2003 Seattle Annual Meeting (November 2–5, 2003)

Paper No. 7
Presentation Time: 1:30 PM-5:30 PM


SHUNK, Aaron J., DRIESE, Steven G., UHLE, Maria E., MORA, Claudia I. and CLARK, G. Michael, Earth and Planetary Sciences, Univ of Tennessee, Knoxville, TN 37996-1410, ashunk@utk.edu

Terrestrial records of Neogene climate are rare from the eastern half of North America. The recently discovered Gray Fossil Site (GFS) may be a sinkhole lake fill. Well-preserved vertebrate GFS fossils strongly suggest a Late Miocene-Early Pliocene (Hemphillian) Land Mammal Age (> 4.5-6.1 Ma). Three distinct facies occur within the upper 20 m of GFS lacustrine sediment and may indicate abrupt climate change. The graded facies is characterized by continuous succession of individual graded beds that average 0.8 cm thick. This facies contains < 1% total organic carbon (TOC), has d13C isotope values averaging –25.4‰ (PDB) with excursions as positive as –24.3‰, and is interpreted to represent deposition from sporadic storm events in a water-stressed environment. The laminated facies overlies the graded facies. The transition between the two facies is marked by the development of a depositional pattern that eventually grades into the laminated facies, which is characterized by a rapid increase in % TOC, and the steady decrease of d13C values to those typical of the laminated facies. The laminated facies consists of laterally continuous couplets that contain statistically significant 5-couplet periodicity; the couplets are therefore interpreted as annual varves, with variable thickness related to ENSO cycles. Couplets average 0.4-0.6 cm thick and each consists of a thin, coarse-grained, and organic-rich terrigenous (A) lamina added to the apparently continuous deposition of silty-clay terrigenous matrix that comprises the (B) lamina. The laminated facies averages about 8% TOC with average d13C values of –29.3‰, and is interpreted to represent a wetter overall climate with a distinct high-energy wet season. Using the varve couplets at the base of the laminated facies to establish a sedimentation rate (which likely underestimates the sedimentation rate at the top of the graded facies), the majority of the facies change that records climate change may have been deposited in less than 500 years. A capping subaerial facies records soil formation and filling of the sinkhole lake. The climate shift documented at the GFS may have been caused by a eustatic rise documented at 5 Ma, which caused brief upwelling along the Atlantic coast of North Carolina, and possibly caused a shift from a drier climate towards a wetter monsoon climate.