A HIGH RESOLUTION RECORD OF LATE-PLEISTOCENE PROVO SHORELINE DEVELOPMENT FROM LAKE BONNEVILLE, WESTERN UTAH

The Bonneville lake cycle has been the focus of many studies involving lake processes and Quaternary climate change. However, little is known about the timing and nature of deposits that formed following the catastrophic lowering of the lake to the Provo level about 14.5 ka ¹⁴C years. Field studies in Tooele Valley, Utah, provide new evidence of the high frequency, oscillatory nature of Lake Bonneville and the deposition of almost 100 beach ridges that formed during the post-flood Provo stillstand. The timing of the initial drop of Lake Bonneville is constrained, but previous attempts at dating the final stages of Provo shoreline development have been relatively unsuccessful. Radiocarbon dates of samples collected from selected beach ridges constrain the termination of the Provo stillstand and the climatically driven fall to the pre-Holocene lowstand.

Gravel pit exposures reveal several 30-cm thick packages of inversely graded coarse sand and gravel. These packages consist almost entirely of shoreface deposits and are interpreted to represent annual cycles. Most of the annual shoreface deposition probably occurred in the spring, at the time of maximum fluvial delivery of sediment to the shorezone. The finer fraction of the sediment load was rapidly incorporated into the longshore transport system and therefore deposited prior to the coarser gravel that arrived slightly later in the annual cycle. Sand was simultaneously winnowed from the shoreface and deposited as an apron that extended into the deeper water of the nearshore zone. The high rate of deposition (30 cm/yr) can be attributed to an abundance of alluvium proximal to the shorezone and is consistent with the total amount of shoreface accretion that occurred within the interval of time represented by the Provo shoreline. Morphological evidence indicates that the direction of longshore transport reversed at least once from a dominantly westerly flow to a dominantly easterly flow toward the end of Provo shoreline development.