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High-Resolution Chronostratigraphy of Offshore Wells Within Pleistocene Through Upper Miocene of Gulf of Mexico

Dwight M. Trainor, Douglas F. Williams

Absolute ages can be determined from the Pleistocene through upper Miocene sediments recovered from offshore well cuttings and sidewall cores in the Gulf of Mexico by using oxygen and carbon isotope stratigraphy. The technique, isotope chronostratigraphy, integrates available biostratigraphic datums with several type models to interpret the unique isotopic signatures contained in the foraminifera and detrital carbonate removed from these sediments. Model one, based on studies from the Orca and Pigmy basins, defines the boundary conditions necessary to distinguish glacial, interglacial, and meltwater events from a given isotopic record. Model two defines isotope stages and events, and calibrates them on the basis of paleomagnetic and biostratigraphic information gathered f om holes drilled by the MV Eureka in the 1960s and several Deep Sea Drilling Project cores. The third model permits the recognition of regional unconformity surfaces along the upper to lower slope. In integrating these models, glacial to interglacial isotope stages have been recognized to stage 90, extending well into the Pliocene. The character of the oxygen and carbon isotopic records in the Gulf of Mexico is exaggerated because the gulf has periodically received extreme volume discharges of meltwater and fluvial products from the ancient mid-continent ice sheets and drainage systems. These isotopic events extend as far back as the early middle Pliocene. Isotope chronostratigraphy thus allows age determinations with a precision of ±20,000 to ±50,000 years in the Pliocene-Plei tocene. Once a chronostratigraphic framework is established, stratigraphic correlations, sedimentation rates, and unconformity surfaces can be determined and related to seismic sequence boundary patterns.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.