Stephen I. Dworkin

Eustatic Control on Alluvial Sequence Stratigraphy: A Possible Example from the Cretaceous-Tertiary Transition of the Tornillo Basin, Big Bend National Park, West Texas, U.S.A.

Paleosol-bearing alluvial strata of latest Cretaceous and earliest Tertiary age are continuously exposed along Dawson Creek, in Big Bend National Park, west Texas, U.S.A., and exhibit a three-tier hierarchy of depositional cyclicity. Meter-scale, fluvial aggradational cycles (FACs) occur as fining-upward successions that are gradation- ally overlain by paleosols or are sharply overlain by the coarser- grained base of the succeeding FAC without an intervening paleosol. FACs stack into decameter-scale, fluvial aggradational cycle sets (FAC sets) that also fine upward, and from base to top contain either a grad- ual upsection increase in soil maturity and soil drainage or a somewhat symmetrical pattern of increasing and decreasing paleosol maturity. Longer-period trends of FAC thickness, lithologic proportions, paleosol maturity, and paleosol drainage indicate that two complete, and two partial, hectometer-scale fluvial sequences occur within the study in- terval. From base to top, each sequence is characterized by an asym- metric increase and decrease in FAC thickness, a decrease in the pro- portion of sand-prone fluvial facies, an increase in paleosol maturity, and better paleosol drainage. Whereas FACs and FAC sets are interpreted to record cyclic epi- sodes of channel avulsion and stability, and longer-term avulsive chan- nel drift within the alluvial valley, respectively, fluvial sequences may coincide with third-order sea-level changes within the North American Western Interior Seaway. As such, the Cretaceous-Tertiary (K-T) transition within the Tornillo Basin may provide an example of me- gascale stratigraphic cyclicity that is controlled by eustatic sea level within a fully fluvial succession. Thickening and thinning successions of FACs record a third-order period of accelerating (transgressive- equivalent) and decelerating (highstand-equivalent) base-level rise, and subsequent base-level fall (falling stage- to lowstand-equivalent). Se- quence boundaries are placed at the sharp inflection between thinning and thickening FACs. Sand-prone facies and immature, more poorly- drained paleosols are associated with the transgressive-equivalent por- tion of each sequence, and mudrock-dominated overbank facies and their associated mature, well-drained paleosols are associated with the highstand- and falling stage-equivalent.

Petrographic and Geochemical Constraints on the Formation and Diagenesis of Anhydrite Cements, Smackover Sandstones, Gulf of Mexico

ABSTRACT Anhydrite cement is common in sedimentary rocks, yet its origin is poorly understood. The high concentration of sulfate in sea water and the lack of appreciable sulfate in most other natural waters suggests that anhydrite cement may have a marine origin, but the relatively late timing of most anhydride cement tends to preclude sea water as the source of sulfate. Anhydrite cement is present in Upper Jurassic sandstones in the Gulf of Mexico as poikilotopic masses in which detrital grains are replaced and as smaller patches that have replaced single detrital grains. Anhydrite is a relatively late cement and postdates all other volumetrically significant authigenic phases, including K-feldspar, dolomite, quartz, and most calcite cements. The expected isotopic composition and trace-element concentration of marine-derived Late Jurassic anhydrite cement is predicted based on analyses of bedded anhydrite and on analyses found in the literature. The expected chemical signature is: 34S ( CDT) = +16, 18O ( SMOW) = +14, 87Sr/86Sr = 0.7069, Sr = 1500-2500 ppm, Ba 200 ppm. When geochemical analyses of Smackover anhydrite cements are compared to the predicted composition, it is apparent that most of the cement is not of marine origin. Two generations of anhydrite cement have been identified in the East Texas basin on the basis of their strontium isotopic compositions and their strontium concentrations. An early cement may have been derived from slightly modified Late Jurassic sea water, whereas a second group of cements may have precipitated later in the burial history or may represent recrystallization of the first cement. The chemistry of Louisiana and Mississippi basin anhydrite cements indicates that the sulfate was derived from dissolved bedded anhydrite and was reprecipitated in the sandstones from fluids that had undergone extensive water-rock interaction after considerable burial. This study suggests that late anhydrite cements in sedimentary rocks are most likely derived from remobilized calcium sulfate de osits.

Ecosystem response to soil biogeochemical behavior during the Late Cretaceous and early Paleocene within the western interior of North America

Paleosol properties are routinely characterized by whole-rock geochemistry, compromising the interpretation of important biogeochemical information in deep time. As a consequence, we employ a new pedotransfer function approach to the characterization of paleosols and apply this methodology to Late Cretaceous (Campanian and Maastrichtian) and early Paleocene (Danian) landscapes from the Dawson Creek study area of the western interior to: (1) reconstruct collodially based physical and chemical soil properties, and (2) assess climate and soil biogeochemical controls on evolving terrestrial ecosystems. Nine paleoseries (i.e., pedotypes) characterize the range of soil properties within the fluvial stratigraphic succession, which includes Entisols, Inceptisols, and Vertisols. All soils had optimal water-holding potential as inferred from low bulk densities, whereas poorly drained and colonizing landscapes likely suffered from poor aeration during seasonal water logging. Even with high water-holding capacity, Maastrichtian soils experienced seasonal moisture stress because of lower rainfall than Campanian and Danian soils. Fertility levels were sufficient for the growth of most plants judging from high cation exchange capacity and base saturation, negligible aluminum toxicities because of nonacid pH, and limited salinity and sodicity from relatively low exchangeable sodium and soluble salts in solution. Unlike warm-temperate and forested paleosols with neutral pH from the Campanian and Danian, subtropical and alkaline paleosols from the Maastrichtian apparently supported a woodland plant formation adapted to low availability of iron and manganese, which were fixed with calcium in carbonate, and low availability of phosphorous because it formed insoluble compounds with iron and manganese. Carbon, nitrogen, phosphorous, and sulfur cycling through microbially mediated mineralization of soil organic matter was limited from low litter inputs in both early and midsuccessional ecosystems, particularly in woodland soils. Results do not reveal demonstrable changes in soil characteristics through the K-T transition.

DETERMINING FLOODPLAIN PLANT DISTRIBUTIONS AND POPULATIONS USING PALEOPEDOLOGY AND FOSSIL ROOT TRACES: UPPER TRIASSIC SONSELA MEMBER OF THE CHINLE FORMATION AT PETRIFIED FOREST NATIONAL PARK, ARIZONA

ABSTRACT The Upper Triassic Sonsela Member of the Chinle Formation is an alluvial succession containing interbedded sandstone and pedogenically modified mudstone. Despite preservation of silicified logs within channel sandstone beds, the Sonsela plant ecosystem is less understood than other intervals due to decreased preservation of nonconifer plant taxa. Sonsela paleosols and rhizoliths are evaluated using macromorphology, micromorphology, and geochemistry to determine the spatial distribution of paleosol characteristics and plant sizes and densities across the study area. Three pedotypes identified within the Sonsela are classified as Inceptisols and Vertisols that exhibit fining of matrix textures (from clayey siltstone to claystone) and reduced drainage with distance from the paleochannel. Overall, Sonsela paleosols are immature, suggesting that the Sonsela fluvial system experienced high rates of lateral migration and cannibalization of overbank sediments in a low-subsidence regime. Rhizohalos within the Sonsela Member are likely diagenetic and commonly include silicified roots (silica root petrifactions). Silicified roots provide information on root size and density that is not commonly afforded by other rhizolith types. Diagenetic rhizohalo diameters may be controlled by paleosol matrix textures within the Sonsela Member. Rhizolith characteristics suggest that channel-proximal paleosols contained only small-stature plants while distal floodplain paleosols may have hosted both small-stature and arborescent plants. Paleosols within the Sonsela Member do not contain rhizoliths whose size or abundance are reflective of a dense coniferous forest. Floodplain plants were commonly small of stature and immature, unable to evolve into more mature communities due to high rates of floodplain cannibalization during fluvial migration.

Origin of organic matter in the Eagle Ford Formation

ABSTRACTThe Upper Cretaceous Eagle Ford Formation is an organic-rich mudrock of economic significance for oil and gas exploration. In order to facilitate a better understanding of paleoceanographic conditions during Eagle Ford deposition, this study integrates the isotope chemistry of bulk organic matter with inorganic geochemical data. Measurements of total organic carbon (TOC), total N, δ13Corg, δ15N and inorganic major, and trace elements were taken from 166 Eagle Ford and Pepper Formation outcrop samples from McLennan County, central Texas. These data reveal the chemostratigraphic character and the evolution of Cretaceous seawater chemistry on the Texas shelf and allowed the identification of six distinct chemofacies that are useful for correlation purposes. Based on these data, changing paleoredox conditions were documented ranging from normal marine (oxic) conditions associated with the Pepper Formation, anoxic conditions associated with the Lower Eagle Ford Formation, suboxic conditions associated ...