Calcrete palaeosols from the lower carboniferous llanelly formation, South Wales

Sedimentary Geology, 33 (1982) 1-33

1

Elsevier Scientific Publishing Company, Amsterdam--Printed in The Netherlands

CALCRETE P A L A E O S O L S F R O M T H E L O W E R C A R B O N I F E R O U S LLANELLY FORMATION, SOUTH WALES

V. PAUL WRIGHT Department of Earth Sciences, Open University, Walton Hall, Milton Keynes MK7 6AA (Great Britain)

(Received September 1, 1981; revised and accepted March 9, 1982)

ABSTRACT Wright, V.P., 1982. Calcrete palaeosols from the lower Carboniferous Llanelly Formation, South Wales. Sediment. Geol., 33: 1-33. Three calcrete-bearing palaeosol units are described from the lower Carboniferous Llanelly Formation in South Wales, and are named the Tyle'r bont Pedocomplex, the Cwm Dyar Pedoderm and the Llanelly Pedocomplex. These units were formed on semi-arid to arid floodplains and are associated with sheet flood, stream flood and meandering-channel deposits. Three major processes operated during pedogenesis; calcification, pedoturbation and solution. Pedoturbation resulted in the formation of brecciated horizons, pseudo-anticlinal and prismatic structures and a predominance of disorthic nodules. The soils developed in smectite-rich clays under a seasonally arid climate resulting in strong argillipedoturbation.

INTRODUCTION C a l c r e t e - b e a r i n g palaeosols, i.e. p a l a e o s o l s c o n t a i n i n g horizons of p e d o g e n i c a l l y f o r m e d c a l c i u m c a r b o n a t e , are n o w widely recognised in ancient s e d i m e n t a r y sequences (e.g. see Allen, 1974a; Steel, 1974; Leeder, 1976; H u b e r t , 1978; M c P h e r son, 1979). T h e y have p r o v e d useful for p a l a e o c l i m a t i c a n d p a l a e o g e o m o r p h i c r e c o n s t r u c t i o n s (e.g. Allen, 1974b; H u b e r t , 1978) b u t there are few d e t a i l e d a c c o u n t s o f their p e t r o g r a p h y ( m i c r o m o r p h o l o g y ) . T h e aims of this p a p e r are to describe a variety of c a l c r e t e - b e a r i n g p a l a e o s o l s from the L o w e r C a r b o n i f e r o u s of South Wales, to d o c u m e n t the v a r i a t i o n s within a n d b e t w e e n these palaeosols, to d o c u m e n t their m i c r o m o r p h o l o g y a n d to discuss the types o f p e d o g e n i c processes which o p e r a t e d d u r i n g their f o r m a t i o n . GEOLOGICAL SETTING T h e p a l a e o s o l s d e s c r i b e d here occur in the L l a n e l l y F o r m a t i o n , of p r o b a b l e A r u n d i a n (Visean) age ( I n s t i t u t e o f G e o l o g i c a l Sciences, 1976), which o u t c r o p s a l o n g 003%0738/82/0000-0000/$02.75

© 1982 Elsevier Scientific Publishing Company

the northeast part of the South Wales coal field synclinorium (Fig. la). Further details of the general background of the lower Carboniferous succession in the area can be found in Wright (1981a) and Wright et al. (1981). The Llanelly Formation consists of four members (Fig. lc), of which the upper and lower are alluvial units (the Clydach Halt Member and Gilwern Clay Member, respectively), while the middle two represent peritidal units (the Cheltenham Limestone Member and Penllwyn Oolite Member). The Clydach Halt Member consists of sheet-flood and stream-flood deposits, claystones and palaeosols interpreted as having been deposited on the floodplain of a large river which intermittently received influxes of locally derived sediment from small interfluvial drainage stream floods (cf. Allen and Williams, 1979). Dolomite

MILLSTONE GRIT (NAMURIAN)

DOWLAIS

°~, o°

0

LIMESTONE

25km. [ ] CARBONIFEROUS LIMESTONE

LLANELLY FM.

a e} @

OOLITE @

GROUP

@

30

/ PENLLWYN

/

OOLITE MR. /

5ml,,/

•

.

•

o

.

o

I_ ._"_._.J b

- LOWER LIMESTONE SHALES OLD RED SANDSTONE (DEVONIAN)

CHELTENHAM/ LIMESTONE /

/

HALT MR. ]

C Fig. 1. Geological setting of the palaeosol calcretes described. (a) position of the outcrop area; (b) major stratigraphic subdivisions in the outcrop area: (c) subdivisions of the Llanelly Formation.

DOWLAIS

~CM

LIMESTONE

LLANELLY PEDOCOMPLEX

RO.M CWM DYAR PEDODERM

CL~

CH.M

TYLE'R BONT PEDOCOMPLEX

OOLITE GROUP

Fig. 2. Calcrete-bearing palaeosols in the Llanelly Formation. Member names are abbreviated (see Fig. 1).

also occurs and has been interpreted as a playa deposit (Wright, 1981a). A number of palaeocalcrete horizons occur in this member and are called the Tyle'r bont Pedocomplex (Fig. 2) (see Wright, 1981 a). The Cheltenham Limestone Member is a peritidal-terrestrial unit with facies of open marine, lagoonal to supratidal type and with thin palaeosols. One prominent palaeocalcrete occurs near the top of the member and is called the Cwm Dyar Pedoderm (Fig. 2; Wright, 1981a). The Gilwern Clay Member varies from a thick (8 m) overbank clay lithofacies to a thick (12 m) fining-upwards sandstone lithofacies interpreted as a high-sinuosity channel deposit (Wright, 1981a). The palaeocalcretes occur in the clay lithofacies and are called the Llanelly Pedocomplex (Fig. 2) (Wright, 198 l a). Details of localities are given in this text for specific features but further details of the localities are given in Wright (1981a, and in press). SOIL STRATIGRAPHIC UNITS

