The biostratigraphy and formational relationships of the upper aeronian and lower telychian (llandovery,silurian) formations of western mid-wales     

David K. Loydell 《Geological Journal》1991,26(3):209-244

The stratigraphical relationships of the upper Aeronian-lower Telychian formations in the Aberystwyth area are examined in the light of a new biostratigraphical subdivision of this interval based on graptolites, in which part of the Monograptus turriculatus Biozone is divided into six subzones and the Stimulograptus halli Biozone is resurrected. In the Aberystwyth area (B.G.S. 1:50 000 Sheet 163 (1984)) there is shown to be greater diachroneity of formational —boundaries than previously recognized. Notably, the Cwmsymlog Formation facies persisted in the northeast of the area far longer than anywhere else, due to the persistence of a bathymetric high into the lower Telychian. Also, the basal beds of the Aberystwyth Grits in the west of the area are shown to be of the same age as the uppermost beds of the Devil's Bridge Formation in the east. Thus for a short period the area was subject to turbiditic incursions from two sources. The degree of diachroneity of the base of the Aberystwyth Grits Formation as a whole is less than previously thought — the oldest beds are approximately one subzone older in the south than in the north. The core of the in the clinal pericline responsible for the crescentic outcrop pattern of the Aberystwyth Grits is demonstrated to lie in the vicinity of Llanrhystud, rather than at Aberarth as previously suggested. Four new graptolite species are described and one new genus is erected.  相似文献   

A revision of the stratigraphy of the longmyndian supergroup,welsh borderland,and of its relationship to the uriconian volcanic complex     

John C. Pauley 《Geological Journal》1991,26(2):167-183

Major revisions are proposed to the stratigraphy of the late Precambrian Longmyndian Supergroup of the Welsh Borderland. The stratigraphic relationship between it and the adjacent, late Precambrian Uriconian volcanic complex is reviewed and reinterpreted. Contrary to previous belief it is proposed that the Wentnor Group of the Longmyndian Supergroup does not overlie the Stretton Group unconformably. An apparent unconformity is interpreted as the result of post-Longmyndian tectonism. The Ragleth Tuff Formation, previously included with the Uriconian volcanic complex, is argued to be part of the Longmyndian Supergroup. The Helmeth Grit Member, previously thought to be the base of the Longmyndian Supergroup, is incorporated within the Ragleth Tuff Formation. These modifications necessitate a revision of the previously held belief that the Helmeth Grit Member is the base of the Longmyndian Supergroup which overlies the ‘Ragleth Tuffs’ of the Uriconian volcanic complex unconformably. The Ragleth Tuff Formation and the Stretton Shale Formation are thought to be faulted against the Eastern Uriconian, and the Wentnor Group is thought to be faulted against the Western Uriconian. Juxtaposition of the Longmyndian Supergroup with the Uriconian volcanic complex across faults is the result of strike-slip movements along the Church Stretton and Pontesford-Linley fault systems. Lithological and petrographical evidence suggests that the Longmyndian Supergroup is partly coeval with, and partly younger than, the Uriconian volcanic complex, which acted as a source. The Willstone Hill conglomerate, previously thought to be interbedded with the Eastern Uriconian, may be a representative of the Wentnor Group which overlies the Eastern Uriconian unconformably. As such, it may represent a late onlap of Longmyndian sediments onto the Eastern Uriconian at the margins of the Longmyndian basin. A new formation, the Linley Formation, is recognized. This occurs as fault-bounded slivers and lenses within the Pontesford-Linley fault system, and correlation with the rest of the Longmyndian is uncertain.  相似文献   

Late Ordovician stratigraphy northwest of the Sugarloaf,mid-Wales — a discussion     

D. M. D. James 《Geological Journal》1990,25(2):199-204

Several modifications are proposed to the Ordovician stratigraphy recently proposed by Mackie and Smallwood (1987), based in particular on the recognition of a major Mynydd Trawsnant anticline on the northwest flank of the Tywi structure and on field relationships indicating the Foel Formation directly to overlay the Caradoc black shale succession and directly to underlay the Craig Las Formation. The Clyn Glas Formation is suggested to have accumulated at a slope base rather than a shelf–slope break.  相似文献   

A white mica crystallinity study of the Berwyn Hills, North Wales   总被引：1，自引：0，他引：1  

