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<jats:p>The Cretaceous records some of the highest sea levels, atmospheric temperatures and extreme events in Earth history. These two ‘Cretaceous Project 200’ volumes cover all the major fields of Cretaceous research from dinosaurs and ammonites to volcanism and ocean-wide anoxia and provide a review of Cretaceous strata across the Earth.</jats:p>

<jats:title>Abstract</jats:title> <jats:p> Improvements in analytical procedures in parallel with intercalibration of <jats:sup>40</jats:sup> Ar/ <jats:sup>39</jats:sup> Ar and U-Pb methods and laboratories, spurred since 2003 by the EarthTime geochronology community initiative, have led to ± 2s uncertainties on the order of 50 to 100 ka, or better for Cretaceous ash beds. Assembled here are fifty-seven <jats:sup>40</jats:sup> Ar/ <jats:sup>39</jats:sup> Ar ages and sixteen <jats:sup>238</jats:sup> U- <jats:sup>206</jats:sup> Pb ages of volcanic ash beds in strata of the Western Interior Basin of North America determined during the last 15 years since these improvements have been made. These age determinations span from 108 Ma in the middle Albian to 66 Ma in the latest Maastrichtian. Five of the <jats:sup>40</jats:sup> Ar/ <jats:sup>39</jats:sup> Ar ages from Campanian and Maastrichtian strata are newly reported here, whereas the remainder are from the literature. Building on the pioneering work of John Obradovich and Bill Cobban, where possible these age determinations are tied to ammonite and inoceramid biostratigraphy. For most ash beds the temporal uncertainties, unlike earlier time scales for the Western Interior Basin, are much shorter than the duration of fossil biozones. Proposed ages for stage boundaries based on this review of the radioisotopic ages include: <jats:list list-type="bullet"> <jats:list-item> <jats:p>Maastrichtian-Danian 66.02 ± 0.08 Ma</jats:p> </jats:list-item> <jats:list-item> <jats:p>Campanian-Maastrichtian 72.20 ± 0.20 Ma</jats:p> </jats:list-item> <jats:list-item> <jats:p>Santonian-Campanian 84.19 ± 0.38 Ma</jats:p> </jats:list-item> <jats:list-item> <jats:p>Coniacian-Santonian 86.49 ± 0.44 Ma</jats:p> </jats:list-item> <jats:list-item> <jats:p>Turonian-Coniacian 89.75 ± 0.38 Ma</jats:p> </jats:list-item> <jats:list-item> <jats:p>Cenomanian-Turonian 93.95 ± 0.05 Ma</jats:p> </jats:list-item> <jats:list-item> <jats:p>Albian-Cenomanian 100.00 ± 0.40 Ma</jats:p> </jats:list-item> </jats:list> </jats:p> <jats:p> Five bentonites that occur within the <jats:italic>Vascoceras diartianum, Prionocylus macombi, Scaphites preventricosus, Scaphites depressus</jats:italic> , and <jats:italic>Desmoscaphites bassleri</jats:italic> ammonite zones, dated using both <jats:sup>40</jats:sup> Ar/ <jats:sup>39</jats:sup> Ar and U-Pb methods, yield ages in agreement to within 150 ka and form the backbone of the Western Interior Basin time scale. In parallel, improvements in the taxonomy of ammonites, inoceramids, and foraminifera, and recent field work, is better establishing the biostratigraphic framework for these age determinations. These efforts each contribute to the progressive refinement of the chronostratigraphic framework of the Western Interior Basin, and enhance its utility for global correlation. </jats:p> <jats:p content-type="supplementary-material"> Supplementary material at <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" specific-use="dataset is-supplemented-by" xlink:href="https://doi.org/10.6084/m9.figshare.c.6895395">https://doi.org/10.6084/m9.figshare.c.6895395</jats:ext-link> </jats:p>

