Podogenic columnar calcite from the Oolite Group (Lower Carboniferous), South Wales

Sedimentary Geology, 62 (1989) 47-58

47

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

Pedogenic columnar calcite from the Oolite Group (Lower Carboniferous), South Wales ALISON

SEARL

School of Earth Sciences, Unioersity of Birmingham, P. 0. Box 363, Birmingham B15 2 TT (UK)

Received June 21, 1988; revised version accepted January 18, 1989

Abstract Searl, A., 1989. Pedogenic columnar calcite from the Oolite Group (Lower Carboniferous), South Wales. Sediment. Geol., 62: 47-58. Calcrete palaeosols in the Oolite Group have microspar hardpan layers overlying clay-rich nodular layers which, in turn, overlie the degraded host oolite. Within and immediately below hardpan layers there are extensive sheet-like and fan developments of columnar calcite with individual crystals up to 30 cm long. Crystals range from those with straight extinction and normal growth zonation through feathery crystals to those showing sweeping extinction. The calcretes accumulated within a seasonal climate with hardpan accumulation probably occurring through evaporation during the dry season. The columnar calcite precipitated from more dilute solutions with maximum growth probably occurring during the wet season. Precipitation may have been partly or wholly bacterially mediated, or induced by the reduction in Pco2 of fluids as they migrated out of the extremely restricted microporosity of the hardpan. Columnar fabrics reflect low supersaturations relative to those at which the hardpan accreted, inhibition of crystal nucleation and of growth on c-axis parallel faces, and an excess of CO 2- availability over C a 2÷. Growth and nucleation may have been inhibited by the presence of particular organic compounds or clays. The range in crystal form is related to the degree of poisoning of crystal growth faces by organic compounds.

Introduction T h e fabric a n d c h e m i s t r y o f p a l a e o s o l s have b e e n the focus o f m u c h recent research (e.g. Southgate, 1986; Beier, 1987; W r i g h t a n d Wilson, 1987; Saigal a n d W a l t o n , 1988) a n d W r i g h t (1981, 1982, 1983, 1984, 1986a, 1987a, b) has des c r i b e d several of the L o w e r C a r b o n i f e r o u s p a l a e o s o l s of S o u t h Wales. T h e p a l a e o s o l s o f the O o l i t e G r o u p have two n o t a b l e features which are largely a b s e n t in the p a l a e o s o l s d e s c r i b e d b y Wright. F i r s t l y there are extensively d e v e l o p e d h o r i z o n s of p e d o g e n i c d o l o m i t e (Searl, 1988a) a n d secondly, s p e c t a c u l a r h o r i z o n t a l sheets of colu m n a r calcite (Fig. 1). Similar p a l i s a d e calcite occurs in the D i n a n t i a n of Belgium ( S w e n n e n et al., 1981), b u t this a p p e a r s to b e the o n l y d i r e c t l y 0037-0738/89/$03.50

© 1989 Elsevier Science Publishers B.V.

equivalent c o l u m n a r calcite r e p o r t e d from the geological record. In o r d e r to p u t discussion o f the c o l u m n a r calcite into context, a general d e s c r i p tion of the O o l i t e G r o u p p a l a e o s o l s p r e c e d e s c o n s i d e r a t i o n of the c o l u m n a r calcite. T h e final section o f the p a p e r integrates a m o d e l for c o l u m n a r calcite d e v e l o p m e n t with an overall m o d e l of p a l a e o s o l s evolution in the O o l i t e G r o u p .

Palaeoenvironmental setting of the Oolite Group D u r i n g the early C a r b o n i f e r o u s times, S o u t h W a l e s c o m p r i s e d a s u b s i d i n g c a r b o n a t e r a m p imm e d i a t e l y south of an a r e a of e m e r g e n c e t e r m e d St. G e o r g e ' s L a n d (George, 1974). T h e O o l i t e G r o u p was d e p o s i t e d o n the m a r g i n s o f St. G e o r ge's L a n d (Fig. 2) to the n o r t h of the a r e a of

