Pacific Section AAPG Convention

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California Margin-Wide Cherts And Organic-Rich Phosphates During A Major Sea-Level Fall: What Happened During Middle Monterey Deposition?


The middle member of the Miocene Monterey Formation across California is widely characterized by either an unusually organic-rich and phosphatic mudstone facies or by the chertiest succession in the unit. Although both lithofacies are associated with intense upwelling and planktonic productivity, they also represent siliciclastic sediment starvation. Their origin is enigmatic because – in spite of their concentrated richness of silica or organic matter – they record lower mass accumulation rates of biogenic silica and organic carbon than other members of the formation. These units, locally part of the Carbonaceous Marl, Middle Chert, Sandholt, McDonald members, etc., were for the most part deposited during the late Luisian to Mohnian stages, between 14 Ma and 9 or 10 Ma during Antarctic ice sheet growth and eustatic sea-level fall. This is puzzling because phosphates and sediment starvation are broadly associated with transgressive systems tracts or maximum flooding surfaces, not eustatic sea-level fall and forced regression. Increased latitudinal thermal gradients with polar cooling would have amplified winds, upwelling, and current velocities that may have combined to increase diatom productivity and simultaneous winnowing and condensation of elevated banktops, upper slopes and shelves; this could concurrently deposit both facies as shown in the Santa Maria and Santa Barbara basins. Intensified currents possibly explain the linked, lateral facies variability, but not the near margin-wide decrease in sedimentation rate during one of the largest sea-level falls of the late Cenozoic. This presentation explores and evaluates the tectonic, climatic, and paleoceanographic framework in which this perplexing facies assemblage was deposited.