Jurassic raft tectonics in the northeastern Gulf of Mexico

Abstract

The northeastern Gulf of Mexico is dominated by the 900–1800-m Florida Escarpment, which forms the bathymetric expression of the Cretaceous carbonate shelf edge. Outboard of the escarpment lies a region of salt-detached raft blocks, which are closely analogous to type examples in the Kwanza Basin, Angola, in terms of structural style, scale, and amount of extension. We undertook the first detailed structural interpretation of an emerging petroleum exploration province. The rafts detached and translated basinward by gravity gliding on the autochthonous Louann salt in the late Jurassic to early Cretaceous. The Upper Jurassic source rock (lime mudstones) of the Smackover Formation and eolian sandstone reservoir intervals of the Norphlet Formation are structurally segmented and entirely contained within the raft blocks. The rafts are separated by salt ridges and/or extensional fault gaps containing expanded uppermost Jurassic and lower Cretaceous strata of the Cotton Valley Group. The main episode of rafting occurred after deposition of the Smackover and Haynesville Formations and broke the Jurassic carbonate platform into raft blocks 2–40 km in length, which were then translated 25–40 km basinward from their original position. Map-view restoration of the raft blocks suggested a minimum extension of 100%, with basinward transport directions indicating a radial divergence of rafts. In the north of the study area, the transport direction was westerly, whereas in the south, translation was southerly. This pattern, which mimics the Florida Escarpment, suggested that the morphology of the Jurassic slope controlled the style of gravitational tectonics and the location of subsequent Cretaceous carbonate buildups. As with other linked systems on mobile substrates, the observed extension and translation must be balanced by downdip contraction. In the case of the northeastern Gulf of Mexico, the contraction is largely cryptic, being accommodated by salt evacuation, compression of salt walls/stocks, and possibly open-toed canopy advance.