TECTONIC LOADING-UNLOADING AS CONTROLS ON SILURIAN SEQUENCE STRATIGRAPHY IN THE APPALACHIAN FORELAND BASIN

The Silurian Red Mountain Formation of the southern Appalachians comprises five distinctive members consisting of seven sedimentary sequences separated by subtle unconformities. Sedimentation was dominated by fine-grained clastics deposited on a storm-dominated shelf. Similar facies were repeated through several sequences with thick proximal deposits in the east (towards the Taconic orogen) and thin distal deposits marginal to the craton in the west. The thick stratigraphic section at Ringgold, Georgia is characteristic of the former, while the thin but more complete type section at Birmingham, Alabama, typifies the latter. The sedimentary ironstones, for which the formation is famous, are best developed on transgressive surfaces, and in incised channels at the base of transgressive systems tracts on the western side of the foreland basin. These are the shallowest, highest energy, and coarsest grained facies; deposited along sediment-starved, reworked shorefaces. By contrast, highstand systems tracts are thickest in the east. They consist of coarsening-upward, progradational sequences dominated by shales with graded storm-laid, geostrophic, sandstones at the base and hummocky-crossbedded sandstones above. Rhuddanian and Late Aeronian-Late Telychian sequences thicken towards the orogen and bevel increasingly older beds towards the craton, which suggests they were initiated by flexural subsidence related to tectonic loading of the orogen. Thin Aeronian and Wenlockian sequences, restricted to the type area near Birmingham, may be related to unloading of the orogen through erosion with consequent isostatic uplift in proximal terranes in the east and basement sag due to the collapse of a peripheral bulge in the west. Eustatic changes related to glaciations in Gondwana were also a contributory factor.