The Gasport Member is a normal-marine unit of the Lockport Formation, and records a period of patch-reef growth on the southeast flank of a crinoidal, submarine-bar complex along the Algonquin axis in western New York. Reefs, averaging 25 ft high and up to 100 ft in diameter, are exposed in a 60-mi zone in the western part of the state. Associated facies of the Gasport are exposed westward along the Niagara Peninsula of Ontario and eastward almost to Rochester, a distance of 110 mi.

Detailed lithologic and paleontologic study of these patch reefs and associated facies has shown that all were deposited under normal-marine conditions in a shallow, subtidal and intertidal area. The reefs first began growing as coral thickets on a crinoidal-sand substrate (crinoidal bar facies) that spread eastward from the barrier-bar complex. These coral thickets (initial reef subfacies) consisted of delicate, branching tabulate and rugose corals, bryozoans, and encrusting girvanellid algae. These were replaced upward by large, massive, and encrusting stromatoporoids which served as the organic framework, as well as the binder of interstitial carbonate mud (reef-core subfacies). Organic debris torn loose from the tops of the reefs was deposited on the flanks (reef-flank subfacies), which grade laterally into the interreef facies of dolomitized biomicrite and sparsely fossiliferous micrite. As the reefs grew into shallower, more agitated water, a slight lowering of sea level resulted in the termination of reef growth, allowing low, domal stromatoporoids to spread laterally over the reef zone and to form a biostromal layer of organic rubble (stromatoporoid cap facies).