REVERSE PALEOMAGNETIC POLARITY IN THE MIDDLE EOCENE BLUE BLUFF MARL AT PLANT VOGTLE, BURKE COUNTY, EASTERN GEORGIA

The upper part of the middle Eocene Blue Bluff marl records reversed polarity where collected on the south wall of the excavation for Unit 4 at Plant Vogtle, about 6-8 feet below the top, in Burke County (eastern Georgia) near the Savannah River. The Blue Bluff marl includes the zonal bivalve Cubitostrea sellaeformis, which in the Gulf Coast region occurs in calcareous nannofossil zone NP16 and lower nannozone NP17. The Blue Bluff marl at Plant Vogtle underlies deposits of the Utley “limestone” unit. A molluscan fauna in the Utley (submitted for these abstracts) correlates with the Gosport Sand in Alabama, which there includes an NP17 nannofossil flora. Common to abundant, well preserved, calcareous nannofossils in the Blue Bluff marl at Plant Vogtle indicate placement in NP16-18, most likely NP16. Reticulofenestra umbilicus (first occurrence marks the base of NP16) and common Cribrocentrum reticulatum (last occurence at the top of NP 18) provide stratigraphic control. Rare specimens (2) of atypical Chiasmolithus solitus (last occurrence marks the top of NP16) further restrict the zonal placement. The relatively poorly preserved, mostly fragmentary dinoflagellates assemblage from the Blue Bluff marl contains common Homotryblium plectilum, which indicates correlation with the upper part of NP16 or higher in the middle Eocene. Pentadinium goniferum (typically present in NP16 and more sporadically present in NP17) was not recorded, perhaps due to poor preservation. The nannofossils and dinoflagellates represent a relatively warm, nearshore paleoenvironment. The fragmentary nature of the dinoflagellates suggests they were size-sorted and transported by currents.

Biostratigraphic and lithostratigraphic evidence constrains the age of the upper part of the Blue Bluff marl at Plant Vogtle to nannofossil zone NP16 or lower nannozone NP17. Within that age range, reversed paleomagnetic polarity in this Blue Bluff site most likely corresponds to Chron C18r of Gradstein et al. (2004). This same reversed polarity has been reported for the middle Eocene from the Santee Limestone at Santee State Park, SC (where correlative with lower NP17). It also is consistent with reversed polarity that overlies normal polarity in the Santee Limestone at the Martin Marietta Orangeburg quarry, near Eutawville, SC (Liddicoat et al., 2000, 2001).