Gordon Walkden

Basal dinosauriform remains from Britain and the diagnosis of the Dinosauria

A new genus and species of dinosauriform is described from the Triassic of south-west England. The description is based on isolated elements recovered from an assemblage of other dissociated tetrapod remains that include crocodylomorphs, rauisuchiforms and sphenodontians. The key elements in the new taxon are the ilium, astragalus, and the humerus, and these exhibit five synapomorphies of Dinosauria. Three of these, namely a largely to fully perforate acetabulum, the presence of a brevis fossa, and a reduced astragalus with an ascending process, are considered to be particularly relevant. The definition and diagnosis of the Dinosauria are restated and the positions of the new form, herrerasaurs and Eoraptor relative to true dinosaurs are discussed.

Palaeokarstic surfaces in upper Visean (Carboniferous) limestones of the Derbyshire Block, England

ABSTRACT Undulating, hummocky and often deeply pitied discontinuities, commonly up to 1 m in amplitude, occur throughout D1 and D2 subzones in the Carboniferous Limestone sequence of the Derbyshire Block, England. Previously termed mammillated surfaces and potholed surfaces, these horizons usually underlie a thick K-bentonite clay and are shown to be Carboniferous erosion features that formed by subaerial solution beneath a cover of soil derived from volcanic ash. These palaeokarstic surfaces present evidence for contemporaneous lithification of the limestones, and brown laminated coatings are interpreted as caliche-type crusts similar to some modern examples. Surfaces of average development may each represent subaerial exposure of between 30,000 and 100,000 years, and certain karstic morpholo ies may reflect the influence of a near-surface water table.

Late Quaternary highstand deposits of the southern Arabian Gulf: a record of sea-level and climate change

Abstract The southern Arabian (Persian) Gulf is at present the site of extensive carbonate sedimentation, as was the case during Pleistocene interglacial marine highstands. During glacial lowstands the basin was subaerially exposed, and aeolian sedimentation predominated. Most of the southern Arabian Gulf floor is underlain by Quaternary carbonates, and scattered outcrops may be found onshore. These belong to three formations: the aeolian Ghayathi Formation, the continental Aradah Formation and the marine Fuwayrit Formation. The Fuwayrit Formation consists of three members, separated by subaerial exposure surfaces. These are, from the base upwards, the shallow marine Futaisi and Dabb’iya Members, and the aeolian Al Wusayl Member. Offshore, at least six Quaternary sequences are present within the uppermost 50 m of sediment. No reliable direct age dates have been acquired from Pleistocene shallow marine or coastal deposits in the southern Arabian Gulf. It has therefore been necessary to infer the ages of these sediments by a comparison of their stratigraphy and elevation with deposits known from other parts of the world. We regard this approach as valid because the southern Gulf coastline lacks evidence for significant widespread neotectonic uplift, and halotectonic effects are localized. This comparison indicates that the Fuwayrit Formation was deposited during the last interglacial (oxygen isotope substage 5e), as (1) these sediments represent the youngest pre-Holocene marine deposits, and (2) they are found at an elevation correlative with many substage 5e deposits from other parts of the globe. Sedimentary evidence reveals two highstands during this period, peaking at around 1.5 m and 6 m above present sea level, respectively. Offshore sediments indicate that sea level did not fall as far as 24 m below present level in the intervening regression. Following the second highstand, sea level fell to more than 23 m below present level, before briefly rising once again (late isotope stage 5). This later highstand probably peaked between 17 and 7 m below present level. The sequence underlying the Fuwayrit Formation was probably deposited during the penultimate interglacial (late oxygen isotope stage 7). It is also likely that the Ghayathi Formation aeolianites were largely sourced from this sequence. Facies analysis of offshore core sediments indicates that sea level reached at least 15 m below present level during this period. Widespread evidence exists for a Holocene sea level higher than at present in the southern Arabian Gulf, indicating that it peaked at 1–2 m above present level, c. 5.5 ka bp. Pleistocene deposits preserved in the southern Arabian Gulf provide a record of changing palaeowinds and palaeoclimates. Currently, the region experiences a hyper-arid to arid climate, with facies patterns dominated by the northwesterly shamal wind. The Ghayathi Formation was originally deposited under an arid climatic regime, which allowed the sediments to remain unconsolidated. The dunefield was later remodelled under conditions of increasing wind speed, with a change in wind direction from NNW to WNW. These changes are thought to reflect the onset of glaciation. Palaeocurrent directions from the Al Wusayl Member, combined with sedimentary evidence from the Futaisi and Dabb’iya Members, indicate that during the peak of the last interglacial the prevailing wind (the ‘palaeo-shamal’) blew from the NE. Compelling evidence for a pluvial episode during this period is provided by abundant and widespread dissolution (palaeokarstic) pits found in the top surface of the Futaisi Member, believed to represent the former positions of abundant trees or large plants.

