2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 249-12
Presentation Time: 4:30 PM


HUBBARD, Dennis, Department of Geology, Oberlin College, Oberlin, OH 44074, dennis.hubbard@oberlin.edu

As reefs decline, we wonder how well core-based models of Holocene reefs inform our understanding of reef building in an increasingly stressful world. Outcrops in the western Dominican Republic expose in-place marine communities that allow us to address this question. Twenty-five vertical transects and 58 quadrats revealed a diverse community of corals (26 species; 17 genera) and molluscs (56 species; 44 genera) within four well-defined and depth-related facies. Data from four other outcrops reflect similar reef environments reaching paleo-depths of ~30m.

Coral abundance reached 80%; the dominant corals were Acropora cervicornis in shallow water and Orbicella, Siderastrea and Montastraea spp. along the forereef. The bay was a protected environment flanked by steep slopes and sedimentation was high but episodic. Hemispherical colonies were rare and morphology varied from columnar to conical. Bioeroders were dominantly molluscs (Lithophaga sp.), contrasting with sponges typical of oligotrophic systems. Episodic burial is reflected in ragged colony edges and fluctuating growth rates suggesting ENSO-mediated sedimentation.

Detailed transects and radiocarbon ages were used to reconstruct a three-dimensional facies architecture that reflects early transgression until ~ 7 KaBP when rising sea level slowed and accommodation space was increasingly overfilled. This triggered regression as the A. cervicornis facies prograded over a forereef dominated by massive colonies similar to "organ-pipe" Montastraea that had gone extinct in the Pleistocene.

In western Cañada Honda, vertical accretion was characterized in 12 "virtual cores" in which coral, sediment and rubble were logged vertically following the protocols used for reef cores collected by submersible drilling systems. Calibrated 14C ages were use to constrain timing (10-5 KaBP) and rate (<5 m/Ka) of reef building. Vertical patterns within a single "virtual core" were similar to those revealed by traditional reef cores. However, the shift from transgression to regression was not obvious without a suite of cores that crossed the entire system, suggesting that studies with limited cores may not fully characterize facies architecture and the relationship between reef building and the rate of sea-level rise - either in the deeper past or in the 21st century.