M. A. AWAN  N. H. WOODCOCK 《Journal of Metamorphic Geology》1991,9(6):765-773

Abstract White mica crystallinity studies have been carried out on 90 samples of mudrocks, six of spotted slate, and five of accretionary lapilli tuff from the area around the Berwyn Hills, North Wales. Strain was measured for some of the spotted slate and tuff samples. The metamorphic grade increases from southeast to northwest, with values of the Kübler index varying from 0.64 to 0.20Δ2θ. Metamorphic zonal boundaries follow the strike of bedding and cleavage, but crystallinity values increase into stratigraphically younger rocks on the northwest side of the Berwyn Dome. This effect is attributed mainly to a rapid increase in the thickness of synmetamorphic overburden to the northwest, comprising exposed Silurian turbidites and inferred Lower Devonian non-marine sediments. Strain variations have a more local influence on crystallinity, and lateral variations in the contemporary geothermal gradient cannot be ruled out. However, only with unrealistically high gradients would the need for a thick Lower Devonian component to the overburden be removed. This reasoning implies that the metamorphic peak was coeval with the Acadian (late Caledonian) event, rather than with an early diastathermal event.  相似文献   

National and transnational borderlanders’ attitudes towards the security fence between Israel and the Palestinian Authority     

Alon Gelbman  Ofra Keinan 《GeoJournal》2007,68(4):279-291

The subject of the security fence between Israel and the Palestinians in the West Bank has become a major issue in Israel and in the world in the last several years. The main aim of this research is to reveal the attitudes and thoughts about the fence held by local residents living in settlements (borderlanders) in the proximity of a part of the security fence that has already been completed. The research concentrates on the western-Israeli side of the fence, as it aspires to delve into and understand the meaning and implications of the security fence on matters such as personal security, safety of property and freedom of movement, the possibility of maintaining social and economic ties between the two sides and feelings about living in the area in the future. Underlying this research is the transformation occurring in the border area as a result of its closure by construction of the security fence, after many years in which it was open partially. This process has many diverse consequences, some of them contradictory, on the two populations residing near the border in Israel: the majority Jewish population (the national borderlanders), and the minority Arab population (the transnational borderlanders).  相似文献   

Sedimentary and faunal events revealed by a revised correlation of post‐glacial Hirnantian (Late Ordovician) strata in the Welsh Basin,UK     

Jeremy R. Davies  Richard A. Waters  Mark Williams  David Wilson  David I. Schofield  Jan A. Zalasiewicz 《Geological Journal》2009,44(3):322-340

The discovery of a previously unrecognized unconformity and of new faunas in the type Llandovery area underpins a revised correlation of Hirnantian strata in mid Wales. This has revealed the sedimentary and faunal events which affected the Lower Palaeozoic Welsh Basin during the global rise in sea level that followed the end‐Ordovician glacial maximum and has allowed their interpretation in the context of local and global influences. In peri‐basinal shelfal settings the onset of post‐glacial deepening is recorded by an unfossiliferous, transgressive shoreface sequence (Cwm Clyd Sandstone and Garth House formations) which rests unconformably on Rawtheyan rocks, deformed during an episode of pre‐Hirnantian tectonism. In the deep water facies of the basin centre, this same sequence boundary is now recognized as the contact between fine‐grained, re‐sedimented mudstones and an underlying regressive sequence of turbidite sandstones and conglomerates; it is at a level lower than previously cited and calls into question the established lithostratigraphy. In younger Hirnantian strata, graptolites associated with the newly recognized Ystradwalter Member (Chwefri Formation) demonstrate that this distal shelf unit correlates with the persculptus graptolite‐bearing Mottled Mudstone Member of the basinal succession. Together these members record an important macrofaunal recolonization of the Welsh Basin and mark a key event in the post‐glacial transgression. Further deepening saw the establishment of a stratified water column and the imposition of anoxic bottom water conditions across the basin floor. These post‐glacial Hirnantian events are consistent with the re‐establishment of connections between a silled Welsh Basin and the open Iapetus Ocean. However, a comparison with other areas suggests that each event records a separate deepening episode within a pulsed glacio‐eustatic transgression, while also reflecting changes in post‐glacial climate and patterns of oceanic circulation and associated biotic flux. British Geological Survey © NERC 2009. All rights reserved.  相似文献   

