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.