48

Fig. 1. Columnar calcite in an Oolite Group palaeosol (the Daren Ddu Beds, east end of Cwar yr Ystrad (see Figs. 2 and 3): in (a) individual columnar crystals can be distinguished whereas in (b) individual crystals are less easily distinguished but an overall radiating fabric is apparent.

maximum subsidence. Globally, South Wales was at equatorial latitudes (Turner and Tarling, 1975) and, like modern South-East Asia, was situated at the southern margin of a large, northern hemisphere landmass (Anderton et al., 1978). The palaeoclimate was therefore probably warm with a strongly seasonal (monsoon) rainfall pattern, the

dry season coinciding with the northern hemisphere winter (Spalton, 1982). Net rainfall during emergence of the Oolite Group was sufficient to promote extensive plant colonisation (Searl, 1988b) and to mobilise sufficient C a C O 3 to precipitate large volumes of early meteoric cements in emergent limestones (Raven, 1983; Searl 1988b).

St. G e o r g e ' s ~ Land ~

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Fig. 2. Location and geology of the study area

~

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49

NE C w a r yr H e n d r e

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oolite with g r a d e d bioclastic lenses 1 biosparite nm peloidal limestone palaeosol dolomite

[rougn cross stratification /// planar cross s t r a t i f i c a t i o n , ~ , h u m m o c k y cross s t r a t i f i c a t i o n ~q climbing ripples ~

Fig. 3. Sedimentary facies within the Oolite Group: BOO = Blaen Onneu Oolite, DDB = Daren Ddu Beds, PYCO = Pyll-y-Cym Oolite

The Oolite Group (Fig. 3) largely comprises mixed oolitic, peloidal and bioclastic sand deposited in a dominantly backshoal environment (Searl, 1988a). Variable sediment thicknesses and facies reflect differential subsidence along northwest trending faults that cut the area (Figs. 2, 3). Sedimentation was also affected by regional sealevel changes (Wright, 1986b) and deposition was interrupted by numerous phases of sediment emergence and accompanying pedogenesis. Many of these emergence events appear to have been quite localised in extent (Fig. 3) and often several distinct palaeosol horizons within areas of maximum subsidence equate to a single, polyphase palaeosol elsewhere (Fig. 3). This suggests that elevation of emergent sediment was low and that topography was subdued. Despite extensive quarry outcrop of the Oolite Group, no alluvial channels are exposed, suggesting that drainage during sediment emergence occurred dominantly by subsurface flow. The watertable during emergence

appears to have been (seasonally) fairly close to the sediment surface as palaeosols extend down to zones of meteoric-phreatic cementation (Searl, 1988a). The Oolite

G r o u p palaeosols

Description

Palaeosol horizons are generally about 1 m thick but vary between 0.3 m and 2 m: the thickest palaeosols are polyphase and occur in condensed parts of the Oolite Group. The general form of the palaeosols is closely analogous to m o d e m carbonate hosted calcretes (e.g. James, 1972) and they are described below (base upwards) in terms of Klappa's (1982) idealised carbonate-hosted calcrete (Fig. 4). (1) The host rock and transitional zone: the degraded grainstone host is frequently red-stained and is cemented by brown-tinged microspar. AI-

50 Oolite Group Palaeosol

~

clay parting _ erosive_ _ba_s_e. . . . . . . . . micrite/microspar

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~lY~ columnar calcite . . . . . . . . . . . ~,~,>,~1 mottled clay with CaCO3 ~ ] ~ L ~ ?_°ncreti°ns

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(Klappa, 1983)

oolitic grainstone

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Idealised Calcrete

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-.7:i.i~-: .-..\ micrite/microspar I~o.°:l°oOc?~- oomolds in brown microspar I~°r~,l-oee[

oil hardpan sheet calcrete glaebular calcrete chalky calcrete transitional

horizon

oolite with e x t e n s i v e

I early me t e o r , c

cement

host material

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weathering profile

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Fig. 4. Comparison of Oolite Group Palaeosol profile with an idealised calcrete profile (Klappa, 1983).

