HIGH-PRECISION CHRONOSTRATIGRAPHY OF A LATE PLEISTOCENE SHELF-EDGE DELTA, LOUISIANA

The accurate dating of depositional sequences and sea-level changes is essential in the search for hydrocarbons and in the study of global c limatic changes. Graphic correlation is a precise and accurate techniq ue that integrates diverse types of time-significant data and compares the events in a new section with the complete range of those events i n a reference data base. Graphic correlation is a quantitative method of analyzing complex data to evaluate and test hypotheses of chronostr atigraphy, During the Late Pleistocene glacio eustatic lowstands, sand -prone deltas were deposited at the Louisiana shelf edge. Regional sei smic stratigraphic boundaries traced on the seismic data have been tie d to important sequence stratigraphic contacts in four coreholes on th e outer shelf and upper slope, Fossil abundances and diversities contr ibuted 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 region al unconformities and five transgressive surfaces were cored in two we lls within the uppermost 250 m of section on the shelf margin and uppe r slope. These contacts were graphically correlated with a composite s tandard reference section containing oxygen isotope events dated in de ep sea cores (DSDP 502, 552A, 572, ODP 607 and 625), Lowstand events a re 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 14-1 6 ka, These data define the timing of sea-level changes recorded south east of the ancestral Mississippi River. The timing of these events co mpares 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 mag be a result of the incompleteness of the sediment preserve d at each site. Although the sequence boundaries record glacio eustati c events, their timing is not periodic and does not represent a single Milankovitch frequency. This seeming inconsistency may be a clue to t he influence of complex sedimentary processes. Perhaps not all the eve nts have been recorded in this section. Application of continental gla cial stages to these events is not recommended.