Oligocene Calcareous-Nannofossil Biostratigraphy from Leg 101, Site 628, Little Bahama Bank Slope

Drilling at Hole 628A, Leg 101 of the Ocean Drilling Program, recovered a thick, relatively complete Oligocene section of nannofossil-foraminifer chalk and ooze. Sediments from Cores 101-628A-16H through 101-628A-29X were examined using the light microscope to provide information about biostratigraphy of calcareous nannofossils. Several authors noted a problem in distinguishing the first appearance of species within the Sphenolithus predistentus-S. ciperoensis lineage of the mid-Oligocene, particularly Sphenolithus ciperoensis, because of the gradual evolution of its members. Successful separation of species in this lineage is critical for accurately dating relative age of mid- and upper-Oligocene nannofossil sediments. Length and taper of the apical spine, along with width of the proximal shield, were reported by various authors as the best criteria for separating S. ciperoensis from Sphenolithus distentus in light microscopy. We find that the extinction line characteristics of the proximal shield, as originally proposed, are the best criteria for separation in light microscopy. These criteria provide results that correlate well with dating using other microfossils. Scanning electron microscopy (SEM) study shows that this feature directly results from the widening of the proximal shield during evolution from S. predistentus to S. ciperoensis. Transitional forms between S. distentus and S. ciperoensis show features of both species. The last appearance datums (LAD) of Lanternithus minutus and Helicopontosphaera compacta were found to correspond with the first appearance datum (FAD) of S. ciperoensis in Hole 628A. The LAD of H. compacta has a high confidence rating for reliability as an alternative datum to the FAD of S. ciperoensis. The usefulness of L. minutus to approximate this datum is much less. Other potentially useful Oligocene nannofossil datums (with varying degrees of usefulness) also are given