Abstract

A thin bed of calcareous shale in the Middle Devonian Moscow Formation of western New York marks a submarine discontinuity of considerable regional extent. This bed, characterized by reworked phosphatic pebbles and steinkerns, has been traced in outcrops for 150 kilometers and has been found to mark the boundary between two major marine biotopes. Although no erosion surface is found associated with the bed, an erosion event is indicated by the presence of reworked diagenetic structures within it. Prior to erosion, phosphorite (carbonate-apatite) formed as a diagenetic cement in buried, sediment infilled shells and as isolated nodules within near-surface soft muds. These sediments were subjected to erosion resulting in exhumation of the shells and nodules. Many phosphorite infilled shells were broken to produce a variety of phosphatic steinkerns. A new fauna later colonized the erosion surface and epizoans encrusted both reworked shells and phosphatic material. Shallow burial by muds was followed by a period of soft-sediment bioturbation which variably obscured evidence of the erosion surface. This study shows that significant discontinuities in the geologic record need not be characterized by discrete erosion surfaces. Future recognition of similar features within apparently homogeneous sedimentary sequences should prove valuable in stratigraphic mapping and paleoenvironmental reconstructions.

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