<jats:title>Abstract</jats:title> <jats:p> Offshore CO <jats:sub>2</jats:sub> sequestration in basaltic formations of the North Atlantic Igneous Province (NAIP) may allow permanent storage of large volumes of CO <jats:sub>2</jats:sub> through rapid carbonate mineralization. Characterizing the internal architecture of such reservoirs is key to assessing the storage potential. In this study, six photogrammetry models and three boreholes on the Faroe Islands have been used to characterize the internal lava sequence architectures as a direct analogue to potential offshore NAIP storage sites. The studied formations are dominated by ca. 5 m to 50 m thick simple and compound lava flows, with drill core observations documenting a transition from pāhoehoe moving towards ‘a’ā lava flow types interbedded with thin (&lt;5 m thick) volcaniclastic rock units. The identification of flow margin breccias is potentially important as these units form excellent reservoirs in several other localities globally. Stacked, thick simple flows may present sealing units associated with dense flow interiors. Connected porous and permeable lava flow crusts present potential reservoirs, however, the degree of secondary mineralization and alteration can alter initially good reservoir units to impermeable barriers for fluid flow. Large-scale reservoir volumes may be present mainly within both vesicular, fractured pāhoehoe and brecciated flow margins of transitional simple lava flows. </jats:p> <jats:p content-type="supplementary-material"> Supplementary material at <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" specific-use="dataset is-supplemented-by" xlink:href="https://doi.org/10.6084/m9.figshare.c.6949132">https://doi.org/10.6084/m9.figshare.c.6949132</jats:ext-link> </jats:p>

<jats:title>Abstract</jats:title> <jats:p>The Browse Basin is one of Australia's major hydrocarbon provinces, where significant discoveries have been made in recent decades including the Ichthys and Prelude fields, which accounted for ∼15% of the cumulative Australian LNG production in 2019-2020. This rift basin hosts extensive Mesozoic intrusive and extrusive igneous rocks, having been identified from both well and seismic data, and which are recognised as one of the key challenges for exploration and production activities in this region. Their impact on petroleum exploration is demonstrated by the number of wells which encountered unpredicted or thicker than expected igneous rock units both within and adjacent to target sections. This study therefore aims to document the reasons of such unexpectedness, and to develop capability to predict the occurrence of igneous rock units prior to drilling in the Browse Basin and other rift settings that contain igneous rocks. Multiple case studies of uncommercial exploration wells are developed by integrating petrophysical and seismic reflection data, focussing in particular along the outboard part of the basin where igneous rocks are most prevalent. Our study highlights the importance of understanding petrophysical, spatial and chemical heterogeneities of igneous rocks in basins to explain their emplacement and distribution, and thereby predict their occurrence prior to exploration and development activities.</jats:p>

<jats:title>Abstract</jats:title> <jats:p> A model is presented for the generation of natural hydrogen from cratonic basement rocks and its migration into the sediments of overlying cratonic basins. It is based on the ‘discovery’ of hydrogen at Bourakebougou in the Taoudeni Basin of Mali. In the <jats:italic>Cratonic Greenstone Model</jats:italic> hydrogen is generated by the serpentinisation of olivine-rich, ultramafic rocks contained within Precambrian ‘greenstones’. The model requires a protolith (in greenstones), a supply of water (from groundwater), connecting faults to act as a plumbing system and an indurated sediment cover to retard hydrogen movement. Hydrogen is expelled into the overlying basin sediments which form the host for hydrogen accumulations. The model describes a continental ‘hydrogen system’ which can form the basis for petroleum-type play-based hydrogen exploration in cratonic settings. Using play elements derived from the model, the Bourakebougou play fairway can be extended across the Taoudeni Basin &gt;700 km northwards of the discovery. </jats:p>

