Seismic Imaging of Pre-Salt Half-Grabens Along Low-Angle Normal Faults in the Northeastern Gulf of Mexico

Abstract:

The northeast-trending Apalachicola rift (AR) in the northeastern Gulf of Mexico (GOM) is a ~220-km-long half-graben that contains 5-8 km of undrilled, syn-rift clastic deposits inferred to be of late Triassic-early Jurassic age. Previous workers have interpreted pre-salt (Triassic-early Jurassic), basinward-dipping, seismic reflectors within the syn-rift half-graben of the AR as either: 1) volcanic layers or sills erupted and tilted within Mesozoic half-grabens; or 2) as layered, volcanic flows ("seaward-dipping reflectors") erupted during GOM phase 1 rifting of late Triassic- early Jurassic age.

This study uses a 35,000 km grid of 2D, pre-stack depth-migrated, seismic reflection data, tied to 6 wells in the overlying sag basin to map these enigmatic reflectors to distinguish these two previous interpretations and to improve structural and stratigraphic interpretations of the AR syn-rift. Mapping reveals that the AR is a failed composite rift composed of two major en echelon half-grabens: 1) a 136-km-long, northwest-trending half-graben with dips around 30° to the northeast; and 2) a 75-km-long, northwest-trending half-graben with dips around 31° to the north. Both half-grabens contain basinward-dipping reflectors with observable stratigraphic wedging controlled by the dips of low-angle normal faults (LANFs).

Mapping of the seismic grid shows a similar trending rift, called the Elbow rift, located 200 km to the southeast of the AR. Together the AR and Elbow rift form a 300-km-long and a 40-km-wide belt of rifting that crosscuts the northeast-trending Paleozoic orogenic and earlier Phase 1 rift fabric beneath the West Florida shelf and the mainland of Florida. I interpret the parallelism of the AR-Elbow rift trend with the marginal rifts bordering oceanic crust ~200 km to the southwest as indicating that both sets of rifts are part of the second phase of rifting in GOM that was related to the late Jurassic, counterclockwise rotation of the Yucatan block.