PALEOLANDSCAPES IN FLORISSANT FOSSIL BEDS NATIONAL MONUMENT (LATE EOCENE), 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. 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 a volcano west of modern-day Florissant, burying the ancient stream valley. This event preserved in situ petrified stumps of Sequoia affinis that mark the paleotopography of the area. Five paleosol profiles within the lower mudstone unit at Florissant Fossil Beds National Monument (FLFO) were trenched, measured and sampled during the 2023 field season. Three distinct paleosols were identified: vertisols in the paleovalley, andic soils in floodplain deposits, and inceptisols in upland areas. X-ray diffraction (XRD) revealed high amounts of smectite, quartz, and potentially allophane, a common byproduct mineral of weathered volcanic material. Mineralogy of the profiles indicates weathering of primary volcanic minerals via hydrolysis, consistent with depositional history. The potential presence of allophane suggests a short duration of pedogenesis, a feature of andisols. This was observed in weakly developed soil profiles of upland and floodplain deposits of pumaceous mudstones. This suggests pedogenesis was constantly reset by the deposition of intermittent ashfalls/lahars. Within the paleovalley, paleosols were more developed, displaying slickensided structures indicative of in situ periodic wetting and drying resulting in expansion and contraction of smectitic soil and formation of vertic features. Hand samples from paleovalley profiles contained abundant rooting structures and plant debris, suggesting it was heavily forested. Geochemical analysis of profiles and thin section petrography will be used to further classify FLFO paleosols and determine paleoclimate conditions of Eocene Florissant Fossil Beds National Monument.