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Abstract. The Late Cretaceous Epoch was characterized by major global perturbations in the carbon cycle, the most prominent occurring near the Cenomanian–Turonian (CT) transition marked by Oceanic Anoxic Event 2 (OAE-2) at 94.9–93.7 Ma. The Cretaceous Western Interior Seaway (KWIS) was one of several epicontinental seas in which a complex water-mass evolution was recorded in widespread sedimentary successions. This contribution integrates new data on the main components of organic matter, geochemistry, and stable isotopes along a north–south transect from the KWIS to the equatorial western Atlantic and Southern Ocean. In particular, cored sedimentary rocks from the Eagle Ford Group of west Texas (∼ 90–98 Ma) demonstrate subtle temporal and spatial variations in palaeoenvironmental conditions and provide an important geographic constraint for interpreting water-mass evolution. High-latitude (boreal–austral), equatorial Atlantic Tethyan and locally sourced Western Interior Seaway water masses are distinguished by distinct palynological assemblages and geochemical signatures. The northward migration of an equatorial Atlantic Tethyan water mass into the KWIS occurred during the early–middle Cenomanian (98–95 Ma) followed by a major re-organization during the latest Cenomanian–Turonian (95–94 Ma) as a full connection with a northerly boreal water mass was established during peak transgression. This oceanographic change promoted de-stratification of the water column and improved oxygenation throughout the KWIS and as far south as the Demerara Rise off Suriname. In addition, the recorded decline in redox-sensitive trace metals during the onset of OAE-2 likely reflects a genuine oxygenation event related to open water-mass exchange and may have been complicated by variable contribution of organic matter from different sources (e.g. refractory/terrigenous material), requiring further investigation.

Abstract. The principal palynological proxy for the Cenomanian–Turonian Stage boundary, the top of consistent/common Litosphaeridium siphoniphorum (a dinoflagellate cyst), occurs in Greenhorn Bed 73 at the international stratotype section, west of Pueblo, Colorado, USA. This datum occurs in the same position, as indicated by planktonic foraminifera (a few beds higher than the range top of R. cushmani), ammonites (upper part of the S. gracile/M. geslinianum Zone) and geochemistry (immediately below maximum δ13C values), at Pueblo (Western Interior Basin) and localities in southern England (Wessex–Paris Basin) and northern Germany (Lower Saxony Basin). Of over 100 dinoflagellate cyst taxa recorded from Pueblo and a correlative section at Lulworth, southern England, possibly as few as six do not range into the Turonian. In the uppermost Cenomanian – lowermost Turonian succession at Pueblo, there are no consistent absences of any common taxa (with four exceptions) and there is no evidence for a collapse in cyst-forming dinoflagellate populations during the Cenomanian–Turonian boundary mass extinction interval/‘oceanic anoxic event’. However, the composition of palynological assemblages from the Upper Cenomanian appears to reflect palaeoenvironmental stress and/or an increase in the supply of land-derived and relatively nearshore palynomorphs.

This study describes the detailed palynology of the Eagle Ford Group at Lozier Canyon, its principal outcrop reference section in west Texas. Prominent marine phytoplankton assemblages are consistent with a proposed depositional setting for Lozier Canyon on a submarine platform within the Cretaceous Western Interior Seaway (KWIS) of the USA. The Lower Eagle Ford Formation (middle to upper Cenomanian) was deposited under restricted marine conditions with oxygen-deficient, probably anoxic bottom waters, as indicated by the black, organic-rich (ca. 1 to > 6% total organic carbon) shale lithology. The formation comprises two fourth-order stratigraphical sequences, each showing sedimentary evidence of increasing water depth during deposition. Palynomorph assemblages from the sequences are dominated by green algal prasinophyte phycomata in the lower parts and peridinioid dinoflagellate cysts in the upper parts. The productivity of these marine phytoplankton groups may have been stimulated by availability of ammonium (prasinophytes) and nitrite (peridinioids) from upwelling, or vertical expansion of the oxygen-minimum zone. The transition from prasinophyte-to peridinioid-dominated assemblages may reflect elevation of the photic zone to a level less regularly encroached by deeper, ammonium-enriched waters of the denitrification zone. The overlying Upper Eagle Ford Formation (upper Cenomanian to upper Turonian or Coniacian) was deposited under deeper, relatively open marine conditions, as indicated by a decrease in organic carbon content and an increase in carbonate content. This is supported by the presence of diversified dinoflagellate cyst assemblages, including inter-regional (high-to mid-latitude) marker events; in the upper Cenomanian, base and acme Cyclonephelium compactum-membraniphorum, top consistent and frequent Litosphaeridium siphoniphorum, and top Adnatosphaeridium tutulosum; in the middle to upper Turonian, range bases of Heterosphaeridium difficile and Senoniasphaera rotundata. The stratigraphical resolution offered by palynology indicates potential application to wellsite 'geosteering' of commercial shale oil and shale gas wells drilled in the Eagle Ford Group.

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Abstract. Kerogen extracted from rock samples for palynological studies frequently requires oxidative treatment in order to liberate palynomorphs from amorphous organic matter (AOM). Treatment with nitric acid (70% HNO3) is sometimes adequate though many Mesozoic organic-rich samples require more severe oxidation to break up clumped AOM. A widely used method involves the repeated oxidation of a preparation in Schulze’s solution (70% HNO3 supersaturated with KClO3), followed each time by rinsing with 2% potassium hydroxide solution (KOH) (Jones, 1994).The mid-Cretaceous Greenhorn Formation, exposed near Pueblo, Colorado, USA, is composed of interbedded bioturbated limestones and laminated calcareous shales (Cobban &amp; Scott, 1972). In the middle part of the Bridge Creek Limestone Member, kerogen extracted from these two lithologies is markedly different. Diverse, predominantly gonyaulacineaen dinocyst assemblages are found in kerogen from the bioturbated limestone samples. These either require five minutes of nitric acid treatment or no oxidation at all. Most of the AOM is finely disseminated and passes through a standard 10 μm sieve mesh.Dinocysts in the >10μm kerogen fraction from the laminated calcareous shale samples are outnumbered by clumped AOM at a ratio of between 10 and 1000:1. Prior to oxidation, gonyaulacineaen and peridiniineaen dinocysts are seen to be present. However, if Schulze’s solution and KOH are applied until most of the clumped AOM has disintegrated (a process which takes from 1 to 48 hours), Gonyaulacineae are rare (<5%) in resulting dinocyst assemblages (which are dominated by well-preserved Peridiniineae). . . .