摘要:

AbstractThe collection of papers in this volume results from a joint meeting of the Tectonic Studies Group and the British Sedimentological Research Group of the Geological Society of London, held at the University College of Wales, Aberystwyth on 10–12 April 1986. About half the papers presented at that meeting were offered for publication. A report of the full proceedings has appeared in the Journal of the Geological Society (Fitches and Woodcock 1986) as have abstracts of all the contributions.The stimulus for the conference was the recent upsurge in structural and sedimentological research in Wales, following and partly stimulated by new work on Welsh volcanic rocks (reviews by Allen 1982; Bevinset al.1984; Kokelaaret al.1984). The papers here reflect the breadth as well as the quantity of the activity, particularly in previously underresearched areas such as the Mid Welsh Late Ordovician to Early Silurian sedimentary rocks.The papers are arranged in approximate time order of the events they describe. Naturally those on sedimentation in the Early Palaeozoic basin precede those concentrating on Caledonian basin inversion and deformation. The areas that they describe are located on Figure 1. Here we briefly set the papers in their regional context and suggest some tentative conclusions that can be drawn from the conference presentations as a whole. More extensive reviews of sedimentation and/or tectonics in the Welsh basin are given by Coward and Siddans (1979), Hollandet al.(1979), and Woodcock (1984

ISSN:0072-1050    

DOI:10.1002/gj.3350220502

出版商:John Wiley&Sons Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractManganese mineralization occurs throughout the greywackes and pelites of the Harlech Grits Group (Lower/Middle Cambrian) of the Harlech area, North Wales. It is most intense at the base of the Hafotty Formation where it forms a discrete ore bed, and in the upper half of the Gamlan Formation where it forms coticules (quartz–spessartine metapelites). Low‐grade mineralization also occurs in sediments of equivalent age at St. Tudwal's Peninsula. The mineralization is syngenetic/syndiagenetic. Manganese was probably supplied to the basin by submarine exhalative hydrothermal solutions, and may have precipitated directly as an oxide/carbonate sediment, or by replacement of pre‐existing carbonate. Early diagenetic remobilization of manganese oxides, precipitation of manganese carbonates, and replacement of clastics and volcaniclastics has played an important role in manganese concentration in the sediments. In the Harlech ore, many original depositional textures are preserved in spite of lower greenschist facies metamorphic recrystallization of the diagenetic mineral assemb

ISSN:0072-1050    

DOI:10.1002/gj.3350220503

出版商:John Wiley&Sons Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractRefined biostratigraphy for the Arenig Series has permitted the use of the three stages recognized in South Wales, and hence the recognition of structurally‐bounded areas of North Wales with substantially different depositional histories. During the Moridunian and Whitlandian, continuous subsidence was restricted to the western margin of the present mainland, and in many areas deposition did not commence until the Fennian. Volcanics, mass flow deposits, and synsedimentary deformation indicate two minor phases of tectonic activity. Late in the Moridunian, uplift around the Harlech Dome caused the absence of the Whitlandian. During the Fennian rapid fault‐controlled subsidence occurred in the west allowing the incursion of an open ocean biofac

ISSN:0072-1050    

DOI:10.1002/gj.3350220504

出版商:John Wiley&Sons Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractPost‐Arenig ironstones and associated sediments of North Wales have yielded Upper Llanvirn, Llandeilo and Lower Caradoc acritarch assemblages. The occurrence of most ironstones coincides with a eustatic regression and subsequent transgression, and the emergence to the northwest of the Irish Sea Landmass, the probable source of the iron. The majority of ironstone deposition within the basin was restricted to short‐lived local submarine rises most likely produced by synsedimentary fault

ISSN:0072-1050    

DOI:10.1002/gj.3350220505

出版商:John Wiley&Sons Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractAn Ashgill and Llandovery sedimentary succession north‐west of the Tywi Axis has been remapped at a scale of 1:10000. A revised lithostratigraphy is suggested, based on more rigorous sedimentology, and is integrated with earlier biostratigraphic work. The lithostratigraphy is compared with that from other parts of the Welsh basin and the sediments are related to palaeogeography and tectonics.Ashgill sediments indicate a shelf–slope environment which became shallower during the Hirnantian due to glacio‐eustatic regression. During the early Llandovery, sedimentation took place in deeper water mainly on a slope, while middle and upper Llandovery sediments record deposition in a slope to basin enviro

ISSN:0072-1050    

DOI:10.1002/gj.3350220506

出版商:John Wiley&Sons Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractWenlock age turbidites in central Powys are subdivided into the mudstone rich Llanbadarn Formation and the underlying more sandy Castle Vale Formation. Sand turbidites were deposited by NNE directed flows along the base of a fault controlled basin slope dipping WNW. The resulting Castle Vale Formation thins laterally onto the slope, first onlapping it then offlapping it to be overlain diachronously by silt–mud turbidites of the Llanbadarn Formation. A finely laminated hemipelagic facies occurs between the turbidites throughout. The fine laminae are interpreted as having an annual origin. Bioturbation is entirely absent suggesting that sediment pore‐waters may have been ano

