ABSTRACT The Eagle Ford Group is a mudrock system deposited during the Late Cretaceous across a flooded carbonate platform (referred to as the Comanche Platform in south and central Texas) that rimmed the Gulf coast region of the southern United States. During the Cenomanian–Turonian the Comanche Platform was situated at the confluence of the Western Interior Seaway and the Gulf of Mexico. Bathymetric features on the Comanche Platform include: 1) the San Marcos Arch, which separates the East Texas Basin to the northeast from the Maverick Basin further to the southwest, 2) isolated fault-bound troughs including the Karnes, Atascosa, and Gonzales Troughs, and 3) structural remnants of the down-dip Aptian through middle Cenomanian Stuart City and Sligo Reef margins. Collectively, bathymetric features and oceanographic circulation patterns resulted in a complex depositional network that affected sediment source input, bottom water redox potentials, and carbonate productivity. The onset of Oceanic Anoxic Event-2 (OAE-2) at the Cenomanian–Turonian boundary (ca. 93.5 Ma) coincides with the transition from lower Eagle Ford (LEF) to upper Eagle Ford (UEF) deposition and is marked by a positive ~2–3‰ carbon isotope excursion measured from bulk organic matter. OAE-2 also reflects a transition from anoxic organic-matter rich mudrocks typical of the LEF to oxic, carbonate-rich, and organic-matter poor mudrocks typical of the UEF. As part of a regional study of the Eagle Ford Group, 42 core have been characterized using high resolution X-ray fluorescence core scanning coupled to lithofacies, biostratigraphy, volcanic ash dating, and stratigraphic wireline log analysis. In south Texas, onset of LEF deposition at ca. 96.25 Ma is dominated by compositionally banded TOC-rich argillaceous–calcareous mudstones to wackestones deposited under anoxic bottom water conditions (elevated concentrations of Ni, Cu, Zn, Mo, and V). These TOC-rich anoxic mudstones are interbedded with diagenetic calcite cemented limestones and concretions. Importantly, anoxic conditions and maximum TOC preservation occurred in the LEF approximately 2–4 million years prior to the onset of OAE-2. At the top of the LEF, initiation of OAE-2 coincides with an increase in clay content, decrease in TOC content, decrease in redox-sensitive trace elements Mo and V, and increase in bioturbation, all of which are indicative of an increase in oxidation of bottom water. Not all cores preserve the 2–3‰ carbon isotope excursion associated with OAE-2, indicating that either erosion or non-deposition occurred during the UEF, likely the result of ocean current reworking, clinoform deposition during the high stand system of the UEF, or the result of bathymetric geometries. ‘Recovery’ from OAE-2 is marked by a decrease in carbon isotope values measured from bulk organic matter back to values typical of LEF, but the lithofacies that dominate the UEF are more oxygenated, contain less TOC, and are more carbonate-rich than observed in the LEF. Collectively, integrated core data from the Comanche Platform in south Texas demonstrate that oxic bottom waters persisted during OAE-2, which is different from the Tethys Ocean that experienced increased TOC-preservation and anoxic bottom water conditions contemporaneous with the OAE-2 carbon isotope excursion.
