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
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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 thesouthern 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 Trenchescut into the base of the clay indicated thatthe 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.
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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 sandstone dykes (Fig. 4). These grain-size trend reflects eluviation related to the overlying dykes are up to lOcm wide and taper towards their base. Quaternary soils. Their vertical extent attains 2 m but their lateral continuity is difficult to assess. The spacing varies from several metres to only a few centimetres, but most are normallya few Llanelly Quarry centimetres or tens of centimetres apart.The intervening The Gilwern Clay Member here (Fig. 5) comprises 6.5 m of mudstones lack any fossil rootlets or carbonate nodules. At softmudstones and, for convenience, can be divided into the western end of the section these dykes connect with the two units. The lower 4.5 m contains red and green mottled overlying thin-beddedsandstones. However, over most of mudstones with carbonate plates or nodules. The upper 2 m the section this third horizon is sharply overlain by boulder comprisesgreen mudstones with abundant rootlets and clay and peat. small carbonate nodules, passing upwards gradationally into Grain-size analyses were carried out on the profile using a grey ferroan-dolomite horizon overlain by a thin coal or a Malvern Instruments 3600 laser particle sizer (Fig. 3). The erosively capped by a thin sandstone. sandstone dykes were not includedin the analyses. A The lower unitcontains several prominent carbonate coarsening-upwards trend occurs from silt (as defined in horizons (Fig. 5). The base of the mudstone unit, above the United States Department of Agriculture system) within the irregular and fissured top of the Penllwyn Oolite Member, middle horizon to silt-loam in theupper horizon. The consists of a platy horizon, 50cm thick, with irregular, increase in the sand fraction is real and not a reflection of contorted plates of fine grained calcite, up to 5 cm thick and mixing from the sandstone dykes because the sand fraction 20cm inwidth. These plates areseparated by thin
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to 1.5 cm in diameter.The upper 40cm of the profile consists of a well-lithified, brown-weathering, massive grey DOWLAIS ferroan-dolomite layer with a gradational base. This horizon LIMESTONE sandstone contains abundant fossil rootlets, preservedas carbonized thin coal films, and pyrite which has commonly replaced the rootlets. Locally this dolomite is capped by a thin coal (W. Barclay pers. comm.) but at the accessible outcrops it is overlain by a thin (10-20 cm) lenticular, erosive-based, coarse-grained sandstone which is itself overlain by 30 m of partially dolomitized,thick-bedded, bioclastic wackestones of the Dowlais Limestone Formation. Grain-size analyses were carried out on the upper partof the sequence. The mudstonesplot in the silt field and slickensided detailed sampling between 5 and 5.9 m abovethe base of the pseudo-anticlinal sequence revealed no changes in grain-size. slip planes Clay mineralogy The samples were disaggregated in an ultrasonic tank and i, the <2 pm (e.s.d.) fraction separated using a centrifuge and calcite nodules subsequent filtrationtechniques. Orientated clay mounts were preparedand X-ray diffraction (XRD)undertaken using a Philips 2 kW diffractometer with Ni-filtered Cu-K, platy limestone radiation at a setting of 40 kV and 40 mA. An automatic divergence slit and graphite monochromator were employed PENLLWYN m l+/ l+/ and calibration of the instrumentchecked by using OOLITE MEMBER tetradecanol. Within the two profiles two basic assemblages have been Fig. 5. Sequence exposed at Llanelly quarry (see text). found, whose mineralogical characteristics have been discussed in detail by Robinson & Wright (1987). The first type is dominated by ordered mixed-layer,illite/smectite mudstonesa few millimetresthick. The rest of the lower (I/S), with minor kaolinite, chlorite and quartz, and occurs unit is composed of alternatinggreen and redmottled in association with the carbonate nodulehorizons. Thus, it is mudstones and horizons of carbonate nodules, identical to found only in the lower part of the Blaen Onneu section but those at Blaen Onneu. The first nodular horizon is 70 cm extends upto the rootlet-bearinghorizons at Llanelly thick, with densely packed nodules up to 9 cm in diameter. Quarry. The I/S shows a typical sequence of periodic basal The overlying two horizons are thinner and contain smaller reflections thatare representative of a fully ordered IS and less densely packed nodules. The intervening mudstones structural type of mixed-layered structure (Srodon 1980, also contain scattered carbonate nodules, rarely more than 1984), having a smectite content of some 25%. 3 cm indiameter. All thesenodules are generally The second, more varied,assemblage is found associated subspherical to ellipticalin shape,are rounded, and have with the upper parts of the two profiles where calcrete is sharp margins allowing the nodules to be removed easily. absent. The diffraction peaks of the clay material are These nodules and the platy horizon consist of mosaics of generally weak and diffuse, making positive identification anhedral, non-ferroan calcite microspar, with small irregular more difficult. The main difference between this and the first veins of coarser sparrycalcite. Rareeuhedral rhombic assemblage is that ordered I/S is normally absent but there calcite crystals also occur, which are to to 60pm along the is a mixed-layer phase of kaolinite and smectite, with some long diagonal axis. Illustrations and further details of these 15% smectite. Also athird component, of a chlorite-like carbonates can be found in Wright (19826). material,appears in this mixed-layer mineral in material The mottledmudstones exhibit prominent curved fromthe samples fromthe upper part of the profile at slickensidedplanes (Fig. 5), which define gently curved Llanelly. There the mixed-layer mineral consists of kaolinite synclines and cuspate anticlines identical in shape and size to (60%), chlorite-likematerial (25%) andsmectite (15%) the better exposed examples from the Blaen Onneu section. (Robinson & Wright 1987). Discretekaolinite, dolomite, The upper unit also exhibits curved slickensided planes some I/S and quartz have also been recorded in material andscattered carbonate nodules, but these are small, having this second assemblage type. averaging 2 mm and rarely 6 mm in diameter. These nodules comprise a maximum of only 10% by volume and decrease in size and abundance up the profile. This upper unit lacks Interpretation redmottling and contains abundant sub-vertical,down- The main sandstone lithofacies atBlaen Onneu hasbeen wardly bifurcatingcarbonized rootlets up to 10 mm in interpreted asa high sinuosity channeldeposit (Wright diameter. Carbonate nodules are absent above 6.6m from 1981) while the smaller exposure at the western end of the the base of the unit, while the fossil rootlets become more section (Fig. 2) is problematic, but may represent the fill of abundant upwards.Pyrite also becomes prominent above a small channel, possibly a crevasse system. The tapering the last carbonate nodule occurrences, its first appearances sandstone dykes are interpreted as filled desiccation cracks, marked by a layer of nodules of coarse crystal aggregates up and the flooding event responsible for the crevasse system
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may also have filled and preservedthese cracks. The difficult to assess in the absense of clearly defined upper soil sandstones, and the intervening and underlying mudstones, horizons. represent floodplain sediments. There is no evidence of any The curved slickensided structures are a distinctive wave or tidalreworking, orother features, that might feature of boththe Blaen Onneuand Llanelly Quarry indicate a lacustrine or deltaic depositional setting. sequences. Similar structures have been described from The abundant carbonate nodules, which occur in both Devonian floodplain deposits from south-west Britain by sequences, have been discussed by Wright (19826) and were Allen (1974, 1986) and from the Jurassic of Israel by interpreted ascalcrete (caliche) nodules because of the Goldbery (1982). Such structures can be compared to those similarities of their macro- and microscopic featuresand developed in present-day soils with high shrink-swell associated structures. They were clearly sufficiently early in capacities such as Vertisols (Dan et al. 1968; De Vos & origin to reworkedbe intothe associated channel Virgo 1969; Ritchie et al. 1972; Yaalon & Kalmar 1978; sandstones, which have basal scour-surface lags containing Knight 1980; Ahmad 1983). In such soils considerable identical nodules. contraction occurs during desiccation and wide cracks open, The nodules have predominantly very sharp boundaries while during wetting of the profile, either by rainfall or by and are not gradational into the mudstone matrix. Similar flooding, the clays expand. Stresses areset up within the nodulesin present-day soils havebeen termed disorthic profile, both because of the swelling of the clays and nodules by Wieder & Yaalon (1974) andare believed to because the bulk volume of the subsurface zone is increased develop theirsharp boundariesbecause of movement by surfacematerial collapsing intothe cracksand being (pedoturbation) within the soil profile. Such noduleshave trapped as the cracks close on wetting. The details of the beenrecorded in present-day, smectite-rich Vertisol soils resulting stress fields and subsequent deformation have been fromSudan by Blokhuis et al. (1968). In such soils the discussed by Yaalon & Kalmar (1978). As a result of these seasonalshrinking and swelling of the expandable clays stresses large curved slickensided slip planes form with an creates sufficient movement to displace