Quantifying the Temporal Evolution of Eocene Lake Gosiute

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

ABSTRACT Lacustrine deposits provide some of the best available archives of many critical Earth surface processes, due to the relatively continuous sedimentation in lakes compared to other terrestrial environments. In addition to preserving various paleoclimatic proxies, cyclic sedimentary facies successions may provide records of periodic forcing of climate. Lacustrine strata also offer a unique and underutilized window on the complex interactions between climate, landscape denudation, and rising orogenic topography. The rapid biological speciation that can occur in large lakes may make them important evolutionary incubators. However, the remarkable utility of lake deposits to measure rates of change in all of these processes has been hampered by generally poor chronostratigraphic control. The Eocene Green River Formation (GRF) is perhaps the most studied system of lacustrine deposits in the world. Eocene climate records are of particular interest because the Early Eocene represents the warmest interval of the Cenozoic, and as such may shed light on the potential long-term effects of modern global warming. Recent studies suggest that deposition of the GRF may have also coincided with unexpectedly rapid uplift of adjacent Laramide mountain ranges. Numerous tephras interbedded in lacustrine facies have long aided in correlating these classic deposits, but until recently the best age resolution was one million years or more. Our intitial NSF-funded project has vastly improved the geochronologic framework for the GRF in Wyoming by determining laser fusion 40Ar/ 39Ar ages of sanidine and/or biotite from seven tuffs. Ages for six of the tuffs are between 50.700.14 and 48.940.12 Ma (2 sigma), thus we can resolve temporal differences on the order of 100 kyrs over most of the 2.3 myr span of the dated deposits. To accurately place these tuffs within their broader sedimentologic and stratigraphic context, we have also constructed basin-scale cross sections of the Wilkins Peak and Laney Members. Our results have already permitted several significant new conclusions: 1) We have found that the GRF was deposited 3-4 myr earlier than suggested by previous K-Ar ages, and therefore coincides with the the latter half of the globally warmest period in the Cenozoic. 2) The age of the Bridgerian-Wasatchian faunal transition can now be constrained to 50.55 0.43 Ma. 3) The evaporative facies of the Wilkins Peak Member accumulated three times more quickly than the deeper lacustrine facies of the Tipton and Laney Members. 4) The average duration of lake cycles in the Tipton and Laney Members is consistent with precession, but previously interpreted "precession" cycles in the Wilkins Peak Member are actually of much shorter duration (~10 kyr). 5) Although our initial study focused on climatic issues, we fortuitously discovered a desiccation event that appears to record catastrophic emplacement of the Heart Mountain Detachment. We have submitted three papers to top journals to document our prior results. To capitalize and build upon these achievements, we propose the following new objectives: _ Increase accuracy and temporal resolution via U-Pb and 40Ar/ 39Ar dating. By conducting U-Pb analysis of volcanic zircon grains we hope to cut our already small relative dating errors in half, and to reduce fully propagated errors by an order of magnitude. _ Test the ability of statistical models to delineate lacustrine cycles. Radioisotopically-measured accumulation rates will be used to scale bed thickness spectra into time, and to determine permissible cycle periods for different GRF Members. Also, we will evaluate the effect of varying stratigraphic "completeness" on such spectra. _ Explore the temporal record of basin-margin alluvial fans. Coarse-grained alluvial strata that interfinger with the GRF offer a unique record of the timing of uplift of basin-bounding ranges. We will correlate these deposits with dated tuffs to provide a high-resolution record of Laramide fault movements. _ Extend temporal correlations into adjacent basins. By dating selected tephras from the Uinta, Piceance Creek, Fossil, and Bighorn basins we will establish a regionally consistent chronostratigraphic framework that will facilitate the correlation of important tectonic, climatic, and biologic events. This project will draw upon cross-disciplinary expertise from the fields of sedimentology, stratigraphy, and geochronology to address fundamental questions concerning rates of Earth surface processes. This integrated approach to one of the worl's best known lacustrine deposits will provide a new benchmark for reading the terrestrial record of paleoclimate preserved in the deposits of large lakes.