THE EXXON 564-1 WELL: CONTINENTAL CRYSTALLINE BASEMENT ON THE FLORIDA SHELF

Previously proprietary industry data from offshore oil exploration drilling on the Atlantic OCS document the only known penetration of crystalline basement rocks in the offshore of the southeastern U.S. The Exxon 564-1 well, drilled in late 1979 to early 1980 to a total depth of 3,920 m (12,863 ft), passed through ~3,734 m (12,250 ft) of post-rift Cenozoic and Mesozoic strata before encountering ~64 m (210 ft) of inferred lower Paleozoic strata overlying ~122 m (400 ft) of intrusive granitic rock at the bottom of the well. Examination of thin sections of cuttings from the lower section of the well confirms observations from the geophysical well and mud logs that (1) the base of the post-rift section consists of carbonate-cemented, matrix-supported, lithic sandstones, (2) the sequence of inferred Paleozoic strata consists predominantly of recrystallized orthoquartzitic sandstones, and (3) the intrusive rocks are dominated by an assemblage of quartz and altered feldspar, typically exhibiting a granophyric or graphic granite texture. The contact between these intrusive rocks and the immediately overlying sedimentary strata remains enigmatic, as do the ages of the respective units. The marked contrast in degree of recrystallization of the sedimentary rocks across the post-rift unconformity supports the interpretation that the underlying strata are Paleozoic in age, despite the lack of paleontologic or palynomorphic data. Similarly, the absence of evidence for significant recrystallization in the basal post-rift sequence ~65 m above the top of the intrusive body suggests intrusion pre-dates deposition of the post-rift sequence. While the contact may yet be structural between the igneous and overlying sedimentary rocks, we interpret these data to record the basal Paleozoic unconformity above intrusive rocks of Precambrian age at a depth of ~3,800 m on the Florida shelf. If correct, this interpretation of the Exxon 564-1 well would represent the first known penetration of Precambrian crystalline basement in the offshore of the southeastern U.S., and would have significant implications for tectonic evolution of the continental margin. Ongoing geochronologic studies of the granitic rocks should clarify the validity of this interpretation.