INFERRED ENVIRONMENTS OF ANCIENT LAKE ALAMOSA EXPOSED AT THE BACHUS PIT, ALAMOSA COUNTY, COLORADO

This research examines and describes sediments deposited within ancient Lake Alamosa and associated environments exposed in the Bachus Pit, Alamosa County, Colorado. Ancient Lake Alamosa occupied a portion of the San Luis Valley in the Rio Grande rift. Lake Alamosa and associated sediments were first described by Machette and Thompson in 2005. This research expands the work of Machette and Thompson by describing exposed sediments throughout the pit and using this information to infer paleo-depositional environments. Lake Alamosa is believed to be a principal sediment source for eolian deposits in the Great Sand Dunes National Park. Lake Alamosa likely controlled the movement of paleo-mammals and humans, and related lacustrine clay deposition in Lake Alamosa is believed to be responsible for a complex series of confined and unconfined aquifers within the San Luis Valley. Sediment analyses in several stratigraphic sections provide compelling evidence for at least two discreet fluvial non-marine environments. The abundance of very fine sand and silt in lower levels of the exposed sections and samples are indicative of offshore lower depositional energy. Coarser gravelly material in adjacent areas suggests a beach or fluvial system. Preliminary paleocurrent analysis suggests a sediment source to the west. Minimal fauna and flora are present. This research provides additional information on the extent of Lake Alamosa and associated depositional setting, which in turn adds to paleoclimate, hydrogeologic, and perhaps paleoarcheologic understanding of the San Luis Valley.