ABSTRACT: Inorganic Geochemistry and Diagenesis of Organic-Rich Marine Sediments, Monterey Formation, Shell Beach, California

Christian S. Laber, Paul A. Baker

Over 300 samples from the Shell Beach section of the Monterey Formation were analyzed for their mineralogical and chemical composition. This basinal section includes a variety of lithologies, mainly organic-rich shales, siliceous shales, porcelanites, dolomitic shales, and dolomites. According to Khan and co-workers, the undecompacted sedimentation rate at this site is 100 m/m.y.

The organic C content (carbonate free) of these sediments is very high, averaging 8.3 wt. %. Assuming an average porosity of 33% and a grain density of 2.2 g/cm³ results in a calculated organic C accumulation rate of 13 g/m²a, about 1% of primary productivities in oceanic upwelling regions and at least 10% of the organic C that reaches the sea floor in such regions. The average N content (carbonate free) of the sediments is 0.35 wt. % and the C/N weight ratios are almost invariant at 24/1 (r²=0.941), values typical of thermally altered kerogen and much higher than the Redfield ratio of 6/1. The average total P content (carbonate free) of the sediments (excluding phosphatites) is 0.78 wt. % (0.64 wt. % inorganic P, 0.15 wt. % organic P). The C/P weight atio of organic matter averages 55/1, higher than the Redfield ratio of 41/1, but the C/total P ratio averages 10/1. These data imply that some P in the sediments must have been derived from a source besides organic matter, possibly directly from seawater. Much of this P occurs as small lenses or nodules of nearly pure apatite embedded in an organic shale matrix. The total S content (carbonate free) of these sediments averages 1.91 wt. % and the C/S ratio is very high, averaging about 4/1, and constant. Most of this S occurs as pyrite. Volcanic tuffs that occur in the lower part of the section contain high S contents (up to 6.7 wt. %), but contain virtually no organic C. In general, the degree of pyritization is very high, averaging 92%, that is, almost all available Fe is used for pyrit formation.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990