--> ABSTRACT: Sedimentation and Early Sulfide Diagenesis of Harmon Member, by John Bloch; #91022 (1989)

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Sedimentation and Early Sulfide Diagenesis of Harmon Member

John Bloch

Pyrite is ubiquitous in the Albian Harmon Member (Peace River Formation) in concentrations from less than 0.2 to 8.5 wt. %. Petrographic observations show two distinct modes of occurrence that are stratigraphically controlled. In neritic laminated mudstone, pyrite occurs dominantly as very fine-grained (< 20 µm), disseminated framboids. In moderately bioturbated pro-delta (transition zone) mudstone and siltstone, pyrite occurs as framboidal aggregates, burrow-filling cement, and rarely as a cement in peloidal muds.

Pyrite formation may be limited by the availability of organic carbon, sulfate, or reactive iron. Iron-sulfur ratios exceed stoichiometric pyrite and show that iron is not a limiting factor in pyrite formation. This conclusion is further supported by the presence of iron-rich carbonates and silicates in all Harmon sediments.

Laminated mudstone carbon-sulfur (C/S) ratios range from approximately 0.8 to 2.5 and show no distinct correlation. Transition zone C/S ratios are extremely variable (0.4-60.0) and show no meaningful trends. Total organic carbon content is consistent throughout all Harmon rocks (m = 1.75 ± 1.1), however, implying variable sulfate availability or retention.

The mean ^dgr34S value for disseminated pyrite in laminated mudstones is -28.9^pmil CDT (2^sgr = ± 4.0). The mean isotopic composition of transition zone pyrite is -8.0^pmil CDT (2^sgr = ± 19.0). The morphology and isotopic composition indicate all pyrite formation was primarily from bacterial sulfate reduction.

The preservation of primary sedimentary structures and the absence of bioturbation in neritic mudstones indicate deposition beneath anoxic to euxinic bottom waters. Elevated C/S values may further indicate a euxinic depositional environment. Consistently low 34S/32S values indicate unrestricted availability of sulfate for bacterially mediated reduction.

Excess iron (relative to stoichiometric pyrite), a lack of C/S correlation, and elevated 34S/32S values show that sulfate availability was the limiting factor in transition zone sediments. Sulfate availability was probably controlled by episodic sedimentation and bioturbation.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.