SEDIMENTOLOGY AND CHRONOLOGY OF POST-GLACIAL VALLEY FILL FROM DRY GULCH, UINTA MOUNTAINS, NORTHEASTERN UTAH

Clastic and organic sediments collected immediately upstream from the middle of a set of three late Pleistocene moraines record over 5,000 years of sedimentation following deglaciation in Dry Gulch in the southern Uinta Mountains. The sedimentary package in the 2.9-m high exposure is defined by alternating layers of organic-rich and organic-poor silt and silty clay; individual layers are between 2- and 12-cm thick. The base of the section rests on quartzite cobbles and boulders as large as 0.5 m diameter. The organic-rich layers range in color from very dark grey (Munsell color 5 Y 3/1) at the base of the section to dark grey (7.5 YR 4/2) at the top of the section; the organic-poor layers range from light bluish grey (5 B 7/1) at the base to light grey (5 YR 7/1) at the top. The package is interpreted to represent the repetitive expansion and contraction of a wetland/lake impounded behind the moraine.

A series of five radiocarbon samples collected from organic detritus of riparian and aquatic species indicates that sedimentation at this site began no later than 9980 ± 100 ¹⁴C yr BP (11,760 to 11,190 cal yr BP) and continued until at least 5390 ± 80 ¹⁴C yr BP (5970 to 5950 cal yr BP). Extrapolation of the dates suggests that local deglaciation may have commenced as much as 1000 years before the oldest radiocarbon date. Sedimentation at the site apparently ceased after the distal margin of the moraine was breached. These data provide a basis for ongoing studies of the timing and pattern of deglaciation in the Dry Gulch area and across the southern Uinta Mountains.