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Exeter, United Kingdom

Enay R.,University Claude Bernard Lyon 1 | Gallois R.,92 Stoke Valley Road | Etches S.,Ashfield
Revue de Paleobiologie | Year: 2014

Kimmeridgian perisphinctid faunas from England and eastern France have been studied in order to try to clarify the relationships between the Late Kimmeridgian perisphinctids of the Boreal Faunal Realm, whose forerunners are unknown in the Subboreal Province, and their possible ancestors in the Submediterranean Province of the Tethyan Faunal Realm. From Late Oxfordian times onward eastern France was occupied by the Western Europe Swell, a shallower water area that acted as a transitional faunal region between the Subboreal and Submediterranean Provinces. Different biozones have been developed within the provinces, and these are still imperfectly correlated. They are discussed in the present account. Here, we describe the stratigraphical and geographical distributions of the perisphinctid genera that form the basis for the study. They include the Submediterranean forms Crussoliceras and Garnierisphinctes, the Subboreal Subdichotomoceras and Pectinatites, and Tolvericeras and Pseudogravesia of the Western European Swell. Many of the English specimens studied are preserved as crushed impressions, but their positions are accurately located within a well-defined chronostratigraphical succession. In contrast, the stratigraphical relationships of much of the material from France, mostly museum specimens in solid preservation in old collections, are uncertain or have yet to be clarified. The palaeontological descriptions include 18 species or subspecies of which 8 species (Crussoliceras dubisense, lamberti, lotharingicum, petitclerci, ? Subdichotomoceras praecursor, Tolvericeras anglicum, popeyense, robertianum) and 1 subspecies (Subdichotomoceras lamplughi dorsetense) are new. The last part of the paper discusses the origin of the Subboreal perisphinctids, including the possibility that Subdichotomoceras evolved from Crussoliceras and that Pectinatites may have evolved from Subdichotomoceras. However, with the possible exception of ? S. praecursor n. sp. from the Western Europe Shelf of eastern France, there is no published description of a transitional form between Crussoliceras and Subdichotomoceras. An evolution quantum jump is needed to explain the sudden morphological differentiation of Subdichotomoceras. This probably occurred via peripatric speciation.

The type sections of the Kimmeridge Clay Formation and the upper part of the Kimmeridgian Stage are cliff and foreshore exposures close to the village of Kimmeridge, Dorset. The succession is made up of rhythmic alternations of mudstone, organicrich mudstone and calcareous mudstone that contains only minor sedimentary breaks. The exposures provide the only complete section in the UK of the succession adjacent to the Kimmeridgian-Tithonian and Kimmeridgian-Volgian boundaries, and are one of the most complete in Europe at this stratigraphical level. Accurate thickness measurements can be made in the cliffs, but palaeontological collecting is difficult due to their weathered state. In contrast, there are extensive outcrops in a relatively unweathered condition in the intertidal zone. This is where almost all the better preserved fossils recorded to date have been collected from. The cliff and foreshore outcrops are mostly separated by beach deposits that make precise correlation between them difficult. The stratigraphical accuracy with which specimens from the intertidal outcrops have been recorded with respect to the succession exposed in the cliffs has mostly been not better than ± 2 m. The use of ortho-rectified air photographs of the intertidal-shallow subtidal areas combined with digitally rectified photographs of the cliff sections has made it possible to produce more accurate correlations between the cliff and foreshore exposures. These have been combined with a revised description of the cliff sections based on rhythms to divide the succession into numbered units that can be recognised in all the outcrops. The revised classification makes it possible to place samples and/or specimens collected from the cliff and intertidal exposures in the stratigraphical succession with an accuracy of ± 0.1 m or better.

