Purbeck Beds Research Articles

Sauropod dinosaur tracks from the Purbeck Group (Early Cretaceous) of Spyway Quarry, Dorset, UK.

Dinosaur tracks have a long history of discovery and study in the UK, but track sites for sauropodomorph dinosaurs-the group that included the giant, graviportal herbivorous sauropods-are comparatively rare. Here, we provide a description of a sauropod dinosaur track site at Spyway Quarry in Dorset, southern England. The tracks at Spyway were discovered in the late 1990s and occur in the Stair Hole Member of the Durlston Formation in the Purbeck Limestone Group, of earliest Cretaceous age. More than 130 individual tracks of large sauropod dinosaurs are present at the site, but they are generally poorly preserved and do not form clear trackways, although it is likely that they represent multiple individuals. They provide further evidence for sauropods living in or passing through coastal lagoonal environments. Although poorly preserved, Spyway represents the largest in situ dinosaur track site currently accessible within the Purbeck Group, with considerable potential for further discoveries through ongoing quarrying in the surrounding area.

Lost seas, lakes, and lagoons: terminal Jurassic strata and environments in Buckinghamshire, English south midlands

The Jurassic strata running through the county of Buckinghamshire, English south midlands, culminate in the marine to non‐marine Upper Tithonian (formerly Portlandian) Portland and Purbeck formations. Despite poor exposure, this richly fossiliferous and strongly regressive carbonate‐clastic succession allows reconstruction of shallow marine, coastal lagoonal and ultimately lacustrine environments, against the backdrop of a hot, greenhouse climate.

Multi-scale characterization from the Hard Cap, Mupe Member, Lower Purbeck Limestone Group, Wessex Basin, Dorset, UK

Multi-scale characterization from the Hard Cap, Mupe Member, Lower Purbeck Limestone Group, Wessex Basin, Dorset, UK

New evidence of the architecture and affinity of fossil trees from the Jurassic Purbeck Forest of southern England

ABSTRACT We document the habit and affinity of the most complete Mesozoic Era tree to be excavated in the UK. The fossil was found in situ in a palaeosol of the Upper Jurassic Purbeck Group of southern England (Tithonian: ca. 150–145 million years). It comprises over 100 permineralized (silicified) pieces that represent a rooted stump and fallen trunk, together weighing more than two tonnes. This exceptional specimen was excavated in a manner that retained the original associations among its parts, providing a unique insight into the overall habit and mode of growth. A laser scanning approach was used to facilitate the investigation, producing the largest 3D reconstruction of a plant fossil. Anatomical details reveal that the wood belongs to the fossil-genus Agathoxylon. Despite an estimated growth age of more than 200 years, the tree was of modest size, not greatly exceeding 12 m in height. The main trunk bifurcated, developing into a decurrent, spreading crown. Its habit differed significantly from most modern arborescent conifers, which have pole-like central trunks and narrow, conical crowns, and from known growth forms in the Cheirolepidiaceae, an important extinct group of Mesozoic conifers. These findings extend our knowledge of the growth architecture of Jurassic conifers, which were prominent and diverse elements of seasonally arid, low to mid-latitude coastal communities during the Late Mesozoic.

Carbonated Water Injection Effects on Lacustrine Carbonates of Mupe Member, Lower Purbeck Group (Upper Jurassic), United Kingdom

Summary This paper describes the study of dissolution and mineralogical alteration caused by saline carbonated water injection (CWI) and its effects on the petrophysical properties (porosity and permeability) of limestone samples from the Mupe Member, composed of lacustrine microbialites from the Upper Jurassic, part of the Purbeck Group lower portion. These limestones are a partial analog of the Brazilian presalt Aptian carbonates, the most important oil reservoir in Brazil. These reservoirs present large amounts of CO2 that are reinjected into the formation, which given the high reactivity of carbonate rocks in the presence of carbonic acid generated by the reaction between CO2 and water, can cause damage to the rock’s pore space. To achieve the proposed objectives, four laminated/massive samples with very low permeability (<5 md) and two vuggy/microbial samples with very high permeability (>1,700 md) underwent laboratory tests carried out before, during, and after CWI, including gas porosity and permeability measurement, nuclear magnetic resonance (NMR), microcomputed tomography (micro-CT), and ion chromatography. X-ray diffraction (XRD) analysis and petrographic thin-section observations were also performed. The experimental results showed that samples with high permeability showed a small decrease in permeability, possibly indicating formation damage, while low-permeability samples presented a significant increase in permeability with little change in porosity, indicating feasibility for carbon capture and storage (CCS) in similar samples in likewise experimental conditions (20°C and 500 psi). For samples with more pore volumes injected, the pressure stabilization seems to have favored dissolution in the later injection stages, indicated by the highest output of calcium ions. In all samples occurred salt precipitation during injection, especially in the more heterogeneous rocks, presenting a possible issue.