24Assessing the contribution of the La Luna Sea to the global sink of organic carbon during the Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE2)
Assessing the contribution of the La Luna Sea to the global sink of organic carbon during the Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE2)
Read more4High-resolution oceanic anoxic event 2 (OAE2) records from the north of eastern Tethys and evidence for short-term sea regression and wildfire at its early phase
High-resolution oceanic anoxic event 2 (OAE2) records from the north of eastern Tethys and evidence for short-term sea regression and wildfire at its early phase
Read more68Heterogeneous oxygenation states in the Atlantic and Tethys oceans during Oceanic Anoxic Event 2
Heterogeneous oxygenation states in the Atlantic and Tethys oceans during Oceanic Anoxic Event 2
63The Cenomanian–Turonian anoxic event in southern Tibet
The Cenomanian–Turonian anoxic event in southern Tibet
2Recent Advances in Research on Terrestrial and Marine Sequences From the Mid-Cretaceous Oceanic Anoxic Events (OAEs)
The Cretaceous period was a time of greenhouse climates characterized by sea-surface temperatures of at least 15°C warmer than today. During the climax of Cretaceous warming, anoxic conditions in the oceans expanded globally, resulting in accumulations of organic-rich sediments (socalled black shales). These events, called Oceanic Anoxic Events (OAEs), occurred at least eight times during the Cretaceous period (Fig. 1). Three important aspects make the study of OAEs so interesting: (a) the OAEs acted as a global thermostat because carbon to a large extent was sequestered in the sediments (black shales) instead of adding to the CO2 content of the atmosphere, (b) the expansion of anoxic conditions in the oceans caused mass extinctions of marine biota, and (c) most hydrocarbon source rocks from the Cretaceous formed under anoxic oceanic environments. The discovery of the Cretaceous OAEs (Schlanger and Jenkyns, 1976) was an important achievement of the Deep Sea Drilling Project (DSDP), and understanding Cretaceous OAEs is one of the major scientifi c objectives of the extreme climate initiative outlined in the Integrated Ocean Drilling Program (IODP) Initial Science Plan. To support this initiative a symposium entitled “Recent Advances in Research on Terrestrial and Marine Sequences from the midCretaceous Oceanic Anoxic Events (OAEs)” was held at Hokkaido University, Sapporo, Japan during 21–23 September 2005. The symposium was hosted by the 21 st Century Center of Excellence (COE) Program for the Neo-Science of Natural History, and was organized by Hisatake Okada (COE program leader) and Hiroshi Nishi. Thirteen talks were given, eight by invited speakers, and eight posters dealt with land and marine OAE sections shown in Figure 2. Discussion of land sections concentrated on two types of regions: (a) those with black shale, such as Umbria-Marche Basin, Western Interior, Northwest German Basin and Vocontian Basin; and (b) those devoid of black shale, such as the AngloParis Basin, Yezo Basin and Great Valley Basin. For regions of the fi rst type, the group discussed results of high-resolution studies on a millimeter to centimeter scale of microfossils and organic and inorganic geochemical analyses. It was demonstrated that the black shales in continental interiors and on the continental margins were deposited during marine transgressions, with individual laminae corresponding to the increased runoff from the land areas. For regions of the second type, attention was paid to the integrated analysis of macro- and microfossil and carbon isotope stratigraphies for identifi cation of the precise horizons of OAE levels. Although fully anoxic environments apparently did not develop in these basins, clear evidence of remarkable faunal turnover, involving various taxa, was shown at the OAE levels. Studies of the marine sections dealt with core data mainly from Blake Nose (ODP Leg 171B) and the Demerara Rise (ODP Leg 207). Both sections have yielded extremely well-preserved calcareous fossils suitable for isotopic geochemical analysis. Oxygen and carbon isotope data on planktic and benthic foraminifers revealed the development of two types of water-column structures during
Read more14Seismic multiattribute analysis for shale gas/oil within the Austin Chalk and Eagle Ford Shale in a submarine volcanic terrain, Maverick Basin, South Texas
We conducted seismic multiattribute analysis by combining seismic data with wireline logs to determine hydrocarbon sweet spots and predict resistivity distribution (using the deep induction log) within the Austin Chalk and Eagle Ford Shale in South Texas. Our investigations found that hydrocarbon sweet spots are characterized by high resistivity, high total organic carbon (TOC), high acoustic impedance (i.e., high brittleness), and low bulk volume water (BVW), suggesting that a combination of these log properties is required to identify sweet spots. Although the lower Austin Chalk and upper and lower Eagle Ford Shale intervals constitute hydrocarbon-sweet-spot zones, resistivity values and TOC concentrations are not evenly distributed; thus, the rock intervals are not productive everywhere. Most productive zones within the lower Austin Chalk are associated with Eagle Ford Shale vertical-subvertical en echelon faults, suggesting hydrocarbon migration from the Eagle Ford Shale. Although the quality factor (Q) was not one of the primary attributes for predicting resistivity, it nevertheless can serve as a good reconnaissance tool for predicting resistivity, brittleness, and BVW-saturated zones. In addition, local hydrocarbon accumulations within the Austin Chalk may be related to Austin TOC-rich zones or to migration from the Eagle Ford Shale through fractures. Some wells have high water production because the water-bearing middle Austin Chalk on the downthrown side of Eagle Ford Shale regional faults constitutes a large section of the horizontal well, as evidenced by the Q attribute. Furthermore, the lower Austin Chalk and upper Eagle Ford Shale together appear to constitute a continuous (unconventional) hydrocarbon play.