the nodules and to anticlinal-synclinal morphology identical to those in the two create sharp nodule-matrix boundaries (Wright 19826). palaeosol sequences. The pseudo-anticlinal slip planes at The various concentrations of carbonates in present-day Blaen Onneu might owetheir origin to cryoturbation and fossil soils havebeen classified into a maturity index processes related tothe formation of the overlying (Gile et al. 1966; Steel 1974; Machette 1985). The Blaen periglacial deposits; identical structures do occur, however, Onneu calcrete would be classified as a Stage 3 horizon in at Lanelly Quarry wherethey are overlain by 30m of the sense of Machette (1985). Dowlais Limestone. Carbonate nodules form typically in the lower horizons of Thedeep desiccation cracks exposed at Blaen Onneu soils atthe depth of seasonalwetting (Weider & Yaalon also owe their origin tothe drying of the smectite-rich 1974), which is generally in the depth range of 0.5-2 m. At floodplain clays. Similar deep cracks are distinctive features Blaen Onneu the nodular horizon occurs near the base of in modem Vertisols (Blokhuis et al. 1964; El Abedine & the sequence and if the vertical sequence represents a single Robinson 1971; Ahmad 1983). The large slickensided soil profile it must have been at least 5 m thick. This depth is surfaces occur beneath the sandstone-filled cracks at Blaen unrealistic for a single profile, when compared with the Onneu.In present-day Vertisols the optimum depth for depthsat which such nodulesform in present-day soils, slickensiding is just below the level of cracking at the depth unless the nodules represent a capillary fringe calcrete. of seasonal wetting (Yaalon & Kalmar 1978). The fact that Nodularcalcrete can format the capillary fringe dueto these slickensided structures extend another 2-3 m below evaporation (Seminiuk & Meagher 1981), but it is difficult to the cracks is not exceptional formodern Vertisols evaluate this idea aslittle information is available on the (Komornik 1974; Yaalon & Kalmar 1978), but it may morphology of such calcretes. indicate that some of the lowest structureswere already The top of the nodular horizon is sharp and does not relict features, as has been inferred for deep slickensides in show the typical gradational upper surface that such some present-day soils (Blokhuis et al. 1964). The depths of horizons show in present-day soils. This may indicate that the cracks may also be significant, for, in the Vertisols of the this horizon is a relict feature, unrelated to the overlying Sudan, El Abedine& Robinson (1971) noted that the depth profile, and that it may have an erosional top. The case for of the cracks was inversely related tothe amount of such an interpretation is strengthened both by the lateral precipitation or surface water added by irrigation. The deep variationin thickness of this horizon and by theabrupt cracks represented at Blaen Onneu must indicate prolonged change in clay mineralogy immediatelyabove the nodule andsevere desiccation. The variations in the density of horizon. Thecarbonate nodules at Llanelly Quarryare cracking noted is a common feature of present-day Vertisols similar tothose at Blaen Onneu asregards macro- and and relates to variations in the kind and density of microstructures.However thecarbonate horizons show a vegetation (El Abedine & Robinson 1971). Regrettably, this progressive decrease in their respective stages of maturity cannot be tested for the Blaen Onneu example because of (in the sense of Machette 1985). The lowest, platy horizon the absence of rootlet structures.Alternatively, if corresponds to a Stage 4 profile, having greater than 60% micro-relief had been present on the floodplain surface this carbonate and a platy or tabular structure. The overlying could also have affected crack density. Such relief commonly horizons have smaller, less dense and progressively thinner occurs on swelling clay substrates where gilgai form (Paton carbonate accumulations, and pass from a Stage 3 to 2 and 1974; Knight 1980). In present-dayVertisols, the surface 1. Soils actively formingtoday have a single horizon of widths of the cracks can be as much as 15 cm (Blokhuis et al. carbonate accumulation, andthe presence of fourin the 1965), but are often less. Crack development is controlled Clydach Gorge sequencesindicates that several separate by the length of the drying period, the vegetation cover and soils must be present. Whether these soils are compound or the clay content of the soil (El Abedine & Robinson 1971). compositein nature (in the sense of Morrison 1978) is The widths of the cracks at Blaen Onneu must have reached