Broken Beds, but better science; using multiple hypotheses to interpret geological data

AbstractIn this study, we propose a multiple hypotheses approach to improve interpretations of limited remotely sensed datasets, such as sparsely exposed outcrops, subsurface datasets, or planetary objects using semi‐quantitative scoring and ranking of observable features. This method is demonstrated using an outcrop example from the Broken Beds of the Upper Jurassic–Lower Cretaceous Purbeck Limestone Group exposed along Britain's Jurassic Coast. Four published hypotheses regarding their origin are refined, represented in matrix form, scored and ranked based on carefully selected outcrop features. Semi‐quantitative scoring utilises knowledge of likely processes governing the occurrence of a range of features, some of which might be ignored or down‐played to favour a single hypothesis. Furthermore, by integrating expertise from different sub‐disciplines (e.g. basin analysis, sedimentology, diagenesis), we also consider the combined evidence of multiple features. This new method results in an interpretation that favours a multi‐process origin for the Broken Beds due to evaporite dissolution, overpressure release and tectonic folding, with identified uncertainty, all useful to guide further data collection.

An alternative interpretation of small-bodied turtles from the “Middle Purbeck” of England as a new species of compsemydid turtle

A series of small-sized fossil turtles were collected from Beckles’ Pit, Durlston Bay, Dorset, United Kingdom in 1856 from a sediment package referable to the Early Cretaceous (Berriasian) Purbeck Group. The two primary accounts that previously documented these turtles concluded that they represent the juveniles of the coeval early pleurosternid Pleurosternon bullockii. A brief, third account, however, suggested that these may represent a new species of compsemydid turtle. We here highlight a series of discrete morphological characters that consistently distinguish the small-bodied turtles from Beckles’ Pit from large-bodied Pleurosternon bullockii, in particular the arrangement of the bones and scutes along the anterior margin of the shell. As these characters are otherwise used to diagnose new species of turtles, in particular compsemydids, and to establish the phylogeny of fossil turtles, we side with the latter interpretation and name a new taxon of early compsemydid, Tongemys enigmaticagen. et sp. nov. The early record of compsemydid is restricted to the Early Cretaceous of Europe, but is extremely fragmentary. We suggest that this may be a bias towards the collection and identification of small turtle remains, but also that a re-study of Early Cretaceous continental turtle faunas is likely to yield further material.

New and little known taxa of the suborder Ensifera (Insecta: Orthoptera) from the Lower Cretaceous of England

This is a continuation of the synoptic monograph of fossil Orthoptera from the English Lower Cretaceous (Purbeck and Wealden groups) published in 2006. A new species belonging to the superfamily Elcanoidea is described from the Purbeck Group: ?Sibelcana websteri sp. nov. One species of the superfamily Hagloidea from the Purbeck Group is redescribed on the basis of a much more complete imprint: Cyrtophyllites cretaceous Gorochov, Jarzembowski et Coram, 2006. Two new genera and two new species of the superfamily Grylloidea are described from the Wealden Group: Transigryllus edmundi gen. et sp. nov. and Proararipegryllus andrewi gen. et sp. nov. One species from the Wealden Group, originally included in the Brazilian Cretaceous genus Araripegryllus Martins-Neto, 1991, is here transferred to the latter new genus: Proararipegryllus orientalis (Gorochov, Jarzembowski et Coram, 2006), comb. nov.

Ontogenetic development of the European basal aquatic turtle <i>Pleurosternon bullockii</i> (Paracryptodira, Pleurosternidae)

Abstract. Several British specimens of relatively complete and partial shells of small pleurosternid turtles, found in the Purbeck Limestone Group (Berriasian, Lower Cretaceous), are analysed in detail here. Despite having been found more than a century ago, most of them remained unpublished until now. Due to the scarce knowledge available to date about the small individuals of Pleurosternidae from the British record, their taxonomic status was doubtful. Thus, some authors proposed their attribution to a new but not defined taxon of small size, whereas others suggested that they could represent juvenile individuals of Pleurosternon bullockii, the adults of which were also found in the Purbeck Limestone Group but at different levels. Knowledge about the shell of the adult individuals of Pleurosternon bullockii has notably increased recently, abundant information on its intraspecific variability being currently available. In this context, a detailed study of the small specimens through qualitative and quantitative approaches is performed here. The results are evidence a significant range of shape variability because of ontogenetic development, as well as other types of intraspecific variation such as inter-individual variation. As a consequence, their attribution to juvenile individuals of Pleurosternon bullockii is justified, providing new data on the ontogenetic development of a basal form.

