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Study of a granule-to-boulder conglomerate of probable middle to late Eocene age provides evidence that the last pulse of uplift of the Wind River Range occurred along a fault or fault zone within the crystalline core of the range. The conglomerate is preserved on the northeast side of the Continental fault between Tabernacle Butte on the west and Horsetrack anticline on the east. It overlies rocks of the Bridger Formation (Eocene), locally overlies Precambrian crystalline rocks, and appears to be overlain by lower Miocene strata.
The conglomerate consists of numerous coarse pebbles, cobbles, and boulders floating in a matrix ranging from coarse sand to granules. Many of the nonequidimensional pebbles and cobbles are imbricated and isolated, suggesting current scour on the upstream side of the pebbles, and are oriented with their long axes between transverse and longitudinal positions, indicating later reworking of orientations developed during initial high velocity flows. Thus, fabric indicates deposition in a fluvial system having highly variable current velocities.
Distributary paleocurrent patterns, proximity to a major uplift, and the presence of large boulders suggest deposition on alluvial fans. These alluvial fans were characterized by deposition of moderately to poorly sorted coarse sand to boulder-size material in a braided stream environment. Although the conglomerate is matrix supported, deposition by fluvial processes is indicated by well-defined stratification, lenticularity of bedding, common scour surfaces, local cross-stratification, channel fills ranging up to 31 m (102 ft) wide, and fabric as noted above. Evidence of mass flows is very rare in this unit.
Composition of pebbles and cobbles in the conglomerate is quite variable, indicating both the complex Precambrian geology and the erosion of Precambrian units within the core of the range as well as those exposed locally. The large amount of finer sediments (sand and granules) in the conglomerate at exposures near the Precambrian paleohighs suggests, at least in part, a distal source. Mapping of other Eocene sedimentary deposits in the Big Sandy Opening area suggests the presence of fluvial tributaries well within the crystalline core of the range. These characteristics suggest a middle or late Eocene pulse of uplift within the core of the range resulting in incision of tributary systems into the Precambrian rocks of the southwestern margin of the Wind River Range. A decrease in strea gradient at the southwestern limit of the uplifted crystalline rocks resulted in deposition of much of this core-derived sediment on basin-margin alluvial fans. This system was probably morphologically similar to but smaller in scale than, the modern Kosi fan system of northeastern India.
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