PALEOSOLS AS CLIMATE INDICATORS FOR DINOSAUR SITES, BRUSHY BASIN MEMBER OF THE JURASSIC MORRISON FORMATION, EAST-CENTRAL UTAH

Three sites in the Jurassic Morrison Formation near Ferron, Utah have produced over one hundred Allosaurus teeth, thousands of individual bones, and dinosaur trace fossils such as prints, trackways, and coprolites. As part of interpreting the depositional environment and taphonomy of the dinosaur fossils, one question was the aridity of the paleoclimate during the Late Jurassic in this particular fossil rich area. Paleosols (fossil soils) form in response to climatic conditions and can archive paleoclimatic information. This study was based on field descriptions, petrography, analysis of polished slabs, photomosaics, and X-ray diffraction analyses of clay minerals. Analysis of paleosols found both humid region and arid region indicators. Arid region indicators included calcisols with stage I-III calcretes and rhizoliths. Other paleosol features suggest humid climate, such as histosols and gleysols with mottling, drab-halo rootcasts, and blocky ped structures. Collectively, the paleosols suggest both fluctuating water tables, and alternating aridity and sufficiently humid to accumulate coal. Possibly the soil humidity is due to fluvial overbank flooding and waterlogged soil conditions in an overall arid paleoclimate. Interbedded, thin tufa deposits indicate groundwater springs or seeps on ancient hillsides. Finally, the better developed calcisols tend to go with intervals of multistory fluvial channels, suggesting better paleosol development is linked to slower subsidence rates.