PALEOENVIRONMENTAL AND PALEOCLIMATIC RECONSTRUCTION OF CRETACEOUS TERRESTRIAL FORMATIONS OF TEXAS AND OKLAHOMA USING PEDOGENIC MINERALS

The clay mineralogy, chemistry, and stable oxygen and hydrogen-isotope compositions from 11 phyllosilicate samples in paleosol profiles that span the Lower to Upper Cretaceous boundary in north-central Texas and southern Oklahoma have been analyzed as a potential source of information for Early to Late Cretaceous paleoenvironmental and paleoclimatic conditions. Samples consist of mineralogical mixtures of illite, smectite, and kaolinite. Samples from the Antlers Formation (Albian) in southern Oklahoma and the Woodbine Formation in Texas (Cenomanian) are dominated by kaolinite, while the remaining samples are dominated by smectite and illite. Chemical data are used to estimate paleoprecipitation for each locality using the CALMAG weathering index. Chemical and mineralogical data are used in conjunction with thermodynamic mineral models to calculate unique hydrogen and oxygen isotope fractionation factors for each naturally-occurring phyllosilicate mixture. Paleotemperature estimates are derived using measured δD and δ¹⁸O values and applying calculated fractionation factors to each sample. Paleoprecipitation estimates range from 268 to 1486 mm/yr with two marked increases in precipitation in the Oklahoma Antlers Formation (Albian) localities and Woodbine Formation (Cenomanian) localities. δD values of the phyllosilicate mixtures range from -50‰ to -67‰, whereas δ¹⁸O values of phyllosilicate mixtures range from 18.5‰ to 21.7‰. Assuming the phyllosilicates preserve a record of isotopic equilibrium with Cretaceous meteoric waters, the measured hydrogen and oxygen-isotope values correspond to phyllosilicate crystallization temperatures ranging from 26 ± 3°C to 31 ± 3°C. Cooler temperatures correspond to periods with markedly higher precipitation estimates which also correlate with the presence of extensive shallow seas upon the Texas craton.