--> ABSTRACT: Depositional Processes Across Carbonate to Clastic Transition in a Middle Devonian Epeiric Sea, by Keith B. Miller and Carlton E. Brett; #91041 (2010)

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Depositional Processes Across Carbonate to Clastic Transition in a Middle Devonian Epeiric Sea

Keith B. Miller, Carlton E. Brett

Deposited in the northern end of the Appalachian basin, the Middle Devonian (Givetian) Hamilton Group of New York state has become a highly fruitful stratigraphic interval for investigating cyclic and event processes. Biofacies and lithofacies gradients indicate a northeast-trending basin axis with a probable carbonate-dominated margin to the north, and a clastic shoreline to the east and southeast. This study concentrates on a single regressive-transgressive cycle, approximately 3 m thick and nearly symmetrical in the west, with numerous widely traceable fossil beds of demonstrable storm-event origin. These isochronous fossil layers have permitted very high-resolution correlation of the western muddy carbonates at a decimeter scale.

Using bundles of event layers and condensed key beds, correlations have been extended across the poorly fossiliferous mudrocks of the basin axis into the siltstone and fine sandstone facies to the east. This correlation has revealed an unexpected pattern: the regressive phase of the cycle expands to 6 m at the basin center and thins eastward into a condensed silty fossil lag. The transgressive phase, however, becomes a 12-m thick siltstone wedge with well-defined asymmetrical subcycles capped by fine-grained sandstone and decreasing in thickness upward. An explanatory model is proposed in which sea level fall results in nearshore winnowing with the erosion and basinward transport of sediment, and maximum redeposition near the basin axis. By contrast, the site of maximum sedimentation s offset source-ward during sea level rise due to the relative rise in storm wave base and increased distance to the source.

AAPG Search and Discovery Article #91041©1987 AAPG Eastern Section Meeting, Columbus, Ohio, October 7-10, 1987.