The lower and upper palaeocalcrete units (see Fig. 2) are referred to as pedocomplexes. A pedocomplex (Fink, 1976) is a sequence where soil profiles occur in

close vertical succession but do not overlap, being separated by intervening sediment. The middle unit is called the Cwm Dyar Pedoderm. A pedoderm is a 'mappable unit mantle of soil, entire or partially truncated at the Earth's surface or partially or wholly buried, which has physical characteristics and stratigraphic relations that permit its consistent recognition and mapping' (Brewer et al., 1970, p. 106). There are many problems associated with the stratigraphy of palaeosols and the reader is referred to Finkl (1980) for an excellent review of them. CLASSIFICATION OF PALAEOSOLS IN THE I_.LANELLY FORMATION

Calcrete profiles result from the progressive calcification of the soil by the illuvial concentration of calcium carbonate (Goudie, 1973) and this calcification progresses from young profiles with scattered carbonate nodules to older, massive limestone beds often with brecciated and laminated tops. Various stages in this sequence can be recognised and a number of morphogenetic classifications have been devised, based largely on the idea of the maturity of the profile (Gile et al., 1966; Netterberg, 1967; Reeves, 1970; Freyet, 1971; Goudie, 1973: Steel, 1974). Most of the profiles in the Tyle'r bont Pedocomplex are at stages 3 and 4 of Steel (1974). At the type locality for this unit, Odynau Tyle'r bont (British Grid Reference S001, 0635, 1125) there are 4 such stage 3 and 4 units (Fig. 3), although the third unit appears to

Cheltenham Limstone Member

Oolite Group

a KEY

~ ~

b

conglomerates sandstones

Calcrete O nodules [--~ massive

{11/ prismatic

-~

(~

blocky

dolomite

~a~~, fenest ral Fig. 3. Comparative sections in the Tyle'r bont Pedocomplex. (a) Clydach Halt Lime Works; (b) Baltic Quarry; (c) Odynau Tyle'r bont; (d) Ogof Fawr.

8LOC,Y

____.,

.

DOLOM,TE

CONGLOMERATE

CLAY ~

\

/

~

o

WITH

~

~tTf_~l~_h~,(~ -

NODULES

O

? o

~/7-/-+~

D G~o

(~)

@

C)

.

~,

~ / ()

t/

/)/

~

/

k:/

PIPES

®

Fig. 4.a. The Tyle'r bont Pedocomplex at the Clydach Halt Lime Works. Here the rubbly (karsted) top of the Oolite Group (1) is overlain by a thin conglomerate (2) of stream flood origin, and locally by a very thin blocky calcrete. This is overlain by a nodule-bearing clay and a limestone with cylindroids (3), which contains large dolomite nodules (4). Laterally another nodular clay and limestone unit occurs beneath the conglomerate (see b). b. Schematic diagram of the above. A lower stage 3 profile is preserved in solution pockets in the top of the Oolite Group.

c o n s i s t of t h r e e l i m e s t o n e b e d s s e p a r a t e d

b y m i n o r e r o s i o n surfaces. A t o t h e r

l o c a l i t i e s s u c h as C l y d a c h H a l t L i m e W o r k s (British G r i d R e f e r e n c e S021, 2343, 1261) t h e r e are t w o u n i t s s e p a r a t e d b y a t h i n s t r e a m - f l o o d c o n g l o m e r a t e c o m p o s e d

Fig. 5. Pseudo-anticlinal structures in the Cwm Dyar Pedoderm at the Clydach Halt Lime Works.

Dowlais )al grey mudstone (seat earth)

~

claystone

•

glaebule platey calcrete rootlets

O /"1 Gr Gr/R

pyrite intraclasts green clay mottled green and red clay

Ooli te Fig. 6. Composite log of the Llanelly Pedocomplex.

Fig. 7. The Llanelly Pedocomplex at Llanelly Quarry. The irregular top of the Penllwyn Oolite Member (1) is overlain by a platey calcrete horizon (2). A buff weathering pyrite-rich zone occurs near the top (3). The coal and seat earth are missing at this particular outcrop and were cut out by the marine limestones of the Dowlais Limestone (4). The pedocomplex is 7 m thick.

of oolite and calcrete cobbles set in a calcrete matrix, which is locally overlain by a very thin compact calcrete bed (see Figs. 3 and 4). At other localities, e.g. Ogof Fawr (British Grid Reference SN 9845, 0964) only a nodular horizon is present (Fig. 3). The Cwm Dyar Pedoderm consists again of a profile at either stages 3 or 4 of Steel (1974). At the Clydach Halt Lime Works (Fig. 5) it has a distinctive platey appearance with pseudo-anticlinal structures (see below). The Llanelly Pedocomplex (Figs. 6 and 7) contains a platey horizon (stage 3) at its base but most of the unit consists of calcrete nodule-bearing clays. This nodular material is at stages 1 and 2 of Steel (1974), depending on the concentration and sizes of the nodules, although burial compaction of the clays may well have created concentrations different from what they were in the original floodplain soil. Near the top of the unit the nodules are absent and a coal and seat earth are developed (see Fig. 6). PALAEOSOL FEATURES Four components can be recognized in these palaeosols: (1) clay; (2) nodules; (3) platey horizons; and (4) limestone beds.

