SNAKE HILL FORMATION—SYNTECTONIC FACIES OF THE EASTERNMOST TACONIC FORELAND BASIN IN EASTERN NEW YORK

A remnant of the easternmost margin of the Late Ordovician Taconic Foreland Basin is represented by the Snake Hill Formation at its type locality on Saratoga Lake, eastern New York. This coherent section within the Taconic deformed belt is dominated by medium- to coarse-grained feldspathic quartz arenites that show decameter-scale shoaling-up alternations. The alternations include distal conglomeratic turbidites, prominent hummocky cross stratified intervals (storm-dominated shelf), and wave rippled with coarse-grained herringbone and trough cross-bedded units (wave- and tide-dominated shelf). Moderate to intense bioturbation is characteristic. Dominance of the Sowerbyella-Onniella brachiopod community, dominance of Cryptolithus tesselatus relative to other trilobites, and diverse bivalves show similarity with benthic faunas of the younger Lorraine Group of central New York. The Snake Hill succession is interpreted as a siliciclastic ramp sequence that prograded into the Taconic foreland basin. Presence of the graptolite Normalograptus mohawkensis (=Orthograptus ruedemanni zone) shows that the Snake Hill sequence is older than the surrounding tectonized shales (Climactograptus spiniferus zone), thus it is best interpreted as a giant parautochthonous block in Taconic mélange. The proximal, shallow-marine Snake Hill type locality has nothing in common with the various heavily deformed, unfossiliferous dysoxic to anoxic shales mapped elsewhere as “Snake Hill Shale” in the past. Any application of the term “Snake Hill” to facies of characteristics that differ from those of the shallow-marine type locality should be dropped. An ongoing challenge is to find other proximal, fossiliferous horizons and slices to determine if the Snake Hill Formation can be extended to other localities, and to better understand the Taconian syntectonic shelf-to-basin facies transition overall.