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up tolOcm, as measured fromthe top of thesandstone horizons occur atother subaerialsurfaces in the Lower dykes. This supportsthe view that desiccation was both Carboniferous succession in South Wales (Wright 1982b, severe and prolonged. 1986~;Raven 1983), and also in the Belgian marginal The particle size analyses must be treated with care, for marineLower Carboniferous (Muchez & Viaene 1987). McCave et al. (1986) haveshown thatthe Malvern Similar horizons have been described fromUpper Instruments 3600 laser particle sizer suffers from analytical Carboniferous deltaic sequences (Curtis & Coleman 1986). problems with clay-sized particles. Furthermore these The microstructure of the dolomite horizon consists of a sediments locally have carbonate cementation which creates very fine dolomicrite showing no typical calcrete microstruc- additional difficulties in achieving complete disaggregation. tures.However the horizon is not considered to be With these reservations in mind, and with the emphasis on secondary in origin because no unreplaced, precursor relicts the coarser fractions, a distinctive coarsening-up trend is have been identified and the style of dolomitization is unlike present at Blaen Onneu, with the appearance of a very fine that found in overlying units in which the dolomite is clearly sand component near the topof the profile (passing from silt replacive of earlier marine limestones. to asilt-loam). Since the upper part of the profile shows The occurrence of ferroan-dolomiteindicates reducing distinctive Vertisolcharacteristics it may be useful to conditions, and the lack of any prominent mottling in this comparethese grain-size trends with those inpresent-day zone suggests permanent waterlogging possibly reflecting a soils. Many present-day Vertisols show no grain-size trends. static rather than mobile water table. The occurrence of this This is generally interpreted to be due to thechurning of the ferroan-dolomitecan be compared tothe presence of soil caused by material being introduced down cracks and siderite in present-dayhydromorphic soils (Duchaufour homogenized into the profile causing continuous mixing of 1982, p. 336), where low Eh, but high pH conditions the soil (pedoturbation). Yaalon & Kalmar (1978) noted develop, associated with groundwaters richin carbonates. thatthe sand-sized fraction does increase systematically The source of the Mg is problematic, but it may have been upwards in some Vertisols and suggested that this was due derivedfrom local marine or brackish waters.Sea-water to coarsergrains being upliftedduring wetting phases by contains dissolved sulphate which may inhibit dolomite differential swelling. An alternative interpretation is that the precipitation (Baker & Kastner 1981) (but see also Hardie change in grain-size reflects a purely sedimentological trend. 1987 for criticism of this view). Lowering sulphate levels, Various studies onmodem floodplains, for example the either by mixing fresh to sea-water, or by theaction of Missouri floodplain (Ruhe 1975) andthe lower Indus sulphate-reducing bateria, can allow dolomite to precipitate (Holmes & Western 1969),have shown thatthere is a (Curtis & Coleman 1986). The association of primary general relationshipbetween the particle sizes of the dolomite with organic matter (rootlets and an original peat sediments and a number of factors including elevation, relief in this case) is common (Hardie 1987), and the dolomite andthe degree of waterlogging. Inthe simplest terms, may have beenprecipitated by organic reduction or floodplain sediments tend to be finer grained away from the methanogenesis reactions. channel zone, while channel margin deposits are coarser The iron content of the ferroan-dolomite implies that (Bridge & Leeder 1979; Bridge 1984). As channel belts this must have formed in waters with a low sulphate content. migrateacross floodplains they create upwards-coarsening As in the case of siderite, a low sulphide activity is required, sequencesin the floodplain sediments(Farrell 1987). The failing which pyrite forms (Postma 1982; Curtis & Coleman channeldeposits atBlaen Onneu show evidence thatthe 1986). However, the occurrence of abundant pyrite in this channel was migratingwestwards (Wright 1981, p. 135), horizon indicates that high sulphide activity did occur. The towards thepresent palaeosolexposures. The coarsening- pyrite was probably the product of sulphate-reducing upwards trend could reflect this increasing proximity to the bacterial processes in the waterlogged soil. Ittherefore source of overbanksediment. The section sampled for seems likely that the formation of ferroan-dolomite occurred particle size analyses at Llanelly Quarry, from5 to 5.9 m either afterthe removal of sulphate by sulphate-reducing above the base, was chosen as an interval which was within bacteria, or before pyrite formation, in brackish groundwa- a zone of change in palaeosol composition, and where the ters with a low sulphide concentration. The sulphate for the disappearance of calcrete nodules might reflect an interval pyrite must have been derived from marine waters. These of illuviation or eluviation. The absence of any change in would have flooded the environmentat the time of the grain-size might suggest thatno prominent eluviation or Dowlais Limestone (Holkerian) transgression.Organic illuviation took place. Such processes are, however, most matter, including rootlets, was the site of sulphate readily detected inpalaeosols using the fine (
