UTIG RESEARCH PROJECTS ARCHIVE

Principal Investigators: Richard T. Buffler and Craig S. Fulthorpe

Funded by: National Science Foundation

Continental margin depositional sequences contain a record of global sea-level change. However, sequence formation is also influenced by numerous local depositional and tectonic processes, e.g., rates of subsidence and sediment supply, sediment source geometry, currents, isostasy, compaction and physiography. In order to decipher the sea-level signature, the roles of these local processes, which vary within basins in three dimensions, must be understood.

An extensive grid of oil industry seismic profiles from the Main Pass area has been interpreted to investigate three-dimensional (3-D) sequence stratigraphic geometries and their response to local geological controls. Well log and biostratigraphic data from four oil industry exploration wells have also been incorporated in the study. Twenty unconformities (SB20-SB1), bounding sequences T through A, have been defined in the Oligocene to Recent section (post 30 m.y.). Sequence boundaries generally display clinoform geometries in the dip direction and some exhibit a secondary clinoform breakpoint on the slope. Most sequences comprise highstand (HST) and lowstand (LST) systems tracts (Type 1 sequences). LSTs were subdivided into early LST (ELST)and late LST (LLST) in some sequences. LLSTs are progradational and downlap onto ELSTs. Transgressive systems tracts were not generally resolved seismically. Isopach maps of sequences F to N between SB14 (~10.5 m.y.) and SB 5 (~3.8 m.y.) illustrate the 3-D geometries and evolution of the Neogene shelf/slope sediment prism at the level, in some cases, of individual systems tracts. For example, an isopach map of the ELST in sequence L reveals it to be an isolated, fan-like sediment body, whereas the LLST is more extensive. The HST covered the paleoshelf and prograded across the LLST. Isopach maps of sequences show that they comprise localized depocenters that migrate with time. Four major depositional episodes, bounded by SBs 20, 14, 9 and 5, document the history of the region. Below SB 14 (10.5 m.y.), deposition of sediment occurred in the southwestern portion of the study area, filling the basin seaward of the Lower Cretaceous carbonate shelf margin. From SB 14 to 9 (10.5 to 5.5 m.y.), a relatively uniform southward progradation occurred and the movement of depocenters was toward the south. At SB9 time (~5.5 m.y.), successive sequence depocenters began to migrate eastward. The rate of southerly progradation in the west decreased, while that in the east increased. After 3.8 m.y. (SB 5), the ELST, which had previously occurred in a basinal setting, moved to the slope and reflection patterns changed from oblique with high amplitude reflections at clinoform toes to sigmoid. These changes may reflect a transition from sandier to muddier sediment. Sequence development was primarily governed by sea level, but changes in depositional patterns are related to local controls, i.e., changes in drainage patterns, sediment supply, subsidence history (including effects of salt tectonics).