Proceedings of the Geologists Association最新文献

This paper presents the results of a detailed macro- and microscale structural study of the glacitectonised sequence at Kilcummin Head on the western side of Killala Bay, County Mayo, northwest Ireland. The sequence comprises two laterally extensive, thrust-stacked rafts of Carboniferous limestone and mudstone emplaced upon the in situ bedrock of the Ballina Limestone Formation. Restored cross-sections reveal that the glacitectonised sequence has undergone extensive deformation and shortening of 61 %. This accompanied the formation of a prominent thrust-moraine associated with a northward ice advance across Killala Bay during the Midlandian (Devensian). A five-stage model is proposed to describe the deformation. Stage 1 comprises detachment of the rafts, and the majority of the sequence shortening. North-directed transport of the rafts occurred on a major décollement surface located at the base of the rafted sequence, marked by a mudstone-rich glacitectonite. Stage 2 accommodated further shortening via a series of imbricate thrusts leading to the stacking of the bedrock rafts (Stages 3 and 4). Stage 5 saw final stacking of rafts within a prominent thrust-block moraine, followed by the deposition of a sequence of coarse-grained, ice-marginal glacigenic deposits.

Scelidosaurus is a herbivorous dinosaur belonging to the ornithischian clade Thyreophora. Until recently, its distribution was limited to the cliffs of Charmouth, on the south coast of Dorset, United Kingdom. In 2021, a fragmentary specimen from Lower Jurassic strata in Northern Ireland (BELUM K3998) was referred tentatively to Scelidosaurus. This bone, the proximal end of a femur, was suggested to be an ornithischian based on several characteristics, including gross morphology and osteohistology. This study reassesses these characteristics, and finds no evidence to support their use for identifying this specimen as either an ornithischian or Scelidosaurus. With the most comprehensive studies on Scelidosaurus finding no autapomorphies of the proximal femur, this study regards BELUM K3998 as Dinosauria indet. Additionally, this study highlights the importance of apomorphy-based approaches in taxonomy and the benefits of such methodologies.

A recent revelation has come to our attention, revealing the remarkably well-preserved post-cranial remains belonging to Protosiren. These remains include vertebrae, ribs, and fragmentary bones, and were discovered from two different horizons located in the Mokattam Formation of El-Galala Plateau, North Eastern Desert of Egypt. The first specimen, GCU0101, is identified as Protosiren sp., whereas the GCU0201 specimen is recognised as Protosiren cf. P. fraasi due to certain distinguishing features. These include vertebrae with wide keyhole-shaped neural canals, reduced tuberculum on the ribs, lack of pachyostotic ribs, and the possession of rugose articular rib head. This discovery is remarkable, as it offers a unique opportunity to study the ancient marine mammal's morphology and osteology, shedding light on its evolutionary history and ecological niche. The study documents the first occurrence of Protosiren from an ornamental limestone of the Middle Eocene (Bartonian) Observatory Formation from El-Galala Plateau, Northeastern Desert, Egypt.

This field study records that the Tortonian phosphatic hardground at S'Algar, Menorca has a diverse fossil macrobenthic fauna dominated by bivalves. These faunal elements occur in four preservational forms (A–D). Each taphonomic form within the hardground is characterised by different concentrations of Entobia isp. (clionaid sponge boring) and encrustation. The preservational forms have specific fossil macrofaunas associated with them, and the hardground represents a melange of bioclasts derived from different environments on the Menorcan Miocene shelf. These habitats are influenced by the upwelling event that provides the conditions for hardground mineralisation.

A Glycymeris-Crassatella-Clypeaster association (A) records part of a fauna that lived on the upper ramp slope that was periodically affected by high energy and low oxygen conditions due to upwelling waters. The smaller sized fauna (B), characterised by an arcid-carditid-venerid association is derived from the middle ramp that was affected by storms and a more persistent westward flowing current that intermittently drove upwelling. The fragmentary and disarticulated nature of both faunas indicates that they were mixed within the hardground via reworking processes driven by these seafloor energy regimes.

Some of the large gastropod taxa, dentition-up Glycymeris and Gastrochaena suggest that the hardground had its own fauna. Preservation A fauna and other opportunistic taxa inhabited an environment where the hardground was exposed on the Miocene seafloor as ‘islands’ surrounded by thin layers of looser sediment. Phosphatic bioclasts in the underlying limestone indicate that similar mineralised beds were formed elsewhere on the shelf prior to the formation of the studied outcrop.

A pulsed mantle convection model has been proposed for regional third-order cyclic sea level changes during time intervals for which robust evidence of extensive polar ice caps is lacking. One such interval, the Late Jurassic–Early Cretaceous, is represented in southern England's Wessex Basin by the marine to non-marine Portland and Purbeck Limestone groups and Wealden Group and Supergroup. Third-order cyclicity in this succession is reviewed against the mantle convection model, involving pulsed growth of a regional uplift centred on the Cornubian Massif, bordering the Wessex Basin. Supporting evidence for the model is forthcoming from cyclicities in the Upper Tithonian marine Portland Group and Hauterivian–Lower Aptian alluvial–lacustrine–lagoonal Weald Clay Group. Third-order cycles in the Berriasian–Valanginian alluvial–lacustrine Hastings Group reflect fault reactivation along the Anglo-Brabant Massif, conventionally linked to North Atlantic opening. Credible cyclicities on comparable scales have not been identified in the Upper Tithonian–Lower Valanginian lagoonal–lacustrine Purbeck Limestone Group or the Valanginian–Barremian fluvial–alluvial Wessex Formation. Purbeck lithofacies are defined largely by climatic and local tectonic signals; cyclicities were potentially masked in the Wessex Formation by dynamic fluvial processes in proximity to the massif. Certain biotic events in the Purbeck Limestone Group and fluvial events in the Wessex Formation possibly reflect vertical movements of the Cornubian uplift.