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Elders 1980). The generation of an ordered I/S is typically described, from present-day soils in Australia(Norrish & attributed to a transformation process, converting a smectite Pickering 1983) and from Vertisols in El Salvador (Yerima into a randomand then ordered mixed-layer structure as et al. 1985). Thegeneration of kaolinite/smectite by temperature increases with burial, and as potassium is fixed weathering under acidic conditions from a parent smectite within the smectite structure(Hower et al. 1976). An material,derived from anEocene ash-fall in Mexico, has alternativeconcept of neoformation involving smectite beeninferred by Schultz et al. (1971). Kaolinite-rich dissolution and illite precipitation and generation of an horizons developed by acidic leaching of smectite-rich clay apparentordered, mixed-layer XRDpattern by an bedshave been reported from a present-day swamp interparticular diffraction process has beenproposed environment in South Carolina by Staub & Cohen (1978). (Nadeau et al. 1984, 1985). Whatever the mode of actual The three-component system comprising kaolinite, smectite ‘illitization’ both concepts appearto concur in that and an interlayer (chlorite-like) material does not appear to XRD-recognized, ordered I/S can betaken to indicate a have beendescribed before. In this situation, the high diagenetic level of alteration. Using thegederal chlorite-like material perhaps marks a transitionbetween correlations of Hoban & Hower (1979), the ordered I/S thealteration of a smectite parentinto kaolinite. The structure would be indicative of temperatures in the order of experimental work of Poncelet & Brindley (1967), which 175-200 “C. If this were correct the clay mineralogy in these showed that hydroxyl interlayers could be introduced into palaeosols would represent a diagenetic imprint and not be smectiteforming a di-octahedralchlorite material, offers related to the pedogenic processes. some support for this suggestion. On heating, this material There is much evidence,however, to suggest thatthe no longer collapsed to the approximately l0A position of clay assemblages are directly related to palaeosol develop- dehydrated smectite, but readily converted to kaolinite. The ment and as such can be used to aid in the interpretation of proposed genesis of the minerals kaolinite and the environmentalconditions. The restriction of the kaolinite/smectite in a subaerial environment is thus entirely different clay minerals to distinctive parts of the profiles and compatible with Tertiary and present-day examples of their the incompatibility of kaolinite/smectite with a high formation.This suggests strongly that they may be diagnetic level are primeexamples (Robinson & Wright interpreted as pedogenic assemblages largely unmodified by 1987). Also, clay mineral and vitrinite reflectance data diageneticchange and representative of the leaching of a shows that there is no pronounced burial diagenetic effect in smectite parent material under acidic conditions. this eastern part of the South Wales Coalfield (Gill et al. 1977, 1979). The detailedarguments for and against the differing modes of generation of the two clay assemblages Discussion are given by Robinson & Wright (1987). While it is quite feasible that the sequence has undergone some small degree of diagenetic modification, the overall weight of the Classification evidence is thatthe basic differences inmineralogy do The palaeosol sequences described here contain a variety of represent the fingerprint of pedogenic processes (Robinson different soil types. The lower parts of the sequences, with & Wright 1987). theirprominent carbonate accumulations and absence of The Vertisol characteristics seen in the present profiles other horizons, could be classified as Aridisols (Soil Survey of the palaeosolsindicate a smectite-rich, floodplain clay Staff1975) or Xerosols (FAO/UNESCO 1974; Wright deposit. The generation of ordered mixed-layer I/S in this 19826). The occurrence of vertic features in these sequences settingrequires an illitization process by K-fixation in the would suggest they be classified as vertic Aridisols. The smectites. The process of wetting and drying of smectite upper sequence at Blaen Onneu also exhibits vertic features clays has been suggested as an illitization process by Mamy andthe lack of any normalhorizonation could suggest & Gaultier (1975). Srodon & Eberl (1984) have shown that classification as a Vertisol. Such present-day soils typically up to 50% illite layersin randon mixed-layer I/S can be lack horizonationbecause of the constantchurning produced from smectite clays as the result of repeated cycles (pedoturbation) of the profile caused by seasonal shrinkage of wetting and drying. Less than 40 cycles are required to and swelling. The lack of horizons could also be interpreted irreversibly collapse the swelling layers,although only as the result of only a brief period of pedogenesis, the soil random mixed-layering