Abstract

Most workers attribute the preferred grain orientation that defines the shale fabric to the mechanical collapse of a high-porosity clay microfabric under increasing overburden pressure. Scanning electron microscope analysis of samples of the Upper Devonian (Frasnian) Rhinestreet shale of western New York State collected from near early diagenetic carbonate concretions provides further evidence of this mechanism. Specifically, mudstones recovered from pressure shadows immediately adjacent to lateral edges of concretions are characterized by an open fabric of randomly arranged clay domains—the cardhouse fabric. Laterally equivalent shale samples collected only 0.2 to 0.3 m distant the pressure shadows, however, reveal a low-porosity, strongly oriented shale fabric in which clay particles mold around incompressible quartz silt and euhedral pyrite grains and pyrite framboids. These observations suggest that the organic Rhinestreet shale accumulated as water-rich flocculated clay. The open clay fabric observed in pressure-shadow mudstones was preserved by the incompressible concretions during burial. Flocculated clay beyond the shielding effect of the concretions, however, collapsed during progressive burial to form the strongly oriented shale fabric.

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