PALEOSOLS OF FLORISSANT FOSSIL BEDS NATIONAL MONUMENT, CENTRAL COLORADO

The Late Eocene Florissant Formation (ca. 34.9 ma) of central Colorado is predominantly composed of lacustrine shale and mudstone with minor channel sandstone and floodplain mudstone. The Florissant Formation records the last phase of the Eocene greenhouse before transitioning to the Oligocene icehouse in North America. The two million year history of lacustrine deposition was intermittently disrupted by deposition of lahars and volcanic airfall from nearby volcanoes. The lower mudstone unit is capped by a lahar originating from the Guffey caldera, southwest of modern-day Florissant, that buried an ancient stream valley. This event preserved in situ Sequoia affinis petrified stumps that mark the paleotopography of the area. Eight samples collected from foundation anchor holes for the shelter construction project for three stumps at Florissant Fossil Beds National Monument (FLFO) in 2001 were found to have evidence of paleosols. The samples preserved carbonized rootlets and slickensided structures indicative of in situ periodic wetting and drying resulting in expansion and contraction of ancient soil material. Petrographic thin sections revealed soil fabrics. X-ray diffraction detected the high amounts of smectite, indicating weathering of volcanic minerals via hydration, consistent with the depositional history of FLFO. Further analyses (fieldwork, petrography, geochemistry) will be used to classify FLFO paleosols and compare them to modern soils that host Sequoia sempervirens, the last extant Sequoia species, in northern California to investigate the evolutionary responses to climate change.