Lithological controls on the deformation mechanisms operating within carbonate-hosted faults during the seismic cycle

A significant proportion of moderate-large earthquakes, plus aftershocks, nucleate within and propagate through upper-crustal carbonate-dominated sequences, where the effects of lithological variations on fault behaviour are poorly understood. The Gubbio fault is an active (1984, Ms = 5.2) normal fault in Italy, hosted in Mesozoic–Cenozoic limestones and interbedded marls. Fault core domains derived from limestone at the studied outcrop are characterised by fractures/hydrofractures and breccias and host a number of localised (<1.5 mm wide) principal slip zones (PSZs). The majority of displacement of up to 230 m is concentrated in these PSZs, which comprise cataclasites, gouges, and calcite veins. Degassing bubbles, ‘quenched’ calcite, and the transformation of smectite to illite, are also observed within PSZs, implying frictional heating and seismic slip. In contrast, marl-rich domains exhibit distributed shear planes bounding a continuous and pervasive foliation, defined by phyllosilicate-rich pressure-solution seams. Microstructures in the seams include folds/kinks of phyllosilicates and pressure shadows around clasts, consistent with aseismic fault creep. A model is proposed for the behaviour of lithologically complex carbonate-hosted faults during the seismic cycle, whereby limestone-dominated fault core domains behave in a predominantly seismic manner, whereas phyllosilicate-rich domains behave in a predominantly aseismic manner

A new subsurface record of the Pliensbachian–Toarcian, Lower Jurassic, of Yorkshire

Here, we describe the upper Pliensbachian to middle Toarcian stratigraphy of the Dove’s Nest borehole, which was drilled near Whitby, North Yorkshire, in 2013. The core represents a single, continuous vertical section through unweathered, immature Lower Jurassic sedimentary rocks. The thickness of the Lias Group formations in the Dove’s Nest core is approximately the same as that exposed along the North Yorkshire coast between Hawsker Bottoms and Whitby. The studied succession consists of epeiric-neritic sediments and comprises cross-laminated very fine sandstones, (oolitic) ironstones, and argillaceous mudstones. Dark argillaceous mudstone is the dominant lithology. These sediments were deposited in the Cleveland Basin, a more subsident area of an epeiric sea, the Laurasian Sea. We present a set of geochemical data that includes organic carbon isotope ratios (δ13Corg) and total organic carbon (TOC). The δ13Corg record contains a negative excursion across the Pliensbachian–Toarcian boundary and another in the lower Toarcian that corresponds to the Toarcian Oceanic Anoxic Event (T-OAE). Below the T-OAE negative excursion, δ13Corg values are less13C-depleted than above it. We find no evidence of a long-term δ13 Corg positive excursion. TOC values below the T-OAE negative excursion are lower than above it. Sedimentary evidence suggests that, during much of the Pliensbachian–Toarcian interval, the seafloor of the Cleveland Basin was above storm wave-base and that storm-driven bottom currents were responsible for much sediment erosion, transport, and redeposition during the interval of oceanic anoxia. The abrupt shifts observed in the δ13Corg record (lower Toarcian) are likely to reflect the impact of erosion by storms on the morphology of the δ13C record of the T-OAE

Facies characterisation and stratigraphy of the upper Maastrichtian to lower Danian Maastricht Formation, South Limburg, the Netherlands

