REVISING THE EARLY PLEISTOCENE LACUSTRINE HISTORY OF PALEO-LAKE TECOPA: NEW INSIGHTS FROM STRATIGRAPHY AND GEOLOGIC MAPPING

Pleistocene lacustrine deposits exposed in the Tecopa Basin, southeastern California, are reminiscent of many other pluvial lakes in the Great Basin and Death Valley Region. However, early Pleistocene strata exposed in the east-central part of the basin allow continuous temporal reconstructions to be made of Lake Tecopa through much of the Quaternary. Previous studies interpret the early Pleistocene paleoenvironments to be dominated by dry alluvial conditions, with periods of shallow lake expression. Recent efforts, focusing on stratigraphy and geologic mapping, have identified ~15 meters of open-water lacustrine mudstone north of the sections described in other studies, associated with the early Pleistocene Huckleberry Ridge ash bed (2.1 Ma; HR ash bed). A NW-SE anticlinal structure has exposed these older deposits at the surface. Sedimentation rates constrain these deposits to span from ~2.5 to ~1.7 Ma. These sections (LT-16ab, LT-18) contain abundant authigenic potassium feldspar and other evolved authigenic mineral species and lack evidence of subaerial exposure, indicating continuous lacustrine conditions throughout the early Pleistocene. Additional sections to the south capture the previously described “shallow lake” phases, which are now interpreted as transgressive episodes in a lake-margin environment. These may represent the 41-kyr obliquity cycles that dominate early Pleistocene paleoclimate. Ongoing tephostratigraphic work may help resolve the timing of these lake phases in a time of high-frequency climatic oscillations. In the southeast extent of the lake beds, the five basin-filling lakes present after 1 Ma described in other studies are evident as shallow lakes and lake-margin environments. Furthermore, mapping efforts have identified missing stratigraphy associated with the 1.25 Ma Tecopa Tuff in the east-central part of the basin, whereas the same strata are present with minimal deformation in the western side of the basin. This, along with evidence of syndepositional tectonic and seismic activity, suggest an east-west depocenter shift, likely due to continued basin extension and basin fill. A similar depocenter shift has been observed in the northern part of the basin during the middle Pleistocene.