ORIGIN OF PYRITE CONCRETIONS FROM THE HURON MEMBER OF THE OHIO SHALE (DEVONIAN), CENTRAL OHIO, USA

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2017-05

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The Ohio State University

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Concretions in sedimentary deposits form through chemical processes mediated by the decay of organic matter. Concretions often contain conspicuous macrofossils (Maples, 1986; Borkaw and Babcock, 2003). Use of X-ray computerized tomography (XTC) and scanning electron microscopy (SEM) reveals body fossils of microscopic organisms and pyrite framboids (Borkow and Babcock, 2003), which may be secondary to organic material, in some concretions. By exploring the biological origins of the concretions we can learn how concretions form, develop insight into the processes of exceptional preservation, and enhance understanding of the environmental conditions at the time of sedimentation. A field emission gun (FEG) SEM, EDX, and XCT have been used to study pyrite concretions from black shale layers of the lower Huron Shale Member of the Ohio Shale (Devonian) from central Ohio, USA. Pyrite occurs in the concretions in three forms: 1, small pyrite framboids evidently formed around bacterial cells; 2, cone-in-structure; and 3, large euhedral crystals. Studied concretions contain remains of spores referred to Protosalvinia. Two sizes classes, probably reflecting individual spores (50-100 μm in diameter) and spore capsules (~200 μm in diameter), have been observed. Pyrite framboids are commonly present on the spores, whereas more extensive pyritization and cone-in-cone structure is associated with the presence of spore capsules. A consistent pattern of occurrence of euhedral crystals was not observed. Unrecrystallized bacterial cells and fungal hyphae were not observed. The black shale matrix surrounding the concretions is composed of primarily illitic clay. Protosalvinia spores are present in the matrix, and normally have small pyrite framboids on their surfaces. Studied pyrite concretions show no evidence of an origin with traces in sediment or the decay of animals.

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Pyrite, Pyrite concretion, Ohio Shale, Huron member

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