The Maastricht Formation is a mostly calcarenitic unit that belongs to the Chalk Group but is unlike the typical North Sea chalk in that it is much coarser and at times contains a significant terrigenous component. The formation was deposited between the late Maastrichtian and the early Danian in a proximal zone of the Chalk Sea immediately north of the Anglo-Brabant and Rhenish massifs. The formation crops out in South Limburg, the Netherlands, in the German state of North Rhine–Westphalia, near Aix-la-Chapelle (Aachen), and in the Belgian provinces of Liège and Limburg, and it is present in the subsurface in the Campine Basin and in the Roer Valley Graben, in Belgium and the Netherlands. The often spectacular fossil content of the Maastricht Formation has been extensively studied but its sedimentological aspects remain understudied. Indeed, the lithostratigraphy of the formation is largely informal and based on the abundance and morphology of flint and on the quantification of fossil content. The paucity of facies studies and the lack of a lithostratigraphic framework based on modern facies studies hampers stratigraphic correlation between outcrops and, more importantly, boreholes. A facies characterisation and facies-based lithostratigraphic framework of the Maastricht Formation in South Limburg is particularly urgent because groundwater is abstracted from aquifers in the formation and geological models currently in place fail to predict facies heterogeneity and, consequently, aquifer properties. We studied eight outcrops of the Maastricht Formation across South Limburg and carried out a (micro)facies analysis of the outcrops. We show that the Maastricht Formation can be subdivided into three lithofacies and five microfacies. The lithofacies reflect the traditional subdivision of the formation into Maastricht and Kunrade limestones. Our results suggest that the current subdivision of the Maastricht Formation into six members is untenable. The formation is best subdivided into lower and upper members. The Kunrade limestone should be afforded the status of formation. We interpret the Maastricht Formation as having been deposited in an epeiric ramp, in which facies distribution was controlled by water temperature, nutrient levels and storminess. The (micro)facies of the Maastricht Formation can be organised into two depositional stages: stage 1, representing the lower part of the formation, is characterised by heterozoan carbonates deposited under cooler, mesotrophic conditions in a nutrient-rich, more proximal region of the epeiric sea; stage 2 is characterised by heterozoan-photozoan carbonates deposited in a warmer and stormier environment with slightly lower nutrient levels

Thallium isotopes reveal protracted anoxia during the Toarcian (Early Jurassic) associated with volcanism, carbon burial, and mass extinction

For this study, we generated thallium (Tl) isotope records from two anoxic basins to track the earliest changes in global bottom water oxygen contents over the Toarcian Oceanic Anoxic Event (TOAE; ∼183 Ma) of the Early Jurassic. The T-OAE, like other Mesozoic OAEs, has been interpreted as an expansion of marine oxygen depletion based on indirect methods such as organic-rich facies, carbon isotope excursions, and biological turnover. Our Tl isotope data, however, reveal explicit evidence for earlier global marine deoxygenation of ocean water, some 600 ka before the classically defined T-OAE. This antecedent deoxygenation occurs at the Pliensbachian/Toarcian boundary and is coeval with the onset of initial large igneous province (LIP) volcanism and the initiation of a marine mass extinction. Thallium isotopes are also perturbed during the T-OAE interval, as defined by carbon isotopes, reflecting a second deoxygenation event that coincides with the acme of elevated marine mass extinctions and the main phase of LIP volcanism. This suggests that the duration of widespread anoxic bottom waters was at least 1 million years in duration and spanned early to middle Toarcian time. Thus, the Tl data reveal a more nuanced record of marine oxygen depletion and its links to biological change during a period of climatic warming in Earth’s past and highlight the role of oxygen depletion on past biological evolution

A revised northern European Turonian (Upper Cretaceous) dinoflagellate cyst biostratigraphy: Integrating palynology and carbon isotope events

Organic walled dinoflagellate cyst (dinocyst) assemblage data are presented for a new Turonian regional reference core (Bch-1) drilled at Běchary in the Bohemian Cretaceous Basin, east-central Czech Republic. The detailed stratigraphic framework for the section is summarised based on calcareous nannofossil and macrofossil biostratigraphy, regional e-log correlation, sequence stratigraphy and carbon isotope chemostratigraphy. Dinocyst results obtained for 196 samples from the 405 m long core offer the highest resolution (~ 22 kyr) stratigraphically well-constrained data set available to date for the Turonian Stage, 93.9–89.8 Ma. A dinocyst biostratigraphic framework is presented based on the evolutionary first and last occurrence, first common occurrence, and acmes of key species. Published dinocyst data from English Turonian Chalk successions in East Sussex, Berkshire, Kent and Norfolk are reviewed within a stratigraphic framework provided by macrofossil records and carbon isotope event (CIE) chemostratigraphy. Critical analysis of existing published Turonian dinocyst zonation schemes shows them to be untenable. Correlation of the English Chalk data to Bch-1 provides a basis for defining a regional dinocyst event stratigraphy with 22 datum levels, and a revised dinocyst zonation scheme constrained within a chemostratigraphic framework of 10 major CIEs. The new zones consist of a Cenomanian Litosphaeridium siphoniphorum Zone, followed by the Cauveridinium membraniphorum Zone spanning the uppermost Cenomanian to Lower Coniacian. This is subdivided into: Senoniasphaera turonica (Lower–mid-Middle Turonian); and Raetiaedinium truncigerum (mid-Middle Turonian–mid-Lower Coniacian) subzones. The Oligosphaeridium pulcherrimum Zone (Senonisphaera rotundata Subzone) characterises the Lower Coniacian. The new stratigraphy offers a basis for improved correlation and dating of Upper Cretaceous successions

