Amoco Exploration and Production Technology Group, United States

The accurate dating of depositional sequences and sea-level changes is essential in the search for hydrocarbons and in the study of global climatic changes. Graphic correlation is a precise and accurate technique that integrates diverse types of time-significant data and compares the events in a new section with the complete range of those events in a reference database. Graphic correlation is a quantitative method of analyzing complex data to evaluate and test hypotheses of chronostratigraphy. During the Late Pleistocene glacio-eustatic lowstands, sand-prone deltas were deposited at the Louisiana shelf edge. Regional seismic stratigraphic boundaries traced on the seismic data have been tied to important sequence stratigraphic contacts in four coreholes on the outer shelf and upper slope. Fossil abundances and diversities contributed to the interpretation of depositional systems tracts. The ages of these contacts are dated by the graphic correlation integration of fossil tops and oxygen isotope events in two of the cores. Four regional unconformities and five transgressive surfaces were cored in two wells within the uppermost 250 m of section on the shelf margin and upper slope. These contacts were graphically correlated with a composite standard reference section containing oxygen isotope events dated in deep-sea cores (DSDP 502, 552A, 572, ODP 607 and 625). Lowstand events are dated at 332 ka, 254 ka, 168 ka, and 70 ka. Transgressive sea-level-rise events are dated at 475 ka, 313 ka, 209 ka, 125-133 ka, and 14-16 ka. These data define the timing of sea-level changes recorded southeast of the ancestral Mississippi River. The timing of these events compares with sequence boundaries elsewhere in the Gulf of Mexico dated at 400 ka, 200 ka, and 90 ka by Martin et al. (1993). The differences in ages may be a result of the incompleteness of the sediment preserved at each site. Although the sequence boundaries record glacio-eustatic events, their timing is not periodic and does not represent a single Milankovitch frequency. This seeming inconsistency may be a clue to the influence of complex sedimentary processes. Perhaps not all the events have been recorded in this section. Application of continental glacial stages to these events is not recommended.