THE EXXON 564-1 WELL: NEW GEOCHRONOLOGIC AND STRUCTURAL CONSTRAINTS FOR CONTINENTAL CRYSTALLINE BASEMENT IN THE SOUTHEAST GEORGIA EMBAYMENT

The Exxon 564-1 well is the only known penetration of crystalline basement rocks in the offshore of the Southeastern United States. Located within the Southeast Georgia Embayment, the well was drilled to a total depth of 3,920 m (12,863 ft), passing through both post-rift strata of Cenozoic and Mesozoic age and formations of inferred Paleozoic and Precambrian origin. Examination of thin sections and observations made from both geophysical well and log data suggest the inferred zones of Paleozoic and Precambrian origin consist of predominately recrystallized orthoquartzitic sandstones and an intrusive igneous rock that exhibits a granophyric texture, respectively. The contact between the inferred units of Paleozoic and Precambrian origin is still unknown due to the lack of reliable ages for these units. Differences in the degree of recrystallization and the nature of the pore-filling cement between the inferred Paleozoic unit and the overlying Mesozoic unit support our interpretation that the underlying unit is indeed Paleozoic in age. In addition, the lack of evidence for melting at the basal contact with the intrusive body supports our interpretation that the intrusive body pre-dates deposition of the overlying stratigraphy. The ~65 m (200 ft.) interval of inferred Paleozoic strata dip homoclinally 15-20^o to the southeast, concordant with the underlying contact imaged on seismic reflection data, and in contrast to the overlying, subhorizontal strata of Jurassic age. Ongoing geochronologic studies of the intrusive body through single grain U-Pb zircon geochronology should provide a crystallization age to clarify the validity of our interpretations. Regional relations onshore suggest there are intrusive bodies of Precambrian, Paleozoic, and Jurassic age. If correct, the Exxon 564-1 well would represent the first known penetration of Precambrian crystalline basement in the offshore of the Southeastern United States, and would have significant implications for the tectonic evolution of the continental margin.