indicate that: pretation of the sequence of early Tertiary (1) a sedimentary source to the north supplied events in central-western Wyoming. The inves- quartzite cobbles and associated garnet-bearing tigation of the Pass Peak was undertaken with sand and (2) an igneous or metamorphic source, the following objectives: (1) to describe the or both, to the northeast supplied abundant formation and to map its distribution; (2) to arkosic debris. Regional geology suggests that determine and describe the provenance and the Pinyon Conglomerate to the north was the dispersal pattern of the sediments; and (3) source for Pass Peak sediments and the Pre- to determine the environment or environ- cambrian core of the Wind River Range to the ments of deposition. northeast was the source for the arkosic sedi- ments to the east. Previous Investigations Uplift in the Mt. Leidy Highlands, the Gros The Pass Peak Formation was named by Ventre Mountains, and the Wind River Range Eardley et al. (1944) from Pass Peak, a high during the early Eocene resulted in the deposi- point on the Hoback-Green River divide, tion of fanglomerate in the northern part of the where the rocks are exposed in a slide scar 1800 basin and coarse arkosic alluvial plain sedi- ft high (Fig. 1). The lowest yellow sandstone ments in the eastern part. The finer debris from was defined as the base of the formation and the north was carried southward onto a flood- was mapped over much of the southern and plain where it mixed with the finer sediments eastern part of the Hoback Basin and north- from the east. ward between the Gros Ventre and Hoback Ranges. Dorr (1958, Fig. 1) remapped the INTRODUCTION Pass Peak using the lowest conglomerate as the base. He placed the sandstones beneath this in Objectives the Hoback Formation or in his so-called The sequence of Laramide sedimentational "transition zone." The distribution of the Pass events in central-western Wyoming is com- Peak Formation, therefore, was limited to a Geological Society of America Bulletin, v. 82, p. 156-176, 11 figs., January 1971 159 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/1/159/3428262/i0016-7606-82-1-159.pdf by guest on 02 October 2021 I Tertiary sedimentary rocks V / J Poleozpe and Mesozoic \/s\ sedimentary rocks Precombnan rocks EXPLANATION HOBACK Pass Bon d u r a n t Big Pmey . I Utah Colorado Figure 1. Major physiographic and geologic features of central western Wyoming. Area of investigation enclosed by dashed line. Adapted from Love et al. (1955). Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/1/159/3428262/i0016-7606-82-1-159.pdf by guest on 02 October 2021 INTRODUCTION 161 smaller area in the southern and eastern part was broken, and its lithology determined. Ap- of the basin. proximately, 50 were examined at each location. Keefer (1964) demonstrated that the Pass The "largest size" method, suggested by Petti- Peak Formation was involved in the faulting john (1957, p. 249), was used to obtain a clast- along the southernmost margin of the Gros size index. The maximum axes of the ten largest Ventre Mountains. Antweiler and Love (1967, clasts found at each outcrop were measured p. 7) described the occurrence of gold in the with large calipers. The ten measurements were Pass Peak Formation. On the basis of verte- averaged, and this result was used as the clast- brate evidence, Dorr (1969) determined that size parameter for that location. the age of the Pass Peak ranges from late Heavy mineral concentrates were obtained Graybullian (late early Wasatchian) through by a centrifugation process described by Steidt- Lysitean (middle Wasatchian; that is, middle mann (1969b). The "line method" described early Eocene), and possibly into early Lost- by Ramesam (1966, p. 630) was used for making cabinian (early late Wasatchian, that is, late grain counts. In the present study an average of early Eocene). 200 nonmicaceous grains were counted in each sample. Sandstone, conglomerate matrix, and Techniques limestone samples were examined in thin sec- Mapping, sampling, section measuring, and tion. Where possible, 200 to 250 counts on light collection of detailed information concerning minerals were made on each sandstone and sedimentary structures were conducted for a conglomerate matrix thin section. These petro- total of seven months. One hundred seventy- graphic data are on file in the Department of one sandstone samples were examined in hand Geology, The University of Michigan, Ann specimens. The heavy minerals from 131 of Arbor, Michigan. these samples were studied, and grain counts A maximum grain-size parameter was ob- were made on 142 sandstone thin sections. tained from 70 of the sandstone samples arbi- Both heavy minerals and thin sections were trarily selected to cover the geographic distri- studied in 102 of the samples. Thin sections of bution of the Pass Peak Formation and associ- other rock types were also examined and de- ated arkose as evenly as sample coverage would scribed. allow. The observed maximum diameter of the The distribution of sampling over the area ten largest grains was measured and averaged. was not randomized because of the irregular The result was used as an index of maximum distribution of exposures. Instead, localities grain size for respective samples. were chosen to cover the geographic and strati- A grid of 1-sq mi units was superposed on graphic range of the Pass Peak Formation and the base map showing the locations of current associated arkosic deposits as completely as direction data. Thus each paleocurrent direc- time and accessibility would allow. Samples tion was located by its coordinates. A computer used for comparative purposes were collected program, utilizing an automatic plotter, was from the Pinyon Conglomerate (Paleocene), used to correct data for tectonic tilt where the Camp Davis Formation (Pliocene), and the necessary, to calculate statistical parameters, LaBarge Member and the New Fork Tongue and to plot a map of the smoothed data. of the Wasatch Formation (Eocene). Trends in the area distribution of composi- Approximately 950 paleocurrent directions tion and grain size were determined using com- were determined from cross-stratification. Ex- puter trend-surface mapping. This technique posure was rarely sufficient to permit determi- fits first-, second-, and third-degree surfaces to nation of the orientation of axes of trough the data using the least squares method. The cross-bedding and many directions of maximum surface which best fits the data is indicated by dip were measured on trough flanks. Measur- the amount of reduction of the sum of squares. able pebble imbrication was observed, and Howarth (1967) described the basis for using twelve readings were taken at three locations. the percent sum of squares to test the signifi- Large accumulations of plant debris were seen cance of a trend. on bedding planes, but no preferred orienta- The samples used for both compositional and tion could be determined. size trends were selected by superposing a grid Pebble and cobble-size and composition of 1-sq mi units on the sample location map and counts were made in the field. Samples were randomly choosing one sample from those taken at 6-in. or 1-ft intervals depending on available in each unit segment. Stratigraphic the coarseness of the conglomerate. Each clast separation of samples is small over much of the Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/1/159/3428262/i0016-7606-82-1-159.pdf by guest on 02 October 2021 162 I. R. STEIDTMANN—PASS PEAK FORMATION, WYOMING area studied, particularly that part south and and in the northern Green River Basin. Con- east of The Rim where there is little relief. glomerates and sandstones of Pliocene age crop However, in the Hoback Basin north and west out north of the Hoback Basin between the of The Rim, stratigraphic separation of the Hoback and Gros Ventre Ranges. samples may be as great as 1900 ft. Separate trend maps were computed for the Hoback Stratigraphy and Structure Basin and for the area south and east of The The distribution of the Pass Peak Formation, Rim and made it possible to assess the effect of its constituent lithofacies and their relation to stratigraphic changes in composition and grain the arkose are shown in Figure 2.