Abstract

Submarine, fossiliferous conglomerates in the Upper Devonian Rushford Formation of the Canadaway Group crop out in Allegany County, western New York State. Characteristics of the conglomerates and an erosional disconformity at the base of the conglomerates suggest that the origin of the Rushford Formation conglomerates can be explained by the transgressive incised shoreface model of Bergman and Walker (1987). Evidence for the erosional disconformity separating the conglomerate from the underlying coarsening-upward sequence includes; (1) a sharp change in lithology, as evidenced by the fine-grained sandstone of the Rushford Formation overlain by a coarser unit that varies from a fossiliferous, quartz-pebble, coarse-grained quartzose arenite to a fossiliferous, quartz-pebble, quartzose paraconglomerate (hereafter referred to as "conglomerate"); (2) a sharp change in composition from feldspar-rich quartz sandstone below the disconformity to fossil-rich, feldspar- and mica-poor conglomerate above the disconformity; (3) a sharp change in average grain size from approximately 0.06 mm below the disconformity to approximately 0.2 mm above the disconformity. Trace fossils found at the contact between the fine-grained sandstone and the conglomerate indicate the presence of a Glossifungites firmground and a depositional hiatus. In the transgressive incised shoreface model, the erosional disconformity is formed during periods of stillstand in an overall transgression. The outcrop pattern of the conglomerates with respect to faults known to be active in the Late Devonian suggests that the erosion and emplacement of the lag may have developed as a result of interaction between local fault-block motion and a regional transgression. This relation between local faulting and the transgressive surface of erosion promotes a modification of Bergman and Walker's (1987) model for deposition of conglomerates during a stillstand induced by fault-block motion; a fault-influenced transgressive incised shoreface model.

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