ABSTRACT

The Albian Harmon Member comprises three lithofacies. LM (laminated mudstone) resulted from hemipelagic sedimentation under dominantly anoxic conditions. LBS (laminated to bioturbated siltstone) was deposited primarily by distal storm or turbidity currents under dysaerobic to anoxic conditions. ICM (interbedded conglomerate-mudstone) represents deposition of turbidites or tempestites in more proximal basin margin facies under dysaerobic to perhaps aerobic conditions.

The rapid and episodic deposition of LBS and ICM, compared to the slower, steadier rate of deposition for LM lithofacies, is interpreted to be a primary control on the carbon, sulfur, and iron systematics of the sediments and the morphology and isotopic composition of authigenic pyrite. Pyrite in LM lithofacies is homogeneously distributed and occurs almost exclusively as discrete framboids less than 20 µm in size. LM lithofacies is further characterized by relatively constant carbon-sulfur ratios, iron-sulfur ratios, and ³⁴S values for pyrite that indicate virtual open system conditions with respect to sulfate. Evaluation of organic matter (OM) maturity and hydrogen indices (HI) suggests that OM reactivity limite sulfate reduction in these sediments.

Pyrite in LBS and ICM lithofacies (transition zone) generally is distributed heterogeneously and occurs as framboidal, zoned, or massive aggregates up to 400 µm in size. Transition zone sediments have extremely variable carbon-sulfur and iron-sulfur ratios and pyrite ³⁴S values. Pyrite in these sediments is enriched in ³⁴S relative to LM sediments, indicating that the exchange of sulfate between pore-waters and the overlying water column was restricted, with sulfate reduction sometimes occurring under virtual closed-system conditions with respect to sulfate.