Correlation of onshore–offshore Oligocene through lower Miocene strata using 87Sr/86Sr isotopic ratios, north flank of Cape Fear Arch, North Carolina, USA

Abstract

Abstract Cores from onshore and offshore of the Onslow block reveal at least three depositional sequences above the Eocene Castle Hayne Limestone and below the Miocene Pungo River Formation. Through use of 87Sr/86Sr ratios and their resultant dates, these sequences are correlated from onshore to offshore, thus clarifying the geologic history of the inner continental shelf of North Carolina. The oldest depositional sequence consists of calcareous quartz sand and sandy, pelecypod-mold grainstone updip and yellowish gray, very fine to fine calcareous quartz sand with minor clay and silt downdip. Most of the carbonate in the downdip part of the sequence consists of foraminifers, ostracods, echinoid spines and other small allochems. Eight hand-picked foraminiferal samples, including two replicate analyses, from this sequence selected from two onshore cores and one offshore core yield 87Sr/86Sr ratios from 0.707901 to 0.708006 equivalent to LOWESS (Locally Weighted Regression Scatterplot Smoother) dates ranging from 29.2 to 32.0 Ma. This indicates this sequence is Rupelian (early Oligocene) in age and assignable to either the TA4.4 or TA4.5 global coastal onlap cycle. The youngest sequence consists of oyster-bearing quartz sand and laterally equivalent clay with mollusks onshore and very fine to fine calcareous quartz sand offshore. Twelve foraminiferal and one ostracod sample, including five replicate analyses, from three cores on the inner shelf of Onslow Bay yield 87Sr/86Sr ratios from 0.708332 to 0.708386 equivalent to LOWESS dates ranging from 21.1 to 22.2 Ma. This indicates the sequence is Aquitanian (early Miocene) in age and assignable to the TB1.4 global coastal onlap cycle. Although Chattian (late Oligocene) sequences are recognized onshore in outcrop and core and are projected to continue into Onslow Bay, no cores provided Sr samples for analysis interpreted to be late Oligocene in age.