A Prograding Margin Model for the Monterey Formation, California
The Monterey Formation--marine sedimentary deposit of Miocene (and locally Pliocene) age widespread in California, characterized by organic-carbon-rich fine-grained strata--is generally regarded as a classic petroleum source-rock resulting from high productivity linked with bottom-water anoxia.
Current interpretations generally presume that the Monterey was deposited in steep-sloped isolated silled basins similar to modern Southern California Borderland basins, such that the stratigraphic sequence represents a "pancake stack" recording broad paleoceanographic changes neatly preserved in an anoxic bottom-water environment. Our review of evidence on the depositional setting, however, indicates a dynamic prograding margin, with facies highly controlled by local water depth and bottom configuration. We suggest that (1) Monterey strata were deposited on gentle slopes of broad irregular depressions along an open continental margin; (2) minor irregularities in bottom topography exerted major controls on facies, thicknesses, rates of silica dissolution, and ultimate accumulation rat s of fine-grained sediment at a local scale; and (3) the large-scale sequences in most Monterey sections represent upsection shoaling (from 1500-1000m to 500-200m depths) and "shoring" (from 100 km or more to about 20 km offshore), rather than oceanwide changes in productivity. The richest source-rocks--zones of 8-20% organic carbon in the Santa Maria-Ventura region--are slowly accumulated transgressive shales deposited off sediment-starved margins during the major mid-Miocene sea level rise.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California