Clays The clays are soft, light olive-green or mottled liver-red, purple and green in colour. The clays in the Tyle'r bont Pedocomplex are usually green in colour whereas those in the Llanelly Pedocomplex are often mottled (see Fig. 6) with red and purple mottles forming up to 40% of the mass. These mottles are several centimetres in size and have irregular shapes. In thin section, the clays contain quartz sand and silt, and sand-sized lithoclasts. Some of the quartz grains have embayed margins suggesting that solution has taken place (Fig. 8), but such grains are not common. Clay-mineral analyses of the clays were carried out by Dr. G. Brown of the Soils and Plant Nutrition Department, Rothamstead Experimental Station. The dominant clay mineral in the Tyle'r bont Pedocomplex, the Cwm Dyar Pedoderm and the nodule-bearing part of the Llanelly Pedocomplex, is a mixed-layer, near fully-ordered illite-smectite with variable but generally small amounts of illite, chlorite and quartz. In the lower two units, the mixed-layer component contains about 30% swelling layers, but only about 20% in the Llanelly Pedocomplex. The upper part of the Llanelly Pedocomplex contains clays lacking nodules, and is capped by a seat earth (see Fig. 6) which has different clay-mineral assemblages. Samples from one metre below the seat earth consist mainly of interstratified kaolin-smectite or possibly kaolin-illite-smectite, probably accompanied by an ordered illite-smectite similar to that in the underlying horizons. The seat earth consists of a mixture of illite-smectite similar to that below, and a dioctahedral chlorite.

Fig. 8. Embayed quartz grain (centre) from a nodular clay horizon, Tyle'r bont Pedocomplex, Ogof Fawr; 30×.

Fig. 9. Pseudo-anticlinalstructures in the Llanelly Pedocomplexat Blaen Onneu. The clay here contains very few calcrete nodules.

At the time of writing (August 1981), a temporary exposure was made in the Llanelly Pedocomplex at Blaen Onneu (British Grid Reference S01 l, 1555, 1696). The exposure was an 80 m-long and 8 m-high strike section, composed of clay containing very few carbonate nodules. The clay was dominantly buff and green and had been gleyed near the surface by the overlying present-day peaty soils. The exposure was traversed by large horizontal fold-like structure defined by curved planes with reddened borders. The folds had wavelengths of 2-6 m and the planes were 20-30 cm apart, in sets up to 2 m thick (Fig. 9). The synclinal parts of the folds were gently curved but the anticlines were cuspate in form with some planes having overridden others at the crests (Fig. 9). The planes had slickensided surfaces and these sets of planes were cross-cut by other sets. The beds below these folds show no evidence of folding or slumping. Strikingly similar fold-like structures were described by Allen (1973, 1974b) from pedogenic horizons in the Old Red Sandstone of Wales and the Welsh Borderland (e.g. compare Fig. 9 with fig. 1 of Allen, 1973). These Carboniferous structures differ from some of those described by Allen in lacking nodules although Allen also described some nodule-poor folds.

Interpretation These clays, by virtue of being interbedded with stream-flood and sheet-flood deposits, by being lateral equivalents to, and cut into by channel sands, and by containing quartz sand, silt and sand-sized lithoclasts, are interpreted as floodplain overbank deposits (see Wright, 1981a). The lower part of the sequence in the Llanelly Pedocomplex has been interpreted as being deposited on a semi-arid to arid floodplain while the upper part was deposited in a back-swamp (Wright, 1980). The clays, by virtue of containing pedological features such as calcrete nodules, were part of a soil profile, and thus represent the matrix for the pedalogical features, i.e. a soil S-matrix in the sense of Brewer (1964, pp. 107 and 302). The clay mineralogy has been discussed in detail by the author elsewhere (Wright, 1981a) and two origins for the mixed layer illite-smectites were considered: firstly that they were derived from aeolian, bentonitic, volcanic ashes and that the addition of K + was due to contemporaneous weathering or t o the relative enrichment of K

10 and other salts in shallow marine waters as has been suggested for similar lower Carboniferous clays by Walkden (1972) and Somerville (1979). A second explanation is that the smectite component of these clays may have been pedogenically derived from the weathering and alteration of illite. Watts (1980, p. 674) has described mixed illite-smectite clays from Recent calcretes in the Kalahari and attributes them to the pedogenic degradation of illite, that is, the clays are a partial weathering product (see Robert et al., 1974). Smectite can form in base-rich soils and typically occurs in semi-arid areas. Circumstantial evidence comes from the fact that the palaeosols with more mature calcrete profiles (hence having undergone more prolonged pedogenesis) have higher smectite proportions. This offers a completely opposite interpretation for such K-bentonites to that of Walkden (1972) and Sommerville (1979), suggesting that the clays are not due to the alteration of smectite to illite but to the pedogenic alteration of illite to smectite. Further work is in progress by me on these clays, but whatever their original composition, they almost certainly possessed a higher smectite content before burial, for as Gill et al. (1977) have shown, there has been illitisation of the mixed illite-smectite clays in the Lower Carboniferous of South Wales related to burial diagenesis at depths in excess of 1 km and at temperatures of 100-200°C (Velde, 1977). The kaolinite at the top of the Llanelly Pedocomplex reflects leaching of the smectites by acid waters derived from the overlying peats (coal) (see Staub and Cohen, 1978, for a present-day analogue). The colour mottling of the clays is a typical feature of calcrete palaeosols, and colour mottling and reddening in fluvial sequences has been the focus of much attention (Walker, 1967; Van Houten, 1973). No detailed geochemical analysis on these clays has been carried out, but the green colouration is probably due to the interlayered iUite and smectite (cf. McBride, 1974; Braunagel and Stanley, 1977). The red colouration is probably due to the presence of haematite in the matrix and as coatings. The more liver-like red colours may be a late diagenetic effect (Folk, 1976, p. 612). The absence of reddening in some units might reflect a lack of exposure time and Walker (1967) has shown that prolonged periods are needed for full reddening to occur. This explanation seems unlikely because the clays with the most mature calcretes are often the least red. Perhaps diagenetic processes have altered the colours. The fold-like structures in the Llanelly Pedocomplex differ from those in the Cwm Dyar Pedoderm, for the latter are composed of contorted plates of banded calcrete, while the former occur in thick clay horizons with very few calcrete nodules. The origin of such folds (pseudo-anticlines) in palaeocalcretes is discussed at length below, but Allen (1973) interpreted identical structures to those in the Llanelly Pedocomplex from the Devonian, as probably representing gilgai. These are contorted horizons within soils displaying troughs and peaks created by the seasonal expansion, on wetting, of soils with a high expandable clay content (Vertisols) (see Buol et al., 1973; Bridges, 1978). This process is known as argillipedoturbation