<jats:title>Abstract</jats:title> <jats:p> Intruding magma can create space by uplift and elastic bending of the overburden, which locally fractures the deforming volume and produces flat-topped or dome-like forced folds. Here, I map such fracture networks and quantify their geometry and connectivity across a range of natural and modelled intrusion-induced forced folds. I show that there is a positive relationship between forced fold length and amplitude, and all fracture networks comprise traces, with variable lengths and orientations, that are more intense and denser where fold curvature is greatest. Fracture length populations are typically best described by power-law distributions, but some fit better to log-normal or exponential distributions. Connectivity of fracture networks is low and generally increases with folding, but resurfacing by eruptive products can disrupt this trend. My work supports previous analyses of forced folds and fractures, suggesting that we can use the fracture characteristics of exposed forced folds to predict that of buried forced folds. Due to their geometry and fracture network, intrusion-induced forced folds make ideal fluid traps. As these forced folds are common in many volcanic settings and sedimentary basins, we should consider their potential as exploration targets for water, magmatic-related mineral and metal deposits, and particularly CO <jats:sub>2</jats:sub> storage. </jats:p> <jats:p content-type="supplementary-material"> Supplementary material at <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" specific-use="dataset is-supplemented-by" xlink:href="https://doi.org/10.6084/m9.figshare.c.6946809">https://doi.org/10.6084/m9.figshare.c.6946809</jats:ext-link> </jats:p>

<jats:title>Abstract</jats:title> <jats:p>Magmatic activity can severely alter the thermal structure of a sedimentary basin, with variable effects on the petroleum system. The Namibe Basin of Angola (Cretaceous South Atlantic rift) contains well-exposed magmatic and petroleum system elements, and allows integrated assessment of how magmatic activity can modify the petroleum system. The basin was affected by syn-rift and post-rift magmatic events, and bitumen is observed within both Pre- and Post-Salt stratigraphic sections. In the Pre-Salt, fluorescent bitumen has a lacustrine signature and is associated with calcite and quartz cements. Onshore Pre-Salt units are thermally immature, and therefore the source rock that generated the Pre-Salt bitumen is likely located offshore. Hydrocarbons migrated or re-migrated via magmatically driven fluids, reaching the present-day onshore. Closer to magmatic units, non-fluorescent pyrobitumen was instead observed, evidencing hydrocarbon cracking processes following emplacement. In the Post-Salt, bitumen is in-situ and shows marine-like signatures compatible with an immediate Post-Salt source rock depositional environment. In the immediate Post-Salt, units with very high Total Organic Carbon (TOC; up to 13.8 %) and excellent source rock properties (Hydrogen Index &gt; 600 mg HC/g TOC) have reached thermal maturation. Within the Namibe Basin these Post-Salt source units lie in proximity to major Turonian-Coniacian-Santonian volcanic centres and associated shallow intrusions, which are likely to have caused thermally forced maturation processes and generation of the Post-Salt hydrocarbons. This paper demonstrates the importance of an integrated field, petrographic and geochemical approach in unravelling the influence of magmatic activity on basin thermal structure and petroleum systems.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Soft sediment deformation structures may form when denser sediments or fluids are deposited on or flow over unlithified and less dense sediments. This study presents a seismic geomorphological study of the basal contact between an extrusive volcanic sequence and underlying sediments, defining the ‘Base Basalt’ surface, on the Mid-Norwegian Margin. This contribution focuses in particular on the development of geomorphological features related to the rapid loading of a several hundred-meter-thick lava delta package of hyaloclastite onto poorly consolidated sediments of the pre-volcanic sedimentary basin fill. Seismic horizons, sequence boundaries, volcanic facies units, and attribute maps are used to characterize the seismic geomorphological features imaged within a high-quality 3D seismic cube. The ‘Base Basalt’ horizon and attribute maps reveal incised channels and a network of polygonal to irregular depressions and ridges described here as an ‘egg-box network’. More than 150 depressions, with a typical diameter of 1 km and a depth of 100 m, have been mapped. The deformation features, which are restricted to the base of the Lava Delta seismic facies unit, are interpreted to be the result of rapid loading of the Lava Delta onto poorly consolidated unlithified pre-volcanic sediments. This study presents new evidence for the dynamic nature of the transition between sedimentary basins and large-scale volcanism found along volcanic margins and basins associated with rapid volcanic deposition.</jats:p> <jats:p content-type="supplementary-material"> Supplementary material at <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" specific-use="dataset is-supplemented-by" xlink:href="https://doi.org/10.6084/m9.figshare.c.6946691">https://doi.org/10.6084/m9.figshare.c.6946691</jats:ext-link> </jats:p>

