Paper No. 2
Presentation Time: 9:15 AM
REINTERPRETING THE STRATIGRAPHY OF THE FLORISSANT FOSSIL BEDS NATIONAL MONUMENT THROUGH CORRELATION BY MAGNETIC SUSCEPTIBILITY AND GEOCHEMICAL COMPARISON
PROTHRO, Lindsay O., Department of Earth Science, Rice University, 6100 Main Street, Houston, TX 77005, ELLWOOD, Brooks B., Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803, MEYER, Herb, National Park Service - Florissant Fossil Beds National Monument, P.O. Box 185, 15807 Teller County Road 1, Florissant, CO 80816, RATCLIFFE, Kenneth T., Chemostrat Inc, 6700 Portwest Drive, Houston, TX 77024, TOMKIN, Jonathan H., School of Earth, Society, and Environment, University of Illinois, Urbana, IL 61801 and ABBOTT, Lon D., Geological Sciences, University of Colorado, UCB 399, Boulder, CO 80309, lp15@rice.edu
In the summer of 2012, samples were collected for stratigraphic analysis from the Late Eocene lacustrine Florissant Formation at Florissant Fossil Beds National Monument, Colorado. The formation has informally been divided into six units—the lower shale, lower mudstone, middle shale, caprock conglomerate, upper shale, and upper pumice conglomerate. Because of the discontinuous and isolated nature of many of the volcanic mudstone and shale exposures at the Monument, lithostratigraphic correlation has been difficult for previous researchers. The primary objective of this study was to characterize and interpret the stratigraphy based on correlation using magnetic susceptibility (χ) and geochemical data. Thermomagnetic susceptibility (κ) also proved to be a useful correlation aid.
The two sections that were analyzed displayed remarkable similarities and were easily correlated due to the high-resolution sampling in this study. They correlated especially well through the middle shale unit; however, the caprock conglomerate that appeared in one section was virtually absent from the other, but its stratigraphic location is suggested by magnetic susceptibility data. Both sections include a portion of the upper shale, but not the complete sequence.
This study also presents two scenarios supported by time-series for depositional duration and sedimentation rate for the lake beds. The precise stratigraphy that has been produced will be useful for all future studies of the Florissant Formation, and due to its temporal proximity to the Eocene-Oligocene boundary, may even be beneficial to understanding terrestrial response to climate change.
