Magnetostratigraphic Dating of Cenozoic Platform Carbonates from Bahamas and Florida

D. F. McNeill, R. N. Ginsburg

An earlier study of the magnetic reversals in a single core of late Cenozoic shallow-water carbonates from the Bahamas found that the sequence of reversals, measured with a SQUID magnetometer, correlated with the standard magnetic polarity time scale. This initial application of magnetostratigraphy to date shallow-water carbonates with little or no terrigenous components has now been confirmed by study of two additional cores from the Bahamas and an older outcrop sequence from Florida.

The results of the earlier study of the core from San Salvador are confirmed by measurements of two other cores from localities in the Bahamas, Andros, and Williams Islands. The Bahamian cores record the upper section of the Gauss chron, the reversed Matuyama, and the normal Brunhes. Further support for the application of this comes from measurements of an outcrop section in Florida some 25 m thick across the Eocene-Oligocene boundary. Here as in the Bahamas, the sequence of reversals can be matched to the standard polarity time scale.

The magnetic minerals separated from the Bahamian carbonates are biogenic based on the similarity of their size (0.04-0.3 µm), composition (magnetite), and crystal morphology with those same attributes of known biogenic magnetite. Surprisingly, the original polarity from the biogenic magnetite grains is unaffected by recrystallization and dolomitization.

The late Eocene section, previously dated with only a generalized biostratigraphic age, is now dated at between 36.5 and 37.5 Ma (chron 15 and 13r). For the Bahamas, confirmed preservation of the reversal sequence provides more precise dating of the changes from a late Pliocene reef-rimmed atoll to a Pleistocene flat-topped bank, the localized last appearance of numerous mollusks and corals, as well as the chance to calculate detailed rates of sediment accumulation.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.