A geologic history of the north-central Appalachians, part 2: The Appalachian basin from the Silurian through the Carboniferous

The north-central Appalachians occupy a critical position within the 3000+ km-long Appalachian orogen, lying southwest of the boundary between the central and northern Appalachians (CNAB). The one-billion-year-long history of tectonic activity in the north-central Appalachians includes the assembly and breakup of a late Proterozoic supercontinent, the creation and evolution of the Appalachian orogen during the Paleozoic, and the Mesozoic transformation of the active orogen into a passive margin during Pangea's disassembly. An important part of the Middle and Late Paleozoic history is the formation and growth of the Appalachian basin, an enormous, elongate continental basin lying cratonward of the active Appalachian internides along Laurentia's eastern margin. The Appalachian basin developed out of the Taconic orogeny, the second of the four orogenies that formed and modified the character of the Appalachian orogen. Prior to the Taconic orogeny, the eastern margin of Laurentia consisted of a broad carbonate shelf facing the Theic Ocean. The Taconic orogeny obduced various Theic components, including microcontinents, magmatic arcs, and accretionary prisms, onto the carbonate shelf. The resulting Taconic highlands formed a topographic barrier between Theia and the craton's interior. Because tectonic activity in the internides continued more or less throughout the remainder of the Paleozoic, the Appalachian basin never had a direct connection with Theia. The coarse-grained molasse from the newly uplifted Taconic highland spread westward over most of the basin during the Early Silurian. Carbonate deposition gradually encroached eastward over the basin as the siliciclastic input from the southeast waned. A resurgence in the earliest Late Silurian, perhaps related to orogenesis in Newfoundland and the Maritimes, expanded the clastic wedge somewhat. Before long, however, carbonate deposition once again dominated most of the north-central basin for the remainder of the Silurian and into the Early Devonian. The Early-to-Middle Devonian Acadian orogeny began introducing siliciclastic material into the eastern part of the Appalachian basin, thereby ending the largely paralic environment that persisted from the Late Silurian. A number of deltas formed around local sediment-input centers during the Middle Devonian. Continued uplift and/or orogenesis must have continued into the Late Devonian because a vast amount of terrigenous sediment was introduced into the Appalachian basin to form the vast Upper Devonian Catskill delta. Although the principal tectonic activity appears to have centered in New England, the large volume of sediment input suggests that orogenesis may have extended southward to the north-central Appalachians. However, direct evidence of actual Acadian deformation and metamorphism is lacking at this latitude. The north-central Appalachian basin underwent a significant change during the Carboniferous. Prior to this time, the shoreline shifted laterally through transgressions and regressions largely as a function of sediment input and regional subsidence. In the Carboniferous, vertical movements in the basin became more important. During the Early Carboniferous, much of the Mauch Chunk delta was eroded in the central and western parts of the basin. At the end of the Early Carboniferous, an unconformity truncated progressively older rocks to the north toward New York. New sediment transport patterns and depositional environments within the basin during the Late Carboniferous probably reflect significant topographic and tectonic changes, not only in the hinterland, but in the basin itself, perhaps in conjunction with climatic changes. Extensive interfluvial swamps on a lower delta plain accumulated thick organic deposits which would become coal. This delta plain graded southwestward into tidal and marine environments. The Alleghany orogeny in the Early Permian interrupted deposition in much of the north-central Appalachian basin and profoundly altered its structure.