PLUVIAL LAKES IN THE WESTERN U.S.—A VIEW FROM THE OUTCROP

Huge strides have been made during the last 40 years in deciphering the geologic and climatic records of Pleistocene pluvial lakes in the western U.S. using cores from extant lakes and from desiccated lakes such as Lake Bonneville and Owens (dry) Lake. Sedimentology, stable isotopes, pollen and other microfossils, and physical properties from cores provide proxies for changes in lake level, water temperature and chemistry, and ecological conditions in the surrounding landscape. Commonly, such core data do not directly record lake level or reveal geologic factors that may confound interpretations of proxy data, and most core records do not extend beyond the last ~40 kyr. In contrast, outcrop studies of incised basin fill provide much longer lake histories, indicate absolute lake level, and yield clues to geologic events such as earthquakes, floods, or drainage-basin changes that can clarify interpretations of lake sediment and stratigraphy.

Notable advances made using outcrop studies include recognition of: (1) many pre-late Pleistocene lakes in the western Great Basin that were significantly larger and record wetter conditions than the youngest lakes; (2) drainage-basin changes caused by tectonic or volcanic damming, in some cases triggering lake overflow or catastrophic floods and altering the depositional setting of previously deep-water locations; (3) complex relations among basins and subbasins controlled by changing threshold altitudes; and (4) rapid lake-level fluctuations in dated records. Combined outcrop and core data from Lake Manix yield a 500-kyr lacustrine history indicating persistent lakes during both glacial and interglacial periods, and recognition of significant change in depositional setting of the core by a drainage integration event. Outcrop studies should be coupled with core data to provide the most comprehensive understanding of lake records of environmental change.