AN EARLIER INCEPTION FOR THE ACADIAN OROGENY IN THE CENTRAL APPALACHIANS? POSSIBLE EVIDENCE FROM MIGRATING HELDERBERGIAN (LATE SILURIAN–EARLY DEVONIAN) BLACK-SHALE BASINS

Dating the onset of orogenies can be a rather inexact undertaking, not only because dating methods are imprecise and vary with different geologic situations, but also because orogenies may be diachronous across their distribution. For example, onset of Acadian orogeny in the northern Appalachians is commonly considered to have begun in Late Silurian time, whereas farther south in the Appalachians the orogeny was largely considered to have been a Devonian event; differences of opinion even exist about when in the Devonian the orogeny began. My own earlier work has suggested an Early Devonian (Pragian) onset, based on the formation and migration in time and space of large, black-shale foreland basins that apparently formed in response to deformational loading accompanying the orogeny. Moreover, this and later phases of basin migration correspond well to temporal and spatial requirements necessitated by convergence of the Carolina Terrane with Laurussia via dextral transpression from Early Devonian to Early Mississippian time, which is the most accepted interpretation for origin of the Acadian Orogeny.

Different interpretations exist about the timing of Carolina’s convergence with Laurussia. However, the presence of two Helderbergian black-shale basins, reflected in the Upper Silurian Big Mountain and Lower Devonian Mandata shales, which successively migrate northward in time through Virginia, West Virginia and Maryland behind the Virginia promontory, suggest a Late Silurian time for the inception of docking between Carolina and Laurussia followed by a period of northward, sinistral movement by Carolina. A Helderbergian docking event is also supported by the age and sources of clastic debris that accumulated in the Cat Square remnant ocean basin between Carolina and Laurussia. Likely interactions to the north with Avalonia in Pragian time apparently ended Carolina’s sinistral movement and sent it southward (dextral movement) to effect later parts of the orogeny. Although the described scenario does not necessarily preclude other possibilities, it does add support for a Late Silurian–Early Devonian docking by Carolina. In the broadest sense, this could mean a Late Silurian onset for the Acadian Orogeny in the central Appalachians, as in the north.