Jackie A. Lees

Calcareous nannoplankton evolution and diversity through time

Planktic microfossils arguably provide the most complete (stratigraphic and taxonomic) record of biodiversity of any group of organisms. The phytoplankton record is of particular significance as it most likely tracks global changes in the climate-ocean system and, in turn, influenced biodiversity and productivity of higher trophic levels of the biosphere. Coccolithophores and associated calcareous nannoplankton first appear in the fossil record in Upper Triassic sediments (~225 Ma) and, despite significant extinctions at the Triassic/Jurassic boundary, the Mesozoic diversity record is one of relatively uniform increase punctuated by short periods of turnover and decline. Rates of speciation that are significantly above background were restricted to the Late Triassic, Early Jurassic and Tithonian-Berriasian intervals. Enhanced rates of extinction occurred at the Triassic/Jurassic, Jurassic/Cretaceous and Cretaceous/Tertiary boundaries.

Evidence for global cooling in the Late Cretaceous.

The Late Cretaceous ‘greenhouse’ world witnessed a transition from one of the warmest climates of the past 140 million years to cooler conditions, yet still without significant continental ice. Low-latitude sea surface temperature (SST) records are a vital piece of evidence required to unravel the cause of Late Cretaceous cooling, but high-quality data remain illusive. Here, using an organic geochemical palaeothermometer (TEX86), we present a record of SSTs for the Campanian–Maastrichtian interval (~83–66 Ma) from hemipelagic sediments deposited on the western North Atlantic shelf. Our record reveals that the North Atlantic at 35 °N was relatively warm in the earliest Campanian, with maximum SSTs of ~35 °C, but experienced significant cooling (~7 °C) after this to <~28 °C during the Maastrichtian. The overall stratigraphic trend is remarkably similar to records of high-latitude SSTs and bottom-water temperatures, suggesting that the cooling pattern was global rather than regional and, therefore, driven predominantly by declining atmospheric pCO2 levels.

A Paleogene calcareous microfossil Konservat-Lagerstätte from the Kilwa Group of coastal Tanzania

Microfossil assemblages and their shell geochemistry are widely used in paleocean-ography, but they can be significantly altered by subtle variations in preservation state. Clay-rich, hemipelagic sediments of the Paleogene Kilwa Group of coastal Tanzania host calcareous microfossils that are exceptionally preserved, as evidenced by morphological, taxonomic, and geochemical data. The planktonic foraminifera are preserved as glassy, translucent tests with original microgranular wall textures that resemble well-preserved modern specimens, and they arguably yield geochemical values that are relatively unaffected by recrystallization. The calcareous nannofossils are extraordinarily diverse and represented by unique assemblage compositions that include dissolution-susceptible taxa, especially holo-coccoliths and rhabdoliths, and fragile and very small (< 3-mu m) heterococcoliths, many of which are new taxa. Notably, the extant, deep-photic-zone taxon Gladiolithus is documented for the first time in the pre-Quaternary fossil record. The Kilwa Group calcareous nannofossil diversities are consistently higher than all coeval assemblages and provide a benchmark against which to compare other Paleogene biodiversity data. Highest diversities are preserved in hemipelagic, clay-rich lithologies and the greatest losses occur in lithified, carbonate-rich sediments. Most of the lost diversity, however, is confined to distinct taxonomic groups (holococcoliths and Syracosphaerales), and in general the preservational potential of Paleogene coccolithophores was greater than in the modern oceans because a larger proportion of the biodiversity fell within the larger size fractions. For both foraminifera and coccolithophores, incorporation into impermeable clay-rich sediments that have never been deeply buried appears to have been critical in producing this Konservat-Lagerstatte preservation.

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.

Jake Hancock: reminiscences

At the Geological Society memorial meeting held to celebrate Professor Jake Hancocks extensive career, and to mourn the passing of this legendary geologist, we three nannopalacontologists were invited to talk on nannofossils at the Cretaceous/Tertiary boundary. This talk, which detailed new evidence for rapid nannofossil extinctions and the role of neritic taxa in the subsequent recolonization of the oceans, was preceded by some reminiscences about Jake, whom we all knew. These are reproduced here.

(Table T11) Calcareous nannofossil stratigraphy of ODP Hole 198-1212B

Sediment depth is given in mbsf. Abundance estimates as follows: A = abundant (>10 specimens per field of view [FOV]); C = common (1 -10 specimens per FOV); F = few (1 specimen per 2-10 FOV); R = rare (1 specimen per 11-100 FOV), 1-2 = 1 or 2 specimens, ? = questionable occurrence, - = not found. Biostratigraphy is described with reference to the lowermost Cretaceous NK zones of Bralower et al. (1989, doi:10.1016/0377-8398(89)90035-2), Lower Cretaceous NC zones of Roth (1978, doi:10.2973/dsdp.proc.44.134.1978; 1983, doi:10.2973/dsdp.proc.76.125.1983) with subzones after Bralower (1987, doi:10.1016/0377-8398(87)90003-X) and Bralower et al. (1993), and Upper Cretaceous zones of Burnett (1998). The biostratigraphic zones, chronostratigraphy, and timescale correlations are after Shipboard Scientific Party (2002a, doi:10.2973/odp.proc.ir.198.102.2002).