DIFFERENTIATING THE EFFECTS OF BASEMENT STRUCTURES, ELASTIC FLEXURE, AND EUSTASY IN THE EVOLUTION OF THE EARLY TACONIC (BLOUNTIAN) FORELAND BASIN OF SOUTHEASTERN LAURENTIA (NORTH AMERICAN CRATON)

Middle and Upper Ordovician strata in the southernmost Appalachians document initial collision along the southeastern margin of Laurentia. Stratigraphic correlations, using a bentonite-graptolite-conodont time framework and a palinspastic map, suggest that fault reactivation, flexural subsidence, and eustasy variously controlled uplift, subsidence, and deposition at different sites within the foreland basin.

Abrupt variations in depth of erosion of passive-margin strata and in thickness of Middle-lowermost Upper Ordovician deposits across the boundaries of the intraplate Birmingham graben (IBG) document an early episode of basement-fault inversion in the distal foreland. The magnitude of the Middle Ordovician lacuna (Knox unconformity) was locally controlled by fault reactivation, not by forebulge migration. Local carbonate deposits adjacent to the inverted IBG are of the same age as synorogenic clastic deposits in the proximal foreland. Furthermore, the inverted IBG initially restricted the cratonward progradation of the Blountian clastic wedge to the southeast and favored carbonate deposition to the northwest. Clast composition and distribution of conglomerates in the distal to middle foreland indicate local uplifts of the Cambrian-Middle Ordovician succession. Fault inversion and flexural extension modified the paleobathymetry of the foreland plate as response to compression and tectonic loading along the continental margin.

Distinction between flexural subsidence and eustasy in the foreland stratigraphy is based on cratonward extent of changes in depositional conditions and stratigraphic trends. Near the plate margin, rapid drowning of the carbonate ramp and deposition of deeper water synorogenic clastic sediments indicate cratonward migration of the flexural profile. Changes from shallow-water carbonate to deeper water siliciclastic deposition in late Middle-early Late Ordovician (Blountian) and again in latest Ordovician-Silurian (Taconic) record two episodes of flexural subsidence. Relative sea-level rise in early Late Ordovician is recorded by deposition over previously uplifted blocks, cratonward deposition of carbonates over the Knox unconformity, and coeval basinwide marine transgressions in both carbonate and siliciclastic depositional systems.