Evidence and implications of an extensive early Mesozoic rift basin and basalt/diabase sequence beneath the southeast Coastal Plain

Abstract

Regional seismic reflection profiles across the Georgia and South Carolina Coastal Plain provide a detailed view into the internal structure and stratigraphy of the buried Triassic-Jurassic South Georgia basin. Reflection data indicate that the basin is a complex composite of smaller, individual basins which vary drastically along strike. These basins are relatively wide along north-south profile (maximum >100 km) but appear to be more restricted along strike. In cross section, the basins are asymmetric, being typically bounded on their south sides by an interpreted north-dipping master normal falult. The thickness of interpreted basin fill below the Cretaceous-Tertiary overlap also varies drastically, locally reaching 6 km. Analysis and correlation of the reflection profiles, when integrated with drill-hole and offshore reflection data, suggest the presence of an areally extensive basalt flow and/or diabase sill sequence of probable Early-Middle Jurassic age, covering a large part of the Triassic-Jurassic South Georgia basin. The sequence is traceable on seismic reflection profiles across much of the South Georgia basin and may extend as far as from western Georgia to offshore South Carolina. The implied areal extent of this sequence would be more than 100,000 km 2 , ranking it among the great basalt flows and/or sills of the world. On seismic reflection profiles over the South Georgia basin, the basalt/diabase sequence is also important in marking a post-rift unconformity which separates tilted and apparently faulted lower Mesozoic rift-related strata below from a thinner, more widespread, and generally unfaulted sequence above. This configuration is interpreted to mean that basalt extrusion and/or hypabyssal intrusion occurred after the main episode of Late Triassic-Early(?) Jurassic normal faulting and associated localized subsidence (syn-rift phase) and prior to a later, Jurassic episode of more widespread uniform subsidence (post-rift phase) accompanied by relatively minor normal faulting. This two-stage evolutionary sequence is analogous to that of many rift basins throughout the world and suggests a hypothesis that the South Georgia basin was an area of incipient crustal spreading associated with the early formation of the central Atlantic Ocean.