Deep-Water Boulder Conglomerate Deposits in the Upper Cretaceous Panoche Formation (Campanian) of the Great Valley Group, San Luis Reservoir, Central California
Greene, T. J. and Shapiro, R. S.
The Great Valley Group (GVG) of western California contains forearc deposits (mostly deep marine) tectonically linked to Andean-style subduction of the Mesozoic-early Cenozoic Sierran continental arc and the “Franciscan” subduction complex. Capturing the coarse-grained range of deep-water depositional conditions recorded by GVG deposits, however, is challenging due to partially exposure, burial, and unroofing by subsequent San Andreas-related transform tectonics. Although submarine canyons and conglomeratic-sandy channels systems occur throughout the GVG and represent important conduits for sediment dispersal into the deeper water environments, coarse-grained (boulder) end-member systems are under-represented in the geologic literature.
Low water levels in the San Luis Reservoir in the southern Diablo Range of Merced County, CA, have exposed a deep-water boulder conglomerate succession approximately 500-600 meters thick within the Upper Cretaceous (Campanian) Panoche Formation of the GVG. The succession contains at least 5 packages of high-density sediment gravity flows (ranging 15-135 meters thick) interbedded with high- and low-density turbidites (20-60 meters thick). Using nomenclature of Lowe (1982), each of the packages contain three depositional styles: 1) a basal, mostly matrix-supported, boulder conglomerate (R3) with thin, lenticular massive (S3) sandy beds, which are overlain by, 2) medium-bedded, medium- to fine-grained sandy (S3) beds overlain by, 3) thin-bedded, fine-grained sand to silty (Tabcd) beds. Thickness of the boulder conglomerate units decreases upwards through the entire succession. Lateral extent of the boulder conglomerate could not be determined due to lack of exposure but is at least 600 meters in width. However, published geologic maps indicate Panoche-aged conglomerate extends approximately 18 km along strike to the northwest.
In the San Luis Reservoir locality, we interpret deposition occurred in a submarine canyon much broader than the width of the outcrop. Further work will define architectural elements within the overall conglomeratic succession as well as look for provenance indicators that could help track changes in possible sources for the boulders.
AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013