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Peter Crimes 《Geological Journal》1995,30(1):81-81
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The glacial deposits of the eastern Lleyn Peninsula record the advance, coalescence and subsequent retreat and uncoupling of Welsh and Irish Sea ice-sheets during the Late Devensian cold stage. During advance a thick sheet of basal diamict was deposited over much of the area, and during retreat and uncoupling, which occurred after 14.5 ka, the eastern part of the area was dominated by the formation of a large sandur system draining from the retreating margin of the Irish Sea ice-sheet. Subsequent stages of retreat are marked by a series of arcuate cross-valley moraines formed either by ice-contact deposition during stillstand or by structural deformation during minor snout oscillation. In the western part of the area sedimentation was controlled by a series of dead-ice ridges running parallel to the retreating ice-margin and this led to the development of a complex assemblage of localised depositional environments including ice-front alluvial fans, marginal sandur troughs and pro-glacial lake basins, all formed under supraglacial ice-marginal conditions. No evidence of glaciomarine deposition is recorded. © 1998 John Wiley & Sons, Ltd. 相似文献
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The most extensive ichnofauna yet recorded from a deep water Lower Palaeozoic sequence occurs within the distal turbidites of the Lower Silurian Aberystwyth Grits Formation of Central Wales. The strata contain an abundant assemblage comprising 25 ichnogenera: Asteriacites, Bergaueria, Chondrites, Cochlichnus, Cosmorhaphe, Glockerichnus, Gordia, Helicolithus, Helminthopsis, Helminthoida, Hormosiroidea, Lorenzinia, Megagrapton, Monomorphichnus, Neonereites, Nereites, Palaeophycus, Paleodictyon, Planolites, Protopaleodictyon, Spirorhaphe, Spirophycus, Squamodictyon, Subphyllochorda, Taphrhelminthopsis; 36 ichnospecies are described, three of which (Asteriacites aberensis, Helminthopsis regularis, Cosmorhaphe elongata) are new. The inorganic sedimentary structures and trace fossils of some 418 sandstone beds were examined in detail; 16 per cent of the beds commence with Divisions A or B and 84 per cent with Division C of the turbidite sequence. This indicates a relatively distal environment, mainly receiving low velocity turbidity currents, and favouring trace fossil preservation. The most common traces were Helminthopsis, Paleodictyon, and Squamodictyon which were found on 46 per cent, 34 per cent, and 19 per cent of the beds examined. Data from this, and other recently described sequences, confirms that there was a gradual increase in trace fossil diversity in the deep oceans throughout the Lower Palaeozoic, in contrast to the situation in shallow water shelf seas where a peak was reached as early as the Lower Cambrian. 相似文献
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Trace fossils are recorded from eight localities within Upper Cambrian (Ffestiniog Stage) strata in North Wales. Trilobite furrows (Cruziana), resting impressions (Rusophycus), walking and striding tracks (Diplichnites), sideways grazing tracks (Dimorphichnus) and swimming grazing marks (Monomorphichnus, gen. nov.) are described, largely on the basis of over 500 collected specimens, and are figured together with examples of other trace fossil genera including Rouaultia, Phycodes and Monocraterion. The trilobite traces are used to investigate the varied methods of locomotion of the animais. They also indicate the morphology of some of the trilobite hard and soft parts from which it is inferred that most of these traces were probably made by a population consisting of a small number of closely allied species of trilobites belonging to the Olenid family. 相似文献
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Two typical representatives of the Ediacaran biota, Ediacaria booleyi n.sp. and Nimbia occlusa have been found in deep-water turbidites within the Upper Cambrian Booley Bay Formation at Booley Bay in Co. Wexford, Eire. The examples of E. booleyi were transported over short distances by the turbidity currents and then impacted, in some instances at high angles, into the ocean floor muds where they formed moulds but, as their bodies decayed, these were filled by casting sand from the overlying turbidite. They preserved fine detail during transportation and impaction, thereby indicating the existence of a rigid body wall. The three-dimensional nature of their preservation allows reconstruction, which shows that Ediacaria had a discoid form with prominent relief, particularly on its dorsal side, which was divided into three main concentric zones with thin radial features, more evident at the periphery. The ventral side had lower relief, with alternate areas of coarse and fine concentric markings and numerous fine radial lines, some also thicker at the periphery. Nimbia occlusa are disc-like bodies of low relief and occur in profusion on one sole. Some show a circular outline indicating that they were preserved flat on the bedding plane, but others occur as semicircular reliefs oriented in the palaeocurrent direction. This indicates that they too were moved by the turbidity current, but their abundance and excellent preservation indicate that transport distances were short. There is no evidence in either genus for the coelenterate characteristics of a mouth and a two-layered body wall enclosing a single cavity. This, and the evidence for a rigid outer wall, adds to recent doubt on the concept of the world-wide Ediacaran fauna as dominated by soft-bodied coelenterates. 相似文献
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The relationship between depositional grain orientation and current direction is discussed, and a method to assist in distinguishing between depositional and deformational magnetic fabrics is suggested. Magnetic fabric measurements from Middle and Upper Cambrian strata at St. Tudwal's Peninsula are presented. In the Maentwrog Beds (Upper Cambrian) a correspondence is found between the trend of the maximum magnetic susceptibility axes and the trend of the palaeocurrent as deduced from sole markings. In the Caered Mudstones and Flags (Middle Cambrian) the directions of maximum magnetic susceptibility are used to indicate palaeocurrent trends. 相似文献