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The Beaver Rim area comprises about 600 square miles west and north of the Granite Mountains in eastern Fremont and southwestern Natrona Counties, Wyo. Sedimentary rocks as much as 2,000 feet thick of Eocene, Oligocene, and Miocene ages are well exposed along the Beaver Rim escarpment, which crosses the central part of the area and forms the southern margin of the Wind River Basin and the northern margin of the Sweetwater Plateau. The Wind River Formation of early Eocene age is as much as 600 feet thick in the mapped area; it thickens northward toward the structural axis of the Wind River Basin. The basal contact is an angular unconformity. The formation consists chiefly of lenticular deposits of poorly sorted yellowish-gray feldspar-rich sandstone, poorly sorted pebble conglomerate, and variegated mudstone. Locally, thin layers of carbonaceous shale are present. Very large boulders of Precambrian and lower Paleozoic rocks also occur in the upper part of the formation. In the subsurface in the southwestern part of the area, the lower part of the formation apparently is a coarse boulder deposit. Dacitic pumice tuff is present in the upper part of the formation in the northwestern part of the area. Commercial concentrations of uranium minerals occur in poorly so·rted arkose in the upper part of the formation in the Gas Hills area. The Wind River Formation accumulated on low savannalike piedmonts and valley :flats in a warm humid climate. The sediment was derived mainly from Precambrian rocks of the Sweetwater uplift. Near the end of early Eocene time, volcanic outbursts in the Yellowstone-Absaroka region in northwestern Wyoming spread showers of dacitic ash across the area. The Wagon Bed Formation (new name) of middle and late Eocene age is conformable on the Wind River Formation and has a maximum thickness of 700 feet. Local thickness variations are the result of erosion prior to deposition of the overlying White River Formation of Oligocene age. The Wagon Bed Formation is characterized by persistent beds of greenish-gray to yellowish-gray sandstone, siltstone, and mudstone commonly containing considerable volcanic debris. The volcanic material in the formation in the western part of the area was derived from the Yellowstone-Absaroka calc-alkalic volcanic field; that in the eastern part was derived from the -Rattlesnake Hills alkalic volcanic field. The Wagon Bed Formation accumulated slowly duri:ng a warm humid period on poorly drained lowlands, on broad flood plains, and in lakes. Volcanic debris that reached the western part of the area was altered mostly to montmorillonite and silica, but acidic ash that accumulated in a lake during deposition of the middle part of the formation was altered to potassium feldspar, quartz, and zeolite. Alkalic volcanic debris deposited in the eastern part of the area accumulated more rapidly and generally was less altered. 1 2 TERTIARY GEOLOGY, BEAVER RIM AREA, WYOMING The White River Formation of Oligocene age lies unconformably on the Wagon Bed Formation and has a maximum thickness of 650 feet. The White River Formation consists chiefly of massive yellowish-gray to grayish-orange tuffaceous bentonitic mudstone containing local lenses of arkosic sandstone and conglomerate. Unaltered volcanic debris occurs abundantly mixed with fine-grained detrital deposits and in beds of uncontaminated vitric tuff. In the western part of the area, the lower part of the formation includes the arkosic Big Sand Draw Sandstone Lentil (new name) overlain by the extensive very coarse and poorly sorted volcanic Beaver Divide Conglomerate Member. During the Oligocene Epoch the climate was subhumid and warm-temperate, and there was a marked dry season. The area of deposition of the White River Formation was rather poorly drained, and fluvial sediments accumulated slowly on broad flood plains while many prolonged showers of ash from vents in the Yellowstone-Absaroka field spread over the region. A minor erosional unconformity marks the contact of the Split Rock Formation of Miocene age and the underlying White River Formation. The Split Rock Formation is as much as 150 feet thick at the surface along the Beaver Rim, and as much as 300 feet thick in the subsurface. The formation is predominantly massive yellowish-gray and grayish-orange well-sorted volcanic sandstone with well-rounded grains. Light-gray flaggy limestone is present in the western part of the area. Rather persistent beds of coarse conglomerate are common, especially in the lower 50 to 100 feet of the formation. Basic volcanic detritus and a few beds of vitric tuff are also present. During the Miocene Epoch the climate was cooler and drier than in Oligocene time. Arkosic detritus mixed with abundant sand-sized basic volcanic debris that apparently was blown into the area from northwestern Wyoming accumulated on extensive flood plains. The principal structural features of the Beaver Rim area are: 1. The buried southeast-trending Emigrant Trail thrust fault along which the Sweetwater uplift was thrust southwestward. 2. Late Tertiary fractures and normal faults that are alined with the trend of the buried Emigrant Trail thrust fault. 3. A major system of east-trending normal faults and fractures of late Tertiary age along the north flank of the Sweetwater uplift. 4. Gentle folds in beds of the Wind River and Wagon Bed Formations where they overlie basin-margin folds. Four distinct episodes of regional deformation affected the area during Tertiary time : 1. Major Laramide compressional forces during Paleocene and earliest Eocene time elevated and thrust the Sweetwater uplift southwestward while the Wind River Basin was being downwarped. Basin-margin folds were formed and eroded. 2. Marked downwarping of the Wind River Basin continued during Eocene time. 3. Epeirogenic uplift beginning after Eocene time ended in Ia te Cenozoic time. 4. Late Cenozoic normal faulting and reversal of movement tilted the Sweetwater uplift southward when it failed to rise as the surrounding area was elevated. INTRODUCTION The Tertiary sedimentary rocks that crop out on Beaver Rim along the southern margin of the Wind River Basin and on the northern

Tertiary geology of the Beaver Rim area, Fremont and Natrona Counties, Wyoming