Mudrock Succession Research Articles

Sedimentology of the continental end‐Permian extinction event in the Sydney Basin, eastern Australia

AbstractUpper Permian to Lower Triassic coastal plain successions of the Sydney Basin in eastern Australia have been investigated in outcrop and continuous drillcores. The purpose of the investigation is to provide an assessment of palaeoenvironmental change at high southern palaeolatitudes in a continental margin context for the late Permian (Lopingian), across the end‐Permian Extinction interval, and into the Early Triassic. These basins were affected by explosive volcanic eruptions during the late Permian and, to a much lesser extent, during the Early Triassic, allowing high‐resolution age determination on the numerous tuff horizons. Palaeobotanical and radiogenic isotope data indicate that the end‐Permian Extinction occurs at the top of the uppermost coal bed, and the Permo‐Triassic boundary either within an immediately overlying mudrock succession or within a succeeding channel sandstone body, depending on locality due to lateral variation. Late Permian depositional environments were initially (during the Wuchiapingian) shallow marine and deltaic, but coastal plain fluvial environments with extensive coal‐forming mires became progressively established during the early late Permian, reflected in numerous preserved coal seams. The fluvial style of coastal plain channel deposits varies geographically. However, apart from the loss of peat‐forming mires, no significant long‐term change in depositional style (grain size, sediment‐body architecture, or sediment dispersal direction) was noted across the end‐Permian Extinction (pinpointed by turnover of the palaeoflora). There is no evidence for immediate aridification across the boundary despite a loss of coal from these successions. Rather, the end‐Permian Extinction marks the base of a long‐term, progressive trend towards better‐drained alluvial conditions into the Early Triassic. Indeed, the floral turnover was immediately followed by a flooding event in basinal depocentres, following which fluvial systems similar to those active prior to the end‐Permian Extinction were re‐established. The age of the floral extinction is constrained to 252.54 ± 0.08 to 252.10 ± 0.06 Ma by a suite of new Chemical Abrasion Isotope Dilution Thermal Ionization Mass Spectrometry U‐Pb ages on zircon grains. Another new age indicates that the return to fluvial sedimentation similar to that before the end‐Permian Extinction occurred in the basal Triassic (prior to 251.51 ± 0.14 Ma). The character of the surface separating coal‐bearing pre‐end‐Permian Extinction from coal‐barren post‐end‐Permian Extinction strata varies across the basins. In basin‐central locations, the contact varies from disconformable, where a fluvial channel body has cut down to the level of the top coal, to conformable where the top coal is overlain by mudrocks and interbedded sandstone–siltstone facies. In basin‐marginal locations, however, the contact is a pronounced erosional disconformity with coarse‐grained alluvial facies overlying older Permian rocks. There is no evidence that the contact is everywhere a disconformity or unconformity.

Quartz types in the Upper Pennsylvanian organic‐rich Cline Shale (Wolfcamp D), Midland Basin, Texas: Implications for silica diagenesis, porosity evolution and rock mechanical properties

AbstractThe origin and form of quartz in mudrocks has significant implications for interpretation of depositional environments, diagenetic pathways, mechanisms of porosity reduction and rock mechanical‐property evolution. Quartz types in the Upper Pennsylvanian Cline Shale, Midland Basin, Texas, were examined using a combination of field‐emission scanning electron microscopy‐based energy‐dispersive spectroscopy elemental mapping (to determine mineralogy) and scanning electron microscopy‐based cathodoluminescence imaging (to determine quartz types) with the goal of elucidating a high‐resolution imaging protocol at the micrometre scale for shale petrology. Also, the unconfined compressive rock strength of shale samples with contrasting proportions of different quartz types was measured using Equotip Bambino analyses. The results suggest that extrabasinal detrital quartz, which accounts for an average of 26 vol.% of the rock in all analyzed samples, is the dominant form of quartz in the Cline Shale. The intergranular clay‐size microquartz, which accounts for an average of 10 vol.% of the rock in all analyzed samples, is the dominant form of authigenic quartz. Dissolved radiolarians and sponge spicules are likely sources of silica for clay‐size microquartz and other authigenic quartz showing pale‐mauve to dark greyish cathodoluminescence colour. Some authigenic quartz in the form of intragranular pore filling and mollusc skeletal replacement displays bright‐reddish cathodoluminescence colour, which may be associated with silica released at a different time in the rock's diagenetic history, such as during smectite illitization. Porosity reduction in the Cline Shale predominantly resulted from compaction because of extremely low intergranular volume and the general lack of early cementation. Quartz form significantly impacts rock mechanical properties in the Cline Shale: widely distributed intergranular clay‐size microquartz cement is a major factor controlling rock strength. This correlation also applies to other mudrock successions of various geological ages, tectonic histories and lithologies.

Unconventional hydrocarbon prospects in Ordovician and Silurian mudrocks of the East European Craton (Poland): Insight from three-dimensional modelling of total organic carbon and thermal maturity

Three-dimensional, structural and parametric numerical modelling was applied to unravel the unconventional hydrocarbon potential of a W-dipping, Lower Palaeozoic mudrock succession, which subcrops for some 700 km in the Baltic, Podlasie and Lublin basins across the SW margin of East European Craton in Poland. Input data comprised structural and thickness maps of Ordovician and Silurian strata and the results of thermal maturity (mean vitrinite-equivalent reflectance, % Ro) and total organic carbon (TOC, % wt.) modelling. A new, spatial interpretation of vitrinite-reflectance variability indicates that the regional, W-increasing thermal maturity pattern breaks into a series of domains, bounded by abrupt maturity variations. In total, 14 tectono-thermal domains were recognised and their boundaries traced to known and inferred faults, mostly of NW‒SE and NE‒SW orientations. On the basis of a combination of thermal maturity and total organic carbon levels (0.6% > Ro<2.4%, and TOC >1.5% wt.), good-quality, unconventional reservoirs can be expected in the Sasino Formation (Caradoc) and Jantar Formation (early Llandovery) in the central and western Baltic Basin. The Jantar Formation also is likely to be prospective in the western Podlasie Basin. Marginal-quality reservoirs may occur in the Sasino and Jantar formations within the Podlasie and Lublin basins and in the Pasłęk Formation (late Llandovery) across all basins. Poorto moderate-quality, unconventional reservoirs could be present in the Pelplin Formation (Wenlock) in the Lublin and southern Podlasie basins. In spite of a considerable hydrocarbon loss during multiphase basin inversion, the Ordovician and Silurian mudrocks still contain huge quantities of dispersed gas. Successful exploitation of it would require the adoption of advanced fracking methods.

Grain assemblages and diagenesis in the Vaca Muerta Formation (Jurassic-Cretaceous), Neuquén Basin, Argentina

Mudrocks of the Vaca Muerta Formation (Tithonian-early Valanginian; Neuquén Basin, Argentina) contain grain assemblages of mixed extrabasinal and intrabasinal derivation. Mudrocks that contained, at deposition, relatively more abundant carbonate and siliceous biogenic debris display cementation by calcite and quartz that occluded pores and enhanced rock stiffness, perhaps acting to retard further pore loss by compaction. Another notable diagenetic feature in this mudrock succession is grain replacement that includes carbonate replacement of biosilica and feldspar and replacement of calcitic and aragonitic skeletal debris by calcite, dolomite, quartz, albite, chlorite, and pyrite. Organic matter is present as both detrital and diagenetic components that can in some cases be discriminated petrographically. The extensive chemical reorganization of Vaca Muerta mudrocks in diagenesis can be attributed to the high reactivity of the primary grain assemblage. Petrographic evidence suggests a chemical system in which silicate, carbonate, and organic matter components mutually participate in a complex set of reactions in which silicates provide acid buffering that controls the precipitation of carbonate. Despite the prominent role of chemical diagenesis in the post-depositional history of the Vaca Muerta, compaction is the major cause of porosity decline in these mudrocks.While the relatively small size of the sample set in this study (19 samples) does not permit lithologic heterogeneity to be assessed in the context of basin-scale stratigraphy, the observed variety of mudstones suggests that a stratigraphically representative sample set would need to be much larger than the one used here.

Grain assemblages and diagenesis in the tarl-dominated Lower Silurian mudrock succession of the western margin of the east European craton in Poland and Lithuania

Detrital grain assemblages in mudrocks establish a key starting condition for the diagenetic pathways that drive the evolution of rock properties in unconventional petroleum systems. This case study is based on petrographic examination (light microscopy, elemental mapping, and cathodoluminescence imaging) of 41 subsurface samples of Silurian mudrocks from the western margin of the East European Craton (EEC) in Poland. About 2/3 of the samples have grain assemblages composed by >90% of grains of quartz, feldspar, mica, and clay of extrabasinal derivation. Mechanical compaction is the principal cause of porosity decline in these terrigenous-argillaceous mudrocks (tarls). Where thermal maturation has reached a level sufficient to induce smectite illitization, minute amounts of authigenic silica (≪1 vol%) are observed in the small (~100 nm) primary intergranular pores that survived compaction. Organic-rich mudrocks in this succession also display prominent compaction and contain little organic-matter-hosted porosity. Although these organic-rich tarls may have served as effective source rocks as unimpeded compaction drove expulsion of generated hydrocarbons, they do not in general have sufficient porosity and stiffness to serve as effective reservoirs.Around 27% of the sample set has grain assemblages that include biogenic grains in amounts over 10%. Most of these are calcareous tarls (calcareous-terrigenous argillaceous mudrocks), but do not display discernible carbonate cements. Only three of the studied samples contain grain assemblages that originally included a significant component of biogenic silica. These three samples contain microquartz cement (up to 30% of rock volume) and migrated pore-filling organic matter that has developed secondary porosity. Where thermally mature and sufficiently thick, these local intervals of silica-cemented mudrocks may offer interesting targets for development of unconventional gas.

Stratigraphic completeness and resolution in an ancient mudrock succession

AbstractMudrocks are the most common rock type at the Earth's surface, and they play a major role in informing current understanding of the palaeoenvironmental history of the planet. Their suitability for this purpose is at least partly underpinned by the assumed stratigraphic completeness of mudrock successions, and the ostensible fidelity with which they record temporal changes in palaeoenvironment. Mud does not necessarily accumulate, however, as a steady, near‐continuous ‘rain’ under low energy conditions. Advective modes of mud transport and episodic, ephemeral accumulation have been shown to dominate in many ancient successions. This has implications for the completeness of these records and their suitability for high‐resolution sampling and analysis. In this study, a numerical model of mud accumulation, parameterized with data from the Lower Jurassic of Yorkshire (UK) is presented to explore completeness and resolution constraints on ancient epicontinental mudrock successions. Using this model, stratigraphic completeness of the analysed Yorkshire succession is estimated to be ca 13% and ca 98% at centennial and millennial timescales, respectively. The findings indicate that sub‐millennial scale processes and events are unlikely to be accurately resolved, despite the largely unbioturbated and well‐laminated nature of the succession. Epicontinental mudrock successions are a crucial archive of ancient environmental changes, and the findings of this study help to define a plausible upper limit on the resolution achievable in these successions. Even with high‐resolution sampling, sub‐millennial scale records of palaeoenvironmental change may not be attainable in ancient epicontinental mudrocks.

Upper Ordovician and Silurian ash beds in the Holy Cross Mountains, Poland: preservation in mudrock facies and relation to atmospheric circulation in the Southern Hemisphere

Numerous volcanic ash beds (K-bentonite) are recognized globally in Ordovician to Early Devonian sedimentary successions, and are important for regional stratigraphic correlations. Relatively little, however, is understood of the origin and transport of these ash deposits. Here, we examine ash beds preserved in the Upper Ordovician and Silurian dark shales and mudstones of the Holy Cross Mountains, Poland, and discuss their origin in the context of contemporaneous tectonic configurations. The Ordovician ash beds occur in the upper Sandbian and Katian mudrock succession, whereas the Silurian K-bentonites form prominent beds in the upper Wenlock and lower Ludlow graptolite shales. Their various thickness and irregular distribution reflect fluctuation of volcanic activity and contribution of physical and biogenic factors in their preservation. Deposition under dysoxic or anoxic conditions and relatively rapid burial by muddy sediment seem to be dominant factors preventing physical and biological reworking and mixing of the volcanic ash. The Upper Ordovician ash beds in the Holy Cross Mountains, as well as the Baltoscandian K-bentonites, appear to be accumulated from pyroclastic material delivered by westerlies of the Southern Hemisphere from the Avalonian volcanoes. In the Silurian, the distribution of pyroclastic material was controlled by SE trade winds, and therefore the source area for the Wenlock and Ludlow K-bentonites in the Holy Cross Mountains appears to have been an active subduction zone developed close to the southeastern side of Baltica. However, the delivery of volcanic ash to the central part of Baltica might have been strongly affected by climatically driven latitudinal migration of the Intertropical Convergence Zone.

The Offshore Bar Revisited: A New Depositional Model For Isolated Shallow Marine Sandstones In the Cretaceous Frontier Formation of the Northern Uinta Basin, Utah, U.S.A.

Abstract Shallow marine sandstone bodies isolated within marine mudrock successions are still incompletely understood. A robust understanding of such bodies is of interest not only scientifically, but also in order to facilitate successful exploration for and production of petroleum and other natural resources. Such bodies are preserved in the Turonian Frontier Formation of the northern Uinta Basin in Utah and Colorado, U.S.A., where they have been previously interpreted as the products of “offshore bars.” A reappraisal of these bodies in the uppermost Frontier Formation (herein defined as the Turonian Ashley Valley Member) reveals three northeast–southwest elongate, incised highstand, forced regressive, and lowstand offshore sandstone bodies partially to fully encased in offshore marine mudrock. These bodies are herein named the Buckskin Hills, Kane Hollow, and Raven Ridge sandstones (from oldest to youngest). Five lithofacies were identified from 47 measured outcrop sections located along the western and southern margins of Dinosaur National Monument. They are 1) marine shelf claystones and siltstones, 2) lower–upper offshore marine admixed siltstone and sandstone, 3) lower–upper offshore thin-bedded sandstones and siltstones, 4) distal lower shoreface heavily bioturbated very fine-grained sandstone, and 5) transgressive lag conglomerate. Wireline logs from 112 drillhole locations throughout the Uinta Basin were calibrated with nearby outcrop locations to construct cross sections and net-sandstone isochore maps. From these, regionally traceable horizons (flooding surfaces and internal erosion surfaces) facilitated an analysis of stacking patterns, development of a stratigraphic framework, and a depositional-environment interpretation. Overall, the Ashley Valley Member preserves a basinward-offlapping stacking pattern, indicative of shoreline progradation in response to the late Turonian Greenhorn global eustatic regression that was interrupted by two, high-frequency transgression–regression cycles. This nested cyclicity cannot be explained by global eustasy alone, and instead is suggestive of a secondary allogenic control, likely related to Western Cordilleran Foreland Basin (WCFB) geodynamic driving mechanisms. As a result, the Ashley Valley Member provides an example of tectonic forcing on stratal stacking patterns, highlighting the importance of intrabasinal tectonic processes on dispersal patterns and high-frequency relative base-level fluctuations within the WCFB stratigraphic record.

Eustatic controls on stratigraphy, chemostratigraphy, and water mass evolution preserved in a Lower Permian mudrock succession, Delaware Basin, west Texas, USA

Bone Spring (Leonardian) mudrock successions in Delaware Basin vary between silica- and carbonate-rich facies marking depositional responses to sea-level changes. Increased Mo, [Formula: see text], and total organic carbon (TOC) record reduced oxygenation during sea-level lowstands. In a 91.4-m (300 ft) core from Sun No. 1 Houssels well and a 39.6-m (130 ft) core from Shell No. 1 Marsden well (Reeves Co., Texas), lower Bone Spring cycles comprised siliceous-calcareous mudrock couplets. The Marsden core was correlative to the upper part of the Houssels core. Cyclicity was interpreted from mineralogical calculations at numerous core horizons using energy-dispersive X-ray fluorescence (ED-XRF) measurements of elemental content. Elemental abundances enabled predicting mineralogy based on the stoichiometric relationships between elements and dominant minerals (calcite and quartz), and from average values of Si and K in published illite analyses. The average sample spacing for ED-XRF measurements was 0.24 m (0.8 ft) for the Houssels core and 0.15 m (0.5 ft) for the Marsden core. The measurements and interpretations provided: (1) information regarding comparisons in stratigraphic and geographic development of facies over the 10-km (6.2-mi) distance between wells and (2) guidance for petrographic, scanning electron micrograph, and organic matter analyses that address processes that produced different facies. Greater carbonate abundance in the Houssels core reflected the proximity to carbonate shelves on the Central Basin Platform (CBP); the greater siliciclastic abundance in the Marsden core was thought to reflect the greater distance from the CBP. Redox element (S and Mo) and TOC concentrations were greater, and [Formula: see text] values were higher in some siliciclastic-dominated intervals, suggesting that anoxia characterized sea-level lowstands when normal marine water flowing from the Panthalassa Ocean was inhibited through narrow interbasin channels or over sills. Carbon isotope ([Formula: see text]) variations suggested changes in organic matter characteristics, whereby greater [Formula: see text]-depleted organic matter was deposited during sea-level lowstands, marking: (1) reduction of shallow-marine sources or (2) increased terrigenous organic carbon contributions.

Eaglebine play of the southwestern East Texas basin: Stratigraphic and depositional framework of the Upper Cretaceous (Cenomanian—Turonian) Woodbine and Eagle Ford Groups

The Woodbine and Eagle Ford Groups of the southwestern East Texas basin compose an emerging play, which has generated considerable interest because of its potential for new hydrocarbon production from both sandstone and mudrock reservoirs. However, the play’s stratigraphic and depositional relations are complex and directly relate to the play’s exploration challenges. Productive Woodbine and Eagle Ford (sub-Clarksville) sandstones intertongue with a poorly defined, subregional mudrock-dominated interval that thins southwestward toward the San Marcos arch. We propose dividing this succession into two intervals: (1) the Lower unit, a high-gamma-ray unit at the base of this mudrock succession that is inferred to be equivalent to the Maness Shale of the Washita Group and to part of the lower Eagle Ford Group on the San Marcos arch, and (2) an Upper unit, a basinward-thickening zone of consistently lower gamma-ray-log facies inferred to be equivalent to the Woodbine Group, Pepper Shale, and the Eagle Ford Group of the East Texas basin. Because the Cenomanian–Turonian boundary occurs within the Eagle Ford Group of the East Texas basin and the lower Eagle Ford section of the San Marcos arch, most of the Maness-through-Eagle Ford succession exists as a much-thinned section on the arch. Basinwide integration of the Woodbine sequence-stratigraphic framework shows that the number of fourth-order sequences in the unit decreases westward from 14 in the basin axis to no more than 9 in the most active part of the Eaglebine play because of their systematic depositional pinch out approaching the western basin margin. The Eagle Ford Group consists of three fourth-order sequences capped by the sub-Clarksville sandstones that accumulated after the major late Cenomanian–early Turonian flooding event recorded by a basinwide transgressive systems tract (TST) at the base of the unit. Depositional systems of the Woodbine Group vary within the study area, even between stratigraphically adjacent systems. On-shelf siliciclastic systems include fluvial-dominated-delta; incised-valley-fill fluvial and nearshore-marine; and wave-dominated-delta deposits.

Preservation of Mesoproterozoic age deep burial fluid signatures, NW Scotland

Bedding-parallel fibrous calcite is a widely developed feature of mudrock successions, reflecting conditions of fluid overpressure (Stoneley, 1983; Parnell et al., 2000; Cobbold et al., 2013). The calcite preserves signatures of fluids developed during deep burial, including hydrocarbons. Most studied examples are of Phanerozoic (<540 Ma) age. This study reports well-preserved fibrous calcite in the Mesoproterozoic (∼1180 Ma) Stoer Group, NW Scotland. The fibrous calcite occurs immediately above a unit of carbonaceous black shale. If hydrocarbons were generated from the black shales, they could have contributed to the development of fluid overpressure, but there is no direct evidence for this. The calcite reflects the original deep burial fluid, rather than a later overprint, because (i) it has a distribution related to stratigraphy, (ii) the bedding-parallel fibres have not been recrystallized, and (iii) later veining is at high angles to bedding. The calcite contains fluid inclusions, and has yielded stable isotope and entrained volatile data, indicating the potential to record diagenetic processes over one billion years ago.

Polyphase low-grade metamorphism of the Ingleton Group, northern England, UK: a case study of metamorphic inversion in a mudrock succession

AbstractA series of boreholes in Horton Quarry, northwest Yorkshire (Horton-in-Ribblesdale Inlier) penetrated mudstones and slates belonging to the Austwick Formation (Windermere Supergroup) overlying laminated mudstones of the Ingleton Group. Illite (IC) and chlorite (ChC) crystallinity measurements indicate a metamorphic inversion between the two groups of mudrocks. The Windermere Supergroup mudrocks are mostly in the high anchizone or epizone, whereas the Ingletonian samples are lower grade in terms of IC, and are mostly deep diagenetic zone or low anchizone. Hence younger strata at higher grades rest on older strata at lower grades, creating a metamorphic inversion. Ingletonian slates exposed at Pecca Falls on the River Twiss show epizonal and anchizonal IC values, and greywacke samples from Ingleton Quarry contain pumpellyite. This suggests that grade in the Ingletonian may increase to the NW from the Horton to Ingleton inliers. K-white mica b cell dimensions show further differences between the Ingleton Group and the Windermere Supergroup. The Ingletonian samples are characterized by low b cell values (8.989–9.035, mean 9.007 Å), whereas the Windermere Supergroup has higher values in the range 9.022–9.034, mean 9.027 Å. The Windermere Supergroup values are similar to those recorded from the Windermere Supergroup of the southern Lake District, and Lower Palaeozoic rocks from the Scottish Southern Uplands, and are consistent with metamorphism in a low heat flow, convergent geotectonic setting. The Ingletonian b cell values suggest metamorphism in a higher heat flow setting, most likely an extensional basin. The metamorphic inversion at Horton and differences in K-white mica b cell dimensions suggest that the Ingleton Group and Windermere Supergroup strata evolved in different geotectonic settings and record two separate metamorphic events. The discovery of the metamorphic inversion at Horton provides further evidence in favour of an Ordovician rather than Neoproterozoic depositional age for the controversial Ingleton Group.

Black Mudrocks: Lessons and Questions from the Mississippian Barnett Shale in the Southern Midcontinent

Although largely unstudied until recently, the Barnett Shale of the Ft.Worth Basin,Texas, has become one of the better known black mudrock successions in the world. Emerging results of new research on these rocks, based mostly on subsurface data, offer intriguing insights into fundamental aspects of mudrock formation. Among these insights is the recognition that mudrock facies are probably not widely continuous, that organic matter conversion to hydrocarbon may control distribution and abundance of nanoscale pores and thus permeability, that fracture distribution may be affected by mineralogical variations along a proximal-distal gradient, and that, based on trace element chemistry, the Barnett ocean may have been much more stratified than most other anoxic basins. Developing concepts of processes and products of mudrock formation being derived from Barnett research offer great potential for comparison to other mudrocks worldwide.

The White Stone Band of the Kimmeridge Clay Formation, an integrated high-resolution approach to understanding environmental change

The Kimmeridge Clay is a Jurassic mudrock succession that shows Milankovitch Band climatic cyclicity. A key issue is to determine how the subtle changes that define this cyclicity result from climatic change. Using material from the Natural Environment Research Council Rapid Global Geological Events (RGGE) Kimmeridge Drilling Project boreholes, the White Stone Band was investigated at the lamination scale using backscattered electron imagery and quantitative palynofacies. Fabric analysis shows the lamination to represent successive deposition of coccolith-rich and organic-matter-rich layers. Individual laminae contain unsorted palynological debris with a consistent ratio of marine and terrestrial components. Such mixed organic matter input is interpreted as the result of storm transport. Linking water column processes to laminae deposition suggests seasonal input with a coccolith bloom followed by a more diverse assemblage including dinoflagellates and photosynthetic chlorobiacean bacteria. As the photic zone extended into the euxinic water column organic matter export to the sea bed underwent minimal cycling through oxidation and subsequently became preserved through sulphurization with greatly increased sequestration of carbon. This was significantly increased by late season storm-driven mixing of euxinic water into the photic zone. Increased frequency of storm systems would therefore dilute the coccolith input to give an oil shale. Hence climatically induced changes in storm frequency would progressively vary the organic content of the sediment and generate the climate cycle signal.

The rhenium–osmium isotope system: applications to geochronological and palaeoenvironmental problems

The rhenium–osmium (Re–Os) isotope system occupies a unique position amongst commonly used radiogenic parent–daughter isotope pairs on account of its siderophilic geochemical characteristics. Re and Os are also concentrated from seawater in organic-rich sediments and have been used to obtain absolute ages for the deposition of marine mudrock successions directly. Additionally, Re–Os analyses of ancient mudrocks have provided the best means so far of determining temporal changes in the Os-isotope composition of pre-Cenozoic seawater. The seawater Os-isotope record has yielded valuable information about several major episodes of past environmental change, and is particularly powerful when used in conjunction with other marine isotopic tracers such as carbon and strontium. The present contribution reviews some recent developments in the application of the Re–Os isotope system to geochronological and palaeoenvironmental problems in the Mesozoic and earlier. New results show that the Os-isotope composition of Callovian seawater ( c . 160 Ma ago) was very unradiogenic ( 187 Os/ 188 Os ≈0.26) at the same time that the seawater Sr-isotope curve recorded its lowest value for the Phanerozoic, indicating that global continental weathering rates were very low at c . 10–15% of today's levels.

