Abstract

Recent seismic reflection profiling by COCORP (Consortium for Continental Reflection Profiling) provides the first detailed view of the internal structure of the Triassic-Jurassic South Georgia Basin beneath the coastal plain of the United States. This basin is the southernmost and largest of the onland Triassic-Jurassic rift basins of eastern North America. The reflection data indicate that the South Georgia Basin is a composite feature, which includes several large half-grabens separated by intervening regions where the Triassic-Jurassic section is much thinner. The largest of these individual half-grabens has an apparent width (along the north-south profile) of 110 km, and contains up to 6 km of basin fill sediment.

The COCORP profiling further indicates that the interior of the South Georgia Basin is vertically divisible into two distinct seismic stratigraphic intervals beneath the Cretaceous-Tertiary cover: a lower, thicker graben-filling (“syn-rift”) sequence exhibiting structural truncation combined with stratal tilting, and an upper, thinner basin-overlapping (“post-rift”) sequence characterized generally by less faulted, concordant reflectors. This two-part division, typical of continental rift basins, is akin to the Mesozoic development of the western North Atlantic margin and the North Sea. As seen on COCORP profiles in the South Georgia Basin, the two sequences are separated by a prominent Lower to Middle Jurassic basalt/diabase interval, which is traceable across most of the South Georgia Basin as far east as offshore South Carolina. The basalt interval is analogous to that observed in exposed Newark-style Triassic basins as far north as the Fundy Basin and is probably the extrusive equivalent of diabase dikes exposed in the Piedmont. This correlation, together with the separation by the basaltic interval of the syn-rift and post-rift phases of the basin, would support the hypothesis that the major diabase intrusive- extrusive event in eastern North America postdates the formation of onland Triassic basins and possibly marks the initiation of Atlantic seafloor spreading.