STATISTICAL ECOLOGY OF A NORIAN SPONGE-CORAL REEF FROM THE LUNING FORMATION IN GARFIELD HILLS, NEVADA

Following the end-Permian mass extinction, reef ecosystems underwent an astonishing recovery and radiation in the Middle and Late Triassic which culminated in the Norian-Rhaetian reef optimum. Extensive and abundant coral-dominated Norian-Rhaetian reefs are well documented in the Tethys Ocean. In the Panthalassic Ocean, Norian-Rhaetian reefs are much smaller in number and volume. Panthalassan reefs developed on small, volcanic island shelves and back-arc ramp settings. Due to their rare occurrence, we have a limited understanding of Triassic reef development in the Panthalassic Ocean. To improve our understanding of reef evolution in the Panthalassic Ocean, a newly measured section and early Norian coral reef in the lower member of the Luning-Berlin assemblage in Garfield Hills, Nevada are described. The stratigraphic context and the reef microfacies are described. Thin sections were point counted to assess the relative abundance of reef and interstitial components and grouped into distinct reef facies using hierarchal clustering analysis. The reef is a 7.5 meter thick, coral-sponge biostrome that developed within the middle ramp of a back-arc mixed carbonate-siliciclastic ramp system. The reef framework is primarily composed of platy and branching coral framestone followed by inozoan sponge framestone. Framestones are 50-80% reef builders and have a micritic and bioclastic mudstone/wackestone matrix. Garfield Hills reef ecology and depositional environment is most like the early Norian deep slope coral-sponge biostromes and reefs in the Pilot Mountains, Nevada and Sierra del Alamo, Sonora. Panthalassan and Tethyan Norian-Rhaetian coral reefs have several ecological differences which reinforces the need for further investigation into the ecology and stratigraphic context of Triassic Panthalassan reefs.