lochems are preserved both as nonferroan calcite spar filled moulds and as amorphous micrite. (2) "Chalky calcrete": micrite/microspar layer of variable thickness which has a gradational contact on the host sediment and a sharp upper boundary. Very rarely this layer is laminated a n d / o r contains elongate horizontal fenestrae filled by columnar calcite. The scattered allochems show poor textural preservation and appear to have undergone variable degrees of dissolution penecontemporaneously with microspar precipitation. (3) Glaebular calcrete: mottled y e l l o w / r e d / green clay, dominantly illite with a little mixed layer clay, containing carbonate nodules. Nodules

Fig. 5. Microspar from the hardpan of a palaeosol within the Blaen Onneu Oolite, east end of Cwar yr Hendre. Subrhombic crystals with interdigitating intercrystalline boundaries show significant variation in size. Width of field of view: 0.85 mm.

are red, green or grey coloured, sharply bounded against the clay and range in size from 2 to 20 cm. Sometimes the smaller nodules occur as nodule clusters within the clay matrix. Internal fabrics range from micrite, through microspar, to spar which occurs as radiating bundles of columnar crystals. Spar, both with equant and columnar fabrics, also fills internal fractures in the m i c r i t e / microspar nodules. (4) Hardpan and associated material: (a) "Subhardpan layer": extensive sheets of columnar calcite which are described in detail below. These are not part of Klappa's (1983) model and only occur in palaeosols with a well developed overlying hardpan horizon. (b) Hardpan: a near continuous m i c r i t e / microspar horizon, formed from coalesced nod-

Fig. 6. Alveolar texture and fenestrate fabric within the same specimen as illustrated in Fig. 5. Scale bar = 0.5 mm.

51 ules. The micrite/microspar is largely non-ferroan calcite with a little ferroan material at crystal boundaries. Individual crystals have modified rhombic shapes usually with interlocking boundaries forming a jigsaw fabric (Fig. 5). Crystals range from 1 to 30 #m with a patchy size distribution. Crystals have non-luminescent rhombic cores surrounded by dully luminescent material, and later irregularly distributed brightly luminescent material. The micrite/microspar displays alveolar texture (Fig. 6; similar to that illustrated by Esteban and Klappa, 1983), dark micrite-encrusted root moulds, heavily micritised crinoid plates and scattered moulds after other marine allochems. Moulds and other vugs are usually filled by columnar calcite, but more equant non-ferroan and strongly ferroan calcite also occurs. Quartz silt is disseminated throughout the hardpan layers and, despite constituting less than 1% total volume, is still more concentrated than in underlying limestones. SEM and cathodoluminescence (CL) have not revealed needle-fibre fabrics associated with root moulds such as those described by Solomon and Walkden (1985) and Wright (1984, 1986a) in other Lower Carboniferous calcretes. (5) Soil: original overlying soil is inferred to have been removed by the subsequent marine transgression.

son when plant growth and soilwater flow rates would have been at a maximum, transpiration would have caused dehydration around plant roots, but soil fluids were probably relatively dilute. Microbial decomposition of rapidly accumulating organic matter, bacterial processes of nitrogen fixation/denitrification and plant respiration would have all led to enhanced Pco~. During the dry season, overall dissolved C a C O 3 c o n c e n t r a t i o n s would have been enhanced by evaporation and lower soilwater flow rates. Soil dehydration would retard microbial processes leading to reduced soilwater Pco2" At times of elevated soilwater flow (i.e. the wet season) the sluggish kinetics of CaCO 3 dissolution probably enabled calcite-undersaturated fluids to percolate through the basal microspar to the host rock. Fluid penetration of the basal microspar was probably locally enhanced by root brecciation and with time the basal microspar would migrate slowly down the sediment profile with an overall net loss of C a C O 3 t o cementation of underlying limestone (Searl, 1988b). Ultimately the basal microspar would aggrade to the point of being almost impermeable, downward fluid migration would cease and soilwater drainage would be largely by lateral flow over the basal microspar.

Discussion

The nodular horizons. The colouration of the nodular clay-rich horizons has probably been modified by recent weathering. The accumulation of clays within these horizons was due both to soil water dissolution of the host limestone and possible input from fluvial sheetflood and aeolian sources (Spalton, 1982.) The nodules were precipitated through the operation of one or more of several mechanisms. Of these direct evaporation would have been unimportant because of the intervening hardpan layer, however, water loss through evapotranspiration, seasonal variability of soil Pco2 (see above), the generation of alkalinity through protein decay (see Berner, 1968) or denitrification could all have contributed to nodule precipitation. The nodule clusters suggest that some nodules were initiated around the root systems of larger plants, either through microbial processes or by soil dehydration.

