13 Monterey Formation, Miocene, California, U.S.A.—A Cenozoic Biosiliceous-Dominated Continental Slope to Basin Setting: A Billion-Barrel Deep-Water Mudstone Reservoir and Source Rock

ABSTRACT The Monterey Formation illustrates the expression of sequence-stratigraphic surfaces and units in continental slope and basin settings that are quite unlike those of most mudstone units considered thus far in this book. These strata span a wide variety of siliceous, calcareous, argillaceous, phosphatic, and kerogenous composition, detrital, biogenic, and authigenic origins, and clay to cobble grain size. The Monterey Formation poses particular challenges with its deposition of dominantly biogenic sediments in a variety of deep-basinal environs in an active tectonic region. This chapter shows how applying the sequence-stratigraphic method and approach (from first principles), even to a relatively unfamiliar setting, can provide insights into the accumulation of rocks enriched in organic matter and biogenic material as well as the complex distribution of its time-transgressive facies. The dominant sediment influx was biogenic, composed mostly of diatoms and controlled by primary organic production. The dominant organic-matter input was almost exclusively from diatoms, also controlled by primary organic production. However, intervals with highest sediment accumulation rates do not have the highest organic-matter content but are dominantly biosiliceous, with little organic matter. This dominantly biogenic distal setting offers an opportunity to examine sequence stratigraphy and its relation to organic-matter content and biosilica accumulation where sediment accumulation rates are strongly related to organic-production rates under relatively constant preservational conditions and input of uniform organic matter with relatively low input of siliciclastic detritus and variable downslope influx of reworked biogenic material. At the macro to meso scale, sequence boundaries are marked by truncation below and onlap above, and maximum flooding surfaces are marked by downlap above and conformable strata below, even in this relatively distal, deep-water setting. Parasequence boundaries are particularly well highlighted by early diagenetic evidence of significant pauses in sediment accumulation—most notably, dolomite cement and phosphorite nodules. This chapter illustrates the systematic approach we used for developing a stratigraphic framework consistent with observations that range from seismic to geochemistry. This framework and a few basic concepts enable one to develop predictive capabilities in continental margins settings.