Proceedings of the Geologists Association最新文献

The Cenomanian deposits of the Eastern Saharan Atlas are characterised by gypsum-dominated series interpreted as coastal sabkha deposits, indicating low-latitude, and tropical to subtropical settings. The evaporitic conditions are interrupted by rapid marine ingressions allowed the deposition of carbonate beds. In the Djebel Ousigna section, fine sandy limestone bed shows abundant burrows of Macaronichnus. These traces indicate the upper shoreface–foreshore contact, affected by storm events, which show similarities with recent huge tsunami waves that facilitated the proliferation of Travisia polychaete populations (Macaronichnus tracemaker). This Macaronichnus-suite represents the most proximal marine setting during a regressive phase. Also, this unique occurrence of Macaronichnus in the North African Cenomanian low-latitude, subtropical to tropical evaporites is interpreted as related to upwelling of cold nutrient rich-waters. The latter seemingly challenge the view that this trace fossil is restricted to colder climatic zones. From an ethological viewpoint, studied Macaronichnus specimens are considered to be a sequorichnial trace (exploratory behaviour) produced by polychaete annelids.

Near the Cenomanian–Turonian boundary, the filament event has been identified worldwide. In Tunisia, filaments are well represented in the laminated black shales of the Bahloul Formation. These black shales were deposited under anoxic conditions in an outer ramp environment. Filaments are particularly abundant in the upper part of the Bahloul Formation, and within the W. archaeocretacea Zone; these correspond to the massive occurrence of fragments of thin-shelled bivalves, whether planktic or nektonic. In terms of sequence stratigraphy, two sampled sections in Central and Central-West Tunisia have led to the interpretation of these deposits as a transgressive interval (TST). In the Oued Ettalla area, the organic-rich laminated limestone and the interbedded dark grey marly claystone constitute the Bahloul Member. The transgressive interval ends with a maximum flooding surface (MFS) included within the lower part of the Kef Formation (Annaba Member) which consists of yellowish-grey marly claystone and marly limestone intercalations, rich in filaments. Whilst in the Oued Sfala section the maximum flooding surface (MFS) is characterised by a thin layer of pebbles (approximate centimetric scale) occurring between Unit 4 and Unit 5. Through the analysis of both oxic and anoxic facies from the Cenomanian–Turonian transition, five bioevent markers have been identified in our sections and in other regions within the Tethyan domain: the extinction of the genus Rotalipora, the Heterohelix shift, the proliferation of globular planktic foraminifera, the filament event, and the first occurrence of Helvetoglobotruncana helvetica. These bioevents are important for establishing high-resolution correlations both regionally and worldwide.

Amongst the most neglected landforms in formerly glaciated mountain environments are the scars of major rock-slope failures where the displaced rock or runout debris has been removed by glacial erosion. The characteristics of 33 such debris-free failure scars (DFFSs) on Torridonian sandstone terrain were analysed. These range from 0.011 to 0.183 km² in ground area, and scar volume implies removal of ~ 0.4–9.9 Mt of rock. Failure planes are planar or stepped at gradients of 37–50°, implying failure as ‘rough’ translational slides; most probably represent former rock avalanches or fragmented rockslides where shear through bedding planes resulted in runout fragmentation. Extension of 85 % to the slope crest is consistent with coseismic triggering during periods of glacio-isostatic crustal uplift. At least ten DFFSs demonstrably pre-date expansion of the last ice sheet at ~ 35 ka, and at least five probably represent failure that occurred in the Lateglacial interval of ~ 16–11.7 ka. Fourteen are located within cirques, confirming the importance of RSFs in cirque extension during ice-free interglacials or interstades; others have contributed to trough widening and the formation of arêtes. Older upper-slope cavities in Torridonian mountains are probably also the results of (possibly recurrent) rock-slope failures. It is concluded that the large-scale morphology of the Torridonian sandstone mountains reflects a long-term synergic relationship between alternating episodes of glacial erosion and rock-slope failure: reduction of rock-slope stability during each glacial-deglacial cycle has predisposed slopes to eventual failure, with subsequent glacial removal of displaced blocks and runout debris.

Barthélémy Faujas de Saint-Fond (Montélimar, France, 1741–1819) is a renowned scientist whose work on volcanoes influenced the birth of volcanology. In the meantime, he developed a strong interest in the use of natural pozzolan which he recognised as a key ingredient of durable Roman mortars. Indeed, he recognised the similarity of pozzolans from Pozzuoli (the cradle of Roman pozzolan, which gave the name to this product) and newly discovered pozzolans from various places in France. Within 18th and 19th century renowned cement scientists, it is of interest to emphasise the work of Faujas de Saint-Fond. In the current trend to develop sustainable binders based on pozzolans and low-CO₂ activators, his work on pozzolan-lime mixes is worth recalling today.

The Late Viséan (mid-Mississippian) anatomically preserved flora from Pettycur, Fife, Scotland is one of the most significant plant assemblages (lagerstätte) from this period anywhere in the world. It has been known from the later 19th century, mainly from loose limestone blocks found on the beach. Thin sections of limestones showing outstanding permineralised plants offered a number of the most eminent British palaeobotanists of their day to describe many of the plants. Most of the thin sections were made commercially so that an understanding of the formation of the deposit as a whole proved difficult. The peel technique developed in the 1950s allowed large slices of limestone to be peeled but only small pieces of the peels were ever mounted and studied by the new authors. Large collections (more than 100) of loose blocks on the beach and from in situ from within basaltic lava flows by the author and colleagues from the late 1970s onwards have been slabbed and peeled and provide the opportunity for new studies and an assessment of the plant assemblages and possible communities present. At least 25 plant organ species are present representing more than 13 whole plant species. Of particular significance is the occurrence of the four main lycopsid tree types that dominate later Pennsylvanian peats and the occurrence of five ferns. It is shown also that a number of the plants may also be preserved as charcoal, especially zygopterid ferns such as Metaclepsydropsis. Of particular importance is the occurrence of true permineralised peats that provide evidence of the botanical composition of the earliest peat-forming mire at a time of rapid global change with a sudden fall of atmospheric CO₂ and rise in O₂ that would have also had an effect upon wildfire systems.