Read moreOnset of Oceanic Anoxic Event 2 in the Lower Saxony Basin – Insights fromhigh-resolution stable isotope stratigraphy and geochemical modelling
<p>The Oceanic Anoxic Event (OAE) 2 spanning the Cenomanian-Turonian boundary (93.5 Ma)<br>represents a major perturbation of the global carbon cycle and is marked by organic-rich<br>sediments deposited under oxygen-depleted conditions. In many studies the eruption of the<br>Caribbean LIP is considered to be the cause for rapidly increasing CO2 concentrations and<br>resulting global warming accompanied by widespread oceanic anoxia. In the Lower Saxony<br>Basin of northern Germany, the deposits of the OAE 2 are exposed in several industry drill<br>cores. In this study, the lower part of the OAE 2 has been studied in the HOLCIM 2011-3 drill<br>core. Sedimentary rocks are composed of limestones, marly limestones, marls and black<br>shales and have been analysed with a high-resolution stable isotope approach<br>(approximately one sample every 2 cm) combined with geochemical modelling. Using stable<br>carbon isotopes, bulk rock parameters and petrographic analysis, the onset of OAE 2 has<br>been investigated in detail. The high-resolution δ<sup>13</sup>C curve exhibits overall stable values<br>around 3 ‰ before the onset of the Plenus event. This background level is interrupted by<br>three short-lived and small but significant negative carbon isotope excursions (CIEs) down to<br>δ<sup>13</sup>C values of 2.5 ‰, 2.7 ‰ and 1.9 ‰. Immediately before the main rise in the Plenus bed,<br>a longer-lasting negative CIE down to 2.8 ‰ is observed, preceding the large positive CIE of<br>the OAE 2 to values of 5.2 ‰ over 33 ka. Thereafter, the δ<sup>13</sup>C values decrease to 3.5 ‰ over<br>a period of approximately 130 ka. The results can be correlated with the lower-resolution<br>data set of Voigt et al. (2008) but enable a more accurate characterization of the subtle<br>features of the CIE and hence events before and during this time interval. Carbon cycle<br>modelling with the modelling software SIMILE using a model based on Kump & Arthur (1999)<br>reveals that the negative excursion before the Plenus bed can be explained by a massive<br>volcanic pulse releasing of 0.95*10<sup>18</sup> mol CO2 within 14 ka. This amount corresponds to only<br>81 % of the calculated volume of CO<sub>2</sub> release during emplacement of the Caribbean LIP by<br>Joo et al. (2020). In the model the volcanic exhalation increases atmospheric CO<sub>2</sub><br>concentrations. This will increase global temperatures, intensify the hydrological cycle and<br>thus increase nutrient input into the ocean, resulting in an expansion of the oxygen minimum<br>zone, the development of anoxic conditions and an increase in the preservation potential for<br>organic material. In the model enhanced primary productivity and organic matter preservation<br>can be controlled by the implemented riverine phosphate input and the preservation factor for<br>organic matter. For the positive anomaly, the riverine phosphate input must be nearly<br>doubled (from 0.01 μmol/kg PO<sub>4 </sub>to 0.019 μmol/kg) for the period of the increasing δ<sup>13</sup>C<br>values (app. 33 ka), with a concomitant rise of the preservation factor from 1 % to 2 %. This<br>model scenario accurately reproduces the major features of the new high-resolution δ<sup>13</sup>C<br>record over the onset of the OAE 2 CIE.</p>
Read more476Carbon-isotope stratigraphy recorded by the Cenomanian–Turonian Oceanic Anoxic Event: correlation and implications based on three key localities
We present new, detailed carbon-isotope records for bulk carbonate, total organic carbon (TOC) and phytane from three key sections spanning the Cenomanian–Turonian boundary interval (Eastbourne, England; Gubbio, Italy; Tarfaya, Morocco), with the purpose of establishing a common chemostratigraphic framework for Oceanic Anoxic Event (OAE) 2. Isotope curves from all localities are characterized by a positive carbon-isotope excursion of c . 4‰ for TOC and phytane and c . 2.5‰ for carbonate, although diagenetic overprinting appears to have obliterated the primary carbonate carbon-isotope signal in at least part of the Tarfaya section. Stratigraphically, peak δ 13 C values for all components are followed by intervals of high, near-constant δ 13 C in the form of an isotopic plateau. Recognition of an unambiguous return to background δ 13 C values above the plateau is, however, contentious in all sections, hence no firm chemostratigraphic marker for the end-point of the positive isotopic excursion can be established. The stratigraphically consistent first appearance of the calcareous nannofossil Quadrum gartneri at or near the Cenomanian–Turonian boundary as established by ammonite stratigraphy, in conjunction with the end of the δ 13 C maximum characteristic of the isotopic plateau, provides a potentially powerful tool for delimiting the stratigraphic extent and duration of OAE 2. This Oceanic Anoxic Event is demonstrated to be largely, if not wholly, confined to the latest part of the Cenomanian stage.
