Abstract

Upper Devonian rocks of the Catskill Mountains, New York, have been classically interpreted as of deltaic origin, and more recently as having formed in a "tectonic delta complex." In central Pennsylvania Upper Devonian rocks have been interpreted as the products of marine coastal deposition analogous to the coastal mudflats which occur down current from the modern Mississippi Delta. Although the rocks of central Pennsylvania are somewhat lower in the section than those in this study, north-central Pennsylvania, our study area is a strategic location between the New York and central Pennsylvania study sites

The rocks of north-central Pennsylvania are of two kinds: (1) continental fluvial and (2) marine tidal facies. Specifically the rocks consist of three lithofacies:

I. fossiliferous, gray sandstones, and siltstones interpreted as of tidal origin;

II. shale-pebble conglomerates, red to grayish-green sandstones, and grayish to pale red siltstones and mudstones that are inferred to have accumulated in and near meandering streams as channel, overbank and marsh deposits; and

III. coarse-grained, olive-gray sandstones interpreted as of braided-stream origin.

The particles composing the rocks were derived from a rising mountain chain to the east which consisted of low-grade metamorphic rocks of greenschist facies.

During Late Devonian time tidal-flat deposits (Lithofacies I) accumulated in north-central Pennsylvania. As rivers from the east deposited sand and silt at their mouths, the shoreline was displaced westward resulting in progradation of coastal-plain deposits over the tidal flats. Meandering-fluvial deposition (deposits of Lithofacies II) dominated during part of Late Devonian times. With increasing tectonism in the mountains to the east, perhaps reflecting the collision of two lithospheric plates, the supply of detritus increased resulting in the deposition of braided-stream sediments (Lithofacies III).