Sources and deposition of organic matter in the Monterey Formation, South-Central Coastal basins of California

The abundant organic matter in the Monterey Formation is commonly considered to be derived from marine algal debris rapidly deposited in anoxic bottom water during a period of high surface plankton productivity. However, many aspects of the distribution of organic matter in the Monterey of the south-central coastal ba- sins are inconsistent with this view. For example, organic matter is generally most abundant in beds representing lowest (not highest) surface plankton productivity and slowest (not fastest) organic-matter accumulation. Moreover, at least in the Santa Barbara coastal area, organic matter is generally most abundant in beds I representing intermediate (not lowest) bottom-water oxygen. The one expected relation that generally holds true is that organic matter is most abundant in beds with the smallest original grain size. The distribution of organic matter in the Monterey Formation of the south-central coastal basins raises several questions about the source and depositional environment of this organic matter. For example, is the organic matter really mainly marine in origin, is it generally a mixture of marine and terrigenous debris, or does the mixture vary from basin to basin? How important is oxygen level in bottom water to preservation of organic matter? What other environmental conditions are important in determining the abundance and character of preserved organic matter? Review of present-day environments thought to be analogous to the deposi- ; tional setting of the Monterey Formation shows that the distribution of organic matter in these environments j is also complex and only partly understood. Thus, depositional and early diagenetic controls on Monterey organic matter should be thoroughly reexamined.