GEOLOGY OF SUNOL REGIONAL WILDERNESS AND VICINITY, NORTHWESTERN DIABLO RANGE, CALIFORNIA: UPDATES FROM NEW GEOLOGIC MAPPING

Doug Yule made seminal research contributions to understanding basement geology as well as paleoseismology and active tectonics of parts of the North American Cordillera, a rare combination for which he stood alone. To honor Doug's contributions to Cordilleran geologic research, I present updates of my geologic mapping of Sunol Regional Wilderness in the northwestern Diablo Range of California. In this area, rocks of the Franciscan subduction complex are tectonically overlain by the Coast Range ophiolite and Great Valley Group along the Coast Range fault that is itself tightly folded at km scale. These rocks and structures are unconformably overlain by Miocene marine sedimentary rocks which are folded and cut by faults of the San Andreas fault system, including the Calaveras fault. Many of the younger faults may define a broader zone of Calaveras fault displacement and may have formed as a result of step-overs that have migrated along the Calaveras fault. Some of these faults appear to deflect drainages and/or pond alluvium upstream of them. The area also has multiple levels of strath terraces that reflect both progressive stream incision and drainage rearrangement, including stream capture. Basement geologic features of note include evidence for interbedding of silicicastic and detrital serpentinite horizons in the Franciscan Complex as well as basal olistostrome horizons with the Great Valley Group that include mixed detrital serpentinite and siliciclastic rocks.