This paper presents the results of a detailed study of a deformed sequence of Late Devensian glacifluvial sands, gravels and diamictons exposed within river cliff sections cut by the Allt (river) Cuaich, northeast of Dalwhinnie in the Grampian Highlands of Scotland. Macroscale sedimentological and structural field observations are combined with micromorphological and microstructural analysis to investigate the ice-marginal processes which led to the deposition and subsequent deformation of these glacifluvial to glacilacustrine deposits. Sedimentological evidence indicates that water-lain deposits in the area are capped by till and record the eastward progradation of ice-proximal fans into water ponded by an ice margin that advanced up valley from the west/southwest. These glacifluvial/ice-marginal fan sequences were deformed and subsequently overridden by ice that advanced (at least 6 km) into the Cuaich catchment. A suite of similar, but undeformed ice-marginal fans and laminated glacilacustrine deposits were laid down during the subsequent withdrawal of ice down-valley. Microstructures present within the variably glacitectonised (folded, faulted) laminated sands, silts and clays reveal a complex history of local glacial readjustments in a dynamic ice-marginal aqueous setting. Brittle–ductile shearing, liquefaction and remobilisation, and extensional brittle faulting accompanied the deposition of an overlying till. A detailed model of brittle–ductile shearing and glacitectonite formation during the overriding of water-rich ice-marginal deposits involves initial proglacial thrusting and folding, followed by continued submarginal to subglacial brittle–ductile shearing and extensional faulting. The study supports growing evidence that in Scotland the British Ice Sheet underwent localised expansion towards the end of the main Late Devensian glaciation (Dimlington Stadial, 26–13 ka radiocarbon years BP). In addition, it clearly demonstrates that the complex polyphase deformation recorded by glacitectonised sequences may not simply be a product of intense subglacial deformation, but a result of progressive proglacial to submarginal deformation, followed by subglacial shear as the ice progressively overrides a preexisting sedimentary sequence.

We report on a new assemblage of coleoid arm hooks found in the Blue Lias (Lower Jurassic) succession near Lilstock (Somerset, UK). As there are no soft tissues or other materials (ink sac, phragmocone, etc.) preserved in the vicinity of the hooks, the genus and species cannot be accurately identified. Unlike the hooks of Clarkeiteuthis, which have distinctive inflated, bilobate bases, the hooks discussed herein are delicate, with a gently curved apex, which makes them quite similar to the arm hooks of Belemnotheutis or Acanthoteuthis. The assemblage of hooks shows a distinct organisation and, in some places, the hooks are clearly aligned as they would have been in life position. Comparable material of late Jurassic age from the Etches Collection (Kimmeridge, Dorset) is compared to the new arm hook assemblage.

Fires have been shown to have been common in many Carboniferous ecosystems worldwide yet we still have little understanding of the detail of what, where and how such fires occur or indeed their effects both on the local ecosystem as well as on the Earth System as a whole. The Kingswood Limestone, of late Viséan (Mississippian age) found near Pettycur in Fife, Scotland, is interpreted as being deposited within a crater lake. The limestone contains a range of volcanic clasts together with charcoalified and uncharred plants. Two distinctive communities existed. One dominated by the small scrambling lycopsid Oxroadia that is permineralised and a second dominated by a range of pteridosperms and other gymnosperms preserved as charcoal. Fires surrounding a crater lake charred the plants that were then washed into the lake by post-fire erosion. All plant organs are found as charcoal including woody axes, leaves, pollen organs and ovules. Oxroadia that was living close to the lake was not affected by fire. Rising lake and sea levels allowed saline water to be introduced to the lake and connection to the sea was established. Many new charcoalified plants have still to be formally described. The Kingswood flora, together with the nearby Pettycur flora represents two of the most important Mississippian plant sites globally.

The El Mers I and II formations (Middle Jurassic, Bathonian) are geological units outcropping in the folded Middle Atlas of Morocco rich in body and trace fossils of dinosaurs. The numerous tracksites of these units have been little studied and are severely affected by ongoing erosion (e.g., seasonal flooding) and, to a lesser degree, human activities (e.g., urbanisation). The aim of this project is to fully document and interpret the dinosaur tracks of two historic sites and four new sites discovered in the El Mers area using digital photogrammetry. The ichnofauna comprises abundant tracks and trackways of theropod and sauropod dinosaurs as well as of probable crocodylomorph tracks. The theropod tracks include several footprints of high anatomical fidelity, but the majority of the discovered tracks are identified to be penetrative tracks, with one site preserving abundant swim tracks. The sites preserve both small and enormous sauropod tracks. The enormous sauropod tracks are among the largest known worldwide, with a pes track length of up to 130 cm. The obtained 3D models are compared with site maps created by analogue mapping performed more than 15 years ago of the historical sites, revealing how erosion has both destructed surfaces and exposed new ones. The application of photogrammetry allows for the rapid collection of accurate high-resolution data with sustainable costs. The resulting 3D models can be used in research to digitally conserve threatened sites, and as a basis for knowledge transfer to the public. The present contribution encourages the intellectual, logistical, and social involvement of the local population to collaborate with scientists for the conservation of the rich geological heritage. Furthermore, scientific investigations in this area could shift from academic-only research to research focused on conservation and geotourism initiatives that involve local communities.