A glacial origin for cirque-like hollows cut into the western escarpment of the Usk valley near Abergavenny, South Wales has become widely accepted. Associated supposed extensive moraine ‘festoons’ have been depicted merging and contemporaneous with Last Glacial Maximum (LGM) deposits formed by ice occupying the adjacent Usk valley. We re-interpret these festoons as the product mainly of rock slope failures (RSFs) emanating from the hollows. A cirque glacier origin is preferred to account for a compact double-ridge feature in one of the hollows. The equilibrium-line altitude (ELA) of the reconstructed glacier (357 m) is > 60 m lower than all similarly small, presumed Younger Dryas Stadial (YDS; c., 12.9–11.7 ka) glaciers elsewhere in South Wales. To test whether this glacier nevertheless might date from the YDS, we apply three approaches to reconstruct annual palaeo-precipitation amounts at the ELA, two based on relationships between accumulation and ablation for modern glaciers and the third on a simple degree-day model (DDM) using likely climatic characteristics for this event. The DDM can be tailored to represent the recognised large-amplitude YDS annual temperature range rather than the much smaller one experienced by modern glaciers, making it our preferred approach. Although conditions along the Usk valley escarpment during the LGM would have been well suited to cirque glacier formation, the DDM approach, using the large-amplitude annual temperature ranges, suggests that a YDS age might also be possible. The results have implications for re-assessing the likely ages of some former small glaciers in South Wales.

Scratch circles, previously defined as ‘bedding plane parallel sedimentary structures formed by the passive rotation of a tethered organism into the surrounding sediment’, have hitherto been identified in the geological record from the Ediacaran to the Paleocene, as well as in modern settings. They have not met the definition of trace fossils, being passively registered by a part of a plant. Several variations of scratch circle morphology have been identified in or on Pliocene and Pleistocene deposits on the Cape south coast of South Africa, allowing for an expansion of the scratch circle temporal range. Furthermore, these novel forms require a redefinition of the term ‘scratch circle’. Anthropogenic origins need to be considered for occurrences in the Pleistocene; guidelines to assist in distinguishing such causes from typical scratch circles are presented. Scratch circles may conceivably have inspired the creation of circular patterns (palaeo-art) in sand by Middle Stone Age hominins. Evaluation of scratch circles in snow allows for subtle features to be determined. A re-evaluation of what the term ‘trace fossil’ constitutes could be considered, in order to include the term palaeo-ichnobotany.

Construction of the Holderness/Flamborough Head moraine belt on the East Yorkshire coast, England, records the oscillatory onshore flow of the North Sea Lobe of the British-Irish Ice Sheet from ~ 25.8 to ~ 19.7 ka BP, during which time a thick sequence of multiple diamictons and associated stratified sediments were emplaced. The sedimentology of a > 40 m thick stratigraphy through the moraine belt at Filey Bay, in combination with local borehole records, is used here to reconstruct the depositional processes associated with glacier ice moving ca., 12 km onshore and damming the mouth of the Vale of Pickering, which resulted in the accumulation of an unusually thick and complex sequence of deposits traditionally classified as the “Filey till”. The base of the sedimentary sequence comprises stratified diamictons, which are interpreted as glacilacustrine deposits emplaced predominantly by sediment gravity flows in an ice-contact ‘mud apron’ on the distal slope of a subaqueous push ridge constructed in the earliest proglacial lake in Filey Bay; a vertical increase in coarse-grained lithofacies records increasing glacier proximity. Glacier overriding of the mud apron is recorded by a stacked sequence of tills that interdigitate with lake sediments inland. A zone of till-lake sediment interdigitation migrated first westward during North Sea Lobe advance and then eastward during its retreat, into and out of Glacial Lake Pickering, respectively. Multiple tills and intra-till stratified beds and lenses at the top of the sequence at Filey represent alternating deforming bed-sliding bed facies (subglacial traction tills and subglacial canal fills) associated with the construction of inset push moraines, constructed by sub-marginal incremental thickening or punctuated aggradation. This depositional scenario addresses the problems arising from genetic classifications of substantial accumulations of glacigenic diamictons as ‘till’ when modern analogues indicate only modest thicknesses of subglacial traction till beneath glaciers. Onshore thickening of glacigenic deposits through subaqueous push moraine construction and mud apron progradation is compatible with glacier surging behaviour, but not necessarily solely diagnostic of a surging North Sea Lobe during the last glaciation.

The fossil remains of a pterodactyloid pterosaur from the Kimmeridge Clay Formation (Jurassic: Tithonian) of Abingdon, Oxfordshire, central England are identified as a partial left first wing finger phalanx. The elongation of the phalanx and distinctive morphology of the proximal articular region, in particular the square outline of the extensor tendon process, permit the specimen to be assigned to Ctenochasmatoidea. Although fragmentary, it is sufficiently well preserved to determine accurately its dimensions when complete. Morphometric analysis reveals the specimen to represent one of the largest known examples of a Jurassic pterosaur, with an estimated wingspan of at least 3 m, and is one of the first pterodactyloids to be reported from the Jurassic of the United Kingdom.