REGIONAL TECTONIC SIGNIFICANCE OF THE ANTES SHALE OF PENNSYLVANIA

The Antes Shale of central Pennsylvania is part of a diachronous yet continuous belt of Middle to Upper Ordovician black shale that overlies shelf and slope carbonates and is present from the central Appalachians to the Maritime Provinces. The Antes Shale and the lithostratigraphically correlative units of the Utica Group record distal deposits in an under-filled foreland basin. Through much of the Valley and Ridge of Pennsylvania, the Antes overlies Trenton Group carbonates: the Salona and Coburn formations. Although much thicker, the Salona is correlative with the Watertown to Napanee formations of New York State, and the Coburn is equivalent to the Kings Falls and perhaps some or all of the Sugar River formations. A disconformity between the Coburn and Antes formations in central PA is recorded by a solution-pitted and mineralized limestone and a lengthy biostratigraphic gap (equivalent to at least the duration of the Poland and Russia formations in NY). In the northern portion of its outcrop belt, the Antes Shale can be divided into four members: a lower calcareous shale (distal expression of the Rust Formation of NY) an argillaceous limestone member (Steuben Formation), an upper calcareous shale (Hillier Formation equivalent), and a capping black papery shale (Indian Castle Shale of the Utica Group of NY). Based upon correlation of these intervals and of key beds within these intervals (i.e., bentonites, concretion beds), we recognize a westward onlap of the shale following submarine solution of the collapsed Trenton platform.

In the southeastern portion of the Antes Shale outcrop belt, approaching the Great Valley, Trenton carbonates are absent and basal Antes Shale belongs to the Turinian C. bicornis graptolite zone. Similar stratigraphic relations are present in northern Virginia and SE New York State, where much or all of the Mohawkian is represented by siliciclastic deposition. These relations suggest that the Taconic Basin included a series of strike parallel belts that differed markedly in the timing and magnitude of subsidence across strike. The successively earlier, step-wise collapse of the carbonate platform approaching the Great Valley suggests that preexisting structural heterogeneities may have exerted significant control on patterns of basin migration during the Taconic Orogeny.