is generated. The presence of vertic being more reasonably classified as a vertic Entisol (Soil features in the palaeosols described here is clear evidence Survey Staff 1975). The presence of kaolinite in the upper that numerous wetting and drying cycles did occur during parts of this profile and the absence of carbonate nodules pedogenesis. suggest possible leaching but this was insufficient to cause The enrichment of potassium in many Precambrian and eluvial or illuvial horizons toform. Vertisols commonly Palaeozoic palaeosols has been noted by Retallack (1986, p. form on the lower parts of floodplains wheredrainage is 6). He speculated that one reason for this might be that the impeded, enhancing the pedoclimatic contrast between the original soils, because theylacked a vascular plant cover, seasonal wetting and drying of the clays (Duchaufour 1982, did not have potassium removed by the plants. The horizons p. 259). Evidence of at least periodic waterlogging may be showing the K-enrichment in this study show no evidence of seen in mottling in the calcrete-bearing layers, but this may plant activity either as rootlets or rhizocretions. However be a secondary feature for colour in palaeosols and is one of this may be more a function of preservation than the lack of the most susceptible features to later alteration,even during a vegetation cover. the earliest stages of burial. The clay minerals of the upper part of the profiles are The upper horizons at the Llanelly Quarry section, with also of rarereported occurrencein palaeosol sequences. kaolinite and the absence of calcrete, again suggest leaching Thereappear to be no published reports of the mineral of a vertic soil without any marked eluvial-illuvial horizons. kaolinite/smectite from palaeosols; it has,however, been The overlying dolomiterepresents hydromorphism. This
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unitcould be classified in theEntisol order (Soil Survey & Kraus 1987). If therate of calcreteformation is Staff 1975),such as anAquent or Sulphaquent soil, or a appreciably greater than the rate of sedimentation, mature Gleysol in the FAO/UNESCO (1974) classification. Vertic profiles will form, but if the converse is true, the horizons at features form rapidly in present day soils (Yaalon & Kalmar which carbonateprecipitates will be progressively isolated 1978) andare therefore probably ‘over-represented’ in from the soil by aggradation and so will not reach a mature ancient palaeosolswhen comparedto other soil features. stage of development. This factor should be borne in mind when classifying such The decrease in maturitythrough the Llanelly Quarry palaeosols. sequence could represent either an increase in the rate of sedimentationordecreasea in therate of calcrete formation, caused by changes in the soil moisture budget or Stages of soil development in the availability of calcium carbonate. Kraus (1985, 1987) The Blaen Onneu section represents a sequence ‘fossilized’ has recorded similar sequential changes in the maturity of by being buried by the fluvial sands. The calcrete horizon is palaeosols from the Eocence Willwood Formation of the Big probablyrelicta unit, as discussed above, while the Horn Basin,Wyoming. She interpretsthese to reflect overlyinghorizons representa soil which underwent episodic avulsion and changes in the proximity of points on increased leaching. A possible explanation for these features the floodplainto thechannel system. Therate of might be as follows: sedimentationon a floodplainincreases with proximity to (1)Initially palaeosoldevelopment occurred under a the channel so that as a channel migrates across a floodplain regime with a moisture deficiency, at least seasonally, which the maturity of the soils developed at any one point should allowedcarbonate toaccumulate in the profile. The reflect the increased sedimentationrate as thechannel pedoclimate consisted of alternatingwetting and drying approaches. Thus the changein calcrete-horizon maturity up phases resulting in pedoturbation with disorthic nodules and through the lower part of the Llanelly quarry section could potassium fixation in the clays generating a mixed-layerUS. reflect increased sedimentationrates caused by increased (2) A phase of erosion occurred, truncating the profile proximity to the channel. and leaving only the lowest carbonatehorizon. This was The increased leaching which occurred in the upper part followed by burial by fine-grainedmuds. Erosion surfaces of the profile could reflect the formation of better drainage separating soil profiles and horizons appear to be acommon associated with an alluvial ridge as envisaged for the Blaen feature of alluvial palaeosolsequences and have been Onneu sequence. However, no coarsening-upwards trend is documented recently from the Triassic Chinle and Eocene apparent in the mudstones. The presence of hydromorphic Willwood Formations of theUnited States by Kraus & features(the ferroan-dolomite and coal)above leached Bown (1986) andKraus & Middleton (1987). Such horizons is clearly