摘要:

AbstractAnalcime (NaAlSi2O6.H2O) forms a characteristic authigenic phase in Andean alluvium of northern Chile. Three stratigraphic units ranging from the lower Cretaceous to the Miocene were studied in order to investigate the presence of analcime in diagenetic assemblages of different ages. The units were deposited in forearc, foreland/back‐arc, and intra‐arc basinal settings.Characteristics of other sedimentary analcime occurrences were examined in order to define conditions favourable for analcime precipitation. Zeolite zonation and evidence for the original presence of volcanic glass, common to many sedimentary analcime occurrences, are absent in the studied units. Analcime precipitation requires a high Na+/H+ratio, relatively low Si4+/Al3+activity and low H2O activity.Diagenetic examination of the three stratigraphic units revealed early quartz and feldspar overgrowths, mechanical clay infiltration, and dissolution of labile rock and mineral fragments, particularly intermediate plagioclase (An30–80). Conditions for these processes are favoured by oxidizing groundwater at near neutral pH. Later diagenesis involved the precipitation of analcime, non‐ferroan calcite, quartz, and authigenic clays as the main cementing phases. These formed due to the concentration of oxidizing groundwater and resulted from its continued reaction with detrital feldspars and rock fragments of andesitic and dacitic composition.Analcime precipitation in Andean alluvium resulted from the evolution of saline, alkaline brines under an arid, evaporative climate in closed hydrographic basins with circulating groundwater

ISSN:0072-1050    

DOI:10.1002/gj.3350260302

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractA thin band yielding conodonts and Foraminifera and therefore demonstrably marine, has been found in mudstone lying immediately above the uppermost coal horizon in Billinge Beacon Quarry. The existence of this newly recognized horizon, which is very likely to be the Langley Marine Band at the top of the Lenisulcata Chronozone, much strengthens the correlation of the Billinge Beacon sandstones with the regionally developed Crutchman Sandstone. An alternative correlation of the Billinge Beacon sandstones with the Dyneley Knoll Flags of Burnley, which was recently proposed by F. M. Broadhurst, is not supported by the new evidence, nor by the evidence of boreholes, since it appears that the Dyneley Knoll Flags die out some distance to the north of Billinge.

ISSN:0072-1050    

DOI:10.1002/gj.3350260303

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractThe 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 theMonograptus turriculatusBiozone is divided into six subzones and theStimulograptus halliBiozone 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 er

ISSN:0072-1050    

DOI:10.1002/gj.3350260304

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractThe Dinantian Edale Basin is located to the north of the Derbyshire carbonate platform and underlies the Upper Carboniferous of the central Pennines. The Edale Basin was thought to be part of a large basin which extended from the Derbyshire carbonate platform to the Askrigg Block. The presence of aggregate grains and ooids in the Alport Borehole suggests that a carbonate platform, possibly located on the Holme structural high, was present underneath the central Pennines. This platform is called the Holme Platform.The Arundian to early Asbian section of the Alport Borehole represents deposition of resedimented shallow‐water carbonates with occasional bioturbated periplatform carbonates and basinal shales on the middle part of a carbonate ramp. Volcaniclastic sediments may have been derived from a volcanic centre within the Edale Basin.A change in sedimentation during the mid‐Asbian to the deposition of basinal shales and distal carbonate turbidites is attributed to starvation of the basin. This may have been caused by a combination of the development of accretionary rimmed carbonate shelves and the repeated emergence of shelf carbonates deposited on surrounding carbonate platforms. The late Asbian/early Brigantian section of the Edale Borehole is interpreted as a distal equivalent of the ‘Beach Beds’ which outcrop at the north margin of the Derbyshire carbonate platform. The ‘Beach Beds’ represent bioclastic turbidites derived from the Derbyshire carbonate platform.Throughout the Brigantian, sedimentation in the Edale Basin was dominated by the deposition of distal carbonate turbidites and basinal shales. Variation of dip through the Alport Borehole indicates the common occurrence of slumps throughout the sequence and the presence of either an angular unconformity or a fault within the early Brigant

ISSN:0072-1050    

DOI:10.1002/gj.3350260305

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350260307

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350260308

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350260310

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350260311

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350260313

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350260315

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY