IDENTIFYING RESULTS OF STREAM CAPTURE AND PIRACY IN THE STRATIGRAPHY OF THE TECTONICALLY ACTIVE LIVERMORE BASIN, CALIFORNIA, USA

Evidence exists for stream capture and piracy of the Arroyo Seco at the Lawrence Livermore National Laboratory (LLNL), located in the southeast corner of the Livermore Basin, California, USA. This active trans-compressional basin is defined on the east by the Greenville Fault and on the south by the Las Positas Fault. The geomorphic evidence for stream capture is that the Arroyo Seco (AS) flows north into the Livermore Basin, bisecting an up-thrown block created by the Las Positas Fault; an abandoned geomorphic AS fan is present south of the up-thrown block; and a modern AS fan exists north of the up-thrown block. Two hypotheses exist regarding the sequence of events related to the stream capture. The first states that the AS flowed north, was later cut off by an up-thrown block at which time it formed a fan south of the block. During this time, Arroyo Las Positas (ALP), a westerly flowing stream which crosses the Greenville Fault, dominated the sediment supply at this site. The AS was then captured by a gully headward eroding through the up-thrown block, re-directing AS into the basin once again. Stratigraphic evidence for this hypothesis would be AS sediments (marked by presence of sediment derived from Franciscan rocks) at depth overlain by ALP sediments (containing no Franciscan-derived sediments) and then capped by AS sediments. The second hypothesis states that during the time the AS flowed south of the up-thrown block, the ALP did not supply much sediment to the basin due to its relatively small drainage basin size. Stratigraphic evidence for this hypothesis would be continuous AS deposits with no or minimal ALP sediments present. Distinguishing these two hypotheses is important for hydrogeologic analysis as well as improving our understanding of basin fill dynamics. To test these hypotheses, a provenance study was conducted. Gravel units from 17 well cores were chosen for this work. The study found Franciscan Formation clasts in all gravel units, indicating AS-derived sediments are present throughout the section. Future work will focus on identifying provenance characteristics of finer-grained sediments for additional analysis of potential stream capture influence.