摘要:

AbstractThe lower part of the Old Red Sandstone in the Dingle Penisula has been previously assigned to one lithostratigraphic group (Dingle Group) despite marked variations in sedimentary facies. However the apparently oldest non‐marine sequence in the northwest of the peninsula has sedimentary and lithological attributes that contrast strongly with those of the late Silurian‐early Devonian Dingle Group to the south. This northern sequence, here renamed the Smerwick Group, evolved independently of the Dingle Group in a separate basin of deposition. Field relationships between the two groups in the north of the peninsula are interpreted as indicating that the Smerwick Group overlies, with angular unconformity, a normal Dingle Group succession. Similarly, it is argued that the Smerwick Group overlies, with angular unconformity, the Dingle Group in the northwest of the peninsula, but there the Dingle Group is attenuated, represented only by a conglomerate unit some 10 m thick. In the absence of biostratigraphic evidence the age of the Smerwick Group is poorly constrained. Nevertheless, we propose a tectonic model that suggests that the Smerwick Group evolved within a small extensional half‐graben on the northern margin of the Munster Basin. This model accounts for the stratigraphic and structural relationships observed, and implies that the Smerwick Group is of Late Devonia

ISSN:0072-1050    

DOI:10.1002/gj.3350230201

出版商:John Wiley&Sons Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractThe Carboniferous succession in southeast County Limerick, on the southeastern margin of the Shannon Trough, is Courceyan to mid‐Namurian in age and over 1900 m thick. The lithostratigraphy is described in detail. Its most important aspect is the presence of two thick volcanic sequences, a Chadian one of the alkali basalt to trachyte suite and one of Asbian age dominated by limburgites and ankaramites. The associated Dinantian carbonates are of shelf or ramp facies throughout, and no fundamental division into shelf and basin facies occurs as in the Dublin and Craven Basins in early Viséan times. Rapid differential subsidence between this area and the Shannon Estuary began during deposition of the late Courceyan to early Chadian Waulsortian facies but was less marked in the remaining Viséan when much of the volcanic topography was preserved by rapid basinal subsidence. There was basinal inversion in the late Dinantian to lower Namurian, followed by renewed subsidence in mid‐Namurian times. This contrasts with the continuous rapid subsidence of the area further west on the Shannon Estuary. This behaviour, together with a comparison of that of nearby Carboniferous basins such as the Dublin, South Munster, and Craven Basins, which lack substantial volcanic sequences, suggests an origin in a transtensional regime rather than one of simple crustal stret

ISSN:0072-1050    

DOI:10.1002/gj.3350230202

出版商:John Wiley&Sons Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractIn general, Triassic rocks throughout the studied areas are dominated by limestones locally containing foraminifers and conodonts. Interbedded terrigenous clastics are present in the Lower Triassic wherever it occurs, but otherwise are restricted to the Upper Triassic sections of Hadim. The Lower‐Middle Triassic rocks of Hadim were deposited in a persistent tidal lagoon, invaded from time to time by marine faunas when the enclosing barrier shoal was breached. At other times, the lagoon became very shallow and hypersaline but never dried out completely. Occasionally the lagoon was filled with clastics deposited by sheet‐floods from the land. At the beginning of the Upper Triassic, tectonic uplift of the hinterland resulted in a massive supply of terrigenous gravels and sand to the marginal lagoon. When this supply ceased, marine influence reasserted itself, forming a semi‐enclose

ISSN:0072-1050    

DOI:10.1002/gj.3350230203

出版商:John Wiley&Sons Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractThe Ordovician (Caradoc, Soudleyan) rocks of Montgomery, Powys are shales interbedded with locally conglomeratic volcaniclastic sediments composed of andesitic detritus. New formal lithostratigraphic units are proposed: Montgomery Volcanic Group comprising in ascending order: Castle Hill Shale Formation, Castle Hill Conglomerate Formation and Quarry Sandstone and Shale Formation. The volcaniclastic strata are reinterpreted as deposits of a submarine volcaniclastic fan system sourced by contemporaneous andesitic island volcanism. The observed diagenetic sequence is typical of marine volcanic sandstones and was dominated by hydration reactions related to the degradation of abundant unstable volcanic detritus. Diagenesis has resulted in the virtual destruction of original porosity in the volcaniclastic rocks.

ISSN:0072-1050    

DOI:10.1002/gj.3350230204

出版商:John Wiley&Sons Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractWithin the Lower Carboniferous Oolite Group of South Wales dolomites occur as 0.1 to 3 m thick stratiform bodies, sharply bounded against both underlying and overlying limestones. They contain rootlets and other plant remains and are often laterally equivalent to calcrete horizons. The dolomites are interbedded with thin coals and organic‐rich shales. In thin section a fine‐grained, xenotopic, slightly ferroan, inclusion‐rich ‘early’ dolomite forms a matrix to scattered moulds after ooids, shells, and evaporite minerals. The moulds are filled with ferroan calcite, early dolomite, and ‘late’ strongly ferroan saddle dolomite. The early dolomite is highly subzoned under cathodoluminescence whilst the late dolomite is nonluminescent. Whole rock analyses have δ18O = −5.95%, to −0.80%, and δ13C = +0.50%, to +2.80%. The early dolomite is believed to have precipitated from low sulphate, low Eh fluids within the waterlogged soil of a schizohaline coastal swamp. The late dolomite may also have formed within the pedogenic environment as a precipitate from highly evolved swamp waters, heavily enriched in organically‐derived compounds. Alternatively the later dolomite may have a hydrothermal origin as is more commonly associated

ISSN:0072-1050    

DOI:10.1002/gj.3350230205

出版商:John Wiley&Sons Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractCriteria by which such sills can be distinguished from block lavas include discordance and fault‐controlled geometry, vesiculation and homogenization of sediments adjacent to upper contacts, and the presence of pillows or peperites at upper contacts. Because sills do not always show these characteristics in the field, an extrusive origin for a sheet should be demonstrated and not assumed. However, it is not always possible to make the distinction, in which case the non‐genetic termsheetcan be usefully applied.Field studies suggest that a substantial part of the Borrowdale Volcanic Group is composed of sills. Thus previously erected lithostratigraphic schemes may not correctly indicate the sequence of magmatic events, and the correlation of locally defined stratigraphies will not be possible until the regional importance of high‐level intrusive bodies has been establ

ISSN:0072-1050    

DOI:10.1002/gj.3350230206

出版商:John Wiley&Sons Ltd    

年代:1988

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350230207

出版商:John Wiley&Sons Ltd    

年代:1988

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350230208

出版商:John Wiley&Sons Ltd    

年代:1988

数据来源: WILEY