II (Jongerius, 1970). Such soil profiles in the Recent contain folded structures defined bY cross-cutting thrust planes with slickensided surfaces (Yaalon and Kalmar, 1978). Bearing in mind the original high smectite content of these Carboniferous clays, a similar origin seems likely. Nodules

The nodules may comprise as much as 80% by volume of the clay horizons but usually comprise between 20-60%, and vary from a few hundred microns to 25 cm in diameter. Their shapes also vary, from subspherical to highly irregular, and the larger the nodule the more irregular is its shape. The nodules may gradually increase in size and number up through a clay unit (Fig. 6), but this is usually only the case in the thickest units such as the Llanelly Pedocomplex. In the thinner units, such as those in the Tyle'r bont Pedocomplex (Fig. 4), this is not seen, although the density and size of the nodules increases abruptly just below the bases of overlying limestone beds, into which the nodules merge. Several varieties of nodules can be recognised, based on their external and internal form, and on the nature of the nodule-clay contact. Using this latter feature, two varieties are recognised; firstly, there are nodules which have gradational contacts with the clay matrix (type 1), and secondly, there are those which have sharp boundaries with, and can easily be removed from, the matrix (type 2) (Fig. 10). Type 1 nodules (Fig. 10) are usually small, under 5 cm in diameter and of variable shape. Internally, they have a uniform structure with a dense, fine-grained, calcite spar fabric and gradational contacts with the surrounding clay matrix (Fig. 11). Type 2 nodules have a variety of forms (Fig. 10). There are small (usually under 2 cm in diameter), round, subspherical forms (type 2a), which internally have a dense fabric similar to type 1 nodules. Secondly, there are larger nodules (type 2b), similar in shape to type 2a forms but possessing a compound structure (Fig. 10) consisting

--,,.J

(

\

1

o 2a

@ 2b

2c

veins (crystallaria)

Fig. 10. Types of nodule in the palaeosols. 1. noduleswith diffusemargins(orthic nodules)(see Fig. 11); 2a-c. nodules with sharp margins (disorthic nodules). (a) undifferentiatedfabric; (b) with nodules (see Fig. 12); (c) with crystallaria.

12

Fig. 11. Orthic nodule (type 1) margin, showing the gradational contact of the nodule ( N ~ with the matrix (M); I1 × (photographed from a peel).

Fig. 12. A nodule (glaebule) with a sharp boundary (disorthic) defined by a spar-filled circumgranular crack (circumglaebular crystallaria); 28 x .

13

Fig. 13. Allothic nodule, i.e. a nodule which has been reworked by sedimentary (not pedogenic) agents and concentrated at a sandy horizon within a clay unit; 22X.

of small nodules like type 2a, set in a similar dense spar matrix. The smaller internal nodules can be recognised as separate entities because they are defined by one or more coarser spar-filled cracks (Fig. 12). A third type (type 2c) consists of irregular shaped nodules of variable size, but commonly over 10 cm in diameter, which often have a very hackly surface texture. Internally they have been highly brecciated and the angular fragments are separated by coarsely crystalline, spar-filled cracks similar to those which occur in the limestone beds, which vary from a hundred microns to up to several millimetres in width and have lengths of up to 2 cm. A fourth type of nodule with sharp boundaries occurs (type 2d), which may have an internal structure like types 2a, b or c but also has a very well rounded shape (Fig. 13). They occur in thin horizons in the clays, and are associated with coarse quartz sand and oolite lithoclasts, and resemble the calcrete-clasts in the associated stream-flood conglomerates.

Interpretation Nodules similar to these have frequently been described from ancient sequences and have been interpreted as being pedogenic in origin, (e.g. Allen, 1974a, b; Steel, 1974; Hubert, 1978; McPherson, 1979). Such pedogenic nodules are termed glaebules (Brewer, 1964). Those horizons containing only nodules (i.e. at stages 1 and 2 of Steel, 1974) represent calcic horizons (Soil Survey Staff, 1975). The type 1 nodules, with their diffuse margins, are similar to the type 2 nodules

14

described by Blokhuis et al. (1968) from Recent vertisols from the Sudan, and also to the orthic nodules described by Wieder and Yaalon (1974) from Recent soils from Israel. The type 2a nodules, with sharp edges, are similar to the type 3 nodules described by Blokhuis et al. (1968) and to the disorthic nodules described by Wieder and Yaalon (1974). Analogues for the type 2b are also to be found in the descriptions of Recent glaebules by Blokhuis et al. (1968) and Wieder and Yaalon (1974), (see fig. 3 in both papers) and would be called compound glaebules with glaebular haloes by Blokhuis et al. Type 2c nodules are also similar to forms figured by Wieder and Yaalon (1974, fig. 2), and in general types 2a-c would be termed disorthic nodules by Wieder and Yaalon, because of their sharp contacts with the surrounding S-matrix. The type 2d nodules described were probably reworked from pre-existing soils by sedimentary agents and are termed allothic nodules by Wieder and Yaalon.

