Episodic dolomitization of late Cenozoic carbonates in the Bahamas; evidence from strontium isotopes

The minimum stratigraphicage 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 lnagua and Mayaguana)using the Sr-isotope composition (87Sr/86Sr)of dolomites. Most dolomite sequencesinvestigatedare compositesof severaldolomitizationepisodes. Five phases of dolomitization have been recognized, the most extensive of which are early Late Miocene and Late Pliocenein age. Two minor phases occurred later during the latest Pliocene/earlyPleistoceneand late Pleistocene.The fifth phase (late Early Miocene)is only recognizedin San Salvador. Commonly,dolomitization episodesare stacked, indicatingan increasingage with depth, Only in San Salvadorare youngerdolomites interspersed with older ones. The Late Plioceneepisode on LBB is the best constrained,lastingprobably less than 0.6 Ma and causingextensivedolomitization. The shape 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. Variabledepth of burial of MiddleMiocenedolomitizedstrata indicatesnon-uniform subsidencebehavior of Bahamiancarbonate platforms. Strontium-isotope ratios of other late Cenozoicdolomites (Hope Gate Formation, Jamaica; Seroi Domi Formation, Netherlands Antilles; Rangiroa Atoll; Niue Atoll-Aharon el al. 1987) also indicate early Late Miocene and Late Pliocene dolomite ages. Simultaneousdolomitization in the Bahamas, the Caribbean and the Pacificappears to occur during times when the marine 87Sr/ 865rratio is either constant or decreasing. This suggeststhat dolomitization of carbonate platforms and the evolution of Sr-isotope ratios in seawater are linked, possibly via eustatic sea-levelfluctuations.