Host rock alteration and deoelopment of basal carbonate accumulation. The red colouration of the host rock is typical of disseminated haematite, indicative of intense oxidation. Precipitation of the basal microspar and of cements in the oolite resulted from the reduction in fluid Pco2, as fluids left the restricted microporosity of the clay- and organic-rich soil zone and mixed with relatively degassed groundwater within the oolite. The mouldic fabric of the oolite suggests that precipitation of micritic and microspar calcite cement was interspaced with dissolution of metastable carbonate. This implies that soil-derived fluids had highly variable CaCO 3 saturation levels, which were probably largely controlled by seasonally variable parameters: Pco2, total dissolved CaCO3 concentrations and alkalinity. During the wet sea-

52

Development of the hardpan The hardpan layers developed within the surfical layers of the soil through precipitation of individual calcite rhombs within a clay/humus-rich matrix. Possibly some calcite precipitation was induced microbially around plant roots but hardpan development is more generally due to the evaporative concentration of soil-water (Klappa, 1983). Given that calcite precipitation within playa lakes can be due to CO 2 degassing rather than evaporation (Risacher and Eugster, 1979) hardpan formation may also have been enhanced by CO 2 degassing of surficial soilwater. The hardpan was initially fairly plastic due to the large amount of organic matter and clay caught between crystals. Seasonal dehydration and rehydration of this intercrystalline material caused splitting of the calcite layer, giving rise to the larger horizontal vugs, now filled by columnar calcite (see below). The absence of SEM evidence for the former presence of needle fibre calcite or other indications of microbial activity is probably due to the low preservation potential of such features. The decomposition of b a c t e r i a / fungae/algae could well have induced localised recrystallisation of the hardpan, destroying any characteristic textures associated with microbially induced carbonate precipitation (see Chafetz and Folk, 1984). Source of CaCO3 in calcretes. The CaCO 3 within the Oolite Group palaeosols was probably derived largely by in situ remobilisation of marine carbonate, with a net loss of C a C O 3 t o seaward flowing ground/surficial waters and to cementation of underlying lime sediment (Searl, 1988b). This contrasts with the calcretes in the overlying Llanelly Formation (Fig. 2) in which CaCO 3 was derived from aeolian dust, rainfall and sheet wash (Wright, 1982).

Fig. 7. Columnar calcite from the base of the Blaen Onneu Oolite, east end of Cwar yr Ystrad, with a granulated appearance. Scale bar = 0.3 ram.

void fills but occasionally the presence of "exploded" bioclasts is indicative of displacive growth. The columnar calcites are largely non-ferroan, with small irregular ferroan filled vugs in optical continuity with the parent crystal. Crystals that grew down from the base of the hardpan are occasionally broken and disoriented (Fig. 7). In general, intra-hardpan voids filled by columnar calcite are larger than those filled by equant calcite cements, but there are a number of exceptions to this rule. The columnar calcites show a range of crystal habits: one end member has planar bounded crystals (Fig. 8) with normal extinction, triangular cross sections and rhombic terminations. These crystals have normal rhombohedral growth zones highlighted by thin ferroan, or brightly luminesc-

The columnar calcites

Description The columnar calcites form large fans and planar "beef-like" sheets (individual crystals can be as long as 30 cm), within and growing down from the hardpan layer. They are generally passive

Fig. 8. Columnar calcite within a possible rootmould in the same section as illustrated in Figs. 5 and 6. Crystals are planar bounded and a layer of diagenetic sediment (d) deposited during a hiatus in growth outlines former crystal boundaries. The cavity centre is filled by organic matter. Scale bar = 0.5 mm.

53

Fig. 9. Colunmar calcite showing "normal" growth zonation in luminescence. Width of field of view: 2 ram.

