MAPPING THE MIDDLE AND UPPER DEVONIAN MARINE TO TERRESTRIAL TRANSITION IN THE APPALACHIAN BASIN

An overview of the depositional architecture of the “The Catskill Delta” and the challenges with defining its stratigraphy were outlined in the classic GSA Special Paper 210 of 1985. In particular, the Middle and Upper Devonian strata representing the transition from the marine to the terrestrial environment in the Appalachian Basin have proven to be problematic for consistent geologic mapping and stratigraphic correlation. This transition is composed of various facies that represent intertonguing shallow subtidal, intertidal, and supratidal depositional environments and is many tens of meters thick. We maintain that this transition interval should be mapped separately from the deeper marine and fully terrestrial strata wherever feasible in Appalachian Basin. The stratigraphic names of the deeper marine units include the Lock Haven (lower part), Trimmers Rock, and Brallier Formations in Pennsylvania, Maryland, West Virginia, and Virginia, while the fully terrestrial strata include the Hampshire Formation and the mid to upper Catskill Formation in these states. In New York, this transition interval would equate to the upper “Chemung” and the “Cattaraugus” magnafacies, and would include such units as the Towanda, “New Milford”, and Ashokan Formations. Units already recognized as transitional between marine and terrestrial environments include the Foreknobs, Chadakoin (upper part), and Venango Formations, and the Irish Valley, Towamensing, Beaverdam Run, Walcksville, and Long Run Members of the Catskill Formation.

The lower boundary of the marine-terrestrial transition is generally marked by shallow subtidal delta fan or platform deposits dominated by cross-bedded sands often containing rounded quartz pebbles that sometimes occur in conglomeratic lenses. The upper boundary of the transition from intertidal to fully terrestrial strata is more gradational, marked by an increase in grain size in fluvially-deposited sands in fining upward sequences, red subaerial muds, and the disappearance of marine faunas. Mixed marine and terrestrial strata likely represent a subaerial delta plain environment, with embayments that can be several kilometers in length and width. A useful modern analogue for the environments and scale of the Catskill marine-terrestrial transition is the Ganges-Brahmaputra delta.