Deep reflectors dipping seaward have been known for a long time in the eastern Gulf of Mexico (GOM) below the Jurassic Louann salt. So far, they have been interpreted as half-graben fills preceding the opening of the Gulf in the Callovian. Recently acquired seismic data help delineate their geometry and suggest that they are seaward-dipping reflectors representing subaerial flood basalts associated with the early opening of the GOM.

The SDRs in the eastern GOM form two parallel belts trending NW, each about 50 km wide and 200 km apart. The outer belt shows an evolution from convex-up reflectors that extend very deep proximally and less deep farther seaward. The top of the deep reflectors is rather smooth and planar, whereas the shorter reflectors exhibit a more irregular top surface of several hundred meters relief.

The outer limit of the SDRs marks the transition to a belt characterized by the presence of a thick unit (outer high) with complex, bi-directional internal reflections above the chaotic basement. Seaward of the belt appears the normal, hummocky oceanic crust lacking organized reflectors. In the eastern GOM, the Louann salt pinches out distally near the transition to the normal oceanic crust.

The oceanic crust is not covered by the Louann salt and could therefore be mapped in detail. It shows a high relief, with seamounts exceeding 1 s TWT in height above the regional trend and lows of the same amplitude filled in onlap by sediments. The top of the crust exhibits two orthogonal directions, trending SE and NE respectively. The geometries observed indicate that the NE direction corresponds to transform faults whereas the SE direction would be that of the axis of the opening.

These observations suggest that the eastern GOM opened as a volcanic rifted margin, having (1) an initial phase of subaerial lava extrusion; (2) progressive subsidence close to the level of the hypersaline Louann “lake” (below the level of the world ocean) producing lava deltas with steep, irregular fronts during salt deposition; (3) subsidence below lake level resulting in explosive volcanism forming the outer belt; (4) production of oceanic crust without salt when the GOM communicated with the world ocean, ending salt deposition. Thereafter, seafloor spreading continued normally until the GOM opened fully in the late Jurassic or early Cretaceous.