Calculations of Sulfate Diffusivity for Lithofacies of the Miocene Monterey Formation

ZABACK, DOREEN A., and LISA M. PRATT, Indiana University, Bloomington, IN

Integration of stable sulfur isotopic data, total sulfur content, uncompacted porosity, and estimated sediment density of the Miocene Monterey Formation have been used to calculate the extent of sulfate diffusion in the upper tens of centimeters of sediment during early diagenesis. Extent of sulfate diffusion, expressed as the number of pore volumes of water filtered through a volume of sediment, varies from three to infinity with the majority of values in the range of three to seven.

Diffusion of sulfate into sediments is strongly influenced by bioturbation. Interbedding of laminated and massive Monterey strata within the Santa Maria basin indicates sporadic presence of bioturbating organisms during deposition. In addition to a negative correlation between bioturbation and calculated sulfate diffusivity, a positive correlation exists between calculated sulfate diffusivity and clastic input. Clastic indices are highest for the upper siliceous facies, intermediate in the lower calcareous facies, and lowest for the phosphatic facies. The upper siliceous facies with its abundance of diatoms, represents a period of intense upwelling and is associated with onset of glaciation. Enhanced basinal water circulation and lowering of sea level facilitated transportation of cla tic materials and incursion of aerated waters into the basin, creating an environment favorable to benthic fauna. In contrast, deposition of the calcareous and phosphatic facies in an anoxic basin led to chemically reducing conditions and the preservation of highly metabolizable organic matter in the upper centimenters of sediment. Such an environment would be hostile to burrowing fauna and, subsequently, diminished bioturbation limited both diffusion of sulfate into the sediments and diffusion of hydrogen sulfide out of the sediments.

Calculation of sulfate diffusivity may be used to infer the extent of biomineralization within sediments. High sulfate diffusivity indicates extensive recycling of organic matter by aerobes; low values indicate minor recycling by aerobes and/or major decomposition of organic matter by anaerobes provided sulfate concentration in the sediment was high enough to sustain biogenic sulfate reduction.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)