--> Abstract: Basin Dynamics--Controls on Location of Edgecliff Pinnacle Reefs, by T. H. Wolosz; #91012 (1992).

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ABSTRACT: Basin Dynamics--Controls on Location of Edgecliff Pinnacle Reefs

WOLOSZ, THOMAS H., S.U.N.Y. at Plattsburgh, Plattsburgh, NY

Subsurface pinnacle reefs of the Edgecliff member of the Onondaga Formation (Middle Devonian of New York and Ontario, Canada) are generally considered to have formed along a gently subsiding ramp. However, the marked asymmetry of reef distribution on either side of the axis of subsidence within the basin represents a flaw in this simple model. While a number of these structures have been found along a northeast-southwest trend on the western side of this axis, they are unknown on the eastern side. Examination of surface exposures reveals differences in the manner of reef initiation between the eastern and western sides of the basin, which suggest that differences in basinal dynamics are the cause of the observed asymmetry in reef distribution. In large eastern surface exposures, the b ginning of reef growth is marked by the development of coral thickets rooted in a basal calcisilt facies that directly overlies the Schoharie sandstones. The upward transition from the Schoharie through the basal calcisilts and into the more typical packstones and grainstones of the Edgecliff suggest that formation of these thickets was triggered by an initial shallowing in the eastern part of the basin. Further reef development was then controlled by the local degree and rate of subsidence prior to the onset of Marcellus Shale deposition. This resulted in structures ranging from about 15 m thick near Albany to an apparent erosional remnant (preserved thickness 20 m) of a small pinnacle reef to the south of Utica. Along the western side of the basin axis the pinnacle reefs were apparentl initiated as small mounds or carbonate sand banks in shallow water just above a disconformable surface with their growth controlled by early and continued subsidence resulting in thicknesses of up to 60 m. It is suggested that the absence of large pinnacle reefs along the eastern side of the axis of basinal subsidence is due to the initial shallowing in the east having been insufficient to trigger thicket formation in those areas.

 

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)