Cenozoic Evolution of Carbonate Shelf and Ramp Habitats: Insights from Paleoceanography

Understanding biological, geochemical, and oceanographic processes influencing modern shelf and ramp carbonate deposition is essential when interpreting carbonate sedimentation in the geologic record. Yet recognizing the limitations of uniformitarianism is equally important. Cenozoic carbonate-producing ecosystems emerged from the remnants of Cretaceous biotas, evolving in the warm alkaline oceans of a Greenhouse world, then modifying in response to emerging Icehouse conditions. The latter included stronger latitudinal and bathymetric temperature gradients, declining carbon dioxide concentrations in the atmosphere, and declining calcium concentrations and alkalinity in the oceans. Paleocene-Eocene photic-dependent carbonates tended to be dominated by calcitic coralline red algae and larger benthic foraminifers (LBF), with aragonitic corals and calcareous green algae restricted temporally and spatially. Conceptual models suggest that episodic changes in ocean circulation and thermocline stratification that accompanied high latitude cooling during the Cenozoic provided impetus for turnover in light-dependent (chlorozoan) biotas. For example, comparison of Eocene through Miocene paleotemperature data for surface to thermocline gradients with the history of LBF assemblages indicates that the latter were most diverse and productive when deeper waters were warmest and gradients were weakest. Higher extinction rates corresponded with times when surface to thermocline gradients increased. In contrast, zooxanthellate corals, while relatively diverse in the Eocene, were restricted as reef builders. As Icehouse conditions emerged, aragonite production by corals and calcareous algae became more widespread, with a setback in the early and middle Miocene when coralline algae again dominated. Moreover, the proliferation of reef-building coralline algal taxa into shallow-water habitats in the late Miocene paralleled the emergence of shallow-water corals and new clades of zooxanthellae, indicating co-evolution of these critical reef taxa. Implications of these observations indicate that enhanced understanding of deeper photic-zone (30-100 m) carbonate systems can enhance interpretations especially of Paleogene shelf and ramp carbonate depositional environments, and likely older carbonate systems, especially those deposited during Greenhouse conditions.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California