How do thrombolites form? Multiphase construction of lacustrine microbialites, Purbeck Limestone Group, (Jurassic), Dorset, UK

AbstractThis paper examines how non‐marine thrombolites are formed through a complex, multiphase process of microbial framework construction, erosion, cementation, recrystallization and episodes of internal sedimentation. Recognition of such phases of thrombolite construction provides a framework for the interpretation of the fluctuating environmental conditions leading to their formation. Microbialite frameworks are examined in detail from the Purbeck Limestone Group and their affinities and palaeo‐environmental significance assessed. Three types of thrombolite, one stromatolite and a leolite are described and interpreted. The thrombolite frameworks include: a peloidal mesoclotted type, a thrombolite constructed by the filamentous alga Cladophorites and a type with concentrically laminated micritic mesoclots. Physical and chemical erosion led to extensive early cavity formation within the frameworks. Early calcite rim cements with associated spherulites then developed over the microbial frameworks and these were reworked into cavities. Frameworks were also replaced by chalcedonic quartz and calcite spherulites. Internal sediments comprise peloids, intraclasts and brackish‐water molluscs and ostracods, together with their debris. The thrombolites grew in moderate‐energy to high‐energy shallow, lacustrine, microbial mounds whereas stromatolites occurred in deeper‐water settings. A brackish‐water, lacustrine setting is indicated by the preserved macro‐biota, microbes, absence of charophytes and syndepositional evaporites, and negative stable carbon and oxygen isotope ratios. Strontium isotopes suggest that the carbonate‐rich waters were fed from erosion of Mid–Lower Jurassic limestones on the western basin margin with possible mixing with waters from nearby uplifted Upper Jurassic limestones and with Late Jurassic seawater. The research indicates that non‐marine thrombolites have a complex, multiphase origin resulting in a diverse succession of textures and structures relating to microbially induced and influenced construction, dissolution, cementation, recrystallization and mineral replacement which have not been previously recorded and indicate the major differences between marine and non‐marine thrombolites.

Influence of extensional faults and relay ramp on palaeo‐topography and lacustrine carbonate facies: Purbeck Limestone Group (Upper Jurassic – Lower Cretaceous), Wessex Basin, UK

AbstractLacustrine carbonate facies distribution is controlled by multiple environmental parameters including climate, hydrology, and tectonic setting, but few published models address this complexity. In this study, seismic and borehole data, integrated with outcrop logging, correlations, and facies models, are used to create a new tectono‐sedimentary model demonstrating how extensional faults, linked by a relay ramp, control distribution of lacustrine carbonate facies in the Upper Jurassic to Lower Cretaceous Purbeck Limestone Group (Wessex Basin, UK). Accumulation occurred in half‐graben sub‐basins south of two extensional east‐west faults, with widespread subaerial emergence of footwall blocks to the north. The lacustrine limestones of the lowest unit of this Group are characterised by in‐situ microbial mounds within bedded inter‐mound packstones‐grainstones. Mounds occur in three depositional intervals separated by paleosols. The distribution of facies indicates more brackish‐water conditions shoreward to the west, and more hypersaline conditions basinward to the east. The relay ramp hosts extensive microbial carbonate buildups formed in response to carbonate‐rich waters sourced from the northern limestone footwall blocks that fed into extensive shallow‐water areas on the low‐angle relay ramp slope.

Revisiting climate change and palaeoenvironments in the Purbeck Limestone Group (Tithonian – Berriasian) of Durlston Bay, southern UK

Durlston Bay in Dorset, southern UK, type section of the Tithonian-Berriasian Purbeck Limestone Group, has been intensely studied with regard to its marginal marine palaeoenvironments, rich fossil biota and evidence for earliest Cretaceous climate change. Here we appraise aspects of these topics, incorporating new field observations. We offer changes in charcoal density as a further line of support for a mid-Berriasian moistening of the climate, but suggest that the primary driver of palaeoenvironmental and biotic changes observed through the Purbeck succession was varying input of marine water, with climate change having an overlying and often more subtle influence.