Platey calcrete horizons This variety is not common, occurring only in the Cwm Dyar Pedoderm at one locality (Fig. 5) and at the base of the Llanelly Pedocomplex (Fig. 6). These units average 50 cm in thickness and in the Llanelly Pedocomplex consist of almost horizontal plates of limestone, 1--5 cm thick and up to 20 cm long, separated by thin clay seams up to 2 cm thick. In the Cwm Dyar Pedoderm, it consists of highly

Fig. 14. Photomicrograph of the contorted platey calcrete horizon shown in Fig. 5; 6.5 x .

15 contorted strips and irregular lumps of limestone, up to 12 cm thick (Fig. 5). The horizontal plates consist of horizontally laminated fine spar, with the laminae averaging a few hundred microns in thickness. The contorted forms have a more complex fabric, internally showing highly contorted laminae of fine spar (Fig. 14). Some laminae contain peloids and quartz silt, probably derived from the underlying peloidal limestones. Many of the folds have been fractured and sealed either by drusy spar of fine spar matrix. Some of the contorted plates have slickensided surfaces. These folds are not tectonic in origin for they have a different style to the more open folds in the area and there is no evidence of folding above or below. Furthermore, they do not appear to have resulted from slumping, and the folds are truncated by the overlying beds.

Interpretation The contorted forms resemble some types of pseudo-anticlinal structures described from many Recent calcretes (Watts, 1977a; Klappa, 1980, and references therein), and from palaeocalcretes (Allen, 1973, 1974a, b). Their origin remains controversial (Klappa, 1980), but four possibilities exist: (1) They represent gilgai-like structures (see above). (2) They may have been caused by the activities of plants (Klappa, 1980); a process called floral pedoturbation by Jongerius (1970). (3) They may have been caused by the displacive growth of calcite (Watts, 1977a, 1978). (4) Or they may have been caused by the displacive growth of evaporite crystals, e.g. gypsum. Of these four possibilities, the second seems the most unlikely because of the absence of any evidence of plant activity such as rhizocretions or pedotubules, which do, however, occur in other palaeosols in the Llanelly Formation (see Wright, 198 l b). The occurrence of these platey units in smectite-rich clays, which probably originally had a higher smectite content, and had therefore a shrink-swell potential, suggests that the folds may be analogous to gilgai. The possibility that the displacive growth of calcite was responsible for setting up the forces causing folding is more difficult to assess. Watts (1977a) described a variety of pseudo-anticlinal structures from the Recent of Botswana, and later (1978) discussed the role of displacive calcite in calcrete formation. Displacive calcite can occur as micrite or as fibrous calcite. The latter is a distinctive feature, and occurs also in Devonian calcretes (Watts, 1977b, p. 150) but is absent from all the calcretes in the Llanelly Formation. The possibility of evaporite crystallisation having caused the folding seems unlikely as evaporite pseudomorphs are not common and are absent from the contorted horizon. The probability is, therefore, that the folding was caused either by the displacive growth of calcite or by the action of the swelling clays, or both. Watts (1977a) attempted to identify the mechanisms operating in different pseudo-anticlinal struc-

16 tures in his study of Recent forms and has suggested some geometrical criteria for identifying the causes of deformation, but with the small exposure available at the Clydach Halt Lime Works, it was not possible to apply Watts' criteria. Limestone beds

These beds vary in thickness from a few centimetres to over 1.5 m (Fig. 3), although such thicker forms are probably composites of two or three smaller units. The beds may occur as discrete units or may be underlain by nodule-bearing clays into which they grade. Most of the exposures of the Cwm Dyar Pedoderm consist of discrete beds whilst those in the Tyle'r bont Pedocomplex overlie nodule-bearing clays (Fig. 3). The beds have a variety of forms and may be hard and massive, or poorly consolidated and blocky (Fig. 3) with blocks up to 20 cm across separated by clay seams. Others have a distinctive prismatic structure in which the prisms have 4 to 8 sides of varying widths, are up to 60 cm long, and are separated by clay such that they can be removed easily from the beds. Cylindroid forms also occur which consist of knobbly, vertical limestone cylinders up to 25 cm long and 4 cm in diameter and separated by clay, but this last type is not common and only occurs in the Tyle'r bont Pedocomplex at Clydach Halt (Fig. 4). Fenestral limestone beds also occur, in the Tyle'r bont Pedocomplex at the Baltic Quarry (Fig. 3) and in the Cwm Dyar Pedoderm at Pwll Du (British Grid Reference S021, 2495, 1170). Most units are cut by spar-filled veins which are most common in the blocky and massive forms. These veins must be of early origin for they occur in calcrete clasts in the stream-flood conglomerates and at the base of the overlying Cheltenham Limestone Member. At one locality only, at Blaen Onneu (British Grid Reference S011, 1555, 1696; and see Wright, 1981c), the massive Cwm Dyar Pedoderm contains small rosettes after evaporites (see below). Of unusual interest is the occurrence of clay-covered slickensided surfaces in many of the units, but especially in the Tyle'r bont Pedocomplex at Odynau Tyle'r bont. Here large prismatic structures can be removed from the face and are found to be cut by curved slickensided surfaces. These surfaces are at high angles to bedding. Similarly, some blocky units at the same locality also contain numbers of curved slickensided surfaces. Many nodules in the Llanelly Pedocomplex also have similar surfaces. These slickensides differ from local tectonic forms in two important ways: firstly, all the tectonic slickensides in the area occur at low angles (less than 10 °) to bedding; and secondly, all the tectonic slickensides have essentially planar, but striated surfaces. Vertical variations through the limestone beds The prismatic and cylindroid beds contain few veins and show minor brecciation, but the massive and blocky forms display a much greater degree of brecciation. Most

