Use of gravel provenance to define stratigraphic units in the Sandia Windgap trench, Livermore, California

The Sandia Windgap was trenched as part of an ongoing investigation of the stratigraphy, origin, and correlation of fluvial gravels beneath the Lawrence Livermore National Laboratory (LLNL). This windgap, an uplifted and abandoned drainage system, is located on a horst between the north and south branches of the Las Positas Fault. Two trenches exposed nonmarine clastic sediments of the Upper and Lower Livermore Formations, as well as potentially younger gravel deposits associated with paleodrainage systems through the windgap. The purpose of this investigation is to define and correlate individual stratigraphic units, based on the provenance of the gravel fraction. Eighteen different gravel deposits were sampled in the trench. Sampling was done using a bulk sampling method, and the gravels were processed in the laboratory. Several different size fractions were analyzed, and the detrital clasts were identified and counted both macroscopically and using a stereomicroscope. Thin sections of selected lithologies were constructed for accurate lithologic classification. The source of these deposits is compared to subsurface gravels collected from boreholes drilled on the LLNL site. This investigation will aid in the identification and correlation of stratigraphic units in the subsurface and enhance the understanding of the three-dimensional stratigraphic framework which controls the migration of contaminants beneath LLNL