A Prograding Margin Model for the Monterey Formation, California
Caroline M. Isaacs, Timothy R. Baumgartner, Marilyn E.
Tennyson, David Z. Piper, and James C. Ingle Jr.
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