ISSN:0072-1050    

DOI:10.1002/gj.3350220507

出版商:John Wiley&Sons Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractThe mainly Gorstian (Lower Ludlow, Silurian) Bailey Hill Formation, defined here, has previously been interpreted as a ‘bathyal’ turbidite sequence deposited in the north–south elongated Montgomery Trough. It has been seen as the Ludlow equivalent of the Wenlock and Llandovery turbidite systems preserved further west. Slowly deposited background sediments are punctuated by event deposits interpreted here not as turbidites but as distal storm deposits accumulated in an outer shelf setting. The model envisages entrainment of sediment above storm wave base on the inner shelf, lateral transport in the storm forced flow, and redeposition below storm wave base under the residual influence of this same flow. The distribution of the formation reflects the interplay between the palaeoslope of a broad shelf and palaeooceanographic factors such as storm wavebase, during a period of high sea level. Contemporaneous true turbidites to the northwest suggest that a depositional low existed outboard of the area of Bailey Hill Formation deposition at this time, in continuity with earlier Silurian turbidite systems. The revised palaeogeography is consistent with other aspects of Ludlow regional geology.By mid‐Ludlow time a tectonic modification of palaeogeography was underway, marked by extensive submarine

ISSN:0072-1050    

DOI:10.1002/gj.3350220508

出版商:John Wiley&Sons Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractIn the Denbigh Moors area of North Wales some of the Silurian sedimentary rocks are intensely disrupted. Recently the British Geological Survey has confirmed that this is largely the result of extensive slumping of the sediments, which was part of a long sequence of deformation events ranging from what they called ‘penecontemporaneous movements’ and related ‘additional processes’ to at least three periods of regional tectonism. New knowledge of the microstructural behaviour of deforming sediments obtained from laboratory studies allows the prospect of assigning field structures to a position within this long scenario and a better understanding of the complex pre‐lithification deformation processes.Experimental deformation of argillaceous sediments has shown that they deform, over a wide range of water content, by intense slip within narrow shear zones rather than by pervasive grain slip. Shear zones of strikingly similar appearance are common in parts of the Denbigh Moors succession, and, being demonstrably pre‐diagenetic, can be used as indicators of pre‐lithification disturbance.The shear zones give a polished, finely striated, and crenulated appearance to many of the exposure surfaces in the area. The diverse orientations of the zones reflect to some extent different locations within the sliding sedimentary masses. The style of the zones gives an indication of the water content of the sediment and hence the burial depth at the time of deformation. Some beds show features such as truncated upper contacts and are therefore thought to have been disturbed before lithification, yet the rocks lack shear zones. These are interpreted as having been deformed while very near surface, with a water content too high for the shear zone mechanism to have operated. The possibility remains to be explored that some of the shear zones in the area are the result of regional tectonism acting on unlithif

ISSN:0072-1050    

DOI:10.1002/gj.3350220509

出版商:John Wiley&Sons Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractA regional strain study of Snowdonia, North Wales, has used clast orientation data from Caradoc volcanic rocks. The data, analysed using theRf/Φ technique, have revealed that while the local strain pattern is very heterogeneous in intensity and style, it can be processed to define a regional ‘average’ strain.This regional strain closely approximates to plane‐strain in both rigid and ductile markers. This, when considered with the style of folds, leads to the tentative suggestion that the deformation mechanism in Snowdonia is one of gently inclined and ductile simple shear from the nor

ISSN:0072-1050    

DOI:10.1002/gj.3350220510

出版商:John Wiley&Sons Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractRemotely sensed lineaments are often used as indicators of fractures in near‐surface rocks. Previous lineament interpretations of Wales have used aerial photographs and relatively unenhanced satellite images. New images of part of Wales have been produced from Landsat MSS data using several digital image processing techniques intended to enhance the visibility of linear features. A lineament interpretation of these images is presented and described. Five populations of lineaments are classified, primarily on the basis of their azimuths. The most numerically significant of these has a mean azimuth of 075° and is here termed the ‘Ystwyth Lineament Set’. Typically the lack of rock exposure on the line of lineaments makes direct geological investigation difficult. However, field evidence and literature have been used to shed light on the origin of the Ystwyth Lineament Set. It is concluded that this set of lineaments is the visual representation of a set of fractures which have had a long and tectonically varied h

ISSN:0072-1050    

DOI:10.1002/gj.3350220511

出版商:John Wiley&Sons Ltd    

年代:1987

数据来源: WILEY