ABSTRACT

In western New York and Ontario, the Rochester Shale (Silurian; Wenlockian; Clinton Group) is everywhere divisible into lower (Lewiston) and upper (Burleigh Hill-Stoney Creek) members; thin units within these members are traceable east to west for distances exceeding 100 km, without substantial change in lithology, or fossil content. In contrast, abrupt lateral facies changes occur within the Rochester Shale along short north-south sections (for example, Niagara Gorge). Such stratigraphic evidence suggests that Rochester shale facies belts are elongate east-west, parallel both to a northern paleoshoreline and also to the present outcrop trend.

The Rochester Shale consists of sparsely to richly fossiliferous, gray, shaley mudstone with abundant interbedded carbonates, including intrasparrudites, lenticular biosparites and biomicrites (= calcarenites) and laminated pelmicrites (= calcisiltites). Such interbeds provide evidence for episodic concentration and transport of carbonate sediments by storm-wave action on a gently southwards-sloping shelf. Calcarenite lenses were formed as erosion-lag deposits in regions within storm-wave base. Finer debris, winnowed from such areas, formed suspension clouds which flowed downslope, depositing storm silt- (calcisiltite) and mud layers.

A crinoidal bank facies (Wiarton) accumulated in the area of the Algonquin Arch during Rochester deposition. Adjacent shelf areas, to the south, received a mixture of terrigenous muds from southeastern source areas (Taconics) and detrital carbonate sediments from the Wiarton shoal, forming argillaceous carbonates and/or calcareous shales (Stoney Creek-upper Burleigh Hill). During times of low sedimentation this area became the site of abundant bryozoan growth--"bryozoan belt" (Lewiston B, D and E). Farther southward (basinward), facies comprise a mixture of sparsely fossiliferous mudstones and storm-silt layers (Lewiston C, lower Burleigh Hill).

Vertical facies changes in the Rochester Shale are attributed to north-south shifting of environmental belts, due to migration of the northern paleoshoreline. As a whole, the formation comprises two (eustatic?) trangressive-regressive cycles. The Lewiston Member represents a nearly symmetrical cycle of deepening (units A-C) and shallowing (units C-E), while the upper units (Burleigh Hill, Stoney Creek) reflect aspects of an asymmetrical, shallowing-upward hemicycle. During the last event, allodapic carbonate sedimentation was considerably increased, as a result of migrating crinoidal bars, inhibiting the growth of bryozoans and other organisms.

The Rochester Shale provides a paradigm for interpreting numerous marine units in western New York which exhibit layer-cake arrays of facies.