Devonian of the Appalachian Basin, United States

Although Devonian rocks in the Appalachians have been studied for more than 150 years, they are poorly known in most of the area. The nearly complete Devonian sequence in New York has been established as the North American standard of reference.

The structural belt that includes unmetamorphosed Devonian geosynclinal strata is bonded on the southeast and east by metamorphic and igneous rocks of the Blue Ridge and Piedmont provinces. Some meta-sedimentary and igneous rocks of the Piedmont may be Devonian, but the only rocks known to be of this age southeast of the structural belt are mudstones and siltstones in the subsurface of northwest Florida and adjacent Georgia.

The three oldest stages of the Devonian in the Appalachian area — Helderberg, Deerpark, and Onesquethaw— consist (in ascending order) of carbonate, clastic, and carbonate rocks across an increasingly broad area. The rock units are thin, and facies relations are complex. Most units appear to have accumulated on a relatively stable base in medium- to high-energy environments.

Significant subsidence began in Onesquethaw time and continued into the Carboniferous. Concurrently uplifted source areas to the east shed vast amounts of detritus, most of which was trapped in the trough. Middle and Upper Devonian strata are dominantly shallow-marine, fine-grained clastic rocks. These reach a maximum thickness of over 11,000 feet in eastern Pennsylvania.

Middle Devonian terrestrial rocks occur locally on the east side of the structural belt. By the close of the Devonian they had spread widely over the northern half of the geosyncline.

On the craton, the Middle and Upper Devonian rocks are limestones and shales, but units are thin and discontinuous except for black shale, which, over broad areas, is the latest Devonian deposit. Deposition of this shale persisted into post-Devonian times.