MOLECULAR AND MINERALOGIC PERSPECTIVES ON THE PALEOBOTANY AND DEPOSITIONAL ENVIRONMENTS OF THE LATEST EOCENE FLORISSANT FORMATION AT CLARE’S QUARRY, TELLER COUNTY, COLORADO, USA

The latest Eocene Lagerstätte Florissant Formation (FF, 34.1 Ma) at Clare’s Quarry, Teller County, Colorado, contains a diverse flora and fauna preserved in lacustrine facies. In the late Eocene, during the deposition of the FF, the climate was warm temperate to subtropical and deciduous broadleaved plants and exotic tall conifers were present. This study uses molecular fossils and indices derived from lipid wax n-alkane distributions to reveal the paleobotany and paleoenvironment. Using a set of 84 samples several types of data were gathered: the carbon preference index (CPI) to signify higher odd over even n-alkane abundances, P_aq index (the proxy for measuring types of terrestrial species), Pristane/Phytane (Pr/Ph) ratio to reveal the paleoredox (< 1 anoxic and >1 oxic), average chain length (ACL) to show varying phylogeny, climate and biome, and terrestrial to aquatic ratios (TAR) used to differentiate aquatic (algal and/or bacterial) organic matter. This study also examines 10 FF sediments using X-ray diffraction (XRD), X-ray fluorescence (XRF), petrography, and Cathodoluminescence (CL) to constrain the depositional environment of the ancient lake.

The CPI data from n-alkanes range from 2.2 to 4.5 which indicates that sediments are less likely to be influenced by post-depositional alteration. The P_aq index range from 0.21 to 0.39, implies the dominating submerged/floating aquatic macrophytes relative to emergent and terrestrial species. Pr/Ph ratios range from 0.55 to 1.2 constrains the paleoenvironment to primarily reducing and/anoxic. The ACL range of 26.1 to 30.74 is consistent with angiosperms. TAR shows there were very little bacteria or aquatic input to make short-chain n-Alkanes.

XRD data of a representative sample establishes the presence of 47% quartz, 24% montmorillonite and illite, 11% sanidine, 9% albite, and 9% other minerals. XRF bulk composition is 68% SiO₂, 17% Al₂O₃, 5.9% Fe₂O₃, 2.5% MgO, 1.9% K₂O, and 1.8% CaO. Volcanic lithic fragments were identified from the petrographic study. CL imaging shows the presence of calcite suggesting calm depositional environment. Periods of stable lake sedimentation were frequently interrupted by rapid influxes of suspended fine clays, which propagated into the lake at intervals of high runoff triggered by climatic, volcanic, or tectonic events.