17

/

f

/

\ © olj

~ /

~ - -

/

A

.o

O B

nodules crystallaria clay

Fig. 15. Schematic diagram of the vertical differences within a massive calcrete horizon. A. zone of veining (crystallaria) (see Fig. 20) in which the amount of veining increases upwards; nodules, defined by circumglaebular crystallaria occur near the base of this zone. B. zone of intense brecciation where all the planes are filled by coarser crystic plasmic fabric (see Fig. 16); m a n y clasts (peds) near the top have rounded edges (Fig. 16).

Fig. 16. Brecciated calcrete from the top of a blocky calcrete horizon. Here the planes are filled with a slightly coarser crystic plasmic fabric than the material i n t h e clasts (peds). This material appears darker because of its higher clay content. Some of the peds have rounded faces; 7.5 x (photographed from a peel).

18

Fig. 17. Planes filled by quartz sand. The peds are composed of crystic plasmic fabric. Note rounded edges of many of the peds. These are not glaebules but fragmented calcrete: 9.5 × (photographed from a peel).

of the massive and blocky units contain nodules in their lower parts, delimited by curved spar-filled veins. The nodules are set in a fine spar matrix which may or may not be cut by veins, but some beds contain no nodules. Beds which show nodules generally have a more brecciated matrix and beds which overly clays with nodules of type 2c are more brecciated than beds overlying nodules of type 2a. Variations in the degree and type of brecciation are very marked (Fig. 15). The lower 2 / 3 of the beds contains from relatively few to many spar-filled veins whereas the upper 1/3 is often highly brecciated with the gaps between the angular fragments filled with fine, frequently clay-rich, fine spar matrix (Fig. 16). Some cracks also contain clay or quartz sand (Fig. 17), which was probably washed in from above. The fine spar filling the cracks is slightly coarser than that in the fragments and often appears darker because of the higher clay content. Many fragments in the breccia are highly irregular in shape and resemble an exploded jigsaw, but many others, especially near the top of the beds, are partially or almost wholly rounded such that it is difficult to visually fit back the various pieces of the jigsaw (Fig. 15).

Interpretation The massive limestone beds are analogous to petrocalcic horizons in Recent soils (Soil Survey Staff, 1975), i.e. they are strongly indurated pedogenic carbonate

19 horizons. Such horizons are also called hardpans (Netterberg, 1967) or K2m horizons (Gile et al., 1965). They represent calcretes at stage 4 of Steel's classification (1974). The blocky, prismatic and cylindroid units lack this overall well-cemented structure and can be classified as being at stage 3, i.e., showing clastic material with the carbonate. The fenestral beds probably represent a stage 4 form, but a suitable analogue is lacking. The prismatic forms are typical of many Recent calcretes where the prisms have formed by the calcification of pre-existing prismatic structures, i.e. soil peds (Brewer, 1964). The existence of such structures indicates that severe desiccation of the soils occurred during pedogenesis, to form the deep cracks that define the peds. The cylindroids are also common in Recent calcretes and often form around roots (rhizocretions), but sections through these Carboniferous cylindroids revealed no central tube nor any other evidence of root activity. These cylindroids probably represent coalesced nodules and Klappa (1978, p. 304) has described similar elongate glaebules from Quaternary calcretes from Spain, also not associated with rootlets. The slickensides are problematical and the following arguments apply equally to those defining the folds in the clay horizons described above. They could have formed by a number of processes: they could be tectonic (but of a different style to the tectonic slickensides in the sequence); they could have resulted from stresses set up during clay compaction, or during pressure release associated with unloading. Slickensides are common in many soils today, especially those with a high proportion of swelling clays such as vertisols. They are another product of argillipedoturbation for the seasonal shrinking and swelling of these clay soils creates movements which generate slickensides (Buol et al., 1973; Yaalon and Kalmar, 1978). The clays originally had a high smectite content and, therefore, had a shrink-swell potential suggesting that the slickensides could have been pedogenic in origin. Slickensides in soils possess distinctive stress cutan fabrics (Brewer, 1964), but tectonic or any other type of slickenside would possess the same oriented clay fabrics. The difference in style between the tectonic and palaeosol slickensides is important. The orientations of the palaeocalcrete slickensides, at angles of 40-60 ° to bedding are similar to those of slickensides in Recent vertisols where the shearing associated with clay expansions leads to the formation of slickensided surfaces at angles of 45 ° (Fitzpatrick, 1971, p. 67; Yaalon and Kalmar, 1978). Their exact origin, however, remains uncertain.

