GSA Connects 2021 in Portland, Oregon

Paper No. 61-4
Presentation Time: 2:30 PM-6:30 PM


GRIFFING, David1, GLUMAC, Bosiljka2 and CURRAN, H. Allen2, (1)Dept. of Geology and Environmental Sciences, Hartwick College, Oneonta, NY 13820, (2)Department of Geosciences, Smith College, Northampton, MA 01063

A new 34 m-long (54 mm dia.) core drilled into the Grotto Beach Formation at The Gulf, a sea cliff near the southern platform margin of San Salvador Island, provides additional insights into reef facies and reef developmental history during the Last Pleistocene Interglacial (MIS5). The overall core recovery was 86% with the largest losses in the friable, cross-stratified ooid-peloid grainstones (eolianites) with common rhizoliths near the Modern surface. Below the upper 7+ meters of eolian dune and beach grainstones, the entire core is composed of coral-bearing subtidal facies.

The core does not penetrate underlying Grotto Beach eolianites and thus may not represent the entire subtidal portion of the Cockburn Town Member, but it still contains four reef coral concentrations separated by skeletal-ooid-peloidal grainstones (peri-reefal sands) or by scalloped hardgrounds. The uppermost reef deposit contains coral-red algal-foraminiferal-microbial bindstone with Acropora cervicornis as the dominant coral and stromatolitic coatings up to 14 cm thick, in sharp contact with an overlying A. cervicornis-dominant rudstone bearing skeletal encrusters but no microbialite. This upper reef interval is strikingly similar to previously described reef stages exposed in quarry and surface exposures of the Cockburn Town Fossil Reef (west coast of San Salvador). Below this, however, a second coral interval comprises >6 m of coral floatstone and rudstone (reef rubble) featuring Orbicella cavernosa, O. annularis and Porites porites fragments (some with substantial skeletal encrustations). A third coral interval makes up an 8+ m-thick complex of framestone, bindstone and rudstone with a more diverse assemblage of corals, including O. annularis, Colpophyllia natans, Pseudodiploria sp., and Porites astreoides. Many corals bear extensive borings and skeletal encrustations (some with microbialites), and this interval also contains many small internal cavities with geopetal fills. This diverse reef has some carbonate mud near the base and originates on a series of closely spaced scalloped hardgrounds. The hardgrounds separate the overlying reef from the lowermost coral-bearing interval distinguished by fenestral carbonate mud supporting A. cervicornis fragments with thin skeletal and microbial coatings.