Stratigraphy and Facies Relationships of Silurian (Wenlockian) Rochester Shale: Layer Cake Geology Reinterpreted: ABSTRACT

Abstract

Along its east-west trending outcrop belt in western New York and Ontario, the Silurian Rochester Shale displays classic layer cake stratigraphy. Lower and upper members and their component beds are traceable for distances exceeding 62 mi (100 km) east-to-west, without substantial change in lithology, fossil content, or relative thickness. In contrast, abrupt facies changes occur within the Rochester along short, 3 to 6 mi (5 to 10 km) north-south oriented sections (e.g., Niagara Gorge). Fossil-rich calcareous mudstones and thin limestones tongue out southward and are replaced by sparsely fossiliferous shales. Similarly, the frequency of storm-generated coquinites and calcisiltites decreases to the south. These observations indicate that facies belts are elongate east-w st, perpendicular to a gently south-dipping paleoslope, and subparallel to the modern outcrop belt. Vertical facies changes in the Rochester Shale at local sections reflect lateral (north or south) shifting of environmental tracts, due to migration of the northern paloeshoreline. The entire formation appears to comprise two transgressive-regressive sequences; the lower (Lewiston) member represents a symmetrical deepening-shallowing cycle, while the upper units (Burleigh Hill-Stoney Creek members) record a shallowing-upward hemicycle. Facies tongues in the north-south sections confirm these interpretations. Layer-cake stratigraphy in the Rochester Shale is thus an artifact of parallelism between the outcrop belt and depositional strike. As such, the Rochester provides a useful paradigm for understanding numerous similar stratigraphic units in the northern Appalachian basin. End_of_Article - Last_Page 1165------------