Read more42CALCAREOUS NANNOFOSSILS FROM EASTBOURNE (SOUTHEASTERN ENGLAND) AND THE PALEOCEANOGRAPHY OF THE CENOMANIAN-TURONIAN BOUNDARY INTERVAL
The Cenomanian–Turonian (C–T) boundary interval is marked by one of the most prominent perturbations of the Mesozoic carbon cycle, Oceanic Anoxic Event 2 (OAE2). Increased fertilization of surface waters caused by greater fluvial input of nutrients may have caused the widespread deposition of organic-rich black shales during the OAE2 (productivity model). Alternatively, sluggish oceanic circulation may have enhanced stratification of the water column favoring the preservation of organic matter due to anoxic bottom-water conditions (preservation model). In order to gather evidence for the driving mechanism behind the deposition of the OAE2 black shales, calcareous nannofossils from the midlatitudinal Holywell section (Eastbourne, southeastern England) were studied. Ten bioevents, including last occurrences of six species and first occurrences of four, were recognized throughout the 11-m-thick interval. Preservation of calcareous nannofossils was moderate to good in all studied samples. The C–T int...
Read more1Mesozoic Oceanic Anoxic Events and the Associated Black Shale Deposits as a Potential Source of Energy
Oceanic anoxic events (OAEs) are considered as periods of oxygen deficiency in many oceans; accompanied by accumulation of organic-rich black shales. Mesozoic anoxic events were recognized based on the presence of black shales that are rich in organic matter. The most significant anoxic events during the Mesozoic are the Early Toarcian, the Early Aptian, and the Cenomanian–Turonian. The less significant events are the Valanginian-Hauterivian, the Hauterivian-Barremian, the Aptian-Albian, the Late Albian, the Albian-Cenomanian, and the Coniacian-Santonian. The recognized OAEs in Egypt are the Early Aptian (OAE 1a), the Aptian-Albian (OAE 1b), the Late Albian (OAE 1c), the Albian-Cenomanian (Breistroffer, OAE 1d), and the Cenomanian–Turonian (Bonarelli Event, OAE 2). However, the most widely recoded event is the OAE 2. The Cretaceous oceans hosted huge amounts of organic-rich black shales that sufficient to source all of the hydrocarbons. Black shales are considered as the most important source of hydrocarbons. The exploitation of black shales to generate hydrocarbons becomes a vital substitutional resource for energy. Such utilization of black shales may compensate the shortage of hydrocarbons in Egypt. Detailed filed work and analytical data are required before final estimation of black shale resources in Egypt.
Read more337Enhanced productivity led to increased organic carbon burial in the euxinic North Atlantic basin during the late Cenomanian oceanic anoxic event
Three Cenomanian/Turonian (C/T, ∼93.5 Ma) black shale sections along a northeast‐southwest transect in the southern part of the proto‐North Atlantic Ocean were correlated by stable carbon isotope stratigraphy using the characteristic excursion in δ13C values of both bulk organic matter (OM) and molecular fossils of algal chlorophyll and steroids. All three sites show an increase in marine organic carbon (OC) accumulation rates during the C/T Oceanic Anoxic Event (OAE). The occurrence of molecular fossils of anoxygenic photosynthetic green sulfur bacteria, lack of bioturbation, and high abundance of redox sensitive trace metals indicate sulfidic conditions, periodically reaching up into the photic zone before as well as during the C/T OAE. During the C/T OAE, there was a significant rise of the chemocline as indicated by the increase in concentrations of molecular fossils of green sulfur bacteria and Mo/Al ratios. The presence of molecular fossils of the green strain of green sulfur bacteria indicates that euxinic conditions periodically even occurred at very shallow water depths of 15 m or less during the C/T OAE. However, bottom water conditions did not dramatically change as indicated by more or less constant V/Al and Zn/Al ratios at site 367. This suggests that the increase in OC burial rates resulted from enhanced primary productivity rather than increased anoxia, which is supported by stable carbon isotopic evidence and a large increase in Ba/Al ratios during the C/T OAE. The occurrence of the productivity event during a period of globally enhanced organic carbon burial rates (i.e., the C/T OAE) points to a common cause possibly related to the formation of a deep water connection between North and South Atlantic basins.
