URANIUM-LEAD DATING OF A CORAL FROM THE NEOGENE GURABO FORMATION, DOMINICAN REPUBLIC

Closure of the Central American Gateway at approximately 4 million years ago is believed to have changed the temperature and salinity of Caribbean surface waters, resulting in extinctions of several marine taxa including some corals. Extinction rates for this period rely on age models for sedimentary sequences, many of which are generated using biostratigraphy. Differences in these models have resulted in a range in rates of faunal turnover. Here we present the results of our attempt to constrain the age of one of these sedimentary units, the Neogene Gurabo Formation, northern Dominican Republic, using U-Pb dating methods on an extremely well-preserved coral.

U-Pb dating of corals is rarely attempted because their highly porous and aragonitic skeleton is highly susceptible to diagenetic alteration. Coral samples were intially screened using petrographic microscopy, X-ray diffraction, and stable isotope analysis. Using these tests, a sample of Goniopora hilli was determined to be largely free of diagenetic alteration. 238U/234U and 234U/230Th ratios were measured using TIMS in eight chips from this sample and three contained secular equilibrium values for both isotopic ratios. Subsamples from these three chips were dated using 206Pb/204Pb vs 238U/204Pb methods to an age of 7.0 MA +/- 0.66 Mya.

Using biostratigraphy, Saunders et al. (1986) and Vokes et al. (1989) assigned signficantly different ages to the Gurabo formation (5.1-4.2 Mya and 5.6-5.3 Mya, respectively). Our U-Pb date agrees with new paleomagnetic data which indicate that the Gurabo Formation represents a much longer time interval than previously thought. Thus the rates of faunal turnover during this interval must be recalculated.