INSIGHTS FROM MAPPING SURFICIAL DEPOSITS ASSOCIATED WITH THE TRANSGRESSIVE RISE OF LAKE BONNEVILLE IN NORTHWESTERN UTAH

During 2008-2009 the United States Geological Survey's EDMAP program has been funding the surficial mapping of the Hogup Bar 7.5'quadrangle in northwestern Utah. The Hogup Bar quadrangle was selected due to the presence of deposits that exhibit all of the basin's major well established stillstands and highstands and due to the presence of many other transgressive stillstands. The chronology of the major highstands of Lake Bonneville and the basin's broad climatic history is well established; however, the chronology of many the basin's small-scale base-level changes is still poorly defined. With the large drop of base-level in the Bonneville Basin during the Late Pleistocene and Holocene, many lacustrine landforms that were deposited during the transgressive occupation of Lake Bonneville have been deeply incised by gullies. Gully incisions in the Hogup Bar quadrangle of northwestern Utah, express well exposed stratigraphy used to make detailed sedimentological descriptions of the lake's transgressive lacustrine landforms. Based on clast lithology, imbrications, and some other geomorphic proxies of longshore transport within the region, it has been determined that the sediment sources for many of these transgressive landforms are from large relict alluvial fans that were reworked during the transgression of the lake. The mapping of the region's surficial sediments allow for the identification of past localities within the quadrangle that were predominantly depositional and/or erosional environments. The stratigraphy of these transgressive landforms provides basic age relationships for many of the lake's stillstands, and may even suggest that there have also been some significant oscillations in the lake's base-level during its transgressive record. Currently, three radiocarbon dates have been analyzed to date two of these stillstands near the elevation of the lake's Bonneville highstand. Due to the grouping of the localities in which these radiocarbon dates were extracted, it has been determined that many of these landforms were probably deposited relatively quickly. In addition, due to the orientation and fetch associated with the transport of these sediments, it is assumed that the frequency of more persistent and robust storms was higher in the Late Pleistocene than in the modern climate.