Gallois R.W.,92 Stoke Valley Road
Geoscience in South-West England | Year: 2011

Like many small coastal towns in the UK, Sidmouth in Devon was founded on the valley sides adjacent to a river outfall that provided a natural harbour. Subsequent expansion of the town in the late 18th and early 19th centuries, when living by or visiting the sea became popular for health reasons, involved the entrainment of the river and building on land that was subject to marine flooding. Engineering works in the 19th and 20th centuries that were designed to protect the low-lying parts of the town included the construction of sea walls, and groynes and offshore bunds to protect a ridge of storm-beach gravels that acts as a natural sea defence. These works have collectively had an effect on erosion rates in the cliffs of Triassic sandstone and mudstone on the east side of the town. Natural landslide mechanisms in the cliffs adjacent to Sidmouth include rock-block and toppling failures induced by marine undercutting, and hydraulic stoping along faults and major joints at the foot of the cliffs. The principal landslide mechanism is the collapse of unconsolidated Head deposits and deeply weathered mudstones in the highest (mostly 3 to 5 m) part of the cliff. The falling material commonly destabilises the underlying well-jointed sandstones and mudstones. Artificial factors that have influenced erosion rates in the cliffs east of the River Sid outfall in the last 100 years have included the refraction of waves adjacent to the end of the river wall, and interference with the easterly longshore drift of the beach gravels. A secondary factor has been the destabilising influence of a Victorian railway tunnel that was dug parallel to and up to 25 m from the cliff face on the east side of the town. In the absence of quantitative monitoring data, published estimates of the rates of cliff erosion are significantly higher than those obtained in the present study from a comparison of maps made between 1802 and 2006.

Gallois R.W.,92 Stoke Valley Road
Geoscience in South-West England | Year: 2010

The unglaciated part of southern England was subject to periglacial climates that lasted for more than 20 ka on at least eight occasions during the last 750 ka. There are large outcrops of Jurassic mudstones in south-west England, principally the Lias Group and the Oxford Clay and Kimmeridge Clay formations, and extensive exposures of these beds in the cliffs on the Dorset coast. Notwithstanding the susceptibility of this type of mudstone to permafrost damage and deformation, there is no published record of large-scale folding in the region that has been attributed to periglacial disturbance. Three examples of folding are described here, in the Lias Group at Charmouth and Seatown in west Dorset, and in the Kimmeridge Clay on the Isle of Portland that are attributed to intermittent downhill creep of surface layers up to 20 m thick when in a partially frozen condition. The style of folding in the mudstones and the geometry of the disturbed deposits indicates that they are not tectonic in origin, nor were they formed by valley bulging or landsliding. These are the first large-scale structures of their kind to be recorded in southern England: similar folds elsewhere have been interpreted as valley bulges or tectonic in origin. At the Seatown and the Isle of Portland localities, the deformed mudstones have been preserved beneath younger landslides. The absence of similar structures elsewhere on the Dorset coast is attributed to the rapid removal of similarly weakened materials by marine erosion at times of high sea level during the last c. 6000 years.

Gallois R.W.,92 Stoke Valley Road
Geoscience in South-West England | Year: 2010

A proposed gas-storage site at the former Upper Osprey Ministry of Defence site on the north east coast of the Isle of Portland is underlain by landslide debris and man-made deposits that wholly conceal an outcrop of Kimmeridge Clay Formation. Historical accounts of major landslides in the 17th to 19th centuries have been interpreted as evidence of deep-seated failure surfaces in the Kimmeridge Clay. A pattern of continuously cored boreholes, mostly 25 m to 100 m deep, was drilled to determine the stratigraphy and structure of the area beneath and adjacent to the proposed installations. Comparison of the succession proved in the boreholes with that exposed at the type succession of the formation in the cliffs at and adjacent to Kimmeridge Bay enabled numerous laterally persistent lithological marker beds to be identified. These were used in combination with total-gamma-ray logs to show that the successions proved in the borehole cores could be correlated with one another with a stratigraphical accuracy of ± 0.2 m or better. Taken together, the borehole and geophysical data showed that the Kimmeridge Clay dips steadily SSW at c. 02° beneath the site and the adjacent areas. No tectonic folding or faulting was detected, nor any significant disturbance within the rock mass that could be attributed to a deep-seated landslide. A subsequent multibeam sonar survey of Portland Bay, made for reasons unrelated to the gas-storage site, confirmed the uniform low SSW dip in the Kimmeridge Clay in the offshore area adjacent to the Upper Osprey site.

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