PERMIAN PALEOSOLS OF THE LYKINS FORMATION, COLORADO: FIRST INSIGHTS INTO THE PALEOENVIRONMENT OF AN ERG-PROXIMAL EPICONTINENTAL REDBED SUCCESSION

The Permian-Triassic Lykins Formation of Colorado contains interfingering marine and continental facies. The lower part of the 300+ meter succession contains stromatolitic carbonate, gypsum-anhydrite, and rare black shale and chert. The dominant portion of the Lykins Fm. is characterized by redbeds, and despite intensive work in outcrop, there is little diagnostic evidence for a continental vs. marine origin. Here we report on three, likely contemporaneous, paleosols in the basal 10 m (Kungurian-Roadian; Lower-Middle Permian) of the Lykins Formation found in three cores from Ft. Collins, CO, which were described and evaluated for their macromorphologies and mineral compositions. Clay minerals are identified using XRD spectral analysis of <2.0 µm fractions prepared as oriented aggregates. Paleosols have oxidized clay-rich matrices (2.5YR4/6) with slickensides, wedge-shaped peds, mottles (G17/10Y), slightly calcareous matrices, and rare iron oxide nodules. These characteristics are associated with vertic and gleyed conditions which suggests distinct wet and dry periods to allow for redox, but also periods of waterlogging in the soil, respectively. This may indicate that the depth of the water table fluctuated due to seasonality, with wet periods of prolonged saturation due to high rainfall and subsequent increased groundwater recharge (i.e., episaturation). Preliminary XRD analysis indicates the presence of smectite, chlorite, illite, and kaolinite clay minerals. Moreover, one paleosol sample set contains peaks around 14Å, that is unaffected by heat treatments, which may be vermiculite with Al-hydroxy interlayers. This could be indicative of acidic soil environments which implies periods of high rainfall and some chemical leaching. These paleosols occur above interfingering eolian sandstones of the lower Permian Lyons Formation, and just below the middle Permian Blaine Gypsum member of the Lykins Formation. Thus, despite their discontinuity and low preservation potential, these paleosols offer insights into a redbed facies that was laterally adjacent to an erg and a vast salina and suggest an intermittently wet and warm tropical climate. Moreover, these results suggest that Permian tropical paleoclimate evolution may have been quite dynamic during the long-term trend toward hyperaridity.