Paper No. 13
Presentation Time: 11:00 AM
REFINING THE TIMING OF EARLY TRIASSIC BIOTIC AND ENVIRONMENTAL EVENTS: RESULTS FROM THE MOENKOPI FORMATION OF NEVADA
MARENCO, Pedro J.1, GRIFFIN, Julie M.
2, LEPES, Erin M.
1, CLAPHAM, Matthew E.
3 and FRAISER, Margaret L.
4, (1)Department of Geology, Bryn Mawr College, Bryn Mawr, PA 19010, (2)Department of Geology, University of CA- Davis, Davis, CA 95616, (3)Department of Earth and Planetary Sciences, University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, (4)Department of Geosciences, University of Wisconsin-Milwaukee, 3209 N. Maryland Ave, Milwaukee, WI 53211, pmarenco@brynmawr.edu
The Early Triassic was characterized by a delayed biotic recovery from the End Permian mass extinction and unusual rock features interpreted as evidence for prolonged environmental anomalies. The Lower Triassic of the western United States is an important natural laboratory for the investigation of biologic and sedimentary processes in the wake of the End Permian mass extinction. In particular, the Spathian of California and Nevada have revealed a biologic and environmental dichotomy: distal localities and deeper facies in proximal localities tend to exhibit low diversity faunas and unusual rock facies such as stromatolite mounds and inorganic precipitates (Woods et al., 1999; Pruss and Bottjer, 2004), whereas shallower facies tend to exhibit higher faunal diversity and a lack of unusual lithologic structures (Fraiser and Bottjer, 2007; Beatty et al., 2008). The Spathian represents approximately 3 of the Early Triassic’s 5 million years (Ovtcharova et al., 2006), and yet many of the western US Spathian localities used to study the aftermath of the End Permian mass extinction have had insufficient biostratigraphic study to properly constrain the timing of Spathian events.
Initial results from a new conodont and chemostratigraphic study of the Lower Triassic of the western US suggest a reevaluation of the timing, nature and distribution of microbialite facies from the Virgin Limestone Member of the Moenkopi Formation at Lost Cabin Spring, Nevada. Stromatolite mounds from the upper of two previously-documented stromatolite-bearing units (Schubert and Bottjer, 1992) are more complex than previously reported and are better characterized as stromatolite-sponge-Tubiphytes reefs (Griffin et al., this conference). The lower stromatolite unit of Schubert and Bottjer (1992) is actually up to three distinct stratigraphic units containing stromatolite and thrombolite bioherms. The lower and upper stromatolite units can be correlated to more proximal localities in south-eastern Nevada, suggesting that stromatolitic build-ups may have covered much more of the shelf than previously thought. Ongoing conodont biostratigraphy will further elucidate the timing of unusual rock features and the environmental factors that facilitated biotic recovery.