<jats:title>Abstract</jats:title> <jats:p>Cooling subvolcanic igneous intrusions are known to have a tremendous impact on fluid flow in the shallow Earth’s crust. However, the long-term post-cooling legacy of subvolcanic intrusions on fluid flow received much less attention. Here we describe geological examples in the Andean foothills, Argentina, showing that igneous intrusions have long-term effects on fluid flow after their emplacement and cooling. The case study consists of ∼11-million-year-old, eroded andesitic intrusions of Cerro Alquitrán and Cerro La Paloma, northern Neuquén Basin, Argentina, at the rims of which large volumes of bitumen are naturally seeping out at the Earth’s surface. The intrusions exhibit laccolithic shapes with steep-sided contacts with the host rock. Near the intrusive contacts, the andesite is intensely broken along concentric breccia bands and fracture bands, interpreted to result from syn-emplacement brittle magma deformation,which represent high-permeability pathways for the migrating bitumen. Organic geochemical analyses of the bitumen show that the seeping oils were generated from incipiently mature Vaca Muerta sections located in a regional kitchen to the west, implying a lateral migration of ∼10-20 km. The Cerro Alquitrán and Cerro La Paloma intrusions are demonstrative examples highlighting how extinct subvolcanic intrusions have long-term consequences on subsurface fluid circulations in sedimentary basins.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The Calcaires du Barrois Formation is a succession of dominantly micritic limestone of Kimmeridgian to Tithonian age, outcropping in the eastern part of the Paris Basin. This is an active karstic aquifer of interest to Andra (French National Agency for Radioactive Waste Management), who are studying the feasibility of a deep geological repository of radioactive waste in an underground research laboratory located approximately 450 m below the surface. Surface installations of the Industrial Centre for Geological Disposal project are planned in the upstream recharge zone of the aquifer. It is of primary interest to characterize the Calcaires du Barrois Formation to provide guidelines for the planning and sizing of these facilities, to minimize the impact on the aquifer system.</jats:p> <jats:p>An integrated study was designed for this purpose, linking petrography (thin section and scanning electron microscopy), carbon and oxygen stable isotope geochemistry, X-ray diffraction, petrophysics and geomechanics. It is based on the analysis of three key cored wells penetrating the formation at different relative depths.</jats:p> <jats:p>The Calcaires du Barrois have undergone several stages of diagenesis that have defined the current properties. Unconformities associated with the Jurassic–Cretaceous transition led to prolonged early subaerial exposures during which freshwater flowed efficiently through the upper half of the formation. Through mineralogical stabilization, among other processes, microporosity was preserved in micrites in this interval consisting of clean limestone with thin marl layers. The lower half of the formation, more argillaceous, was not or was only slightly affected by this early meteoric diagenesis, and recrystallization and cementation of micrites occurred during burial diagenesis, involving chemical compaction. Later, during the return to the surface associated to the Cenozoic orogens, another phase of meteoric diagenesis affected the uppermost few metres below the outcropping portions of the formation, but without modifying significantly the previously acquired petrophysical properties. Consequently, an intra-formational boundary was progressively developed at around 75 m (from the top reference). This boundary separates (1) a lower half of the Calcaires du Barrois with dense and tight micrites, showing high Young's modulus values, and a moderate intensity of fractures, from (2) an upper half with microporous micrites showing low Young's modulus values, and almost devoid of fractures. A transitional zone of about 30 m thick, with intermediate properties, sitting above this boundary and below the only thin metre-scale macroporous grainstone level of the formation, accommodated most of the deformation linked to the Cenozoic Western European orogens and is intensively fractured.</jats:p> <jats:p>The current hydrogeological model considers a purely sedimentological boundary to delimit two sub-aquifers within the Calcaires du Barrois Formation. This will have to be reappraised since it is here demonstrated that the real boundary is located significantly higher in the formation and is inherited from a multi-stage diagenetic history. These findings will complement and influence planning for the Industrial Centre's project.</jats:p>