ACCUMULATION RATES IN ANCIENT LAKES: RELATIONSHIP TO LAKE TYPE EVOLUTION OF THE GREEN RIVER FORMATION, WYOMING

The Green River Formation in Wyoming preserves a symmetrical succession of sedimentary facies associations deposited over a period of 4-5 my. These grade upward from intervals of fluvial-lacustrine, to fluctuating profundal, to evaporative, and then back through fluctuating profundal and fluvial-lacustrine facies. The relative thicknesses of these intervals suggest that they primarily record a tectonic cycle of increasing and then decreasing potential accommodation, rather than long term climate change. This hypothesis can be tested by examining average accumulation rates, which are limited by potential accommodation (equal to the elevation difference between the basin floor and lowest basin sill). Our recent ⁴⁰Ar/³⁹Ar dating of interbedded tephras permits the first direct measurement of average accumulation rates, and highlights wide variations between different intervals. For fluvial-lacustrine to fluctuating-profundal facies of the Tipton and Laney Members, average accumulation rates are 88 ± 34 and 108 ± 19 mm /yr. In contrast, the evaporative Wilkins Peak Member accumulated at 327 ± 86 mm/yr.

We propose that as rates of potential accommodation increased, the greater Green River Basin evolved from overfilled to underfilled. This interpretation is bolstered by evidence for active fault movement at the northern and southern basin margins during deposition of the Wilkins Peak Member. Rapid accumulation of carbonate- and bicarbonate-rich sediments appears to have been driven by evaporative concentration of runoff and groundwater that transported solutes from adjacent basin-bounding uplifts. High accumulation rates for evaporative lacustrine facies have also been measured elsewhere (Ku et al., 1998; Bobst et al., 2001), indicating that aggradation of such facies can keep pace with rapid increases in potential accommodation. A later decrease in potential accommodation allowed reintegration of regional drainages, which facilitated a shift back toward overfilled conditions during deposition of the Laney Member. Our model implies that the overall style of sedimentation in the Green River Formation is directly related to Sevier and Laramide orogenic events, and that temporal patterns of lacustrine facies may provide a useful new tool for interpreting continental tectonic processes.