Paleogeographic trends in Late Triassic reef ecology from northeastern Panthalassa

Abstract

The Late Triassic was a pivotal period in reef evolution, but the majority of information about reef ecology during this time comes from buildups in the Alps (e.g., the Tethys Ocean). Recent studies of reefs in western North America have recognized unique ecologies along the eastern margin of the Panthalassa Ocean. Although there are numerous (twenty-five) localities with putative reef builders, only four buildups had syndepositional relief and a rigid framework (i.e. true reefs). The most paleo-northern true reefs were microbial patch reefs with only a few large skeletal bioconstructors; hypercalcified sponges and spongiomorphs built the mid-latitude reefs, with secondary microbial encrustation and branching, phaceloid Retiophyllia corals. Corals are the primary bioconstructors in Panthalassa's most paleo-equatorial reefs and calcareous microbes are sparse. When all reefal deposits are analyzed, the N–S gradient is also present, with microbial and bivalve deposits in the north, sponge-coral deposits in the mid latitudes, and coral deposits near the equator. This ecological gradient is not apparent in the Tethys Ocean. Tethyan reefs thrived in oligotrophic, tropical waters without strong latitudinal gradients; by contrast, paleoceanographic considerations suggest that cool, nutrient-rich waters swept south along the western North American borderlands in the Late Triassic. The eastern boundary current is interpreted to have created a strong north–south differentiation of environments in northeastern (NE) Panthalassa that was manifested in both the biotic and abiotic characteristics of eastern Panthalassic reefs. Reefs from equatorial Panthalassa are similar to Tethyan reefs (warm-water, photozoan, coral reef structures), whereas higher paleo-latitude reefs from Panthalassa are interpreted as cool-water (heterozoan) buildups, with abundant calcareous microbes, diminutive biocalcifiers, and few large, framework-building corals.