TECTONIC CONTROLS ON SEDIMENTATION AND STRATIGRAPHY OF PLIOCENE-PLEISTOCENE LAKE TECOPA, SOUTHEAST CALIFORNIA

Exposures of Pliocene-Pleistocene Lake Tecopa in southeastern California are investigated to determine the role of tectonic damming by uplift of the Tecopa Hump in controlling lake history and depositional processes. Late Neogene uplift of the Tecopa Hump is believed to have occurred due to transpressional deformation resulting from fault movement at the intersection of two conjugate strike-slip fault zones (northwest-trending, right lateral faults and northeast-trending, left-lateral faults). Joint data, bedding orientations, detailed stratigraphic measurements, and sedimentological analysis are used to address the history of recent deformation of the Tecopa Hump and possible influences on lake sedimentation and stratigraphy. Ages of the lake beds are constrained using three widespread, diagenetically altered volcanic ash deposits, the Huckleberry Ridge tuff (2.02 Ma), the Bishop tuff (0.76 Ma), and the Lava Creek B tuff (0.665 Ma). Conjugate sets of vertical joints are found on all three ash beds with preliminary data showing a joint set trending ~N30°W±10° in all three ashes whereas the other set changes from ~N20°E±5° on the oldest ash to a more orthogonal orientation of ~N50°E±10° on the youngest ash. The northwest joint set is younger and represents the local maximum horizontal stress, in contrast to the regional northeast oriented maximum. Lake Tecopa beds typically dip ~1° basinward. However, in the southernmost part of the basin beds dip radially away from the Tecopa Hump with average dips of 3° and maximum dips of 7°, suggesting post-depositional tilting. In addition to structural data, ~100 m of lake stratigraphy is described and measured in four sections along a transect perpendicular to the Tecopa Hump. The stratigraphy adjacent to the Tecopa Hump is dominated by alluvial fan and lake margin facies and largely absent of the deep water facies observed in other parts of basin, suggesting that limited basin subsidence occurred during much of the lake history. At one location, however, the age of uplift is constrained to have occurred following Lava Creek B deposition and prior to the latest and highest lake-shore gravels. The preliminary data suggest that the Tecopa Hump was not active proximal to Lake Tecopa during most of the lake’s history, but was active during the final stages of lake basin sedimentation.