New Insights into Dolomite Concretions, Phosphatic Shale Member, Monterey Formation: Carbonate Associated Sulfate and d³⁴S

Sean Loyd, Frank A. Corsetti, and William Berelson
Dept. Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740

The formation of concretions and cemented layers in sediments remains a poorly understood process, despite the fact that they are relatively common in siliciclastic strata and affect the porosity and permeability of the sediment. Concretion growth in organic-rich sediments has been related to organic matter degradation--processes that include bacterial sulfate reduction, anaerobic methane oxidation, methanogenesis, and thermal decarboxylation and may result in elevated pore water alkalinity. While the carbon isotopic systematics of concretions have been extensively studied, carbon isotopes alone cannot differentiate between these modes of degradation and the detailed origin of most concretions remains unknown.

Here, we propose a novel application of Carbonate Associated Sulfate (CAS) to address the conditions under which concretion form. Authigenic carbonates (e.g., concretions) incorporate sulfate into the crystal lattice in trace amounts, preserving the d³⁴S of pore water sulfate. Unlike d¹³C, the trajectory of pore water d³⁴Ssulfate is unidirectional with depth (barring highly unusual circumstances): as bacterial sulfate reduction proceeds, ³⁴S is enriched in pore waters, and sulfate concentration declines. d³⁴SCAS and concentration of CAS constitutes an innovative way to track the conditions under which concretions formed, especially when used in conjunction with more conventional analyses.

Two preliminary samples from the Phosphatic Shale Member of the Monterey Formation were collected from the Shell Beach locality. These two samples consist of one concretion (CON) and one cemented layer (CL), both dolomitic in composition. CAS concentrations of CON and CL are 7900 and 1100 ppm, and d³⁴SCAS values are 25.3 and 33.3‰, respectively. These values are consistent with cements forming in the zone of BSR with CL forming in a zone that experienced more extensive sulfate depletion.

AAPG Search and Discovery Article #90076©2008 AAPG Pacific Section, Bakersfield, California