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Early Carboniferous Floodplain Deposits from South Wales: a Case Study of the Controls on Palaeosol Development

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Early Carboniferous Floodplain Deposits from South Wales: a Case Study of the Controls on Palaeosol Development Journal of the Geological Society, London, Vol. 145, 1988, pp. 847-857, 5 figs. Printed in Northern Ireland Early Carboniferous floodplain deposits from South Wales: a case study of the controls on palaeosol development V. P. WRIGHT & D. ROBINSON Department of Geology, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol BS8 lRJ, UK Abstract: Two palaeosol sequences are described from early Carboniferous floodplain deposits from SouthWales, which exhibit a complex series of pedogenic profiles. Soil types recognized includevertic Aridisols and Entisols, as well as dolomite-bearing Sulphaquents. The sequential and lateral changes in these soiltypes are interpreted as due to both intrinsic factors, suchas channel avulsionand changing proximity to channel belts (causing changes in sedimentation rates), and extrinsic factors, such as possible climatic changes and a marine transgression. In the last two decades there has been much research effort capped by alluvial units with palaeosols,streamflood, directed to the study of alluvial deposits in the geological sheetflood and high sinuosity channel deposits (Wright 1981, record. Most study has been devoted to the coarse member 19826, 1986~).The thickest of these alluvial units is the deposits, that is the channel deposits, but recently there has Gilwern Clay Member of the Llanelly Formation, which is been increasing interest in the finer, mainly floodplain well exposed at Blaen Onneu near Llangynidr, and in the sediments of alluvial sequences. Such deposits typically Clydach Gorge between Brynmawr and Gilwern (Wright contain numerous palaeosols reflecting the episodic nature 1981). of sedimentation (Kraus & Bown 1986). The study of such The Gilwern Clay Member rests on the Penllwyn Oolite alluvial palaeosols is still in its infancy but they have already Member (Fig. 1) which consists of peloidal, oolitic proved extremely useful as environmentalindicators in packstones and grainstones representing shoal and protected studies of palaeoclimates, palaeohydrology, palaeoecology, back-shoal deposits. It is overlain by the Dowlais Limestone timeresolution and sedimentation rates (Retallack 1983; Formation(Holkerian) which represents a mixed oolitic, Kraus & Bown 1986; papers in Wright 1986~). peritidaland low energymarine sequence, possibly Floodplain sequences are controlled by both intrinsic and deposited in a broad lagoon (Wright 19820). The age of the extrinsic processes. The former are predominantly sedimen- Llanelly Formation has not been fully established but it is tary processes inherenttothe immediatedepositional probably of Arundian (Visean) age (Wright 1981); detailed setting, such aschannel migration and avulsion, flooding descriptions of the local stratigraphy and sedimentology are andcompaction. Extrinsic processes include climatic given by Wright (1981). changesand base-level movements caused by tectonics/subsidenceand eustatic sea-level changes. Few Palaeosol profiles attempts have been madeto use palaeosols to elucidate these controls but a recent study by Kraus (1987) on early Aspects of these palaeosols are described briefly by Wright Tertiary sediments in Wyoming has provided an important (1982b) in a general discussion on the palaeosols within the stimulus which we have followed. Llanelly Formation. The following descriptions represent The aim of this paper is to describe in detail two more detailed work carried out recently by both authors. palaeosolsequences developed in a unit of alluvial The Gilwern Clay Member exhibits two contrasting sediments which occurs within the Carboniferous Limestone lithologies with greenand red mudstones with carbonate of South Wales. These palaeosols display changes in profile nodules being the most common, and cross-bedded, coarse characteristics interpreted as being caused by both intrinsic sandstones and conglomerates being the morerestricted. processes onthe floodplain (channelmigration) and Theformer is well exposed in the Clydach Gorgearea, extrinsic factors (marine transgression). This study provides especially at Llanelly Quarry (British National Grid an example of the integration of palaeopedology, sedimen- Reference SO 224 125) and Clydach Halt Lime Works (SO tology and mineralogy in elucidating the pedological 235 127). The successior. at the Clydach Halt Limeworks is responses to drainage changes in alluvial sequences. identical to that at Llanelly Quarry except that it has been truncated by the Dowlais Limestone (Wright 1981). Another largeexposure occurs at Blaen OnneuQuarry, Geological setting some seven kilometres north-west of the Clydach Gorge The