The upper Middle Devonian (Givetian) Leicester Pyrite Member in western New York marks an erosional event associated with the major Taghanic transgression in the northern Appalachian foreland basin; this transgression eventually led to widespread onlap of black muds (Geneseo Shale Member) over variably truncated Tully Limestone or underlying Windom Member. Typical Leicester deposits in western New York State consist of thin (0-10 cm) lenses of reworked pyrite, conodonts, bones, and other insoluble components at the base of the Geneseo black shale. Diachroneity (westward-younging) of Leicester and basal Geneseo deposits across western New York is evident from conodont studies.

Pyritic lag deposits resembling the Leicester have recently been found in central New York above the upper Tully Formation and also in central Pennsylvania above a severely truncated Tully section. In areas between these two occurrences and the classic Leicester sections in western New York, the Tully-Geneseo contact lacks good development of lag debris and may be conformable. Thus, Leicester-type lags overlie truncation surfaces on either side of the inferred basin center. This pattern suggests impingement of an erosive zone on both margins of the basin during sea-level rise. Erosion and concentration of pyritic lags without oxidation of pyrite requires conditions of variable dysoxia, sediment-starvation, carbonate dissolution, and episodic bottom scour during relative sea-level rise in the foreland basin. We infer that mid-slope submarine erosion took place under dysoxic conditions and was focused along a water-mass boundary. Corrosional dysoxic to anoxic conditions were followed by black mud onlap in given areas effected by sea-level rise and/or thrust load-induced subsidence. The Leicester, as well as earlier transgressive lags flooring the Middle Ordovician Utica Shale in eastern New York, owe their origin to a combination of epeirogenic and eustatic processes.