Microfabric The palaeosol clays, the original soil S-matrix, exhibits an anisotropic clay fabric with a flecked extinction pattern (an argillasepic fabric in the sense of Brewer, 1964). This may represent the original soil fabric or a later diagenetic overprint. The microfabric of the nodules and beds is essentially the same and consists of a mosaic of finely crystalline, non-ferroan, calcite spar (crystal size classes after Folk, 1959). The crystals are rarely less than 5/~m or larger than 30/tm, and are generally

20 equant, subhedral in form, but larger rhombic calcite crystals (15--20 /~m in long diagonal) also occur. These rhombic calcite crystals which may reach lengths of up to 200/~m (Fig. 18) are calcite (as revealed by staining) and not dolomite. There is no dolomite in these mosaics and the larger rhombs lack features indicative of dedolomitization. Some of the crystals in the mosaics have a rounded form and these can reach diameters of up to 60 /~m (Fig. 19). Despite the presence of these large crystals, the mosaics are generally very well sorted in crystal size and this fact, together with the occurrence of many euhedral crystals, suggests that the fabric is not a recrystallised micrite. Clay does occur in the nodules as tiny particles between crystals, or as shard-like patches but it is not common, and some nodules have clay-rich rims. Some limestone beds contain up to 10% clay, mainly as shard-like patches set in a calcite mosaic, but clay is completely absent from other beds. The calcite-filled veins occur in nodules, platey horizons and the various limestone beds but are best developed in the blocky and massive units where they reach dimensions of up to 1 cm wide and several centimetres in length. There is evidence of multiple phases of veining with earlier ones offset by later veins (Fig. 20). The veins may occur in parallel sets but usually occur in irregular cross-cutting patterns. These veins are always filled by drusy, fine to coarsely crystalline sparry calcite (Fig. 21), which is usually non-ferroan calcite but some late ferroan calcite also occurs. Tectonic veining in the Llanelly Formation is totally different in style to these veins and consists of very narrow (a few hundred

Fig. 18. Rhombic non-ferroan calcite set in a finer crystic plasmic fabric: 70x.

21

Fig. 19. Rounded 'crystals' surrounded by darker clay matrix; 120x.

Fig. 20. Multiple generations of spar filled veins (crystallaria) defining darker peds with a fine crystic plasmic fabric; 10×.

22

Fig. 21. Drusy, passive, void filling calcite spar in a plane; 28 X.

m i c r o n s w i d e at the most), closely p a c k e d , p a r a l l e l v e i n s a l m o s t always filled by f e r r o a n calcite. T h e f e n e s t r a l units h a v e a v e r y fine g r a i n e d fabric w h i c h is v e r y clay-rich. T h e

Fig. 22. Solution fenestrae in a calcrete. Although these fenestrae often resemble laminoid (algal) fenestrae, these here occur within a calcrete matrix; 6 x.

23 spar-filled fenestrae are up to 1 cm long and several millimetres high (Fig. 22), and may occur in discontinuous trains.They frequently have a stromatactis-like form with flat floors and irregular roofs (Figs. 22 and 23). Many contain geopetal crystal silt and fragments of the matrix which appear to have collapsed into the cavities (Fig. 23). Many fenestrae have roofs which appear to have been formed by dissolution and have scalloped outlines (Fig. 22). Some have thin clay seams at their margins (not due to pressure solution) which may have acted as a barrier to fenestrae growth during dissolution. These fenestrae are absent from all the other three palaeosol units described here, but some nodules with minor palaeosols in the Cheltenham Limestone Member are also fine-grained and clay-rich, and possess similar fenestrae. These fenestral units can be confused in the field with fenestral peloidal (peritidal) limestones which are common in the overlying Cheltenham Limestone Member, but in thin sections and peels these calcrete forms lack any bioclastic material or peloids, and contain thin veins and nodules with spar-filled cracks around them. The rosettes (Fig. 24) in the Cwm Dyar Pedoderm at Blaen Onneu, average 4 mm in diameter and are composed of radiating blades, 50-300/xm wide and up to 1 mm long. These blades are either filled with non-ferroan sparry calcite or with very fine spar matrix, and occur scattered throughout the unit at Blaen Onneu but increase in number upwards and comprise 10% by volume at the top. They are of 'late' origin in

Fig. 23. Stromatactid-like fenestra overlying a fenestra showing evidence of roof collapse. The crystal silt in the upper fenestra aplbears to have been cemented before the collapse and fracturing of the underlying fenestra occurred; 2 9 × .

24

Fig. 24. Replaced evaporites (gypsum rosettes?). These structures were partly replaced by crystic plasmic fabric: 9×.

that they were not affected by the calcrete brecciation but were 'early' enough to be partially replaced by calcrete matrix.

Interpretation The microfabric of the palaeocalcretes is similar to many ancient and Recent calcretes (see earlier references). The rhombic calcites are common in Recent subaerial carbonates and have been described from soils and weathering profiles including calcretes (Brewer, 1964, p. 296; Folk, 1971, 1974; Lattman and Simonberg, 1971, p. 274; Chafetz and Butler, 1980); from tufas (Irion and Muller, 1968) and in subaerially exposed carbonate sands (Perkins, 1968, p. 1371; West, 1973, p. 239). Folk and Land (1975, p. 66) considered that in very dilute waters, without competing cations such as N a + and K + , calcite forms rhombohedra 2 - 1 0 / ~ m in size and they refer to Usdowski (1963) as having shown experimentally that calcite rhombs form in low-salinity waters with M g / C a of 1:4 or less. Folk (1974, p. 50) considered that the maximum size for rhombicity in fresh-water calcites, appeared to be 5-10 /~m, above which the calcites became polyhedral, yet some of the examples described above have much larger rhombic crystal sizes than 5 - 1 0 ~m. Chafetz and Butler (1980) describe rhombs 15-40 # m in size and Lattman and Simonberg (1971) describe forms 20-40/~m in size and even the largest rhombs described here are not anomalous for soils (Brewer, 1964, p. 296). The round crystals are more difficult to

