Neogene/Quaternary Magnetostratigraphy of Shallow-Water Carbonate Sediments, San Salvador, Bahamas

D. F. McNeill, J. L. Kirschvink, R. N. Ginsburg, S-B.R. Chang

For the first time, a detailed sequence of late Neogene/Quaternary magnetic reversals is documented in shallow-water carbonate sediments. The magnetostratigraphy of a continuous core from San Salvador, Bahamas, correlates with the established magnetic polarity time scale.

The remanent magnetism of 140 samples from a 91-m limestone/dolomite core was measured with a SQUID magnetometer. All samples were demagnetized using alternating field and thermal demagnetization to isolate characteristic components of remanent magnetization. Natural remanent magnetism in the carbonates ranged from 1.4 × 10^-7 to 4.5 × 10^-9 A m²/kg. Magnetic polarities were determined based on relative updown direction in the unoriented core. Extraction of magnetic minerals, followed by TEM examination, revealed magnetofossils of predominantly bacterial origin.

The measured section ranges from early Pliocene (Gilbert chron 3) through late Pleistocene/Holocene (Brunhes chron). The reversals provide a minimum of five new major chronostratigraphic markers for the late Neogene-Quaternary of the Bahamas. Previously established biostratigraphic data (last appearance of the coral Stylophora affinis and a molluscan fauna equivalent to the Bowden Formation of Jamaica) are in concordance with the magnetostratigraphy. The timing of the major lithologic change from skeletal to nonskeletal sediments is dated as upper late Pliocene.

The establishment of a reliable magnetostratigraphy provides much refined dating of shallow-water carbonates that contain a record of sea level fluctuations, subsidence, changes in depositional facies and fauna, and possibly climatic events. That the magnetostratigraphic record is still legible in recrystallized and dolomitized carbonates suggests that it may also be present in older strata.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.