palaeosol sequences are in the Gilwern Clay Member of near Llangynidr (SO 155 169), in which a facies transition the Llanelly Formation (Fig. 1) (Wright 1981), which forms from the mudstones to the sandstones is seen. part of the attenuated Carboniferous Limestone succession along thenorthern part of the South Wales Coalfield synclinorium (Fig. 1). The local succession consists of a Blaen Onneu cyclic series of oolitic and peritidal carbonates, containing a The exposure at Blaen Onneu Quarry is the result of recent number of subaerial exposure surfaces. Some of these are quarrying operations and at the time of writing (July 1987) 847 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/5/847/4889363/gsjgs.145.5.0847.pdf by guest on 29 September 2021 848 V. P. WRIGHT & D. ROBINSON RIT (NAMURIAN) Quaternary peat and till - LIMESTONE ~ sandstone-filled desiccation cracks CARBONIFEROUS N E m m slickensided 2. pseudo-anticlinal -m U slip planes LIMESTONE CLAY MR. (DEVONIAN) PENLLWYN OOLITE MR. PENLLWYN OOLITE MEMBER Fig. 1. Locality map, showing the Lower Carboniferous outcrop 3. The profile at Blaen Onneu (see Fig. 4). which contains the Llanelly Formation, the Lower Carboniferous Fig. succession in the area, and the subdivisions of the Llanelly Formation. The Gilwern Clay Member contains the floodplain interval described. 18" NNW while the local tectonic dip is 6" SSW. The contact with the DowlaisLimestone is not exposedfor the the mudstone facies is well exposed in a 100 m strike section sandstone and mudstones are overlain by glacial till and along the southern margin of thequarry. The mudstones blanket peat. havean abrupt contact with the sandstone facies which Themudstone, which reachesa thickness of 6m, occurs in discontinuous exposures to the east of the strike contains three prominent horizons (Figs 3 & 4). At the base section. Thesharp and highly irregularcontact with the there is a layer, up to2 m in thickness, of limestone nodules. sandstones is erosive and locally vertically oriented.The Trenches cut into the base of the clay indicated that the upper part of the sandstone overlies the mudstones forming thickness of this horizon varies laterally, decreasing in places a wing structure typical of a channel margin (Fig. 2). The to less than 1m thick. The top of this horizon is sharply sandstones immediately adjacent to themudstones consist of defined. The nodules, which reachdiameters of 20cm, 8 m of thickly bedded to massive, coarse to medium sand increase gradually in concentration from c. 30% by volume with large, curved internal erosionsurfaces. Other nearthe base to being densely packed with only thin exposures, 100meast of the contact consist of 12m of mudstonestringers between the nodules atthe top. fining-upwards conglomerates andsandstones, exhibiting Concurrent with thisincrease is achange in mudstone typical point-bar features indicating channel migration to the colourfrom olive greento dark purple-red. The nodules west, that is towards the present mudstone outcrop (Wright have sharp margins with the mudstone matrix. 1981). The conglomerates areconcentrated immediately The second horizon, up to 2 m thick, consists of dark above scour surfaces and contain abundant limestone clasts purple-redcoloured clay with prominent stacked, curved identical to the nodules in the mudstones. slickensided slip planes (Figs 3 & 4). The parallel sets have At the western limit of the section (Fig. 2) the mudstone an average separation of 15 cm and define gentle synclines is replaced by 2 m of thin- tomedium-bedded, medium- and cuspate anticlines, with wavelengths ranging from 2.5 to grained sandstonesand lenticularsandstones exhibiting 4 m and amplitudes in the synclines up to 80 cm. No fossil medium-scale cross-lamination. This sandstone package dips rootlets or carbonate nodules occur. Fig. 2. Schematic representation of the facies relationships in the Gilwern Clay Member at Blaen Onneu, seen at the top of the south face of the quarry. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/5/847/4889363/gsjgs.145.5.0847.pdf by guest on 29 September 2021 PALAEOSOL DEVELOPMENTCARBONIFEROUSIN FLOODPLAIN DEPOSITS 849 Fig. 4. Palaeosol features at Blaen Onneu. (a) Exposure showing Quaternary till (1) overlying horizon with sandstone-filled cracks (2), overlying horizon (3)with pseudo-anticlinal slip planes (arrowed). The horizon with sandstone-filled cracks is locally 2 m thick. The basal calcrete nodule horizon is buried in this view beneath talus. Divisions on pole are 50 cm. (b) Sandstone-filled cracks from the upper horizon. Lens cap 5 cm diameter. (c) Calcrete nodules from basal layer. The third horizon consists of up to 2 m of buff coloured in the mudstones is very fine sand in contrast to the coarser mudstonecontaining numerous vertical or sub-vertical, sand in the dykes. There is no field evidence that this medium- to coarse-grained
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