Fig. 12. Growth zones in radiating feathery calcite defined by differences in degree of brown colouration, Daren Ddu Beds, Cwar yr Hendre. Scale bar = 0.2 mm.

11). The " f e a t h e r y " calcite shows frilly crystal t e r m i n a t i o n a n d is i n t e r n a l l y c o m p o s e d of n u m e r ous r h o m b i c crystallines d e l i m i n a t e d b y i n c l u s i o n s (frequently of organic material). Both the feathery calcite a n d fans of calcite showing sweeping ex-

Fig. 10. Calcite showing sweeping extinction (crossed polars), Daren Ddu Beds, Cwar yr Ystrad. Scale bar = 0.2 mm.

ing b a n d s (Fig. 9) a n d b y diagenetic s e d i m e n t which isopachously rather than geopetally drapes crystals (Fig. 8). T h e other e n d m e m b e r shows curved cleavage a n d sweeping extinction, optic axes converging away from the substrate (Fig. 10) a n d in between are various " f e a t h e r y " forms (Fig.

Fig. 11. Columnar calcite with a feathery form, Blaen Ormeu Oofite, Abercriban Quarry. Scale bar = 0.2 mm.

Fig. 13. Feathery calcite in luminescence, showing frilly growth zonation (a) and at higher magnification the presence of numerous rhombic subcrystals (b), Daren Ddu Beds. Cwar yr Ystrad. Scale bars = 0.5 nun and 0.2 mm, respectively.

54 tinction have a brown colouration which varies between successive growth zones (Fig. 12). The feathery and optically distorted crystals are largely non-luminescent with broad frilly growth zones apparently composed of numerous subcrystals invisible in plane light (Fig. 13). SEM imaging of etched polished sections of columnar calcite reveals no additional information about their internal form. Discussion The occurrence of columnar calcites. The restriction of the Oolite Group columnar calcites to calcrete horizons suggests a pedogenic origin and they have some similarities to radial fans of calcite fibres in Triassic calcretes (Assereto and Folk, 1980) and crystal fabrics within modem calcretes (James, 1972; Chafetz and Butler, 1980). They also have some features in common with modem stream tufa (e.g. Braithwaite, 1979), spelothems (e.g. Folk and Assereto, 1976; Kendall and Broughton, 1978; Heckel, 1983) and hot spring travertine (Folk et al., 1983; Chafetz and Folk, 1984). There is, however, no evidence that columnar fabrics developed directly on subaerially exposed surfaces, or for the collapse of former major cave systems or for hot spring activity. The Belgian columnar calcites, of similar age and appearance to those in the Oolite Group, are believed to be pseudomorphs after gypsum (Swennen et al., 1981) and the sheets of columnar calcite in the Oolite Group are superficially similar to Messinian evaporites (e.g. Vai and Ricci-Lucchi, 1977). The Belgian sequence, however, shows a variety of evaporative features which are absent in the Oolite Group: collapse breccias, gypsum pseudomorphs and sabkha dolomitisation due to hypersaline fluids (Swennen et al., 1982). In addition, pseudomorphs after non-trigonal precursor minerals should occur as randomly oriented, anhedral, calcite mosaic filled moulds (e.g. pseudomorphed aragonite rays described by Assereto and Folk, 1976, 1980) and could not show the growth zonation (particularly that preserved under diagenetic sediment) which is displayed by the Oolite Group columnar calcite.

The restriction of columnar calcite to zones within or underlying hardpan limits the importance of evaporation and algae in its precipitation, but not that of bacterial or fungal processes. The mechanical breakage of some of the subhardpan columnar calcite indicates the previous existence of relatively large intra-calcrete cavities which subsequently collapsed during burial. The general preferred growth of the columnar calcite into large cavities ( > 1 ram), compared with intergranular porosity in the oolite, might indicate that rapid fluid transport of ions, or relative ease of CO 2 degassing were important controls on crystal form.

