Magnetostratigraphic dating of shallow-water carbonates from San Salvador, Bahamas

Abstract

Magnetostratigraphic results are reported here from a sequence of late Neogene-Quaternary shallow-water carbonate sediments from a continuous core drilled on the island of San Salvador, Bahamas. On the basis of the remanent magnetism of 136 samples from a 91-m measured section of core, the polarity sequence can be correlated with the magnetic polarity time scale from the Gilbert chron (early Pliocene) through the late Brunhes chron (late Pleistocene-Holocene). Magnetic polarities were determined on the basis of relative up-down direction in the unoriented core. Extraction studies of the magnetic particles reveal the presence of single-domain crystals of magnetite resembling those produced by the magnetotactic bacteria and algae. The sequence of reversals provides a minimum of six new major chronostratigraphic markers for the Pliocene-Pleistocene of the Bahamas; it confirms and refines the local timing of both the lithologic change from skeletal to nonskeletal sediments and the disappearance of coral and molluscan species from the Bahamas as upper late Pliocene (between 2.6 and 2.7 Ma). That the primary magnetic remanence is preserved in shallow-water carbonates, including replacement dolomites, suggests that this technique could be used to date similar Tertiary and possibly even older carbonate sequences. The establishment of a reliable magnetostratigraphy provides refined dating of shallow-water carbonates and regional faunal appearances or disappearances, sediment accumulation rates, subsidence, and depositional events.