MAGNETOSTRATIGRAPHY OF ATLANTIC COASTAL PLAIN AND CONTINENTAL SHELF SEDIMENTS FOR PORTIONS OF THE GEOLOGIC RECORD SINCE THE LATE CRETACEOUS (Invited Presentation)

For more than 40 years, colleagues and I have documented the paleomagnetic polarity in sediments as old as the late Cretaceous on the Atlantic Coastal Plain (ACP) between the Delmarva Peninsula and Central Florida. Samples from the Atlantic Continental Shelf (ACS) also were measured as part of the Atlantic Margin Coring Project of the U.S. Geological Survey (Hathaway et al., 1979). The paleomagnetic polarity complements ages assigned to the sediments by other methods that include amino acid racemization dating, uranium-series dates, invertebrate paleontology, and detailed field mapping. Where possible, fully-oriented samples from natural outcrops or active quarries in the ACP were measured and treated by alternating field, thermal, or both demagnetization methods; samples from the subsurface also were used and treated similarly. The youngest change in polarity (Matuyama Reverse Chron to Brunhes Normal Chron at about 0.78 Ma) occurs between the Waccamaw (Reverse) and Canepatch (Normal) formations near Myrtle Beach, SC. On the ACS east of New Jersey in the 300-m Core 6009B, the Bruhnes Normal Chron is recorded in the upper about 90 m of the sediment. Our most recent investigation was at Plant Vogtle in eastern Georgia where the middle Eocene Blue Bluff marl records reverse polarity that most likely occurred during Chron C18r of Gradstein et al. (2004), as it did in the Santee Limestone in central South Carolina. Elsewhere, the magnetic polarity can be used to narrow the age of younger sediments to within several hundreds of thousands of years or less. An example is the Duplin Fm and equivalent, related marine and fluvial sediments beneath the Coharie terrace in the middle ACP physiographic province of South Carolina that were deposited during the upper Gilbert Chron C2Ar (4.187-3.596 Ma), Mammoth Subchron C2An-2r (3.330-3.207 Ma), or Kaena Subchron C2An-1r (3.116-3.032 Ma).

I will summarize the results of the combined ACP and ACS paleomagnetism investigations as they apply to tectonic activity in possible earthquake-prone areas such as Charleston, SC; the dating of sea-level fluctuations in the Pleistocene; the confirmation of ages on stratigraphy as established by the chronology methods noted above; and the correlation of the onshore sedimentary record with the deep-sea record and a global time scale.