T21 New Developments in Permian-Triassic Paleoceanography

Sunday, 31 October 2010
8:00 AM - 12:00 PM
Room 607

STROMATOLITE-SPONGE-TUBIPHYTES REEFS IN THE VIRGIN LIMESTONE MEMBER OF THE MOENKOPI FORMATION, NEVADA: IMPLICATIONS FOR BIOTIC RECOVERY FOLLOWING THE END-PERMIAN MASS EXTINCTION

Paper number: T21-16
Program: Technical Sessions
Time: 11:45 AM – 12:00 PM

Authors

Abstract

We document the occurrence of sponges and Tubiphytes within the upper of two previously-described (Schubert and Bottjer, 1992) stromatolite beds from the Spathian (Lower Triassic) Virgin Limestone Member of the Moenkopi Formation at Lost Cabin Spring, Nevada. Stromatolite-sponge-Tubiphytes constructions reported here represent the earliest known metazoan reef framework following the end-Permian mass extinction and the only non-microbial reef with significant relief above the seafloor, with the possible exception of 20cm tall late Spathian Placunopsis bivalve buildups, in the Early Triassic. Although Tubiphytes has been reported from the late Spathian in China, it was not thought to have played an important role in reef-building until the Middle Triassic (Payne et al., 2006) and the Lost Cabin bioherm is the only documented example of Tubiphytes binding a Lower Triassic reef.

The fossils occur between and in the cores of the stromatolite heads that make up the majority of the 1.5m tall reefs. Sponges consist of cylindrical, sub-rounded branches, varying in diameter from 1-2 cm in vertical cross-section. Porous sponge walls are 1-2mm thick, contain calcitized spicules and enclose large interior cavities filled with micrite. Thin (0.25mm) concentric sub-circular rinds resembling Tubiphytes encrust the sponges. Placunopsis bivalve biocoenoses nestle in depressions between stromatolite heads, suggesting that the reefs created hard substrate habitats allowing attachment of the cementing bivalves.

Crinoid packstones occur below and between the reefs, suggesting that the reefs formed in a shallow, high-energy environment; the reefs may have thus acted to shelter animals from currents. The presence of metazoan framework builders, binders, and associated dwellers indicates that the reefs were more ecologically complex than any constructions previously reported from the Lower Triassic, and their occurrence in shallow facies supports the hypothesis that biotic recovery began in shallow marine environments (Fraiser and Bottjer, 2007; Beatty et al., 2008).