FLOODPLAIN PALEOSOLS IN THE UPPER CRETACEOUS WAHWEAP FORMATION, KAIPAROWITS BASIN, SOUTHERN UTAH

Paleosols are important paleoclimatic indicators of ancient environments. A paleosol was identified in each of the upper and capping sandstone members of the Late Cretaceous Wahweap Formation in Grand Staircase Escalante National Monument, southern Utah. The members consist of fluvial sediments that are comprised primarily of sandstone along with few preserved floodplain mudstones. The paleosols were identified in such mudstones on the basis of: 1) erosionally truncated upper surfaces that contains numerous root traces, indicating the surface was able to support plant life; 2) vertical color changes and a carbonate nodular concretion layer, suggesting horizonation; and 3) a subangular blocky texture, interpreted as soil structures. The capping sandstone paleosol is 1.3 m thick and the upper member paleosol is 1.5 m thick, and each of which can be traced laterally for a few tens of meters. A high-resolution image of the capping sandstone site can be viewed at www.gigapan.org (keyword: wahweap paleosol). Samples from the paleosols were analyzed using x-ray diffraction and x-ray fluorescence methods to determine the mineralogy and geochemistry of the paleosol profiles. Grain size, carbonate content, and color were also determined.

The paleosols are comprised mostly of silty kaolinite mudstone. Grain size is fairly consistent down the profile, as is clay mineralogy. The upper member paleosol grades from light gray (10YR7/1) to gray (5YR6/1) to reddish brown (5YR5/2). The capping sandstone paleosol is light gray (10YR7/1) in the first 10cm, and rapidly grades to pink (5YR 7/3), then into pale red (2.5YR 6/2) at 40cm from the top. Carbonate content increases with depth. In the capping sandstone paleosol, carbonate nodules are located 90 cm below the top, and are composed of siderite (possibly some rhodochrosite). The presence of siderite nodules indicates pedogenesis under neutral to basic, water-saturated conditions indicating localized groundwater sustained wetlands in this severe monsoonal setting.