RADIOCARBON CONSTRAINTS ON TIMING OF DEGLACIATION AND DEPOSITION OF POST-GLACIAL VALLEY-FILL SEDIMENTS FROM DRY GULCH, UINTA MOUNTAINS, NORTHEASTERN UTAH

Clastic and organic sediments collected immediately upstream of a prominent, sharp-crested late Pleistocene moraine records over 12,000 years of sedimentation following deglaciation in Dry Gulch in the southern Uinta Mountains. The sedimentary package in the 2.9-m high exposure contains 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, which are interpreted to be glacial till. The organic-rich layers range in color from very dark grey (Munsell color 5Y 3/1) at the base of the section to dark grey (7.5YR 4/2) at the top of the section; the organic-poor layers range from light bluish grey (5B 7/1) at the base to light grey (5YR 7/1) at the top.

Seven radiocarbon dates provide age control for the section. A new basal radiocarbon date from the site collected from a conifer cone (BETA 203234) indicates that local deglaciation occurred before 10,990 ± 50 14C yr BP (12860 – 13000 cal yr BP). A new radiocarbon date from 38 cm below the surface (BETA 203233) indicates that lacustrine sedimentation continued at this site until at least 1210 ± 40 14C yr BP (1060 – 1260 cal yr BP). At sometime after this date, the moraine that impounded the lake failed catastrophically, ceasing deposition and causing an onset of gullying that exposed the described section. These two dates confirm that this section represents nearly 12,000 years of constant post-glacial sedimentation at the site.

Grain size analyses and percent loss-on-ignition (%LOI) analyses were conducted on samples collected at 2-cm intervals through the section. From 11,500 to 10,000 cal yr BP, mean grain size (13.4 µm) was significantly smaller (P < 0.001) than in the remainder of the record (23.1 µm). From 13,000 to 11,500 cal yr BP, %LOI (mean 8.3%) was significantly less (P < 0.001) than during the remainder of the record (mean 17.2%). Preliminary diatom analyses indicate the setting was a clear, shallow lake that fluctuated in size and depth through the Holocene. These fluctuations are also indicated by variations in color, grain size, and organic content in the section.