Columnar crystal form. The large size of columnar crystals implies rapid crystal growth rates compared with nucleation and suggests overall supersaturations lower than those associated with precipitation of the hardpan. The columnar fabric may be partly or entirely due to uninterrupted, competitive growth between adjacent crystals (see Kendal and Broughton, 1978). Many crystals showing columnar fabric, however, are no larger than coarse, equant spar cements in the Oolite Group and elsewhere. Comparison of columnar calcite illustrated here, with equant calcite filling large shelter cavities under shells in the host oolite (Searl, 1988b) shows no obvious difference in early nucleation/growth history, between the two calcite types. It would seem likely that if an equant form was sufficiently more energetically favourable than an elongate form, then for adjacent, initially equivalent crystals, as soon as any random inequality of growth occurred between two crystals, the more successful crystal would quickly expand and engulf the growth surfaces of its neighbour. Two controls, additional to competition, may have contributed to columnar growth. Firstly, surface charge effects mean that a more rapid supply of either COg- or Ca 2÷, than the other, favours growth parallel to the c-axis (Lahann, 1978). If the main trigger to calcite precipitation was CO 2 degassing, then possibly there was an excess of CO gavailability over Ca 2+. Secondly preferential adsorption of foreign ions/organic matter on faces parallel to the c-axis may have inhibited lateral growth.

55

Sweeping extinction. Sweeping extinction in carbonates has been ascribed a number of origins which include: (1) Coalescence of bundles of crystallites, in speleothems (Kendal and Broughton, 1978) and stream tuffa (Braithwaite, 1979), through subsequent overgrowth. In the Oolite Group calcites there is no evidence (such as radially oriented inclusion trails) for growth as fibrous crystallites, although there is some evidence for rhombic subcrystals. The CL growth zonation associated with these rhombic subcrystals suggests that coalescence was almost simultaneous with growth. (2) Kendal (1985) has developed a model by which radiaxial calcite is due to split crystal growth: during growth the crystal repeatedly tries to approximate to a length-slow form in fluctuating chemical conditions. It is difficult to see how the energetic advantages of changing form, by actually nucleating a re-oriented lattice as opposed to preferential growth on certain faces, could be sufficient to overcome the extreme lattice strain in the resultant crystal. (3) Sweeping extinction and curved cleavages are characteristic of saddle dolomites which have been generally ascribed temperatures of precipitation of between 60 and 150 ° C (Radke and Mathis, 1980). Lattice distortion has been related to preferential growth at crystal edges in response to pyroelectric effects (Radke and Mathis, 1980). If crystal growth was sufficiently rapid, preferential growth at crystal edges could simply be due to transport rather than surface controlled kinetics of crystal growth (see Kirkpatrick, 1981). In the proposed low-temperature setting for the columnar calcites it seems unlikely that either pyroelectric effects or transport controlled kinetics could have been important. (4) Dixon and Wright (1983) describe calcite with undulose extinction in calcretes from the formation overlying the Oolite Group. It is somewhat different to that described here, being twinned and having optically unoriented subcrystals developed along crystal glide planes. They ascribe deformation to load pressure during burial. This is unlikely to be the primary cause of distortion in the Oolite Group calcites as crystals are often in contact with clay which could have ab-

sorbed loading stress, there is no preferential orientation of distorted fabrics and there is no recrystallisation along potential glide planes. (5) Folk et al. (1985) ascribe crystallographic distortion within crystals from hot spring travertine to the uptake of SO42- by the calcite lattice, which has a considerably greater ionic radius than CO 2-. This model is probably not directly relevant to the Oolite Group as there is no obvious source of SO2 - , except through wind-blown sea spray. Sea spray affects crystal growth in recent calcretes from Barbados (James, 1972), but could only be significant in very near coastal environments. Although some of the emergence events in the Oolite Group were very localised and may therefore have been subject to sea spray, other events were of regional extent (Wright, 1986b). (6) Chafetz and Butler (1980) describe crystallographic distortion in recent calcrete in calcite which has replaced microcodium. In sections parallel to elongation, the calcified microcodium looks extremely similar to the Oolite Group calcite, but in perpendicular cross-section crystal shape is controlled by that of microcodium rather than being trigonal as in the Oolite Group calcites. There may be a biological control on the Oolite Group calcite, but it is less direct than simple calcification of organic matter. None of the above models leads to a full explanation of sweeping extinction in the columnar calcite. It is perhaps more useful to develop an integrated model for the origin of all the columnar fabrics, including sweeping extinction. The recognition of subcrystals in luminescence in apparently optically continuous calcite suggests that the feathery calcite grew by successive budding off of crystallites which were able to grow, partly coalesce and then themselves bud off fresh crystallites. The growth of crystallites from a number of discrete points on the advancing crystal growth front suggests extensive poisoning of growing faces. Soilwater would be rich in various dissolved and suspended organic compounds, some of which would be readily absorbed on calcite, blocking active lattice growth sites. In the extreme case, crystallites were so small relative to the contaminating organic matter that the lattices of adjacent crystallites became very slightly offset, with

