2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 225-8
Presentation Time: 9:00 AM-6:30 PM

GEOCHEMICAL AND SEDIMENTOLOGICAL INVESTIGATION OF A LACUSTRINE TRIASSIC LAGERST√ĄTTE IN THE COW BRANCH FORMATION OF THE DANVILLE-DAN RIVER BASIN


RITZER, Samantha R. and GILL, Benjamin C., Department of Geosciences, Virginia Polytechnic Institute and State University, 4044 Derring Hall, Blacksburg, VA 24061, sritzer@vt.edu

The North American Atlantic passive margin contains a number of early Mesozoic lacustrine basins that partially comprise the Newark Supergroup. These lacustrine basins record cycles of sedimentary facies attributed to Milankovitch forcing, making them an excellent archive of climatic and environmental change. Additionally, the organic-rich facies within these basins make them potential targets for hydrocarbon production. Of these, the Triassic Danville-Dan River basin (DDRB) is of specific interest because of its well-preserved cyclicity and wide range of fossil flora and fauna, including the Triassic Lagerstätte, a 3 cm thick bed of perfectly preserved soft-bodied insects. To date, a depositional model that integrates all of these features has yet to be developed.

Our research aims to investigate the sedimentology and geochemistry through one Milankovitch cycle within the DDRB to better understand the chemical and physical dynamics of the depositional environment. Specifically, we will test whether a chemocline was present, and if so, its behavior relative to the sedimentary cycle(s). Iron speciation, total organic carbon (TOC), and d13Corg analyses were completed on 11 meters of core material, taken from the Cow Branch formation interval at the Virginia Solite Quarry.

TOC values are highest (up to 3.52 wt%) near the middle of the cycle, where facies appear darkest. Unexpectedly, the Lagerstätte beds exhibited fairly average TOC values (1.4 wt%) with respect to the cycle as a whole. Higher TOC values were associated with isotopically lighter d13Corg values (more negative). Preliminary iron speciation data shows that throughout the core, reducible iron oxides and magnetite values are low, but samples contain between 1-2 wt% carbonate iron. Total iron and pyrite iron will be compared to all values and used to determine temporal chemocline dynamics. These data will then be integrated with sedimentological features to develop a clearer model of the depositional environment of the paleolake through time. This model will be beneficial for future work in any of the east coast Mesozoic lacustrine basins both for industry and the scientific community.