Episodic Dolomitization of Late Cenozoic Carbonates in the Bahamas: Evidence From Strontium Isotopes

Abstract

ABSTRACT The minimum stratigraphic age and the timing of dolomitization have been constrained in sediments of late Cenozoic age in the Bahamas (Little Bahama Bank--LBB, Andros, New Providence, San Salvador, Great Inagua and Mayaguana) using the Sr-isotope composition (87Sr/86Sr) of dolomites. Most dolomite sequences investigated are composites of several dolomitization episodes. Five phases of dolomitization have been recognized, the most extensive of which are early Late Miocene and Late Pliocene in age. Two minor phases occurred later during the latest Pliocene/early Pleistocene and late Pleistocene. The fifth phase (late Early Miocene) is only recognized in San Salvador. Commonly, dolomitization episodes are stacked, indicating an increasing age with depth, Only in San Salvador are younger dolomites interspersed with older ones. The Late Pliocene episode on LBB is the best constrained, lasting probably less than 0.6 Ma and causing extensive dolomitization. The shade of the dolomite body, together with the age of diagenesis, provides support for a dolomite origin from seawater in a hydrologic environment characterized by a mixing zone induced seawater circulation. Variable depth of burial of Middle Miocene dolomitized strata indicates non-uniform subsidence behavior of Bahamian carbonate platforms. Strontium-isotope ratios of other late Cenozoic dolomites (Hope Gate Formation, Jamaica; Seroi Domi Formation, Netherlands Antilles; Rangiroa Atoll; Niue Atoll--Aharon et al. 1987) also indicate early Late Miocene and Late Pliocene dolomite ages. Simultaneous dolomitization in the Bahamas, the Caribbean and the Pacific appears to occur during times when the marine 87Sr/86Sr ratio is either constant or decreasing. This suggests that dolomitization of carbonate platforms and the evolution of Sr-isotope ratios in seawater are linked, possibly via eustatic sea-level fluctuations.