contradictory. Similar associations, of sequences represent periods of gullying with subsequent fill coal above albic horizons, have been described from Upper by floodplain sediments.The present outcrops at Blaen Carboniferous deltaic palaeosols from northern England by Onneu do not allow the broader relationships of the erosion Percival (1986). In this case two phases of pedogenesis were surface to be assessed. suggested, with leaching (to give the albic horizon) followed (3) The overlying mudstones showtwo features which by subsequenta rise in thewater table resulting in are believed to indicate formation in a setting proximal to a hydromorphicconditions and peat formation. Percival majorchannel, as clearlyshown by thenearby fluvial (1986, p. 97) suggested thatsuch changes might have sandstones. Firstly thesequence shows a coarsening- reflecteda majortransgression, as the palaeosolswere upwardstrend reflecting the ‘approach’ of the fluvial overlain by marine deposits. Curtis & Coleman (1986) have channel. Secondly the soil shows an increase in leaching for offered a geochemical model to explain such events during it both contains kaolinitic clay minerals and lacks carbonate regressive-transgressiveCoal Measure cyclothems. An nodules. As the fluvial channel migrated towards that point alternative, but unlikely explanation is that phreatic (lateral) on the floodplain, an alluvial ridge would have developed. leaching might haveoccurred in the Llanelly Quarry The increase in relief on this ridge would have resulted in sequence.The horizon of pyritenodules in the section improveddrainage and leaching, as has beennoted in (possibly afteriron oxides or hydroxides) would suggest, present day alluvial settings (Hayward 1985; Farrell 1987). however, that vertical leaching occurred at some stage. The Whether this would have been sufficient to cause both the situation might be compared to that of the Snuggedy Swamp absence of carbonate nodules and formation of kaolinite is in SouthCarolina (Staub & Cohen1978), where debatable. It might be more reasonable to invoke an overall kaolinite-rich clays areforming beneath apeat by the change in the moisture budget due to increased precipita- leaching (possibly lateral, phreatic leaching) of srnectite-rich tion. However periodic drying on these low alluvial ridges, clays by humic acids. probably seasonal, was severe and resulted in the formation By analogy with present-day soils (Singer 1980, 1984) of deep desiccation cracks followed by deformation of the other explanations for the increased leaching are possible. profile caused by thelater expansion of the clays during The climate could have changed with increased rainfall or wetting phases. alternativelydifferences in slope could have resulted in The Llanelly Quarry sectionexhibits several different changes in the degree of leaching, as occurs in present-day phases of pedogenesis. The calcrete nodule horizonsshow a soil catenas(Birkeland 1984). However it wouldbe progressive decrease in maturity up through the sequence especially difficult to accommodate such a change in relief from a stage 3/4 to a stage 1 or 2 development (in the sense within the stratigraphic and sedimentological confines of the of Machette 1985). The type of maturity stage reached in Llanelly Quarry sequence where there is no evidence of an calcrete formation depends ona number of factors but it can alluvial ridge development. specifically be used to evaluate the relationship between the However, the occurrence of ferroan-dolomite and pyrite rates of pedogenesis and sedimentation (Leeder 1975; Bown in theuppermost horizons requires not only reducing
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conditions and hydromorphism but also sources of Mg and palaeosols, andtocompare them with the climatic SO4, probably frommarine waters associated with the palyno-zones of Van der Zwan et al. (1985) haveproved Holkeriantransgression. The ferroan-dolomite possibly unsuccessful as the clays lacked spores (J. E. A. Marshal1 precipitated under low sulphate (brackish) waters while the pers. comm.). pyrite formed in sulphate-rich marine pore waters. Another possibility is that most of these features reflect a Conclusions single process, namely rising base-level caused by the Holkerianmarine transgression. Afterthe initial sea-level Complex palaeosolsequences occur within the Lower fall (following the deposition of the Penllwyn Oolite Carboniferous floodplain deposits of the Llanelly Formation Member), calcrete soils developed on the exposed top of the of South Wales. oolite. Sea-level may havecontinued to fall resulting in At Blaen Onneu two distinctive palaeosolunits occur, fluvial incision, low rates of aggradation and, as a result, the separated by an erosionsurface. The lower unit is a formation of a mature carbonate horizon. As base-level rose calcrete-bearing horizon and is overlain erosively by a thick at the start of a transgression there was an increased rate of vertic soil showing kaolinite-enrichment. Sedimentological aggradation and a decrease in the