PALEOPEDOLOGY AND GEOCHEMISTRY OF MORRISON FORMATION PALEOSOLS, BIGHORN BASIN, WYOMING: IMPLICATIONS FOR THE LATE JURASSIC-EARLY CRETACEOUS PALEOCLIMATE RECORD

The increased abundance of wetland-lacustrine deposits in the Upper Jurassic Morrison Formation has been used to infer a change to wetter climatic conditions at the end of Morrison time, particularly in the northern part of the Morrison basin. However, distributions of modern systems show that wetland-lacustrine deposits may occur in a wide range of climate regimes including tropical semiarid areas. Comparative paleopedological and multi-proxy geochemical analyses of well-drained, hydric, and hydromorphic Morrison paleosols in the Bighorn Basin, Wyoming provide the opportunity to test this hypothesis. In addition to traditional paleopedological and geochemical methods, clay mineralogy and isotopic analyses of pedogenic barite nodules are used to document the dominant paleoclimate signatures in 3 genetic Morrison successions (#1, #2, #3). Pedogenic carbonates in well-drained, composite floodplain paleosols in 1 indicate that seasonally drier conditions dominated early Morrison time. Mottled olive-gray paleosols with FeMn nodules developed adjacent to thick oolite deposits in # 2, which suggests increasing saturation times proximal to an alkaline-saline lake. A mosaic of palustrine-lacustrine and wetland deposits that developed as the lake basin began to fill dominate in #3. ä34S values of pedogenic barite from purple to greenish-gray gleyed horizons indicate that sulfate reduction occurred in imperfectly drained paleosols, whereas ä18O values are consistent with evaporative conditions. Comparative analyses of lacustrine, palustrine, and pedogenic carbonates show a predictable range of ä18O values for a relatively closed wetland-lacustrine system. Increased water alkalinity from altering airfall ash and sluggish drainage conditions resulted in precipitation of zeolites and length-slow quartz in alkaline marshes and fens. Gypsum pseudomorphs in laterally adjacent gleyed paleosols indicate that evapotranspiration was higher than effective precipitation during latest Morrison time. Combined pedological and geochemical analyses do not support a change to a wetter climate through time in spite of the development of a wetland-lacustrine system in the area. Instead, data suggest that semiarid conditions persisted throughout Morrison time in the Bighorn Basin.