Carbonate ramps and the Pleistocene-Recent depositional systems of the Arabian Gulf

Abstract The term ramp was devised to be helpful. It was to provide an interpretative model with genetic significance for a specific category of carbonate body and one with predictive value in oil exploration. The present-day southern Arabian Gulf is commonly cited as a ramp but it fails to provide a good example. In contrast with most of its pre-Miocene history, the Arabian Gulf is a foreland basin with a tectonically, climatically and glacioeustatically controlled mixed carbonate-clastic fill. Because of glacioeustasy and its relatively shallow depth, the Gulf has spent much of the last 2.5 Ma emergent. Three separate and equally important sedimentary systems currently contribute to its development: the Arabian Gulf marine carbonate system, the Mesopotamian fluviodeltaic system and the Arabian continental aeolian system. The Arabian Gulf carbonate system is a ‘high stress’ regime that is in tectonic, eustatic and depositional disequilibrium. Its structural, morphological and sedimentological diversity discount it as a Recent analogue for the thick carbonate bodies in the geological record that show persistent accumulation of depth-related facies developed across gentle slopes lacking a marked shelf-slope break. The Arabian Gulf fails some fundamental tests for whether it should be regarded as a stable carbonate ramp depositional system and its use as an example of a ramp is not helpful.

Carbonate spherules and botryoids as lake floor cements in the East Kirkton Limestone of West Lothian, Scotland

The East Kirkton Limestone is typically a carbonate/organic laminite characterised at many levels by abundant radial-fibrous calcite (RFC) spherules and by less common larger stromatolite-like accretions of laminated botryoidal RFC. The spherules are mostly c. 1 mm in diameter and have cyanophyte and chlorophyte inclusions. Some spherules enclose parallel bundles of complete cyanophyte fibres and probably grew within a living cyanophyte mat. The botryoidal accretions were commonly seeded upon wood and other exposed organic remains such as bone, and they completely enclose twigs and branches where these were held above the sediment surface. Botryoidal accretions commonly contain the remains of a benthos of cyanophytes, chlorophytes and ostracods

The use of burial diagenetic calcite cements to determine the controls upon hydrocarbon emplacement and mineralization on a carbonate platform, Derbyshire, England

Abstract Late diagenetic calcite cements in the Upper Dinantian limestones of the Derbyshire Platform are contemporaneous with both hydrocarbon emplacement and Mississippi Valley-type (MVT) mineralization. Calcite cementation began during the progressive burial of the Derbyshire Platform and the surrounding basins, principally within fractures generated during the waning effects of Upper Carboniferous extension. Six burial calcite cements can be recognized in dilational vein systems. Successive veins contain progressively more mature hydrocarbon inclusions, and calcite cements are intergrown with fluorite, baryte, galena and sphalerite in increasing quantities. Compacting Dinantian-Namurian shales in basins adjacent to the platform offer the most likely sources of fluids, trace elements and hydrocarbons. Fluids entered the platform along major fault systems, and circulated using smaller fracture systems, precipitating calcite. The final phases of calcite cementation and the main phase of MVT mineralization coincided with the onset of the Variscan Orogeny. A model is now established relating fluid flow to Variscan tectonic events in northern Britain.