56 the effect of creating sweeping extinction over the crystal as a whole. Organic compounds incorporated on the interionic scale during growth would cause lattice distortion in a similar fashion to SO42- incorporated in travertine (Folk et al., 1985). The elongate nature of the less distorted crystals may be due to preferential adsorption of organic compounds or clays on crystal faces paralleling the c-axis. Adsorption of some kind of sticky organic matter on crystal faces is also implicated by the isopachous distribution of diagenetic sediment in Fig. 7. Possible microbial influence on calcite precipitation. It is difficult to assess the possible role of microbial mediation in the precipitation of the columnar calcites. Despite the absence of characteristic microtextural features indicative of the former presence of bacterial/fungal colonies, the infilled microporosity of the crystals is compatible with localised calcite dissolution and reprecipitation due to their former presence (see Chafetz and Folk, 1984). The feathery calcites, in particular, are morphologically very similar to bacterial shrubs precipitated through sulphur oxidation in hot spring waters (Chafetz and Folk, 1984). They are obviously not directly comparable, because the feathery calcites grew in the absence of light and there is no evidence for the occurrence of sulphur oxidation. Bacterial calcification may have resulted from the generation of alkalinity through denitrification or through some part of the nitrogen fixation process. Bacterial denitrification only occurs in flooded soils (Fenchal and Blackburn, 1979) which is compatible with the restriction of the columnar calcite to non-evaporative parts of soil profiles and the generally phreatic distribution of columnar calcite within voids. Although calcretes are generally associated with aridity it is possible that the subhardpan soil was seasonally waterlogged. If bacterial influences were of any importance in the precipitation of the columnar calcites, some of the range in crystal form may be due to varying degrees of inorganic and biologically induced precipitation. General model for the Oolite Group columnar calcite. Overall it is difficult to produce a constrained model for the development of the columnar calcite based solely on petrographic and field evidence.

Chafetz and Butler (1980) attribute partly analogous columnar calcite to slow precipitation, about 5 m below the soil surface, within the vadose realm. In the Oolite Group, despite evidence for periodic vadose conditions during crystal growth, there is no strong evidence for very slow columnar crystal growth with repeated drying of advancing crystal faces, and the columnar calcite probably largely grew under water-logged conditions. Precipitation may have been bacterially mediated a n d / o r induced by CO 2 degassing as soilwater passed from the restricted porosity of the hardpan to larger intra- and sub-hardpan voids. CO 2 generation, and therefore CaCO 3 mobilisation and reprecipitation, like bacterial denitrification, would be at a maximum during the wet season. It is therefore, tentatively suggested that the columnar calcite was a wet season precipitate, while the hardpan accumulated during dry seasons.

Conclusions (1) Columnar calcite is found underlying and within calcrete hardpans from the Lower Carboniferous of South Wales. (2) The columnar calcite shows a range of growth forms, including variably distorted crystals. This is attributed to differential degrees of poisoning by organic matter of growing crystal faces. (3) The low preservation potential of microbial structures means that the role of microbial activity in generating columnar fabrics cannot be eliminated. There is, however, no direct textural evidence for microbially induced calcification. (4) The overall development of columnar calcite is related to CO 2 degassing of soil water in the lower part of the calcrete profile a n d / o r bacterial denitrification processes. (5) It is suggested that precipitation of columnar calcite occurred largely during the wet season in contrast to hardpan accretion during the dry season.

Acknowledgements I thank the Sedimentary Geology reviewers, lan Fairchild and Debbie Armstrong for their help in preparing this manuscript.

57

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