STRATIGRAPHY, SEDIMENTOLOGY AND PETROGRAPHY OF THE SILICEOUS LITHOFACIES IN THE UPPER MODELO TURBIDITE SYSTEM, EASTERN VENTURA BASIN, SOUTHERN LAKE PIRU, CALIFORNIA

Lateral and vertical transitions between deepwater clastics and fine-grained biosiliceous mudstone have been poorly studied compared to those in a pure clastic system. In the eastern Ventura basin, the Monterey-equivalent Modelo Formation includes intercalated deposits in the Piru submarine fan where these transitions are well exposed and can be studied. This sedimentologic study identified and characterized facies abundance and relationships between sandstone and siliceous mudstone in the Modelo Formation, Lake Piru area. While both the sandstone and the fine-grained siliceous (originally diatomaceous) lithofacies have been individually well studied at different locations, they have not been in the same depositional setting. The Modelo and Tarzana submarine fan systems, which extend into the Ventura and Los Angeles basin, represent one part of the Modelo Formation, and has similarities with the Stevens Sand system in the San Joaquin basin, along with the Guaso I turbidite system in the Ainsa Basin of Northern Spain, and therefore may provide insight beyond the Eastern Ventura Basin. This combined field and laboratory study included several detailed lateral sections of quantitative stratigraphic data and analysis of sedimentary structures. Lab methods used to identify and characterize siliceous facies include X-ray diffraction, SEM/EDS, and light petrographic analysis of hand samples and thin sections.

This depositional setting reflects sedimentation from two distinct provenances. Most of the siliceous rocks were deposited by lateral, downslope transport processes, indicated by distinct gravity flow features, some of which are unique to diatomaceous sediment (e.g., “speckled beds”, “doomed pioneers”) or hybrid composite beds. Here, in this most proximal submarine fan setting, the combination of provenance and depositional process results in intercalation of relatively pure chert and sandstone, which would not be predicted from general lithofacies models across the entire Miocene Borderland (rare chert in the proximal San Joaquin basin, frequent in the distal Santa Barbara and Santa Maria basins). The nature of the clastic-biosiliceous transition holds important implications for the zone’s ability to seal charged sandstone petroleum reservoirs or to provide a mixed/hybrid reservoir system.