Reconstructing Fluid Expulsion and Migration North of the Variscan Orogen, Northern England

ABSTRACT Lower Carboniferous platform carbonates in northern England host lead-zinc-fluorite-barite deposits that have previously been classified as Mississippi Valley-type (MVT). The controls on the distribution of low-temperature, particularly MVT, minerals have long been debated, but the interplay of basin evolution and fluid movement is often neglected. Early Carboniferous sedimentation on the Derbyshire Platform took place within a back-arc extensional regime, north of the northward-migrating Variscan orogenic front, which increasingly influenced fluid movement on the platform throughout the Carboniferous. It is the interplay of fluid movement, cementation, and Variscan tectonism that is the focus of this paper, which aims to reconstruct fluid movement within a basin moving from an extensional to a compressional regime. In particular, it demonstrates the variability of fluid flux during post-rift subsidence and basin inversion and the corresponding influences of this upon mineral distribution. During early burial of the Derbyshire Platform, pore-filling cements were precipitated from meteoric porewaters, driven downdip from the east under topographic drive. These cements occluded most interparticle porosity, and therefore permeability became fracture-controlled. The waning effects of extensional tectonism during post-rift subsidence permitted intermittent expulsion of small volumes of trace metal-charged, carbonate-saturated fluids from overpressured contemporaneous clastic basins adjacent to the Derbyshire Platform. Overall, however, waning tectonism meant that the volume of fluids released during this period remained minor. With the onset of the Variscan Orogeny, and resultant compressional tectonism and basin inversion, Caledonian basement fault systems were reactivated, channeling large volumes of trace metal-charged fluids from the basins onto the Derbyshire Platform, establishing an east-west component to fluid flow. The reconstruction of fluid expulsion and migration within this tectonically active regime demonstrates variations in fluid source and migration pathways during basin evolution. Consequently, fluid movement is complex, with the platform receiving fluids from a variety of sources. This highlights the importance of understanding both the temporal and the spatial variations in cement chemistry when reconstructing flow. The results of this study have relevance to other mineralized carbonate platforms in extensional and compressional settings worldwide and especially to sedimentary basins that host MVT deposits and hydrocarbons, where it is crucial to understand the timing and mechanisms of diagenetic cementation and hydrocarbon emplacement.

Burial diagenetic evolution of the Lower Carboniferous (Dinantian) of the southern margin of the Askrigg Platform and a comparison with the Derbyshire Platform

In order to predict the style and impact of post-depositional modification of carbonate successions, well-studied and accessible outcrop analogues are invaluable. The Lower Carboniferous (Dinantian) carbonate platforms of the Pennine Basin of northern England have a long history of investigation. As such, they offer the potential to evaluate the mechanisms and timing of fluid flux during extensional tectonism, post-rift basinal subsidence and inversion. This study concentrates upon the diagenetic evolution of the late Dinantian of the southern margin of the Askrigg Platform of North Yorkshire and a comparison with published data from the age-equivalent Derbyshire Platform. A pattern of consistent, diagenetic modification during early diagenesis is evident, but key differences occur in the burial realm. On both the southern margin of the Askrigg Platform and the Derbyshire Platform, patterns of dolomitization, hydrocarbon emplacement and mineralization can be determined on the platform that reflect the diagenetic evolution of the adjacent basins. However, within the study area of the Askrigg Platform, there is only local evidence for a fault/fracture control on the migration of Mg-enriched, hydrocarbon-bearing fluids. In contrast, on the Derbyshire Platform, burial diagenesis is intimately associated with NW–SE- and NE–SW-trending faults and fractures. Data suggest that pervasive cementation in the marine and meteoric realm occluded matrix porosity in both areas, such that fluid migration was almost entirely fracture controlled. With the localization of structural deformation along the Craven Fault Zone, and a low abundance and density of open fault/fracture networks, circulation of fluids on to the southern margin of the Askrigg Platform was inhibited, however. Furthermore, the presence of local aquifers in the Craven Basin may have led to fluid expulsion from the basin during early burial.

PROMOTING ART, MANUFACTURES AND COMMERCE IN ONE—THE SOCIETY'S ROLE IN THE DEVELOPMENT OF A BRITISH MARBLE INDUSTRY

Amongst its promotions at the start of the nineteenth century, the Society for the Encouragement of Arts, Manufactures and Commerce included calls for British marbles. The calls were repeated annually for two decades but what initiated them was more than just an altruistic desire to promote indigenous sources of statuary and decorative stone. Supplies of both, especially statuary marble, greatly relied upon imports from France and Italy. At the time of the first calls these were jeopardised by the revolutionary and Napoleonic upheavals and other sources of stone became necessary, but the Society never cited political pressures as a driver behind their calls for British marbles. The term ‘marble’ was to be interpreted widely, and the response brought limestones, serpentines, granites and true marbles from across the British nations including much from southern Ireland. Two Gold Medals were awarded, one for a spectacular revelation of Devonshire marbles, and one for sheer guts and determination shown in bri...