FLUCTUATING RATES AND EPISODIC LAKES: A NEW VIEW OF THE GREEN RIVER FORMATION
Reconstructing the depositional history of lake basins is an old and difficult problem, due to the potential for extremely rapid environmental change and generally poor age control. In the absence of direct dating, previous studies have often assumed that periodic changes in solar insolation directly forced bed thickness patterns. However, recent advances in our ability to directly measure time in such deposits contradict the conventional view of lakes as faithful recorders of climatic cyclicity, and instead highlight some of the same fundamental stratigraphic questions that Dott (1983; 1996) elucidated for marine strata. Tuff horizons in the Green River Formation now give age resolution down to ±100 ky (2s), and show that sedimentation rates varied dramatically between different members. For example, the net average accumulation rate of the evaporative Wilkins Peak Member was approximately three times that of profundal Laney Member oil shale. The Wilkins Peak also contains abundant diastems, indicating that true sedimentation rates were even higher. For comparison, short-term accumulation rates in the Quaternary Salar de Atacama (Bobst et al., 2001) were up to five times as fast as the long-term Wilkins Peak rate. If instantaneous rates were similar in both cases, hiatuses could account for up to 80% of the time spanned by Wilkins Peak deposition. Lake expansions therefore appear to represent surprisingly rare events in a basin that was normally dry. Improved geochronology also permits one of the first truly objective tests for periodicity of lacustrine episodes. The average duration per episode between two Wilkins Peak tuffs is less than 10 ky near the basin center, or less than half the expected period for precession. This result suggests a need to reassess apparent cyclicity in this and other lake basins, and to look for alternative, non-periodic mechanisms (such as drainage network instability) that could account for expanding and contracting lakes.