摘要:

AbstractThe position of a boring in a cup of the disparid crinoidSynbathocrinus conicusPhillips from Clitheroe, England, suggests various inferences concerning the ecology of the pit‐forming organism. The crinoid cup shows a growth deformity, indicating it must have been alive when infested. The boring is at a triple suture between the E and A ray radials and the EA interray basal, presumably one of five equally weak points on the cup. This site is well removed from the anus and would have been an ideal point of attachment for a filter feeding organism, being well elevated, upcurrent and protecte

ISSN:0072-1050    

DOI:10.1002/gj.3350260102

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractOrdovician stratigraphy between the Sugarloaf and the Wye valley is summarized with particular attention given to the area northeast of Abergwesyn. New data refine thickness estimates for the Cerriggwynion Formation, the Trembyd Formation (new), and the Cefn Cynllaith Formation (new). The major Bron Rhudd Fault is suggested to have been a synsedimentary down‐to‐basin fault which was later reactivated, folded, and suffered local reversal of throw. Roll‐over into this fault controlled deposition of mass‐flow conglomerates in a perched basin. During the late Ordovician‐early Silurian the axis of the Tywi Anticline underlay a non‐depositional slope which developed by northwest‐directed progradation of a shelf margin, possibly accentuated by flexure over a basement fault zone with contemporaneous basin‐down normal movement. The slope angle cannot be estimated from the geometry of the Ordovician because the contemporaneous throw on the Bron Rhudd Fault cannot be determined, but from the rate of Silurian onlap it was

ISSN:0072-1050    

DOI:10.1002/gj.3350260103

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractThe most extensive ichnofauna yet recorded from a deep water Lower Palaeozoic sequence occurs within the distal turbidites of the Lower Silurian Aberystwyth Grits Formation of Central Wales.The strata contain an abundant assemblage comprising 25 ichnogenera:Asteriacites, Bergaueria, Chondrites, Cochlichnus, Cosmorhaphe, Glockerichnus, Gordia, Helicolithus, Helminthopsis, Helminthoida, Hormosiroidea, Lorenzinia, Megagrapton, Monomorphichnus, Neonereites, Nereites, Palaeophycus, Paleodictyon, Planolites, Protopaleodictyon, Spirorhaphe, Spirophycus, Squamodictyon, Subphyllochorda, Taphrhelminthopsis;36 ichnospecies are described, three of which (Asteriacites aberensis, Helminthopsis regularis, Cosmorhaphe elongata) are new.The inorganic sedimentary structures and trace fossils of some 418 sandstone beds were examined in detail; 16 per cent of the beds commence with Divisions A or B and 84 per cent with Division C of the turbidite sequence. This indicates a relatively distal environment, mainly receiving low velocity turbidity currents, and favouring trace fossil preservation. The most common traces wereHelminthopsis, Paleodictyon, andSquamodictyonwhich were found on 46 per cent, 34 per cent, and 19 per cent of the beds examined.Data from this, and other recently described sequences, confirms that there was a gradual increase in trace fossil diversity in the deep oceans throughout the Lower Palaeozoic, in contrast to the situation in shallow water shelf seas where a peak was reached as early as the Lower Cambrian.

ISSN:0072-1050    

DOI:10.1002/gj.3350260104

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractThe Budleigh Salterton Pebble Beds (BSPD) are a 20–30 m thick formation of conglomerates and subordinate sandstones which crop out along the western margin of the Wessex Basin. The formation has previously been interpreted as representing a major conduit for southerly‐derived (Armorican) detritus and as signalling the start of early Triassic rifting. In this paper the role of the BSPB in the evolution of the Wessex Basin is reassessed.In the south of the outcrop, the lower portions of the BSPB are dominated by extensive (>50m) sheets or narrow lenses of planar cross‐bedded conglomerate (sets 1–3m thick). This reflects deposition from linguoid‐shaped bars whose downstream margins were bounded by slipfaces. These accreted at anabranch confluences and as bank‐attached bars in relatively confined and channelized gravel‐bed streams. Most of the rest of the BSPB consists of couplets of horizontally‐bedded conglomerate overlain by large‐scale trough cross‐bedded sandstone. This style of accretion represents deposition from relatively low‐relief gravel bars and large sand dunes in substantial but poorly confined channels. Towards the top of the BSPB in the central parts of the outcrop, there is a change to thinner‐bedded units with more lenticular and ribbon‐like geometries. This represents deposition in smaller and more flashy streams.The BSPB was deposited either on a braidplain or by an antecedent ‘wet’ alluvial fan. In both settings, the BSPB streams were exotic, draining basins to the south of the Wessex Basin. This implies that the BSPB did not necessarily develop in response to differential subsidence and ri

ISSN:0072-1050    

DOI:10.1002/gj.3350260105

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractSequence stratigraphy has produced a number of new concepts and terms which have been described and defined in the Wilguset al.(1988) volume. Many of these concepts and terms can be applied to the two orders of allocycles present in the non‐autocyclic parts of the Limestone Coal Group. The longer‐period allocycles responsible for major Namurian marine transgressions correspond in their duration to ‘sequences’. The shorter‐period allocycles of the distal facies association in the upper part of the Limestone Coal Group correspond in their duration to ‘parasequences’. However, they include deeply incised fluvial channels similar to those cut into shelf sediments at ‘Type 1 sequence boundaries’, and are best regarded as unusually short‐period depositional sequences. The deep fluvial channels, which relate to lowstands, were incised when the rate of fall in sea level (probably glacial‐eustatic) exceeded the local rate of tectonic subsidence.The bulk of the distal facies association belongs to the ‘highstand systems tracts’ and ‘transgressive systems tracts’ of sequence stratigraphy. Lowstand deposits are poorly represented. Erosional breaks within two‐storey sheet sandstones probably reflect scouring by fluvial channels in the prograding ‘highstand systems tracts’ during the early stages of eustatic falls. Coals mostly relate to the rising water table conditions associated with ‘transgressive systems tracts’ and each seam may be preceded by a depositional hiatus. Extensive penecontemporaneous lowstand deposits may be found in the turbidite‐fro

ISSN:0072-1050    

DOI:10.1002/gj.3350260106

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350260107

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350260108

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350260109

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350260111

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350260112

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY