Richard A. Denne

Association of bathyal foraminifera with water masses in the northwestern Gulf of Mexico

Abstract Q-mode cluster analysis and R-mode factor analysis of benthic foraminiferal data from 120 core tops from the northwestern Gulf of Mexico (87–1361 m; 26.4–29.0°N; 88.5–96.4°W) distinguish separate thanatotopes and faunal assemblages related to the five water masses present within the depth range of the samples. These water masses are the Surface Mixed Layer (SML), Gulf Water (GW), Oxygen Minimum Water (OMW), Subantarctic Intermediate Water (SAIW) and Caribbean Midwater (CMW). Reussella atlantica, Bigenerina irregularis and Angulogenerina bella are particularly associated with SML, Bulimina spicata, Planulina foveolata and Cibicidoides umbonatus with GW, Sphaeroidina bulloides, Bulimina mexicana and Siphonina bradyana with OMW, Epistominella exigua with SAIW and Gyroidinoides laevis, Cibicides wuellerstorfi, Pullenia subsphaerica, Bulimina aculeata, Gyroidinoides polius and Laticarinina pauperata with CMW. Cluster analysis also indicates the presence of three faunal groups related to water depth and bottom-water oxygen content within the OMW assemblage. An additional assemblage recognized in the Mississippi River Delta area may be related to low-oxygen conditions above the normal depth for the oxygen minimum. The close correspondence between the results of cluster and factor analyses suggests that similar numerical analyses of fossil data should be useful in deciphering ancient bathyal assemblages influenced by water-mass properties.

Biostratigraphic and Geochemical Constraints on the Stratigraphy and Depositional Environments of the Eagle Ford and Woodbine Groups of Texas

Abstract The 130-year history of study of the Cenomanian–Turonian Eagle Ford and Woodbine Groups of Texas has created a complicated and often confusing nomenclature system. Deciphering these nomenclatures has frequently been hindered by outdated biostratigraphic studies with inaccurate age interpretations. To resolve these issues, a comprehensive compilation and vetting of available biostratigraphic, geochemical, and lithologic data from Eagle Ford and Woodbine outcrops and subsurface penetrations was undertaken, which was then tied to a large network of wells in both south and east Texas. Composite sections were built for four outcrop areas of central and north Texas (Dallas, Red River, Waco, Austin), five outcrop areas from west Texas (Langtry, Del Rio, Big Bend, Chispa Summit, Quitman Mountains), four subsurface areas from south Texas (Webb County, Atascosa County, Karnes County, DeWitt/Gonzales Counties), and two cross sections from the east Texas subsurface (basin center and eastern margin). The resulting datasets were utilized to construct age models and characterize depositional environments, including paleoceanography. In agreement with previous studies, the total organic carbon (TOC)-rich Lower Eagle Ford was interpreted to have been deposited under anoxic to euxinic conditions and the Upper Eagle Ford under dysoxic to anoxic conditions. The Oceanic Anoxic Event 2 (OAE2) interval is missing at all locations north of Atascosa County; when present it is characterized as having been deposited under oxic to suboxic conditions. High abundances of radiolaria and calcispheres identified within recrystallized medial to distal limestones of the Lower Eagle Ford indicated limestone formation during periods of enhanced water-column mixing and increased primary productivity, in contrast to proximal limestones composed of planktonic foraminifera and inoceramid prisms concentrated by bottom currents. Standardized nomenclature systems and age models are proposed for each of the outcrop and subsurface areas. Proposed changes to existing nomenclatures include reassignment of the Tarrant Formation of the Eagle Ford to the Lewisville Formation of the Woodbine in the Dallas area and the Templeton Member of the Lewisville Formation to the Britton Formation of the Eagle Ford in the Red River area. The proposed term “Waller Member” of Fairbanks (2012) for the former Cloice Member of the Lake Waco Formation in the Austin area is recognized with a new stratotype proposed and described, although the Waller Member is transferred to the Pepper Shale Formation of the Woodbine. The Terrell Member is proposed for the carbonate-rich section at the base of the Boquillas Formation in the Langtry and Del Rio areas, restricting the Lozier Canyon Member to the organic-rich rocks underlying the Antonio Creek Member. The south Texas subsurface is divided into the Upper Eagle Ford and Lower Eagle Ford Formations, with the clay-rich Maness Shale Member at the base of the Lower Eagle Ford and the foraminifera grainstone dominated Langtry Member at the top of the Upper Eagle Ford. Use of the term “middle Eagle Ford” for the clay-rich facies south of the San Marcos arch is not recommended.

Regional Depositional Episodes of the Cenomanian–Turonian Eagle Ford and Woodbine Groups of Texas

Abstract Twelve stratigraphic intervals originally defined in the Eagle Ford of south Texas were mapped across the San Marcos arch into the Maness Shale, Woodbine, and Eagle Ford of east Texas. The maps are based on well log correlations of 1729 wells across 22 counties in south and east Texas using biostratigraphic, geochemical, and lithologic data from 99 wells as seed points for the correlations. These mapped intervals were tied to a regional chronostratigraphic framework developed using data from the outcrops of west, central, and north Texas and cores from the subsurface of south and east Texas. Seven regional depositional episodes were identified across the Texas shelf for the Woodbine and Eagle Ford Groups based on the isopach maps, outcrop data, and paleoenvironmental interpretations. The clay-rich Maness Shale was deposited during the Early Cenomanian in east Texas and northern south Texas where it correlates to the base of the Lower Eagle Ford. After a relative fall in sea level, east Texas was dominated by the thick siliciclastics of the Woodbine, whereas in south Texas deposition of the organic-rich EGFD100 marls began during the subsequent transgression. A shift in depositional style to the limestones and organic-rich shales of the Eagle Ford occurred in east Texas during the Middle Cenomanian produced by the continued rise in sea level, correlating to the EGFD200 marls of south Texas and the carbonates of the Lozier Canyon Member (restricted) of the Eagle Ford Group in west Texas. During the EGFD300 interval deposition transitioned to the organic-rich marls and limestones of the Lozier Canyon and Antonio Creek Members of the Eagle Ford Group in west Texas and the Templeton delta became active in northern east Texas. Erosion along the Sabine uplift shifted the focus of deposition in east Texas southward to the Harris delta and deposited the “clay wedge” of the EGFD400 in northern south Texas. Although the lower part of the EGFD500 episode was deposited during OAE2, it is characterized by low total organic carbon (TOC) due to the presence of oxygenated bottom waters, and the Cenomanian–Turonian boundary sea-level high produced a regional hiatus. Deposition recommenced on much of the Texas shelf during the Late Turonian EGFD600 interval with the Sub-Clarksville delta of east Texas and the carbonate-rich Langtry of south Texas and eastern west Texas. Bottom waters became oxygenated at approximately 90 Ma, initiating the transition from the Eagle Ford to the Austin Chalk.

Taxonomic Uniformitarianism in Gulf of Mexico Basin Cenozoic Foraminiferal Paleoecology: Is the Present Always the Key to the Past?

Abstract Taxonomic uniformitarianism, the application of recent foraminiferal data to reconstruction of ancient paleoenvironments (paleoecology), is an important, widely-used biostratigraphic tool in Gulf of Mexico Basin Cenozoic petroleum exploration. However, the application of recent data to ancient faunas has decreased utility through time. For example, faunal turnover renders Cenozoic models useless for Mesozoic paleobathymetry. Species-based models provide greater reconstruction accuracy than genera-based models, as illustrated by environmental range data from three foraminifera, Bolivina, Cibicides, and Uvigerina. Faunal assemblages are shown to be more accurate ecologic predictors than individual marker taxa. Selected case histories of several foraminifera indicate migrations from shallow to deeper environments have occurred. Examples from invertebrate groups (mollusks and corals), vertebrates (coelacanth), and trace fossils indicate this phenomenon is not limited to foraminifera. Because of evolutionary changes in foraminiferal biofacies, it is determined that more age-specific models for paleoenvironments provide greater interpretation potential than previously published generalized models. Paleozonation shifts do not always indicate a change in depth, but may signal some geologic event worthy of investigation. Large increases in faunal diversity and abundance may indicate other events besides paleobathymetric increases or condensed zones. These include salt gouge and rafting, unconformities, faults, flood surfaces, and clear-water events. Faunal declines may indicate shallowing, unconformities, faults, increased turbidity or rapid rates of sedimentation. Ecologic morphotypes are frequently substituted for younger marker taxa upon extinction. However, similar shapes do not alway prove ecological compatibility. The concept of taxonomic uniformitarianism in paleoecological reconstruction using foraminifera and other groups is therefore supported, primarily as an interpretive tool for Late Cenozoic strata, but with several important caveats.