DISCOVERY OF THE CARBON ISOTOPE EXCURSION (CIE) AT THE PALEOCENE-EOCENE BOUNDARY IN THE PICEANCE CREEK BASIN, WESTERN COLORADO

At the Paleocene-Eocene boundary, a rapid warming event called the Paleocene-Eocene Thermal Maximum (PETM) has been identified in strata by a negative carbon isotopic excursion (CIE). Here I report on the location of the CIE in the Piceance Creek Basin on the western side of the continental divide. The Piceance Creek Basin encompasses 26,000 km2 of rugged topography bounded by the Grand Hogback on the east and the Douglas Pass Arch to the west. The basin preserves a thick sequence of Cretaceous, Paleocene and Eocene rocks. Based on fossil mammals and gastropods, I constrained the Paleocene-Eocene boundary interval to about 50-100 meters. I measured four stratigraphic sections across this interval and analyzed 87 rock samples for carbon isotopic ratios. Analyzed carbon isotopic signatures, thus far, indicate that the CIE is identifiable only within the axis of the basin in the Horse Thief Canyon section where the interval abruptly becomes dominated by sandstones. Coined the Molina Member of the Wasatch Formation, the lithology is characterized by fine-grained, sheet-like sandstones interbedded with infrequent mudstone and claystone units lacking soil development. Within the Molina Member, the CIE is constrained to the basal ten meters. Sedimentary structures across the CIE interval include horizontal planar bedding and medium scaled transverse bar cross-beds, indicative of a fluvial environment. The absence of the CIE in the Corcoran Peak section at margin of the basin is interpreted as a paraconformity, resulting from either lack of deposition or erosion of sediments. Based upon comparison with modern eolian environments, previous investigators have suggested an eolian source for the influx of sand during the deposition of the Molina Member. Others have suggested intensive flooding. Because the onset of deposition of the Molina Member includes the CIE at the Paleocene-Eocene boundary, climate was definitely an important contributing factor in this abrupt change of lithology from mudstone to sandstone dominated facies. Vertebrate fossils are absent in the CIE interval, but preliminary study of the fossil mammals immediately before and after the event indicates a pronounced localized extinction event at the Paleocene-Eocene boundary.