DELINIATION OF A BACK-BULGE BASIN AND FOREBULGE FLANK IN UPPER DEVONIAN ACADIAN DEPOSITS OF THE CENTRAL APPALACHIAN BASIN AND IMPLICATIONS FOR THE LOCATION OF THE HINTERLAND

Isopach and sedimentary facies maps of Upper Devonian late Frasnian and early Famennian strata deposited in part of the central Appalachian basin during the Acadian orogeny show a significant change in depositional style through time. Maps of two successive late Frasnian stratigraphic intervals show steady thickening (up to over 200 m) to the east towards the hinterland. Coarser-grained sediment within these sequences was deposited in distinct tongues in front of the Augusta Lobe. Maps of two subsequent early Famennian stratigraphic intervals show distinct depocenters in north-central West Virginia. Strata thin significantly eastward (from around 200 m to around 100 m) from these depocenters over a distance of about 100 km up to the limit of mapping at the Allegheny Structural Front (and towards the Acadian highlands). The axes of these depocenters are stacked on top of each other. Also, coarser-grained sediment is concentrated in strike trends just east of the depocenters, with no tongues developed in front of the Augusta Lobe. The duration of individual mapped intervals is estimated to be between 0.5 and 1.5 m.y. The early Famennian depocenters are interpreted to be the result of back-bulge basin development behind a forebulge (peripheral bulge) in response to intensification of the Acadian orogeny. Earlier deposition may have taken place in a simpler foreland basin without segmentation into a back-bulge basin and foredeep by a forebulge. The axis of the later developed forebulge would have been located to the east of the Allegheny Structural Front. If allowance is made for space for a foredeep between the early Famennian forebulge and the thrust front of the orogenic belt, the Acadian hinterland was most probably located beneath the modern Coastal Plain. This would then account for the apparent absence of an Acadian terrane in the Piedmont of Virginia. If one is indeed present it must be located to the east.