STRATIGRAPHIC EVIDENCE FOR SUBTLE TECTONIC ACTIVITY DURING PLEISTOCENE DEPOSITION IN THE TECOPA BASIN, SOUTHEASTERN CALIFORNIA

Pleistocene Lake Tecopa, located in the southwestern Great Basin of southeastern California, formed in part due to damming of the ancestral Amargosa River by an antiformal uplift known as the Tecopa hump. Previous studies have concluded that tectonic processes were largely inconsequential during lake deposition, as the focus of tectonic activity during the Pleistocene was in Death Valley. However, stratigraphic and structural evidence for subtle syndepositional deformation of the Tecopa Lake beds is documented in the northern and southern ends of the basin. The stratigraphy in the northern part of the basin is characterized by intertonguied lacustrine, alluvial fan, and ancestral Amargosa alluvial deposits. The stratigraphic interval between the Lava Creek B (665 ka) and Bishop (758 ka) tuffs is 40 % thinner along the western side of the basin and characterized by alluvial fan facies, suggesting subtle syndepositional uplift during and briefly following Bishop deposition.  The stratigraphy of the southern Tecopa Basin is characterized by a tectonically controlled fan-delta complex that prograded north from the Tecopa hump and intertongued with lacustrine sediments. Bedding attitudes and elevation profiles collected on three time horizons, the Lava Creek B tuff, Bishop tuff, and a 1.25 Ma unnamed ash, suggest at least 50 m of uplift occurred during the Quaternary. Rising lake level most likely competed with uplift along the southern margin of the basin such that without uplift lake spillover into southern Death Valley would have occurred earlier.