(1) Universidade Federal da Bahia, Salvador, Brazil
Abstract: Tertiary shallow marine deposits can be forever young
The modern tropical Brazilian carbonate shelf is characterized by vast calcareous algae deposits. Coralline algal maerl, rhodoliths and Halimeda plates are volumetrically of greater importance that the localized occurrences of coral reefs. The hydrodynamic regimes are either current- or wave-dominated, and these imprint on the sediments large-scale bedforms leading to heterogeneous facies distributions. The current-dominated portion has mud-depleted sediments, modern carbonate gravelly sands and palimpset siliciclastic deposits. Wave-dominated environments have gravelly carbonate sands, and muds and muddy-sand siliciclastics where there is a large contribution from river discharge. The calcareous algae, particularly the free-living corallines and their fragments are the main carbonate producers, yielding enough sediments to cover thousands of square kilometres. Although many carbonate producing organisms have changed through time, most corallines have been conservative in their genera and mode of contribution to carbonate deposits. Corallines are general favour medium to high energies in both modern environments and the fossil record. We compare modern and Tertiary coralline algal and mixed carbonate-siliciclastic deposits from Brazil and elsewhere, looking at the environments, lateral and vertical facies associations and bioclastic bedforms as means of paleoenvironmental reconstruction. On the present day tropical shelf of Brazil, carbonate to siliciclastic lateral transitions are abrupt. However, similar vertical transitions recorded in the fossil record are generally related to larger shifts of facies or larger sea-level oscillations. These findings can help to understand the morphology and architechture of carbonate platforms (ramps) and help prediction of lithofacies distributions in these settings.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana