The depositional and diagenetic history of the Westbury Formation (Upper Triassic) in South West Britain

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An investigation of the sediment dispersal operating to control lithofacies variability and organic carbon preservation in an ancient mud-dominated succession : a case study of the Lower Jurassic mudstone dominated succession exposed in the Cleveland Basin (North Yorkshire)

In this study the Cleveland Ironstone and Whitby Mudstone Formations have been investigated to characterise: a) the evolving mudstone facies present in a basin that is gradually deepening and developing bottom water anoxia over time and b) what the fundamental geological controls were on this variability. Using detailed facies descriptions obtained from analyses of approximately 151 samples obtained from combined optical, electron optical and geochemical methods 6 lithofacies have been identified. These include: 1) sand and clay-bearing, silt-rich mudstones, 2) siltbearing, clay-rich mudstones, 3) clay-rich mudstones, 4) clay, calcareous nannoplankton-, and organic carbon-bearing mudstones, 5) fine-grained muddy sandstones, and 6) cement rich mudstones. Individually, the samples are highly heterogeneous and typically organised into thin beds (<10 mm thick). These beds contain varying proportions of materials derived from inputs to the basin, primary production within the basin and the effects of diagenesis. In addition, they are microtexturally diverse and preserve primary depositional textures such as: sharp bases, normal graded bedding, starved ripple laminae, triplet fabrics, tempestites, pelleted laminae as well as a variety of burrowing fabrics. With these data three main questions were addressed (each of these questions forms the basis of a paper). These include (1) Identifying the main processes responsible for sediment dispersal in this succession, (2) Discussing the mechanisms of organic carbon preservation when bottom water anoxia was not as prevalent as most authors have assumed and (3) Determining if this succession can be interpreted within a sequence stratigraphic framework. The presence of ripples and triplet fabrics throughout this succession indicate that mud deposition in this succession was much more dynamic than most researchers had assumed. Moreover there was not that much difference between the processes operating to deliver and disperse sediment in the coarser parts of the succession compared with those operating in the finer grained, more production parts of the succession. The large volumes of organic carbon preserved here indicate that the role of bottom water anoxia as a pre-requisite for enhanced organic carbon preservation in sediments has been overstated. Moreover, the presence of marine snow suggests that much of the organic carbon was delivered episodically to the sea floor following phytoplankton blooms. Finally, as a significant fraction of the sediment is being dispersed by advective processes operating to infill available accommodation the building of sequences can be recognised (namely beds, parasequences, and systems tracts at stratal surfaces) this type of succession is reasonably interpreted using sequence stratigraphic principles. This study demonstrates that it is possible to directly link up-dip lithofacies variability in proximal sandy-mudstone deposits with coeval variations down-dip in distal basinal deposits. There is no reason why the processes occurring in basinal settings should be disconnected from those occurring up-dip in the lower shore face and offshore transition environments as their deposition represents a continuum of processes.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Origin and variability of the late Precambrian-Cambrian Athel Silicilyte, South Oman Salt Basin

The Precambrian-Cambrian Athel Silicilyte is an enigmatic chert unit of up to 390 m thick found as slabs (each slab typically 2 × 6 km across) entrapped within salt domes at a depth of 4-5 km in the South Oman Salt Basin. This formation is a prolific self-charged reservoir with high porosity (up to 34 %) and high oil saturation (80 %). Despite its economic value, the origin and the variability of this formation are not fully understood. This study therefore aims to investigate the variability and the origin (silica source and precipitation mechanism) of the Athel Silicilyte. Data obtained from core, wireline log and petrographical analysis were employed to establish the vertical and the lateral variability and, with the assistance of geochemical data, the likely source and precipitation mechanism of silica was determined.The Athel Silicilyte is only present in the deepest parts of the South Oman Salt Basin within the fault-bounded Athel Basin, bounded by two silt-rich mudstone units. Six lithofacies were recognised in the Athel Silicilyte that reflect variability in detrital material contents (three silica-rich facies), sediment remobilisation (slumped and brecciated lithofacies) and diagenetic modification (carbonate-bearing lithofacies), with the silica-rich facies being the most abundant (> 97 % of the total thickness). The Athel Silicilyte exhibits wavy discontinuous lamination and it is predominately (silica-rich facies average = 80 wt. %) composed of connected-networks of microcrystalline quartz (1-5 µm). Other constituent components are detrital quartz (3 wt. %), illite (10 wt. %), pyrite (4 wt. %) and organic carbon (TOC = 3 wt. %). The detrital contents increase towards the Upper Athel Silicilyte and towards the basin margins (ranging from 3 to 30 wt. %). The wavy laminations are interpreted to have a microbial origin. The homogeneity, loose packing of detrital grains and preservation of 390 m thick laminated fabric suggest that the Athel Silicilyte precipitated syndepositionally in microbial layers during low detrital input. The intense detrital input during bounding mudstones sedimentation inhibited Athel Silicilyte precipitation as a result of the rapid burial of microbial layers. The Athel Silicilyte shows strong enrichments of redox-sensitive elements (U, V and Mo), Mn-depletion, positive Ce anomalies, and small framboidal pyrites (3.6-4.0 µm), suggesting that the water column was euxinic during precipitation.Based on the petrographical evidence for the Athel Silicilyte forming as a syndepositional precipitate alongside seawater-like rare earth element (REE) characteristics, silica is interpreted to have been sourced directly from seawater. Mass balance calculations support this interpretation, indicating that silica-rich Precambrian seawater provided the significant silica mass in the Athel Silicilyte. The ability of dissolved silica to form hydrogen bonds with the functional groups (e.g. carboxyl and hydroxyl) in microbial layers was the key for Athel Silicilyte precipitation. The formation of hydrogen bonds was made possible under euxinic conditions, where the pH values were probably lower (< 7) than for the normal seawater as a result of HS- and H+ production by sulphate reducing bacteria and HS- oxidation at the redox boundary by sulphur oxidising bacteria. Consequently, dissolved silica was concentrated in microbial layer microenvironemnts, resulting in silica nucleation and polymerisation.EThOS - Electronic Theses Online ServicePetroleum Development of Oman (PDO)GBUnited Kingdo

Stratigraphic completeness and resolution in an ancient mudrock succession

Funded by NERC Fellowship. Grant Number: NE/I02089X/1 Santander student project scholarship grantPeer reviewedPublisher PD

Stable isotope (δ18O and δ13C) sclerochronology of Callovian (Middle Jurassic) bivalves (Gryphaea (Bilobissa) dilobotes) and belemnites (Cylindroteuthis puzosiana) from the Peterborough Member of the Oxford Clay Formation (Cambridgeshire, England): Evidence of palaeoclimate, water depth and belemnite behaviour

Incremental δ18O and δ13C signals were obtained from three well-preserved specimens of Cylindroteuthis puzosiana and from three well-preserved specimens of Gryphaea (Bilobissa) dilobotes from the Peterborough Member of the Oxford Clay Formation (Cambridgeshire, England). Through-ontogeny (sclerochronological) δ18O data from G. (B.) dilobotes appear to faithfully record seasonal temperature variations in benthic Callovian waters of the study area, which range from c. 14 °C to c. 17 °C (arithmetic mean temperature c. 15 °C). Water depth is estimated to have been in the region of c. 50 m, based upon comparisons between these data, previously published non-incremental sea surface δ18O values, and a modern analogue situation. Productivity in Callovian waters was comparable with that in modern seas, based upon δ13C data from G. (B.)dilobotes, with 13C depletion occurring during warmer periods, possibly related to an interaction between plankton blooms and intra-annual variations in mixing across a thermocline. Incremental δ18O data from C.puzosiana provide temperature minima of c.11 °C for all specimens but with maxima varying between c.14 °C and c.16 °C for different individuals (arithmetic mean values c. 13 °C). Temperatures for late ontogeny, when the C. puzosiana individuals must have been living close to the study site and hence the analysed specimens of G. (B.) dilobotes, are closely comparable to those indicated by the latter. However, for significant portions of ontogeny C. puzosiana experienced temperatures between c. 2 °C and c. 3 °C cooler than the winter minimum as recorded by co-occurring G. (B.) dilobotes. Comparisons with modern seas suggest that descent to a depth of c. 1000 m would be necessary to explain such cool minimum temperatures. This can be discounted due to the lack of deep waters locally and due to estimates of the depth tolerance of belemnites. The most likely cause of cool δ18O signals from C. puzosiana is a cosmopolitan lifestyle including migration to more northerly latitudes. Mean δ13C values from C. puzosiana are comparable with those from G.(B.)dilobotes. However, the incrementally acquired data are highly variable and probably influenced by metabolic effects.The probable identification of migratory behaviour in C. puzosiana calls into question the reliability of some belemnite species as place-specific palaeoenvironmental archives and highlights the benefits of adopting a sclerochronological approach

The Impact of Global Warming and Anoxia on Marine Benthic Community Dynamics: an Example from the Toarcian (Early Jurassic)

The Pliensbachian-Toarcian (Early Jurassic) fossil record is an archive of natural data of benthic community response to global warming and marine long-term hypoxia and anoxia. In the early Toarcian mean temperatures increased by the same order of magnitude as that predicted for the near future; laminated, organic-rich, black shales were deposited in many shallow water epicontinental basins; and a biotic crisis occurred in the marine realm, with the extinction of approximately 5% of families and 26% of genera. High-resolution quantitative abundance data of benthic invertebrates were collected from the Cleveland Basin (North Yorkshire, UK), and analysed with multivariate statistical methods to detect how the fauna responded to environmental changes during the early Toarcian. Twelve biofacies were identified. Their changes through time closely resemble the pattern of faunal degradation and recovery observed in modern habitats affected by anoxia. All four successional stages of community structure recorded in modern studies are recognised in the fossil data (i.e. Stage III: climax; II: transitional; I: pioneer; 0: highly disturbed). Two main faunal turnover events occurred: (i) at the onset of anoxia, with the extinction of most benthic species and the survival of a few adapted to thrive in low-oxygen conditions (Stages I to 0) and (ii) in the recovery, when newly evolved species colonized the re-oxygenated soft sediments and the path of recovery did not retrace of pattern of ecological degradation (Stages I to II). The ordination of samples coupled with sedimentological and palaeotemperature proxy data indicate that the onset of anoxia and the extinction horizon coincide with both a rise in temperature and sea level. Our study of how faunal associations co-vary with long and short term sea level and temperature changes has implications for predicting the long-term effects of “dead zones” in modern oceans

Hadley circulation and precipitation changes control black shale deposition in the Late Jurassic Boreal Seaway

New climate simulations using the HadCM3L model with a paleogeography of the Late Jurassic [155.5 Ma], and proxy-data corroborate that warm and wet tropical-like conditions reached as far north as the UK sector of the Jurassic Boreal Seaway [~35oN]. This is associated with a northern hemisphere Jurassic Hadley cell and an intensified subtropical jet which both extend significantly polewards than in the modern (July-September). Deposition of the Kimmeridge Clay Formation [KCF] occurred in the shallow, storm-dominated, epeiric Boreal Seaway. High resolution paleo-environmental proxy data from the Kimmeridge Clay Formation [KCF; ~155–150 Ma], UK are used to test for the role of tropical atmospheric circulation on meter-scale heterogeneities in black shale deposition. Proxy and model data show that the most organic-rich section [eudoxus to mid-hudlestoni zones] is characterised by a positive δ13Corg excursion and up to 37 wt% total organic carbon [%TOC]. Orbital-modulation of organic carbon burial primarily in the long eccentricity power band combined with a clear positive correlation between %TOC carbonate-free and the kaolinite/illite ratio supports peak organic carbon burial under the influence of very humid climate conditions, similar to the modern tropics. This re-interpretation of large-scale climate relationships, supported by independent modelling and geological data, has profound implications for atmospheric circulation patterns and processes affecting marine productivity and organic carbon burial further north along the Boreal Seaway, including the Arctic

Hybrid event beds dominated by transitional-flow facies: character, distribution and significance in the Maastrichtian Springar Formation, north-west Vøring Basin, Norwegian Sea

Hybrid event beds comprising clay-poor and clay-rich sandstone are abundant in Maastrichtian-aged sandstones of the Springar Formation in the north-west Vøring Basin, Norwegian Sea. This study focuses on an interval, informally referred to as the Lower Sandstone, which has been penetrated in five wells that are distributed along a 140 km downstream transect. Systematic variations in bed style within this stratigraphic interval are used to infer variation in flow behaviour in relatively proximal and distal settings, although individual beds were not correlated. The Lower Sandstone shows an overall reduction in total thickness, bed amalgamation, sand to mud ratio and grain size in distal wells. Turbidites dominated by clay-poor sandstone are at their most common in relatively proximal wells, whereas hybrid event beds are at their most common in distal wells. Hybrid event beds typically comprise a basal clay-poor sandstone (non-stratified or stratified) overlain by banded sandstone, with clay-rich non-stratified sandstone at the bed top. The dominant type of clay-poor sandstone at the base of these beds varies spatially; non-stratified sandstone is thickest and most common proximally, whereas stratified sandstone becomes dominant in distal wells. Stratified and banded sandstone record progressive deposition of the hybrid event bed. Thus, the facies succession within hybrid event beds records the longitudinal heterogeneity of flow behaviour within the depositional boundary layer; this layer changed from non-cohesive at the front, through a region of transitional behaviour (fluctuating non-cohesive and cohesive flow), to cohesive behaviour at the rear. Spatial variation in the dominant type of clay-poor sandstone at the bed base suggests that the front of the flow remained non-cohesive, and evolved from high-concentration and turbulence-suppressed to increasingly turbulent flow; this is thought to occur in response to deposition and declining sediment fallout. This research may be applicable to other hybrid event bed prone systems, and emphasizes the dynamic nature of hybrid flows

Palaeo-environment in an ancient low-latitude, arid lacustrine basin with loessite : The Smith Bank Formation (Early Triassic) in the Central North Sea, UK Continental Shelf

Acknowledgements This work was conducted as part of the Triassic Mudstones Joint Industry Project which was sponsored by BP, ConocoPhillips, EON E&P, Esso Exploration and Production UK Limited, GDF SUEZ E&P UK Ltd, JX Nippon, Maersk Oil, Shell and Total. The guidance of Prof. Steve Hillier of the James Hutton Institute in quantitative mineralogical analysis is acknowledged with gratitude. Dr Stuart Jones and an anonymous referee are thanked for their incisive and constructive reviews that significantly helped in the revision of the manuscript.Peer reviewedPostprin