PALEOCLIMATE OF THE UPPER JURASSIC MORRISON FORMATION IN OKLAHOMA AND TEXAS

Observations and analyses of paleosols in the Upper Jurassic Morrison Formation in western Oklahoma and the Texas Panhandle indicate relatively dry climatic conditions with no evidence of pronounced rainfall seasonality. Samples were collected from a locality east of Kenton, OK, and from a site in the Rita Blanca National Grassland, north of Dalhart, TX. Although currently available evidence for surface exposures of Upper Jurassic terrestrial strata in Texas is ambiguous, detrital zircons isolated from a fluvial sandstone at the site near Dalhart are consistent with a Late Jurassic (Tithonian) age. Calcisols and poorly developed Protosols are common in the Morrison Formation at both the Oklahoma and Texas localities, and vertic features are lacking. Average estimates of soil CO₂ concentration derived from stable carbon isotope analysis of pedogenic calcite and associated occluded organic matter are 3000 ppmV for the Oklahoma locality and 2500 ppmV for the Texas site. Other Upper Jurassic localities exhibit a positive correlation between estimates of soil CO₂ concentration and paleoprecipitation, so forthcoming elemental analyses of B-horizon matrix samples are expected to show higher annual rainfall values for the Oklahoma site relative to the Texas locality. Stable hydrogen and oxygen isotope values of pedogenic phyllosilicates from both sites correspond to temperatures of formation ranging from 40°C to 50°C, which indicate unexpectedly high surface temperatures compared to other Late Jurassic paleotemperature estimates derived from both geochemical proxies and climate models. Ongoing clumped isotope analyses of pedogenic calcite will help determine if the high temperatures indicated by the phyllosilicate results reflect near-surface conditions or not.