Preliminary Age Determinations for New Deep Gulf of Mexico Basin Seismic Sequences

ABSTRACT A detailed seismic stratigraphic analysis of the deep Gulf of Mexico basin has resulted in the definition of twelve new post-mid-Cretaceous, pre-Sigsbee unit seismic sequences. The seismic sequence boundaries mapped are characterized over much of the deep basin by regionally extensive, continuous, high-amplitude reflections, which are interpreted to be condensed sections overlain directly by correlative conformities. Sediments in between consist mainly of basinal lowstand deposits. Age control for five younger sequence boundaries (4.7, 5.5, 10.5, 12.5 and 15.5 Ma) was established by correlating them with published Cenozoic sequences on the northeastern Gulf shelf. The correlation was made across the slope in the DeSoto Canyon area using a dense grid of University of Texas Institute for Geophysics and industry seismic data with minimum interference from salt structures. This correlation has been extended farther to DSDP holes 90 and 91 in the western Gulf, where there exists a good agreement between the correlation and the well information. This correlation allows us for the first time to directly link shelf stratigraphy with the deep basin stratigraphy and to date the youngest sequences in the deep Gulf with some confidence. Some important preliminary conclusions can be drawn from the correlation. The base of the Mississippi Fan defined by previous workers corresponds to the 5.5 Ma boundary (latest Miocene). This is older than inferred by some previous studies, which define the base of the Mississippi Fan as Late Pliocene-early Pleistocene. The main period of deformation in the Mississippi Fan foldbelt can be dated as 15.5 to 5.5 Ma. The initiation of the foldbelt is indicated by a possible locally derived sedimentary wedge downlapping the 15.5 Ma boundary. The major periods of deformation occurred during the middle to late Miocene as indicated by prominent sediment onlap against the folded 10.5 Ma and 5.5 Ma boundaries. In addition, the well developed cyclicity of deposition in the deep Gulf basin provides us more opportunities not only to refine the correlation but also to estimate the ages of the older sequences by correlating the depositional cycles in the deep basin with the depositional episodes proposed by Galloway (1989) and the eustatic sea level curve of Haq et al. (1987). This eventually will lead to the establishment of a more complete Cenozoic chronostratigraphic framework for the entire Gulf of Mexico basin.