Dating diagenetic monazite in mudrocks: constraining the oil window?

Authigenic monazite nodules, increasingly recognized in Pre-Mesozoic mudrock successions, provide a means for dating diagenesis by exploiting trace uranium within the monazite structure. But, dating is difficult in practice because of zonation and abundant host rock inclusions. We report here a refined technique that has yielded a date of 419 ± 14 Ma 2 σ MSWD = 1.2 for a 17-point Pb–Pb regression age from a Telychian (Silurian: late Llandovery) mudrock from central Wales. SHRIMP analyses of monazite nodules from the same horizon are consistent with this, and additionally demonstrate concordance of the U–Pb and Th–Pb systems. These results open the door to providing robust dates for a phase of burial-related diagenesis which, in Wales, seems approximately coincident with, and perhaps was related to, hydrocarbon expulsion.

A Hydro-Geomechanical View of Seal Formation and Failure in Overpressured Basins

The formation of seals, and the consequent retention of overpressure, is a common and expected event during normal basin evolution in active basins that have a significant mudrock succession. Seal failure, and the resultant limitation to fluid-pressure increase (valve action), is also an expected process. Hydro-geomechanical concepts, including poro-elasto-visco-plasticity (PEVP), and discontinuum models, allow the processes of seal formation and failure to be explained, and predict the state of stress in, and behaviour of, overpressured rocks. Importantly, properties that relate to porosity (such as permeability) can be integrated with this approach. These concepts provide a capability to undertake simulations that can serve as predictive tools. The fluid/rock interactions in a basin can be viewed as a self-organizing system, in the sense that overpressure and the undercompaction it permits retard the creation of accommodation space, and hence act as a negative feedback mechanism that slows the loading rate.

Submarine mass‐wasting deposits as an indicator of the onset of foreland thrust loading—Late Permian Bowen Basin, Queensland, Australia

In the eastern part of the Permo‐Triassic Bowen Basin of Queensland, Australia, a transition from passive, thermal subsidence to flexural (foreland basin) subsidence is recorded within the Upper Permian stratigraphy. Two coarse‐grained intervals containing deposits of mass‐wasting processes occur within an otherwise siltstone‐dominated succession over 1500 m thick (the Moah Creek Beds and equivalents). These intervals can be traced over at least 350 km north–south, along the structural eastern margin of the basin. The lower of the coarse‐grained intervals is spectacularly exposed in the banks of the Fitzroy River, west of Rockhampton. Here, interbedded sandstones and siltstones of marine shelf origin are abruptly truncated by a mudrock succession containing evidence of slumping and contemporaneous magmatic activity. This unit passes up‐section into packages of mass‐flow conglomerates and diamictites, interpreted to have formed on an unstable submarine slope. The character of the mass‐flow deposits, their stratigraphic position and lateral extent are interpreted in terms of destabilization of a sloping marine surface by pulsed, subsurface thrust propagation.

Gamma-ray spectrometry as a tool for stratigraphical interpretation: examples from the western European Lower Jurassic

Abstract Portable gamma-ray spectrometry provides an objective and quantitative method of studying sedimentary cyclicity in otherwise-cryptic mudrock successions. Spectral gamma-ray data are presented from the Lower Jurassic sections of the Yorkshire and Dorset Coasts (England), from Peniche (Portugal) and from inland exposures in southern Germany, including a recent road-cut. In Yorkshire, where proximal-distal relationships are readily demonstrated from sedimentological evidence, there is good correspondence between more proximal facies and elevated Th/K ratios. This relationship may be extended to the Dorset succession where the sequence-stratigraphical interpretation of Lower Lias mudrocks has been a source of some controversy. Th/K ratio data here suggest a distal, starved, Hettangian-earliest Sinemurian (Blue Lias) and a prograding Sinemurian interval (Black Ven Marls and Shales-with-‘Beef’). Flooding and further progradation in the Early Pliensbachian (Belemnite Marls) is also suggested. These results provide support for common ‘second order’ stratigraphical forcing mechanisms between the Dorset and Yorkshire successions. Data from mainland Europe suggest that there are systematic regional variations in Th/K ratio, upon which local temporal variations are superimposed. These may reflect climatic or regional sediment transport effects.