Abstract

Upper Devonian black shales of the eastern United States contain in places unusual beds of pyrite ooids. Textural and geochemical studies show that these originated as chamositic iron ooids that were replaced by pyrite during early diagenesis. Pyrite mimics the laminated texture of the precursor grain, yet inclusions of silicate minerals and instances of partial replacement reveal the secondary nature of the pyrite. Pyrite ooids occur above erosion surfaces that are considered sequence boundaries because of large lateral extent. Chamositic precursor ooids indicate an oxygenated water column and wave interaction with seafloor sediments at the time of their formation. This scenario agrees with earlier work that stipulates that erosion surfaces in Devonian black shales reflect lowering of sea level that allowed wave reworking and erosion of earlier-deposited black shales. Pyrite ooid beds thus furnish direct evidence of significant sea-level drops during the accumulation of Upper Devonian black shales in eastern North America.

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