摘要:

AbstractDetailed structural analysis of the northern part of the Cotentin Peninsula provides evidence for major Variscan reworking of Proterozoic structures. Two stages in the regional evolution of the Variscan orogeny are distinguished: the Palaeozoic Siouville Syncline was formed before the development of a south‐facing ductile overthrust. This latter stage is dated as Carboniferous in age from its relationship with the synkinematic intrusion of the Barfleur granitic pluto

ISSN:0072-1050    

DOI:10.1002/gj.3350230301

出版商:John Wiley&Sons Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractMarine fossils includingBroeggerolithus broeggeri(Bancroft) andSalterolithus caractaci(Murchison) collected from strata surrounding the andesitic conglomerate of Castle Hill, Montgomery prove that the volcanic event which produced it, and possibly also the andesite of Moel‐y‐Golfa 10 miles to the NNE, was of Caradoc (early Soudleyan)

ISSN:0072-1050    

DOI:10.1002/gj.3350230302

出版商:John Wiley&Sons Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractTraditional methods of mapping folded regions rely heavily on vergence or minor structures (folds and intersecting foliations) to confine the position of major folds of the same generation. It is argued on theoretical grounds that in certain circumstances the vergence of early structures can become reversed by later deformation. Natural examples of this vergence reversal occur on the NW limb of the Rhoscolyn Anticline in Anglesey. Lack of awareness of the possibility of this phenomenon could lead to serious misinterpretation of data from less completely exposed folded regions.

ISSN:0072-1050    

DOI:10.1002/gj.3350230303

出版商:John Wiley&Sons Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractThe litho‐ and biostratigraphy of the Lower Dinantian succession in a deeper part of the Dublin Basin is described. The sub‐Waulsortian Malahide Limestone Formation (emended) is described fully for the first time, and has proved to be very much thicker than was previously suspected, in excess of 1200 m. Succeeding the ‘Lower Limestone Shale’ unit, which is transitional from the underlying Old Red Sandstone facies, the following six new members are recognized: Turvey Micrite Member, Swords Argillaceous Bioclastic Member, St. Margaret's Banded Member, Huntstown Laminated Member, Dunsoghly Massive Crinoidal Member and Barberstown Nodular Member (top).The Malahide Limestone Formation is overlain by ‘Waulsortian’ limestones of the Feltrim Limestone Formation (new name) which form overlapping and isolated mudmounds with complex relationships with their enclosing non‐mound facies. Though very much thicker, the Courceyan succession is comparable with that elsewhere on the south side of the Basin, and is part of the Kildare Province (Strogen and Somerville 1984). Isopach maps for the region show that this province and the North Midlands are separated by the deepest part of the Dublin Basin, named the ‘East Midlands Depocentre’, in which a shale‐dominant facies is present. The top of the ‘Waulsortian’ is of early Chadian age. Formations younger than this are dominated by basinal calcareous shales (Tober Colleen Formation) and by storm deposits and calciturbidites with appreciable terrigenous input from the east (Rush Formation).The Courceyan main shelf conodont biozones are also greatly thickened in this area. ThePseudopolygnathus multistriatusBiozone (>300 m thick) is succeeded by a very thick (>900 m)Polygnathus mehliBiozone. The base of the Chadian is considered to occur below the top of the Feltrim Limestone Formation and, although equivocal, may be diagnosed in the Dublin Basin by the first appearance of the problematic microfossilSphaerinvia piaiand a primitive form of the calcareous algaKoninckopora.In the late Courceyan, the Swords area was part of a gently sloping shelf extending northwards into the basin. During deposition of the Feltrim Limestone Formation there was major deepening and there is evidence of initial break up of the Dublin Basin by faulting into separate blocks. By Chadian time the Basin was definitely subsiding by fault displacements and basinal limestones contain shallow water faunas and littoral sand and pebbles derived by turbidite flows from the margins of the higher blocks. The early subsidence was apparently by pure flexure, but in the Viséan the Dublin Basin was fault‐controlled, differing from the adjacent Shannon Basin in having bot

ISSN:0072-1050    

DOI:10.1002/gj.3350230304

出版商:John Wiley&Sons Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractThe tectonically isolated Farnacht Formation consists of calc‐alkaline dacitic‐andesitic lavas of volcanic arc affinity. It is situated immediately to the south of the Clew Bay Fault Zone (western continuation of the Highland Boundary Fault Zone of Scotland) in the northeast corner of the Lower Palaeozoic South Mayo Trough in northwest Ireland. It has been metamorphosed to biotite grade greenschist facies following the development of a pervasive, c‐s composite muscovite, quartz, and feldspar schistosity. The Farnacht Formation may comprise a terrane that is directly unrelated to nearby Ordovician and Silurian rocks; its present position was fixed largely by Wenlock times.The age of the Farnacht Formation and the deformational event(s) that produced the schistosity are not known.40Ar/39Ar step heating from four specimens have dated the crystallization of biotite at from 422 ± 2 to 405 ± 14 Ma with a mean age of 413 Ma. These ages date either the post‐D2, pre‐D3 metamorphic peak, or a hornfelsing of the same structural age related to an unseen thermal source, and provide a minimum age for the end Silurian ‐ early Devonian Caledonian tectonothermal activity in the northeast part of the Sout

ISSN:0072-1050    

DOI:10.1002/gj.3350230305

出版商:John Wiley&Sons Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractSoil‐zone microfabrics, alveolar‐septal structure, needle‐fibre calcite, and calcans are described from horizontal calcrete layers, stringers, and infillings in vertical desiccation cracks from an Upper Jurassic limestone in the Lower Purbeck Formation of Dorset. These calcrete palaeosols occur in an oolitic limestone (the Hard Cap) which represents former evaporitic lagoonal to carbonate mudflat environments. The calcretes occur 6‐10 cm below the Great Dirt Bed, a former rendzina soil with rooted tree remains. Desiccation cracks and vugs formed in the oolitic sediment before Great Dirt Bed times. After formation of the Great Dirt Bed, soil‐water rich in dissolved CaCO3preferentially flowed through natural conduits in the underlying sediment, namely the desiccation cracks and vugs. Calcrete precipitated in these cracks and vugs around decaying plant roots, and probably, during more arid (evaporative) climatic periods. These palaeosol microfabrics are among the first to be described from the British Jurassic and were probably preserved due to the semiarid Lower Purbeck climate where rapid oxidation of organic matter limited the amount and strength of carbonic acid generation, thereby limiting extensive dissolution of early formed soil‐zone carbonate. Early diagenetic cementation of the sediment also aided microfabric preservation by sealing off soil‐zone structures from subsequent diag

ISSN:0072-1050    

DOI:10.1002/gj.3350230306

出版商:John Wiley&Sons Ltd    

年代:1988

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350230307

出版商:John Wiley&Sons Ltd    

年代:1988

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350230308

出版商:John Wiley&Sons Ltd    

年代:1988

数据来源: WILEY