Tooth eruption in the Early Cretaceous British mammal Triconodon and description of a new species

AbstractTriconodon mordax, from the lowest Cretaceous (Berriasian) part of the Purbeck Group, Dorset, is known by an ontogenetic series of specimens that document aspects of tooth eruption and replacement. Based on micro‐computed tomography of four specimens we refer one mandible to a new species, Triconodon averianovi, which differs from T. mordax in having a more slender, curved c; p4 notably low crowned with slender main cusp and smaller accessory cusps; and molars with weak cingula, m4 being notably smaller with weak cusps a and c. T. mordax is variable in the number of mental foramina and posterior jaw morphology. Scans reveal an earlier developmental stage (p3 in early eruption) than previously recognized for Triconodon, and demonstrate sequential, anteroposterior replacement of premolars; it remains unclear whether p1–2 were replaced. Scans also support an earlier hypothesis that m4 erupted late in life. Onset of m4 mineralization is likely to have coincided with eruption of p3, followed by replacement of dp4 by p4 and eruption of c. The m4 developed within the lingual side of the coronoid process, well above the tooth row. It remained in position and was subsequently accommodated in the active tooth row through unusually prolonged and localized growth of the posterior part of the mandible. This pattern is seen in some later triconodontids and appears to be unique to the family.

A recommendation of: The last surviving Thalassochelydia—A new turtle cranium from the Early Cretaceous of the Purbeck Group (Dorset, UK)

Background. The mostly Berriasian (Early Cretaceous) Purbeck Group of southern England has produced a rich turtle fauna dominated by the freshwater paracryptodires Pleurosternon bullockii and Dorsetochelys typocardium. Each of these species is known by numerous relatively complete shells and by a single cranium. The two other turtles found in the Purbeck Group (Hylaeochelys belli, a species of uncertain affinities, and the terrestrial helochelydrid "Helochelydra" anglica) are known only from shell remains.

A review of Raymond Casey’s contribution to Jurassic stratigraphy

During the period from 1962 to 1973 Raymond Casey produced several ground-breaking papers on the Upper Jurassic of eastern England. These included the recognition that genera, previously thought to be of Early Cretaceous age, were in fact of latest Jurassic age. This led him to produce new correlations with the latest Jurassic of southern England, to identify the Jurassic-Cretaceous boundary within the Purbeck Limestone Group and to determine new correlations of the uppermost Jurassic succession of Britain with the Volgian Stage of the Russian Platform. These ideas are discussed and an assessment made of their validity in the light of subsequent revision. Re-assessment of the validity of the Oppressus Zone suggests new correlations between southern and eastern Britain in latest Jurassic times.

Comparison of the Palaeomagnetic Parameters of Non-Marine Jurassic-Cretaceous Boundary Sediments in Dorset (SW England), Hebei and Liaoning (NE China)—A Preliminary Study

The Purbeck beds in Dorset, the Tuchengzi Formation in western Liaoning Liaoning Province or the Houcheng Formation in northern Hebei Province are non-marine Jurassic/Cretaceous (J/K) boundary sequences. A Czech-China Inter-Governmental S&T Cooperation Project has been carried out to search for the non-marine J/K boundary in northern China and making international correlation with the Purbeck beds in southern England. The combination of palaeomagnetism and biostratigraphy in northern China and southern England localities proves that these distant places had similar climatic conditions and the same fauna during the Late Jurassic and Early Cretaceous. A preliminary joint research has shown a fruitful result in searching for the non-marine J/K boundary in northern China.

Thrombolites, spherulites and fibrous crusts (Holkerian, Purbeckian, Aptian): Context, fabrics and origins

Carbonates from the Holkerian (Lower Carboniferous) Concretionary Beds of the Bristol area, the Upper Jurassic-Lower Cretaceous Purbeck Limestone Group of southern England and the Aptian Pre-salt succession of the South Atlantic exhibit spherulites and a range of related crystal shrubs and fibrous crusts. Some of these fabrics are associated with framework microbialites, including thrombolites, that developed under varying degrees of non-marine influence. Brackish or fresh waters affected the Holkerian and Purbeckian microbialites, although the Holkerian also experienced marine conditions. The South Atlantic Pre-salt microbialites were deposited under saline lacustrine conditions. The fabrics are a topic of intense discussion and disagreement over the role (if any) of microbes in spherulite formation. Whilst the case for microbial influences on the Holkerian and Purbeckian spherulites is strong, the alternative influence of Mg-Si gel or Mg-clay (particularly stevensite) is appraised in detail for the Aptian spherulites, including reference to studies of modern microbial analogues. Results of published laboratory experiments concerning such gels and studies on contemporaneous Aptian facies in the same basins lacking Mg-clay, but including spherulites, are considered in an attempt to resolve the abiotic-versus-biotic spherulite origin. That spherulites may be either initiated in suspension or reworked and resedimented provides an added dimension for all three cases. The evidence is strongly in favour of spherulite nuclei being directly created by microbial metabolism but the fibro-radial cortices may be entirely inorganic or passively controlled by microbial extracellular polymeric substances (EPS) which apparently occur as relatively abundant inclusions within the spherulite crystals. The body of evidence suggests that the spherulites and their associated shrubs and crusts are at least partly microbially or biotically influenced.