The Sirius Passet Lagerstatte: silica death masking opens the window on the earliest matground community of the Cambrian explosion

The Sirius Passet Lagerstätte (SP), Peary Land, North Greenland, occurs in black slates deposited at or just below storm wave base. It represents the earliest Cambrian microbial mat community with exceptional preservation, predating the Burgess Shale by 10 million years. Trilobites from the SP are preserved as complete, three-dimensional, concave hyporelief external moulds and convex epirelief casts. External moulds are shown to consist of a thin veneer of authigenic silica. The casts are formed from silicified cyanobacterial mat material. Silicification in both cases occurred shortly after death within benthic cyanobacterial mats. Pore waters were alkali, silica-saturated, high in ferric iron but low in oxygen and sulphate. Excess silica was likely derived from remobilized biogenic silica. The remarkable siliceous death mask preservation opens a new window on the environment and location of the Cambrian Explosion. This window closed with the appearance of abundant mat grazers later as the Cambrian Explosion intensified

Microstructures of Early Jurassic (Toarcian) shales of Northern Europe

The Toarcian (Early Jurassic) Posidonia Shale Formation is a possible unconventional gas source in Northern Europe and occurs within the Cleveland Basin (United Kingdom), the Anglo-Paris Basin (France), the Lower Saxony Basin and the Southwest Germany Basin (Germany), and the Roer Valley Graben, the West Netherlands Basin, Broad Fourteens Basin, the Central Netherlands Basin and the Dutch Central Graben in The Netherlands. Outcrops can be found in the United Kingdom and Germany. Since the Posidonia Shale Formation does not outcrop in the Netherlands, sample material suitable for experimental studies is not easily available. Here we have investigated lateral equivalent shale samples from six different locations across Northern Europe (Germany, The Netherlands, The North sea and United Kingdom) to compare the microstructure and composition of Toarcian shales. The objective is to determine how homogeneous or heterogeneous the shale deposits are across the basins, using a combination of Ion Beam polishing, Scanning Electron Microscopy and X-ray diffraction. The work presented here shows that the Toarcian shales of Northern Europe display considerable homogeneity in mineralogy and microstructure in the different investigated samples and formations, where the largest variability is the carbonate content ranging from almost zero up to 80%. We conclude that the outcrop locations in Germany and the United Kingdom are suitable analogues with respect to their mineralogy and microstructure for experimental studies on the Posidonia Shale in the Dutch subsurface

Rising not falling? Differential compaction of shelf-edge trajectories and clinothem geometries

Clinothems document the progradation of sedimentary strata. Their geometries allow us to define shelf-edge trajectories, which are widely used to infer variations in relative sea-level, spatial and temporal partitioning of depositional environments, and the timing of sediment delivery to the slope and basin-floor. Here, we present a novel perspective on trajectory reconstruction of buried successions, applying a decompaction technique that explicitly accounts for down-dip lithology variations within clinothems. We show that preferential compaction of fine-grained foresets and bottomsets results in a basinward rotation of trajectories and a distortion of primary clinothem geometries. In some cases, shelf-edge trajectories change from rising to apparently falling after burial, potentially leading to erroneous interpretations of original basin-margin physiography, relative sea level fluctuations, and incorrect predictions for the timing and volume of sediment transfer to deep water