maturity of each calcrete evidence suggests that the upper unit was deposited on an horizon. The increase in leaching is more difficult to explain alluvial ridge. The changefrom a calcrete-bearing soil to in this model. Eventuallythe transgression resultedin one showing evidence of leaching may reflect a change in hydromorphic conditions. Waterlogging would initially have climate ratherthan acatenary sequence related tothe involved fresh waters, but later brackish waters, and finally migration of an alluvial ridge. marine waters would have inundated the floodplain creating At Llanelly Quarry three main units can be recognized. asulphate-rich marsh ahead of the transgressing marine The lowermost contains four calcrete horizons which exhibit carbonates of the Dowlais Limestone. a progressive decrease in maturity from the lowest to the The differences between the two sections are prob- uppermost. This is interpreted as reflecting the progressive lematic. The lowest unit at Blaen Onneu may correlate with approach of a channel by stages of avulsion. The middle unit the calcretehorizons atthe Llanelly Quarry section, the is a kaolinite-enriched, leached horizon identical to that at erosion event atBlaen Onneu removing much of the Blaen Onneu,and may also reflect achange to a more sequence there.The two kaolinite-bearingunits may be humid climate. The uppermost unit is pyritous, a correlatable but the non-exposure of the contact with the ferroan-dolomite formed initially in brackish water and later Dowlais Limestone at Blaen Onneu prevents further in amarine salt-marsh. This change was the result of comparison. Similar complex lateral relationships are also a flooding by the Holkerian transgression. striking feature of the palaeosol sequences in the Clydach These palaeosolsequences arethe products of both Halt Member of the Llanelly Formation (Wright 19826). intrinsic and extrinsic factors. Channel avulsion and variations in sedimentation rates across the floodplain are features intrinsic tothe depositional system. Extrinsic Palaeoclimate factors include possible climatic changes andmarinea During the development of the earlier parts of the soil transgression. sequences the climate was at least seasonally dry with both Twounusual clay mineral assemblages occur. In the the formation of pedogenic carbonates and deep desiccation calcrete-bearinglayers a mixed-layer illite/smectite occurs cracks. Thissituation occurred when southern Britainsat having a smectite content of c. 25% and IS ordering, These along the early Carboniferous equator (Scotese et al. 1979; clays are interpreted as being the products of the illitization Smith et al. 1981), but clearly a uniform humid-wet of a smectite-rich parent by repeated wetting and drying ‘Equatorial’-type climate did not prevail. Additional cycles causing irreversible K-fixation. evidence for such a seasonally arid climate has been given The kaolinite-smectite and a three component variety by Raymond et al. (1985) and Van der Zwan et al. (1985) with kaolinite/smectite/chlorite-like material are the domin- based on palaeobotanical grounds. Rowley et al. (1985) have ant clays in the horizons lacking prominent calcrete nodules. offered a palaeoclimatic model forthe Visean which Theseare interpreted as the products of leaching on the envokes the development of a strongly seasonal, monsoonal floodplains dueto a possible change tomore humid climate in theequatorial zoneprior tothe docking of conditions. Laurussia and Gondwana. A basic problem, not fully explained at both sections, is The authors acknowledge the help of J. M. Tait and M. J. Wilson of the increase in leaching up the profiles. The development of the Macaulay Institute for soil science. C. Hillassisted with the an alluvial ridge at Balen Onneu would have resulted in a XRD analyses. We thank Malvern Instruments for the particle-size well drained soil, but the soil would still reflect a analyses. J. Hawker, S. Powelland P. Baldaro are thanked for considerable change in moisture budget from the calcrete- assistance. Special thanks to P. Friend, R. Meyer and B. M. Besly bearing soils developedearlier. Similarly, the phase of for their very constructive comments on an earlier draft. This paper increased leaching prior to hydromorphismat Llanelly forms part of a project on the environmental geology of Palaeozoic Quarry remains a problem. Perhaps the climate did change. terrestrial environments partially funded by a grant to V. P. Wright from the Nuffield Foundation. Evidence for humid to semi-arid fluctuations is also seen in the earlier Arundian and Chadian palaeokarsts of the region (Wright 1987). References Certainly a climatic change did occur later, in the latter AHMAD, N. 1983. Vertisols. In: WILDING,L. P., SMECK,N. E. & HALL, G.F. part of the early Carboniferous (Van der Zwan et al. 1985). (eds) Pedogenesir and Soil Tarorny. II. The Soil Orders. 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Received 28 July 1987; revised manuscript accepted 11 April 1988.
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