Discriminating between the origins of remotely sensed circular structures: carbonate mounds, diapirs or periclinal folds? Purbeck Limestone Group, Weymouth Bay, UK

Many sedimentary rock successions contain plan-view circular structures, such as impacts, diapirs and carbonate build-ups. When remotely sensed, it can be difficult to discriminate between their formation mechanisms. Here we examine this problem by assessing the origins of circular structures imaged in high-resolution multibeam bathymetric data from Weymouth Bay, UK. The imagery shows 30–150 m across, concave-down structures within the upper Purbeck Limestone Group on the southern limb of the Purbeck Anticline. Similar structures have not been identified in the extensive outcrops around the bay. The morphology and geological setting of the structures are consistent with three different interpretations: carbonate mounds, periclinal folds and evaporite diapirs. However, none of these structures has been previously recorded in the upper Purbeck Limestone Group outcrops of this internationally renowned geological region. We apply a scoring system to 25 features of the circular structures to discriminate between these three alternative interpretations. This analysis indicates that evaporite diapirs are the least likely and carbonate mounds the most likely origin of the structures. The presence of carbonate mounds revises the upper Purbeck palaeofacies distribution in its type area and provides an analogue for the exploration for hydrocarbon reservoirs in lacustrine mounds. Supplementary material: The methods used in this paper and metrics of the circular structures are available at https://doi.org/10.6084/m9.figshare.c.4103840

Brackish to hypersaline facies in lacustrine carbonates: Purbeck Limestone Group, Upper Jurassic\u2013Lower Cretaceous, Wessex Basin, Dorset, UK

Sedimentary facies and stratigraphic architecture of non-marine carbonates are controlled by a range of environmental parameters, such as climate, hydrology and tectonic setting, but the few published facies models do not account for this variability. Outcrop and petrographic observations from the Mupe Member of the Purbeck Limestone Group (Upper Jurassic–Lower Cretaceous) in Dorset, southern England, are the basis for new depositional models of non-marine microbialites and associated carbonates in an extensional basin. Ten facies are defined, described and grouped into five facies associations. The Mupe Member is characterised by accumulation of in situ microbial mounds developed around tree remains preserved as moulds and silicified wood. Mounds occur within three stratigraphic units, separated by three palaeosoils, characterised by less-porous, bedded, inter-mound packstone–grainstone that commonly onlap mound margins. Mounds are developed mainly in the shallowest areas of the lake, as indicated by their shapes, facies relationships and association with palaeosoils. These microbial mounds are compared to modern (Laguna Bacalar, Mexico and Great Salt Lake, Utah, USA) and ancient (Eocene Green River Formation, Uinta Basin, Utah, USA) analogues to assess their value as palaeoenvironmental indicators. Facies transitions indicate an earlier, brackish-water lake and a later hypersaline lake for the Mupe Member, both within a semi-arid climate setting in an extensional basin. The fact that the microbialites are covered by evaporitic strata, together with sedimentological, palaeontological and stable isotope data, suggest that there was a sharp change from through-flowing brackish-water, to a closed hypersaline, lacustrine system.

The Jurassic–Cretaceous boundary interval in non-marine strata of northwest Europe – New light on an old problem

The non-marine Purbeck- and Wealden-type sediments of latest Jurassic (Tithonian) and earliest Cretaceous (Berriasian) age in northern Germany were deposited in a restricted intercontinental basin. They mark an interval of strong faunal and floral provincialism which makes correlation of the non-marine strata across northwest Europe difficult. The position of the Jurassic–Cretaceous boundary has therefore been debated for decades. Our integrated stratigraphy of four sections in northern Germany, based on palynology (spores, pollen, dinoflagellate cysts) and ostracods, provides new evidence for a precise correlation with contemporaneous strata of other non-marine basins in northwest Europe. Correlation with the marine Boreal Realm and the Tethys is achieved via the Purbeck type section in England. A stratigraphic subdivision of the non-marine Berriasian succession in Germany is possible by using seven short-lived marine flooding events, which are documented by ceratioid dinoflagellate cysts and foraminifera. Our data suggest that a first major transgression took place close to the base of the Boreal Surites stenomphalus ammonite Zone. This level corresponds to a contemporaneous flooding event in southern England (Scallop Beds, Purbeck Group) and in southern Sweden (Annero Formation), thereby providing an excellent marker horizon for interbasinal correlation.