Tectonic Setting, Structure, and Seismic Stratigraphy of the Apalachicola Rift and its Overlying Sag Basin in the Northeastern Gulf of Mexico



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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.
My study uses a 35,000 km grid of 2D, pre-stack depth-migrated, industry seismic reflection data, tied to five wells in the overlying sag basin to map these enigmatic reflectors in three dimensions to distinguish these two previous interpretations and to improve structural and stratigraphic interpretations of the syn-rift and sag phases of the AR. Mapping of a seismic grid reveals that the AR is a composite rift composed of two major en echelon half-grabens: 1) a 136-km-long, northwest-trending half-graben with dips around 33° to the northeast; and 2) a 75-km-long, northwest-trending half-graben normal fault with dips around 31° to the northeast. Both half-grabens contain basinward-dipping reflectors with observable stratigraphic wedging controlled by the dips of low-angle normal faults (LANFs). The observed dip range of bounding normal faults is consistent with previous work on rift-related LANFs in the northeastern GOM and along the eastern coast of North America.
Mapping of the seismic grid shows a similar Phase 1 rift, called the Elbow rift, which is 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 northwest-trending Paleozoic orogenic and earlier 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.



Rifting, extensional systems, salt, Gulf of Mexico, half-graben, low-angle normal fault, Eagle Mills formation