Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Spatial Variation of Extensive Early Diagenesis on Glover’s Reef, Belize
(1) Marine Geology and Geophysics, University of Miami, RSMAS, Miami, FL.
(2) Geosciences, Goethe University, Frankfurt am Main, Germany.
Glover’s Reef is a 260-km2 reef-rimmed platform off the coast of Belize, Central America. Ten rotary cores through the rim and patch reefs of the platform display facies variability and large diagenetic changes in these young strata.
The top of the Pleistocene is between 6.5-11.5 meters below sea level (mbsl). In the underlying Pleistocene sections, three exposure horizons are identified lithologically and with stable isotope excursions of carbon and oxygen. They can be correlated across the platform. Strontium isotope dating and Amino Acid Racemization geochronology of fossil coral and bivalve samples indicate that similarly aged samples span multiple exposure horizons. This strongly suggests that the recorded sea level oscillations are suborbital in nature.
Extensive porosity changes, dissolution, cementation, and re-crystallization occur in these relatively young rocks, highlighting the degree of early diagenesis that can occur in reef settings. In total, four cement morphologies are present in Holocene sections of the cores: 1) acicular, 2) botryoidal, 3) microcrystalline, and 4) isopachous, bladed fringe cement. Some of the diagenetic features are linked to specific reef settings and facies. For instance, Holocene isopachous, bladed fringe cements are found only in the grain-rudstone facies of the most windward core sites. Other Holocene cements are associated exclusively with crustose coralline algae and foraminiferan boundstones, although spatially distributed on both margin and patch reefs sites.
The Pleistocene facies fo Glover’s Reef include head coral framestones, branching coral grainstones, and mollusk-Halimeda wackestones. In all facies, aragonite grains are preferentially dissolved or re-crystallized. In the case of Halimeda grains, complete porosity inversion can occur in a relatively short amount of time as pores are first filled with cement and the grain is subsequently dissolved leaving only its “
ghost
” or micritic rim and cement-filled pores in the rock record. Rare instances of high aragonite (>80%) preservation in coral skeletons are linked to the presence of crustose coralline algae and re-crystallized marine cements. This “shield” of metastable minerals preserves coral in a median stage of dissolution or re-crystallization.