HR: 14:10h
AN: U23B-03    [Abstracts]
TI: Geological Context of Cosmogenic-Nuclide Sampling Sites for the CRONUS-Earth Project on the Shoreline of Paleo-Lake Bonneville, Utah
AU: * Phillips, F M
EM: phillips@nmt.edu
AF: New Mexico Tech, 801 Leroy Place Department of Earth & Env Science, Socorro, NM 87801 United States
AB: One of the objectives of the NSF-funded CRONUS-Earth Project is to obtain geologically and geochronologically well-constrained samples in which the production rates of a large number of cosmogenic nuclides can be compared. One area sampled for this purpose, in July 2005, was associated with the shoreline of Lake Bonneville, a very large Pleistocene pluvial lake that occupied the current Great Salt Lake basin. The Bonneville highstand represented the all-time high lake level in the basin. The timing of this event is well constrained by 14C dating. The lake approached the Bonneville highstand by ~18.9 ka. The highstand probably lasted ~1.7 ka and ended at 17.4±0.2 ka by downcutting of the lake sill at Red Rock Pass. We sampled two localities associated with the shoreline: Tabernacle Hill and Promontory Point. The Tabernacle Hill basalt flow was erupted during the Bonneville highstand and was partially covered by the lake. We sampled the flow at seven locations approximately 500 m south of the northeast margin of the flow. The location of the Bonneville shoreline, marked by wave erosion and tufa deposition, was verified to be to the northeast of, and below, the sampling sites. Samples were collected from the tops of push-up ridges using a rock saw. Promontory Point was sampled from a pronounced wave-cut bench on the west side. At the sample site, the depth of wave erosion was ~140 m. The bench was cut into quartzite. Samples were collected by rock saw and hammer and chisel from wave-planed bedrock outcrops. Based on geological mapping, the sample sites were 18 to 27 m below the lake surface elevation (depending on sample location), with an estimated uncertainty of ±5 m. The geometry of the pre-Bonneville topography can be reconstructed with a fair degree of confidence, enabling correction of the measured nuclide concentrations for pre-Bonneville production by muons.
DE: 1105 Quaternary geochronology
DE: 1130 Geomorphological geochronology
DE: 1150 Cosmogenic-nuclide exposure dating (4918)
SC: Union [U]
MN: Fall Meeting 2005