Read more47Unravelling the sources of carbon emissions at the onset of Oceanic Anoxic Event (OAE) 1a
The early Aptian Oceanic Anoxic Event (OAE) 1a represents a major perturbation of the Earth's climate system and in particular the carbon cycle, as evidenced by widespread preservation of organic matter in marine settings and a characteristic negative carbon isotopic excursion (CIE) at its onset, followed by a broad positive CIE. The contemporaneous emplacement of a large igneous province (LIP) is invoked as a trigger for OAE 1a (and OAEs in general), but this link and the ultimate source of the carbon perturbation at the onset of OAEs is still debated. In this study, we simultaneously assimilate an atmospheric pCO2 reconstruction along with a δ13C record from the Spanish Cau section in an Earth system model to obtain a novel transient reconstruction of emission rates and identify the primary carbon-emitting sources across the negative CIE interval at the onset of OAE 1a. We reconstruct carbon emissions of 4,300–29,200 Pg from a mixture of carbon sources. This estimate is a lower bound, as contemporaneous organic carbon burial is not accounted for. Carbon was first released at slow rates from a 13C-depleted reservoir (e.g. thermo- and/or biogenic methane from sill intrusions). Towards the end of the negative CIE the rate of emissions increased and they became more 13C-enriched, likely from a dominantly volcanic source (e.g. LIPs). New osmium isotope (187Os/188Os) measurements, from the same section as the pCO2 reconstruction and δ13C data, reveal a shift to less radiogenic values coinciding with the change towards mantle-derived carbon emissions as indicated by our modelling results, lending further support to our interpretation. These results highlight that geologically triggered carbon emissions were likely driving the OAE onset.
Read more5Examples of seismic diffraction imaging from the Austin Chalk and Eagle Ford Shale, Maverick Basin, South Texas
Examples of seismic diffraction imaging from the Austin Chalk and Eagle Ford Shale, Maverick Basin, South Texas
217Further evidence for the development of photic-zone euxinic conditions during Mesozoic oceanic anoxic events
Periods in Earth history characterized by extensive organic carbon deposition, so-called oceanic anoxic events (OAEs), are the subject of considerable scrutiny. Insight into the extent of anoxic conditions in the water column has been provided by the reported occurrence of derivatives of isorenieratene, a carotenoid found only in green sulphur bacteria and thus indicative of photic-zone euxinia (i.e. a water column containing free H 2 S), in North Atlantic deep-sea sediments deposited during the Cenomanian–Turonian boundary OAE2 (Bonarelli Event). In this study, we have used the distributions of chlorophyll and bacteriochlorophyll degradation products, maleimides (1- H -pyrrole-2,5-diones) and high molecular weight porphyrins, to examine further the Cenomanian–Turonian boundary OAE2 as well as other OAEs of the early Toarcian (Posidonienschiefer Event) and early Aptian (Selli Event). In particular, methyl isobutyl (Me, i -Bu) maleimide, on structural grounds, appears to be diagnostic of green sulphur bacteria. This compound occurs in five of seven examined marls that record the early Toarcian OAE (Marche–Umbria, Italy), further expanding the geographical range of Toarcian sediments where evidence for photic-zone euxinic conditions has been found. Me, i -Bu maleimide occurs in three of six black shales spanning the Livello Selli (Marche–Umbria, Italy), the type locality for the Aptian OAE1a, providing the first evidence for photic-zone euxinic conditions during this event. With respect to the Cenomanian–Turonian OAE, maleimide evidence for photic-zone euxinic conditions was found in all the North Atlantic sites investigated, including those characterized by relatively organic-lean sediments, several Tethyan sites and one site off western Australia. These data indicate that euxinic conditions were common in the water column during Mesozoic oceanic anoxic events.
Read more3New Data on the Cenomanian–Turonian Boundary Oceanic Anoxic Event in the Biyuk-Karasu River Section (Central Crimea)
Cenomanian–Turonian organic-rich clays deposited under periodic anoxic conditions in the basins of that time are recorded in carbonate sections. Cenomanian–Turonian boundary deposits were studied in the Biyuk-Karasu River section (Central Crimea). Dark organic-rich clays were found to intercalate with carbonate-poor clays in the formation during oceanic anoxic events. Mixed type II–III kerogens are abundant in these black beds which constitute oil source strata that are similar in their mineral assemblages but different in their organic matter contents.
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