摘要:

AbstractShallow water platform limestones of the Chadian–Asbian Milverton Group are restricted to the north‐eastern part of the Lower Carboniferous (Dinantian) Dublin Basin. Here, they are confined to two granite‐cored fault blocks, the Kentstown and Balbriggan Blocks, known to have been active during the late Dinantian. Three areas of platform sedimentation are delimited (the Kentstown, Drogheda and Milverton areas), although in reality they probably formed part of a single carbonate platform. Resedimented submarine breccias and calciturbidites (Fingal Group) composed of shallow water allochems and intraclasts sourced from the platform accumulated, along with terrigenous muds, in the surrounding basinal areas.Sedimentological evidence suggests that the Kentstown and Balbriggan Blocks possessed tilt‐block geometries and developed during an episode of basin‐wide extensional faulting in late Chadian time. Rotation of the blocks during extension resulted in the erosion of previously deposited sequences in footwall areas and concomitant drowning of distal hangingwall sequences. Antithetic faults on the northern part of the Balbriggan Block aided the preferential subsidence of the Drogheda area and accounts for the anomously thick sequence of late Chadian platform sediments present there.Continued subsidence and/or sea‐level rise in the late Chadian–early Arundian resulted in transgression of the Kentstown and Balbriggan Blocks; carbonate ramps developed on the hangingwall dip slopes and transgressed southward with time. Subsequent progradation and aggradation of shallow water sediments throughout the Arundian to Asbian led to the development of carbonate shelves. Several coarse conglomeratic intervals within the contemporaneous basinal sequences of the Fingal Group attest to periodic increases of sediment influx associated with the development of the shelves.Sedimentological processes controlled the development of the carbonate platforms on the hangingwall dip slopes of the Kentstown and Balbriggan Blocks, though periodic increases of sediment flux into the basinal areas may have been triggered by eustatic falls in sea level. In contrast, differential subsidence along the bounding faults of these blocks exerted a strong control on the margins of the late Dinantian shelves, maintaining relatively steep slopes and inhibiting the progradation of the shelves into the adjacent basins.Tectonically induced collapse and retreat of the platform margins occurred in the late Asbian–early Brigantian. Platform sediments are overlain by coarse‐grained proximal basinal facies which fine upwards before passing into a thick shale sequence, indicating that by the late Brigantian carbonate production had almost stopped as the platf

ISSN:0072-1050    

DOI:10.1002/gj.3350290202

出版商:John Wiley&Sons Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractRecent interpretations of the tufaceous limestones from within the ‘Caps Beds’ on the Isle of Portland have suggested a depositional environment of intertidal flats and lagoons with typically higher than normal marine salinity levels, a stark contrast with earlier ideas of a freshwater origin. However, evidence is presented in this paper to show that these deposits are indeed most likely to be freshwater in origin. The micro‐fabrics observed are typical of those seen in freshwater tufas forming at the present day, and contrast with those observed within intertidal/subtidal stromatolites. Furthermore, the Portland deposits lack syndepositional evaporite deposits, they lack recognizable intertidal deposits, and any lagoonal sediments observed are depositionally distinct from the tufas. Finally, the soil horizons observed are clear evidence of periodic subaerial exposure and isolation from marine influences.Four facies types are identified on Portland: (1) tufaceous limestone; (2) littoral grainstones; (3) subaerial stromatolites; and (4) paleosols. Each facies is repeated a number of times through the sequence, and evidence is presented to show that these formed in a marginal marine setting as a response to a series of minor transgressive (soils to freshwater lakes/lagoon to saline marine/lagoonal) and regressive events (saline marine/lagoonal to soils).The tufa stromatolite deposits themselves, which are often developed around cylindrical holes (representing former tree trunks and branches), are shown to have developed seasonally, by the precipitation of carbonate, due to microbial activity within the freshwater lake environment. Precipitation appears to have been most intense around tree bases (and any associated vegetation), where an active biofilm developed on the underlying soil substrate. Two distinct textures are recognized in this material: (1) micro‐porous and (2) macro‐porous. These developed together in a crudely laminated, semi‐concentric pattern around the holes and together may represent a seasons gro

ISSN:0072-1050    

DOI:10.1002/gj.3350290203

出版商:John Wiley&Sons Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractNearly 50 sections through the Llandovery and Wenlock black shales of the Barrandian area (Bohemia) have been examined bed by bed. This has made possible the compilation of an improved and well defined graptolite zonal scheme with much new biostratigraphic data included. A total of 268 graptolite species and subspecies have been found. Their stratigraphic distribution allows the recognition of 27 graptolite zones:ascensus–acuminatus, vesiculosus, cyphus, triangulatus–pectinatus, simulans, convolutus, sedgwickii, linnaei, turriculatus, crispus, griestoniensis, tullbergi, spiralis, grandis, insectus, centrifugus, murchisoni, riccartonensis, dubius, belophorus, rigidus, ramosus–perneri, lundgreni, parvus, nassa–frequens, praedeubeli–deubeli, ludensis, and several subzones. The biozones are defined by the vertical ranges of their ‘index’ species and are characterized by rich accompanying associations. The zonal scheme is correlated with graptolite sequen

ISSN:0072-1050    

DOI:10.1002/gj.3350290204

出版商:John Wiley&Sons Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractThe Rhuddnant Grits turbidite system was deposited within an elongate, fault‐bounded trough in the late Llandovery (Telychian) Welsh Basin. Two groups of sandstones are identified within the system: high‐matrix sandstones and laminated sandstones. The high‐matrix sandstones are medium to very thick bedded, fine to very coarse‐grained muddy sandstones. The high‐matrix sandstone beds are almost entirely structureless and have several features indicative of deposition from high density turbidity currents, probably undergoing late stage debris flow behaviour (e.g. grain size discontinuities, inverse grading, floating clasts). The laminated sandstones are thin to very thin bedded, fine‐grained and have a distinctive mud/silt lamination. Tractional structures and convolution are common in these beds. They were probably deposited by slow moving, dilute turbidity currents. Dissimilar palaeocurrent vectors and estimates of flow properties from the two types of sandstone support the contrasting nature of the depositing flows. A coarsening and thickening upwards trend is identified in the laminated sandstones of the Rhuddnant Grits Formation. This trend is not reflected in the high‐matrix sandstone beds. Although the high‐matrix sandstones appear in packets or groups within the laminated sandstone background, they were otherwise deposited in an entirely random manner throughout the exposed system. This may suggest that the two types of sandstone are the result of different triggering mechanisms at source, or of contrasting flow properties developed early in the

ISSN:0072-1050    

DOI:10.1002/gj.3350290205

出版商:John Wiley&Sons Ltd    

年代:1994

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350290206

出版商:John Wiley&Sons Ltd    

年代:1994

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350290207

出版商:John Wiley&Sons Ltd    

年代:1994

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350290208

出版商:John Wiley&Sons Ltd    

年代:1994

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350290210

出版商:John Wiley&Sons Ltd    

年代:1994

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350290211

出版商:John Wiley&Sons Ltd    

年代:1994

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350290212

出版商:John Wiley&Sons Ltd    

年代:1994

数据来源: WILEY