Dinantian Limestones Research Articles

Carboniferous (Dinantian and Silesian) and Permo‐Triassic rocks in south County Wexford, Ireland

AbstractBoreholes drilled in the Wexford Outlier have proved Permo‐Trias conglomerate and red beds (Killag Formation), Westphalian D sandstone and mudrock. including at least one thin coal (Richfield Formation), and probable Namurian mudrock (Park Formation). The Killag Formation rests on the Westphalian, the Namurian, and on Dinantian (probably Asbian) limestone in different boreholes. The Westphalian contains horizons with bivalves, conchostracans, and plants and has yielded miospore assemblages which, in addition to indigenous taxa, contain reworked material of three different ages: Devonian or early Carboniferous; late Viséan or early Namurian; and Westphalian A. The western end of the Welsh massif of previous palaeogeographical reconstructions was of very low relief during the Silesian and probably was more often a site of deposition than of erosion.

Wall-rock silicification associated with fluorite veins in Carboniferous limestone at Butts Quarry, Derbyshire, England

AbstractWall rocks on either side of narrow fluorite veins cutting Dinantian Limestones at Butts Quarry, Derbyshire, exhibit unusual and spectacular alteration structures. Buff, ‘blackened’ and grey limestone lithologies, containing significant amounts of euhedral quartz crystals, occur in a zoned sequence adjacent to the veins. The alteration structures are attributed to a two-stage process involving silicification and subsequent re-mobilization of silica by fluids passing along the vein.

Optimized similarity matrices applied to the study of carbonate rocks

AbstractThe primary and early diagenetic components of carbonate rocks are commonly arranged systematically in a sort of ‘relay’ along a gradient formed by one or several environmental factors. Such a relay, if present, can be revealed by the optimization of a matrix of similarity coefficients. The technique presented uses presence/absence data and is simple and rapid. An illustration is given using data from some Dinantian limestones in southwest Britain.

Early Tournaisian rocks at Lilleshall, Shropshire

SummaryReidentification of the macrofossils from the lowest Dinantian limestones of Lilleshall, northeast of Telford, indicates that these beds are older than previously reported and that they can be correlated with the early Tournaisian Hastarian Stage. The conodonts recovered confirm this age and are characteristic of the Siphonodella–Polygnathus inornatus Assemblage Biozone. This reassessment affects the Dinantian palaeogeography of the Midlands of England and the early Tournaisian coast-line in the Lilleshall area can be redrawn further to the north than it has usually been placed.

MINOR SEDIMENTARY CYCLICITY IN LATE ASBIAN (UPPER D1) LIMESTONES IN THE LLANGOLLEN DISTRICT OF NORTH WALES

SUMMARY A detailed lithostratigraphical study of the upper Dinantian (Asbian–Brigantian) limestones in the Llangollen district of North Wales has been made. The identification of minor cyclicity of sedimentation has been used to establish a new lithostratigraphical framework to supersede the former lithological divisions of Morton (1870; 1878). The following new formations are recognised: Ty-nant Limestone (early Asbian), Eglwyseg Limestone (late Asbian) and the overlying Trefor Limestone (Brigantian). In the Eglwyseg Limestone the sequence is divided into 9 to 10 minor cycles by prominent master bedding planes. Each minor cycle suggests alternation of transgressive and regressive periods which led eventually to emergence. Repeated subaerial exposure of limestone is recognised by features including palaeokarstic surfaces, calcareous laminated crusts and K-bentonite clays (palaeosols). The palaeokarstic surfaces were formed by contemporaneous chemical dissolution and the laminated crusts developed by in situ modification of the limestone possibly beneath a soil.

PALAEOMAGNETIC STUDIES OF SOME DINANTIAN LIMESTONES FROM THE CRAVEN BASIN AND A CONTRIBUTION TO ASBIAN MAGNETOSTRATIGRAPHY

SUMMARY A palaeomagnetic survey of Dinantian limestones in the Craven basin shows that limestones of Arundian/Holkerian and Asbian age (Skibeden Shales-with-limestones and Draughton Limestone) in the Skipton Anticline area have anomalous directions of magnetization. Although some of the Asbian limestones in this area are proximal turbidites, none of the sampled horizons are conglomeratic or intraclastic limestones and the anomalous directions cannot be accounted for by the derivation of previously lithified (and magnetized) clasts. The anomalous direction also occurs in non-turbiditic Arundian/Holkerian limestones which supports this contention. The directional characteristics of the NRM indicate that the limestones in this area have suffered partial to complete remagnetization. The remagnetization is characterised by the presence of a normal component of magnetization superimposed on an original reversed component. However, some specimens have Lower Carboniferous directions of magnetization which suggests that the remagnetization effect is inhomogeneously distributed throughout the limestones. The remagnetization process stopped prior to the Permo-Carboniferous folding of the area and was probably associated with diagenetic processes such as dolomitization and silicification which took place in the Skipton area prior to the deposition of the Bowland Shales in Brigantian times. The Pendleside Limestone in the Thornton Anticline area is the lateral correlative of the Draughton Limestone but shows completely different palaeomagnetic results. The results agree closely with known Lower Carboniferous directions and it is concluded that these limestones retain their original magnetization. A detailed study of a 40 m section of alternating limestones with shales shows that all the limestone beds are reversely magnetized, a feature taken to indicate that the late Asbian geomagnetic field was reversed for prolonged periods.

Stratigraphic significance of calcareous microfossils from the Lower Carboniferous rocks of the Skipton area, Yorkshire

SummaryAssemblages of calcareous foraminifera and algae have been examined in thin sections of Dinantian limestones from the Skipton, Broughton, Hetton, Eshton, Swinden and Lothersdale anticlines in the Craven Lowlands. Provisional identifications are listed for each assemblage and a correlation of the rock successions is suggested. The assemblages range from late Tournaisian to late Viséan in age, in terms of the Belgian divisions, and from Courceyan to Asbian in terms of the new chronostratigraphic stages of George et al. (1976).

Diagenetic fabrics in some British Dinantian limestones

AbstractDiagenetic fabrics (textures and structures) have been examined in thin sections of Dinantian limestones mainly from the Avon Gorge, North Wales and Yorkshire. Six processes are shown to have been responsible for the change from unconsolidated sediment to limestone. These are: (1) granular cementation and drusy growth, (2) rim cementation (secondary enlargement), (3) pressure solution, (4) grain growth sensu stricto, (5) mechanical deposition in cavities of post‐depositional age, and (6) post‐depositional formation of cavities by erosion and solution in a carbonate mud.Grain growth, a process well known to metallurgists, acts in monomineralic fabrics of low porosity in the solid state. The intergranular boundaries wander so that some grains enlarge while others shrink and disappear, so bringing about a general increase in coarseness.In all the limestones the mosaic between the mechanically deposited particles (skeletal debris, pellets, ooliths) is one of three types. These are granular cement, rim‐cemented detrital crystals, and a mosaic caused by grain growth.

Paleontologie stratigraphique du calcaire dinantien du Tournaisis (Belgique)

Abstract Discusses and tabulates the sequence of faunal zones in the Dinantian (Carboniferous) limestone formation of the Tournai region, Belgium.