New Mexico Geological Society Annual Spring Meeting — Abstracts

Fossil footprints, thought to represent underprints of Rancholabrean proboscideans and camelids, have previously been documented in Pleistocene lakebeds on the floor of the Tularosa Basin. Here we present preliminary results from radiocarbon chronology of associated deposits. The originally documented tracksite is located along the northwest side of the lake basin, about 20 km north of Lake Lucero on the west side of Alkali Flat. The footprints are weathering out of the lowermost exposures of lacustrine beds and similar features can be found at the same general stratigraphic level over a distance of at least a few kilometers parallel to shore. Basal exposures of the lacustrine sequence along the northeast side of Alkali Flat have also been identified that are stratigraphically equivalent to the western-margin track-bearing beds. Lithofacies in the track-bearing deposits include beds of gypsiferous clay, laminated and massive gypsum, carbonate mud, and thin beds containing abundant fragments of aquatic macrophytes. Lithofacies, sedimentary structures, fossil algal mats, and the preservation of aquatic organisms in some beds are consistent with subaqueous deposition and periodic subaerial exposure along the margin of a shallow saline lake. Three samples of aquatic macrophyte fragments from the level of the tracks have yielded radiocarbon ages slightly greater than 31,000 14C yrs B.P. Accuracy of these ages is uncertain because contamination of samples of this antiquity with small amounts of modern carbon would cause the apparent ages to be significantly too young. The track beds are overlain unconformably by sediments containing a relative abundance of siliciclastics and diverse assemblages of ostracode and other aquatic organisms, suggesting input of sediment-laden surface water and relative freshening of the lake. These overlying deposits have yielded four radiocarbon ages from ostracodes valves, aquatic macrophytes, and charcoal ranging from 22,800 to 19,430 14C yrs B.P., comparable with independent chronologies for the onset of significantly wetter climatic conditions in the Southwest during the late Wisconsinan. The stratigraphic relations and radiocarbon dates suggest that the footprints and trackways on the floor of the Tularosa Basin were made before this major late Wisconsinan pluvial episode.