## LSU Historical Dissertations and Theses

1989

Dissertation

#### Degree Name

Doctor of Philosophy (PhD)

Rex H. Pilger, Jr

#### Abstract

Multichannel deep seismic reflection data across a passive continental margin in the northern Gulf of Mexico have been acquired, processed, and interpreted together with three-dimensional gravity modeling. The central Gulf basin is structurally asymmetric from north and south. The northern Gulf is underlain by a 8 to 16 km thickness of sedimentary rocks, significantly greater than the southern Gulf. The top of continental crust occurs at a depth of about 8 km beneath the upper Mississippi-Alabama continental shelf and is characterized by horst and graben structures. The top of oceanic crust occurs at a depth of about 12 km below sea level in the deep Gulf of Mexico. The oceanic crust-transitional crust boundary is interpreted around 27$\sp\circ$ 16$\sp\prime$ N latitude in the profile. The seismic section of continental shelf and continental slope shows four distinct shelf edges; Jurassic, early Cretaceous, mid-Oligocene, and present. Sequence stratigraphic study defines ten seismic sequences since the time of opening of the Gulf of Mexico. The correlation of the sequence boundaries defined in the circum-Gulf region indicates that unconformities with mid-Miocene (10.5 Ma), mid-Oligocene (30 Ma), and mid-Cretaceous (97 Ma), and at the top of the Jurassic (131 Ma) are commonly found as major regional unconformities. The depositional history of this part of the northern Gulf margin can be divided into three main depositional periods: (1) shallow marine deposition from the opening of the Gulf to mid-Cretaceous time, (2) deep marine deposition from Cretaceous to mid-Oligocene, and a return to (3) shallow marine deposition since the mid-Oligocene. The depositional history indicates that characterization of the northern Gulf of Mexico continental margin as a terrigenous sediment wedge province was initiated in late Cretaceous time. Comparison of the location of the seismically defined oceanic crust--transitional crust boundary and the location of steep gravity gradients in the central Gulf of Mexico suggests the existence of outer marginal highs, 20 to 50 km wide. This observation constrains the northern limit of the oceanic crust to 20-50 km south of the steep gravity gradient belt in the north-central Gulf to the west where allochthonous salt inhibits seismic imaging of the deeper structures.

347

COinS