Abstract

A regional stratigraphic synthesis employing subsurface and outcrop data was conducted on a regressive sequence of Middle (Trenton) to Late (Richmond) Ordovician terrigenous rocks in New York and northern Pennsylvania. The sequence is divided into three conformable units: (1) basal shale, (2) medial siltstone, and (3) upper sandstone. Isopachous and lithologic trends of these units reflect tectonic evolution associated with the Taconic orogeny. These trends, coupled with paleocurrent data, suggest that two diachronous source areas influenced Taconic sedimentation: an eastern area (Green Mountain axis) uplifted during Middle Ordovician time, and a southeastern area in Pennsylvania (ancestral Piedmont?) during Late Ordovician time.

Accepting a cordilleran-type model for the Taconic orogeny, it appears that the eastern source was an elongate low-relief orogenic welt resulting from thermal and isostatic disequilibrium. Subsurface trends, however, indicate that the southeastern uplift was of a more rugged relief and limited areal extent, suggesting that it may have resulted from a collision-type mechanism.

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