2Dept. of Geosciences, SUNY Fredonia, Fredonia, NY 14063, USA

Early formed carbonate concretions in the Upper Devonian Rhinestreet shale of western New York State indicate that the organic-rich host sediment accumulated as water-rich flocculated clay. The depositional microfabric of the shale is preserved in strain shadows adjacent to carbonate concretions and within the concretions themselves. Geochemical characteristics of the concretions indicate that they grew by rapid near-uniform precipitation of carbonate cement throughout the concretion body. Thus, the volume percent carbonate in concretion matrix samples - 74 to 93% - is a proxy for the porosity of the host sediment at the time of concretion growth and is consistent with porosities of modern marine clays. The water-rich sediment experienced rapid gravitational compaction and consequent reorientation of platy grains to produce a strongly anisotropic microfabric common to many black shales. The multiple concretion horizons reflect episodic reductions in subsidence rate during burial. Compaction strain measurements conducted on 118 concretions yield an average compaction strain of the Rhinestreet shale of 51.8%, which translates to a paleoporosity of 37.8%, a value markedly higher than that expected for shale normally compacted to the 3.1 km maximum burial depth of the Rhinestreet shale. The calculated paleoporosity of the Rhinestreet shale likely reflects the onset of overpressure at a depth of ~1,100 m, the fluid retention depth, well shy of its maximum burial depth. Early and relatively shallow overpressuring of the Rhinestreet shale likely originated by disequilibrium compaction induced by a marked increase in sedimentation rate in the latter half of the Famennian stage.

AAPG Search and Discovery Article #90059©2006 AAPG Eastern Section Meeting, Buffalo, New York