--> --> Mixed Siliciclastic-Siliceous Succession, Miocene Monterey Formation, Point Dume to Paradise Cove, Malibu, California

AAPG Pacific Section and Rocky Mountain Section Joint Meeting

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Mixed Siliciclastic-Siliceous Succession, Miocene Monterey Formation, Point Dume to Paradise Cove, Malibu, California

Abstract

The Monterey Formation crops out throughout Coastal California and in most of the Neogene sedimentary basins in the State and is known to be highly petroliferous, forming both the source and reservoir rock. The petroliferous Los Angeles Basin's oil is sourced in the Monterey Formation and its stratigraphic equivalents. Yet few sections have been studied in detail in this region. The exposure between Point Dume and Paradise Cove form one of the few continuously exposed successions of the Monterey Formation in southern California and records the evolution of the inner Borderland and the Los Angeles basin through the late Miocene. Stratigraphic data collected includes: meter-by-meter Spectral Gamma Ray (K, Th, and U concentrations), lithologic descriptions and sandstone analysis (percent sandstone and thin section petrography). Select samples from the outcrop are analyzed for total organic carbon data (% TOC), diatoms and X-Ray diffraction data. Based on these data, the Point Dume to Paradise Cove Monterey Formation section can be subdivided into four distinct members and the stratigraphic column can be compared to other subsurface and outcrop sections. In stratigraphic order, the members are the Dolomitic Phosphatic Shale Member, the Porcelanite and Shale Member, the Mixed Clastics Member and the Cherty Diatomite Member. High TOC values found in the Dolomitic Phosphatic Shale range from 4.2 to 7.5%. Opal-CT phase silica is found in cherts and porcelanites and biogenic opal-A phase silica occurs in the diatomites and diatomaceous mudstones. Quartz-phase siliceous rocks do not occur in the section, even in its lowermost extent. Bedding confined fractures sets are common in diagenetic siliceous rocks in the Porcelanite and Shale Member. The base of the Mixed Clastics member contains evidence of high-energy, downslope movement with thick sandstone, conglomerates and debrites. The top of the section contains the Cherty Diatomite member that consists of rhythmically bedded pure diatomite, ‘speckled’ beds, muddy diatomites, siliceous/calcareous mudrocks and opal-CT chert.