From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.
The architecture of basement plays a significant role in the structural evolution, depositional history, and salt migration in the Gulf of Mexico (GoM). That the boundary between continental and oceanic crust (COB) is demarcated by a “step up” in the basement outboard towards oceanic crust is a popular albeit controversial idea. Here, we examine the evidence in the seismic data for the presence and geographic distribution of the basement step, the related salt structures, and its relationship to a transition in crustal type.
The “acoustic basement” in the GoM refers to the base of the autochthonous Louann salt or its equivalent reflector in areas of little to no salt. The basement step is mapped as a discontinuity observed at the last outboard limit of autochthonous salt. The region outboard of the basement step can be defined into 3 geographic areas. 1) The Perdido foldbelt in the west, a large allochthonous salt feature that built out over basement during the Jurassic, lies down-dip to the step. 2) In the central GoM, the J-LK allochthon transgressed the basement step during the Jurassic to Lower Cretaceous, and is characterized by salt feeders and stocks much smaller in scale than those found up-dip of the basement step. A major transfer zone aligning with the Brazos lineament up-dip defines the western edge, while a smaller transfer zone aligning with the up-dip Hackberry lineament lies near the eastern boundary. 3) In the eastern GoM, the edge of autochthonous salt is coincident with the basement step and no significant allochthons are observed down-dip, suggesting the lack of a major up-dip depocenter.
Inboard of the step, a dominant NW-SE orientation in continental basement fabric tends to align with major salt feeders and published lineaments. In the eastern GoM abyssal plain, the dominant structural grain outboard of the step is oriented NE-SW. This difference in structural grain alignment is consistent with a two stage structural opening of the GoM – an NW-SE oriented rift stage followed by a counterclockwise rotation in the drift phase. The basement step does not unambiguously resolve the question of whether the outboard “acoustic basement” is true oceanic crust everywhere in the GoM; however, it places a limit on the possible inboard extent of oceanic crust.