PALEOENVIRONMENT RECONSTRUCTION AND GEOCHRONOLOGY OF THE LOWER AND MIDDLE MEMBERS OF THE DEVIL'S GRAVEYARD FORMATION (TORNILLO BASIN, TEXAS)

The volcaniclastic sediments of the middle and lower members of the Devil's Graveyard Formation preserve a number of important Middle Eocene fossil localities, but research in this area has been hampered by uncertain paleoenvironmental setting and conflicting magnetostratigraphic interpretations. Our study clarifies the paleopedology and geochronology of the Devil's Graveyard Formation.

The informally divided lower and middle members of the Devil's Graveyard Formation are characterized by interbedded green, white, and red layers that reflect pulses of volcanic deposition. During periods of volcanic quiescence, forest-dominated soils formed. These paleosols were not previously distinguished from the many tuffaceous sandstones of the region, but are characterized by abundant calcified roots, insect casings, calcareous layers, and drab haloed root traces. The paleosols of the lower and middle members of the Devil's Graveyard Fm range from immature white-green horizons (andisols and inceptisols) to more mature red horizons (alfisols). Our XRD analysis revealed a basal d-spacing consistent with smectite clays in all soils regardless of degree of development, indicating highly seasonal rain. These new paleoclimatic data will aid paleoecological reconstructions.

The temporal framework of these taxa within the Devil's Graveyard Formation is newly constrained by 243 zircon U-Pb ages collected from 5 tuffaceous sandstones and analyzed via LA-ICP-MS. The U-Pb distributions indicate only minor reworking and inheritance from older tuffs. Using this method, dates are constrained with an accuracy of ~750,000 years. Weighted means of the youngest coherent population were calculated to minimize effects of inheritance and lead loss. Our new calculated ages of the lower and middle members of the Devil’s Graveyard Formation span most of the Uintan, from 45.39 (+0.21, -0.59) Ma to 42.23 (+0.67, -0.38) Ma. The abundance of well-constrained zircons through the section combined with the numerous fossil localities make the lower and middle members of the Devil’s Graveyard an excellent place to test deep-time evolutionary turnover at an unusually fine timescale. Furthermore, these new ages can also aid in the correlation of bio-, magneto- and chronostratigraphy of the Middle Eocene across North America.