SEDIMENTOLOGICAL ANALYSIS OF QUATERNARY FOSSIL-BEARING LAKESHORE SEDIMENTS FROM PILOT VALLEY, NV: IMPLICATIONS FOR CLIMATE CHANGE

Pilot Valley, NV, is a closed-basin remnant from Lake Bonneville and records sediments from the lake's transgressions and regressions over the past ~32,000 years. Terraces from the previous lakeshores are obvious on the nearby mountains and correlate (by elevation) with the Stansbury (4445 ft), Bonneville (5200 ft), Provo (4800 ft), and Gilbert (4275 ft) lake levels (chronologically). An outcrop in the southwestern section of Pilot Valley sits at an elevation of approximately 4610 ft and contains 15 sedimentological units. The lower six units (Units 1 - 6) contain abundant root traces, and large gypsum rosettes that are often concentrated in the top few centimeters of the bed, making a bed-contact “crust”. These beds alternate between clay (Units 1, 2, 4 and 6) and silt (Units 3 and 5), and bulk X-ray analysis of the sediments indicate abundant quartz, gypsum, and calcite, with clays that appear to be kaolinitic or illitic. Unit 7 is an angular, fine to medium quartz-rich sand layer, above which gypsum rosettes are no longer found. Units 9 through 13 contain pebble horizons consisting of rounded dolomitic pebbles, and Units 11 and 13 are sandy with small (<20 cm deep) scour channels. Preservation of Hydrobiidae gastropods is abundant in the layers above Unit 7, with the exception of Unit 14.

Elevation of the outcrop suggests that the sediments record either the transgression from the Stansbury to the Bonneville level, or regression from the Provo to the Gilbert level. The presence of gypsum and root traces in Units 1 - 6 suggests shallow evaporative conditions. The interfingering of the clay/silt layers with sands in the middle of the section may indicate increased rainfall and flash flood events, bringing sands to the lake margin environment. The absence of root traces and gypsum rosettes, as well as the presence of Hydrobiidae gastropods, above Unit 7 indicates wetter conditions, and a flooded lake margin. Therefore, the outcrop shows a transgressional sequence from the Stansbury (ca. 24,400 - 23,300 yrs) to the Bonneville lake level (ca. 18,000 - 16,800 yrs).