Geology of the Pedwardine district,Herefordshire and Powys     

Helen E. Boynton  C. H. Holland 《Geological Journal》1997,32(3):279-292

The stratigraphy of the Pedwardine inlier of Precambrian (Longmyndian), Tremadoc, and what are now shown to be upper Llandovery rocks is described. The inlier is surrounded by Silurian rocks, including upper Llandovery (two very small areas), Wenlock, Ludlow and Přídolí (Red Downton Formation). The Ludlow rocks provide a distal record of the mass movement recorded more fully in the Wigmore Rolls area to the east. The Pedwardine area lies along the Church Stretton Fault Zone. Though ill-exposed, it shows evidence of the history of faulting, including thrusting. © 1997 John Wiley & Sons, Ltd.  相似文献   

Phylogenetic diversity of planktonic bacteria in the Chukchi Borderland region in summer   总被引：1，自引：1，他引：0  

ZENG Yinxin  YU Yong  LI Huirong  HE Jianfeng  LEE Sang H  SUN Kun 《海洋学报(英文版)》2013,32(6):66-74

Planktonic bacteria are abundant in the Chukchi Borderland region. However, little is known about their diversity and the roles of various bacteria in the ocean. Seawater samples were collected from two stations K2S and K4S where sea ice was melting obviously. The analysis of water samples with fluorescence in situ hybridization (FISH) showed that DMSP-degrading bacteria accounted for 13% of the total bacteria at the station K2S. No aerobic anoxygenic phototrophic (AAP) bacteria were detected in both samples. The bacterial communities were characterized by two 16S rRNA gene clone libraries. Sequences fell into four major lineages of the domain Bacteria, including Proteobacteria (Alpha, Beta and Gamma subclasses), Bacteroidetes, Actinobacteria and Firmicutes. No significant difference was found between the two clone libraries. SAR11 and Rhodobacteraceae clades of Alphaproteobacteria and Pseudoalteromonas of Gammapro-teobacteria constituted three dominant fractions in the clone libraries. A total of 191 heterotrophic bacterial strains were isolated and 76% showed extracellular proteolytic activity. Phylogenetic analysis reveals that the isolates fell into Gammaproteobacteria, Bacteroidetes, Actinobacteria and Firmicutes. The most common genus in both the bacterial isolates and protease-producing bacteria was Pseudoalteromonas. UniFrac data showed suggestive differences in bacterial communities between the Chukchi Borderland and the northern Bering Sea.  相似文献   

Palaeotopography of the northern portion of the Telychian (Silurian) turbidite basin in central Wales     

D.M.D. James 《Geological Journal》2005,40(5):593-601

New palaeocurrent data from the country northeast and southwest of Corris (central Wales) indicate that Telychian sandstones in the Devil's Bridge Formation hereabouts were transported from both the northeast and northwest, consistent with topographical control by down‐to‐southeast movement on the Bala Lineament. This conclusion is supported by considerations of thickness variation and by evidence for listric detachment faulting in the formation at Bwlch y Groes. Towards Plynlimon, flow is broadly southerly and suggests confinement by the northward extension of the Bronnant Fault. A facies and palaeocurrent map is presented for the utilis sub‐Biozone of the Telychian which links with work by the British Geological Survey between Aberyswyth and Rhayader where, by contrast, palaeoflow is to the NNW. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

Land cover changes and landscape pattern dynamics in Senegal and Guinea Bissau borderland     

《Applied geography (Sevenoaks, England)》2017

Different levels of climatic, agricultural, demographic, political and socioeconomic change have been detected in West African countries. For many years the human impact on the region was negligible, due to the low population density. However, in the last decades, demographic increase has accelerated exploitation of the natural resources and consequently the degradation of the ecosystems. Land cover degradation and landscape changes are more or less pronounced according to politic and socio-economic conditions of each country. In this study, a region located on the borderland of north Guinea-Bissau and south Senegal, is analyzed with regard to land cover and landscape trends, considering the socio-economic factors that drive them and the borderline effect. In this analysis, remote sensing data (Landsat TM, ETM+ and OLI data) were used to obtain four land cover maps for the years 1990, 2002, 2010 and 2015. The results show a similar landscape behavior in both countries, directly related to similar socio-economic practices. Forest area increase in both countries and there is a reduction in Agriculture/Bare soil areas until 2010. Main causes may be associated to the rural exodus, resulting from the conflict for independence of Casamance, which affects the borderland region of Senegal and Guinea Bissau, or to the conversion of agricultural areas into cashew orchards. In opposition, between 2010 and 2015, Forest area decrease and Agriculture/Bare soil increase, due to the recent trend of conversion of old cashew orchards into traditional fields of cereals and peanut. An increase in landscape fragmentation is observed in the period under analysis, revealing a heterogenization trend, which can be related to the adoption of similar human practices, in the last decades.  相似文献