Stratigraphic and Chronologic Evidence from Offshore Deposits for An Extended Provo Stage, Pleistocene Lake Bonneville, Utah

Changing levels of Lake Bonneville have long been used to interpret late Pleistocene and early Holocene climate conditions in the eastern Great Basin. Throughout most of its history, Lake Bonneville operated as a closed system that responded quickly to changes in the local and regional water balance. The record of the lake at the Provo shoreline is of particular importance because it marks the transition from the colder, wetter conditions that existed during the Last Glacial Maximum (ca. 15-18 ¹⁴C ka) to the warmer, drier conditions present in the region today.

The stratigraphic, sedimentologic and chronologic context of offshore to nearshore deposits was examined at eighteen field sites at modern altitudes 14 to 90 m below the Provo shoreline. At these altitudes, sedimentation was sensitive to lake-level fluctuations and the fine-grained deposits record events not evident in coarse-grained shoreline deposits. The Bonneville flood bed and Pahvant Butte basaltic ash were used as key stratigraphic markers. Radiocarbon ages of shells collected at nine locations were compared to published ages. Ostracode faunas were examined in sediment samples from some stratigraphic sections to help correlate with the basin-wide stratigraphic model.

Results indicate that the final regression from the Provo shoreline occurred as recently as 12 ¹⁴C ka, and therefore, the duration of the Provo stage may have been about 2500 ¹⁴C years. Also, thin, shell and sand beds encased within thick, fine-grained marl beds may represent evidence for Provo-age sub-threshold oscillations. The occurrence of these sub-threshold oscillations of the final regression from the Provo shoreline are significant because they record shifts from open-basin to closed-basin conditions and therefore represent major changes in the hydrologic balance of the lake.