25 explain and require further study, but Wieder and Yaalon (1974) figure some oval particles from Recent glaebules and quote a personal communication with Badiozamani who suggested that the round crystals could have formed because of the presence of soluble organic matter. Bal (1975b, p. 169) described round crystals from Recent soil calcites and Kubiena (1970, p. 220) described round crystals from a Recent calcareous mull horizon. It is possible that the round forms have resulted from solution-rounding, but seem more likely to have resulted from the effects of restraining, surrounding clay minerals. The overall fabric would be described as a crystic plasmic fabric (Brewer, 1964, p. 317) or a K fabric in the sense of Gile et al. (1965), i.e., consisting of a mass of crystals. Bal (1975a) has elaborated the terminology for such fabrics and his terms coarse and fine crystic fabrics would be applicable, the former especially for the coarser crystals commonly filling the cracks at the tops of the massive and blocky units. The larger rhombic and round crystals are also a type of coarse crystic fabric and are not intercalary crystals (Bal, 1975b, p. 169). Similar veins to those described here have been recorded from many ancient calcretes (Allen, 1974a; Hubert, 1978; McPherson, 1979), and have been interpreted as crystallaria. These are crystal fillings of voids in soils (Brewer, 1964, p. 284), which may take the form of crystal tubes, chambers or sheets. The latter originate from the precipitation of crystals in various types of planes, which are planar voids. Three types are recognised by Brewer (1964): (1)joint planes, which occur in regular, parallel sets; (2) skew planes, which are irregular forms; and (3) craze planes which are complex forms due to the interconnection of many flat or curved planes. All three types of plane crystallaria occur in the nodules and limestone horizons but skew planes are by far the most common. The crystallaria which surround the nodules (Fig. 12) have formed in circumgranular cracks (Ward, 1975), and are circumglaebular crystallaria. These are a widespread and characteristic calcrete structure in the Llanelly Formation. Planes result essentially from the cracking of the soil caused by wetting and drying (Brewer, 1964). The fenestral fabrics seem to have resulted from dissolution within the calcretes, and differ from desiccation and crypt-algal fenestrae in the Cheltenham Limestone Member. They may have originated at the sites of earlier evaporite minerals but they do not show any of the typical evaporite pseudomorph forms. Their origin remains a problem but it is striking that they occur in the most clay-rich beds with the finest crystal sizes. Perhaps the finer sizes made the crystals more susceptible to solution but the cause for their finer grain size is unknown. Semeniuk (1971) has described similar solution fenestrae from vadose-altered Ordovician limestones. The calcite rosettes are unusual but resemble some type of replaced gypsum crystals and their common occurrence near the top of the Cwm Dyar Pedoderm at Blaen Onneu would make this a gypsic horizon (Soil Survey Staff, 1975). Gypsic horizons at the tops of calcretes indicate quite marked aridity for gypsum is normally concentrated below the levels of carbonate accumulation, and its presence

26 here may indicate that evaporitic updraw from saline ground waters occurred (Birkeland, 1974). DEVELOPMENT OF THE PROFILES From a study of the forms and internal structure of these profiles, the effects of three processes can be recognised: the calcification of the soil; the brecciation and buckling of the profiles; and the dissolution of the calcrete. Calcretes result from the progressive calcification of the soil due to precipitation of calcite within the soil profile. This carbonate may have a variety of sources, e.g. aeolian dust+ rain water, plants, sheet wash, sea spray, weathering of calcareous parent material and capillary updraw from ground water. Of these, aeolian input is considered the major source (Goudie, 1973). There are a variety of theories to explain calcrete formation but the most widely held explanation for most calcretes is that the carbonate forms by the precipitation of downward-moving solutions carrying the carbonate which has been leached from the upper parts of the soil ('per descencum' model of Goudie, 1973). This carbonate is commonly initially precipitated as nodules which grow and coalesce to form more compact layers which then have the internodular areas filled by carbonate. The tops of the calcretes are often brecciated and show evidence of solution and may be capped by a laminated calcrete horizon (Goudie, 1973). The nodules are a distinctive feature of these Carboniferous palaeosols, but the origin of such nodules is not well understood (Brewer, 1972, p. 86) and Klappa (1978) gives a review of their origin. The orthic nodules (type 1) clearly formed by the progressive precipitation of carbonate around existing nodules, but within the clay matrix, with which they have a gradational contact. This carbonate may have been displacive or replacive (or both) of the clay matrix. Many other nodules show evidence of having had circumgranular cracks around them, formed by the matrix having being pulled away from the nodule (the calcite in the nodule could not have shrunk, but the clays trapped within the nodules could conceivably have contracted, but this would not have accounted for the volume of the resulting crhcks).The formation of the cracks could have provided the sites for the precipitation of new carbonate increasing the size of the nodule. The disorthic nodules (types 2a-c) provide important clues to other processes at work in these palaeosols. These nodules are by far the most common kind and in Recent soils are interpreted as having been subjected to pedoturbation, i.e. they have been displaced and churned about in the soil (Wieder and Yaalon, 1974). The term pedoturbation, refers to this churning (Hole, 1961; Jongerius+ 1970). It is because of this movement in the soil that the nodules have sharp boundaries and often have rounded forms (Wieder and Yaalon, 1974). Pedoturbation can be caused by a variety of factors (Jongerius, 1970, pp. 134 and 316) and two of the most important causes are what might be called biopedoturba-

27 tion (faunal and floral pedoturbation), and argillipedoturbation. The former results from the burrowing activities of animals and plants, but as discussed earlier, there is no evidence of either even though burrows of soil dwellers and rhizocretions occur in other palaeosols in the Llanelly Formation (Wright, 1981a). The occurrence of clays with a shrink-swell capacity does suggest the possibility of argillipedoturbation and disorthic nodules, formed in similar smectite-rich soils, are described by Blokhuis et al. (1968) from Recent vertisols of the Sudan. The brecciation of the nodules and beds also indicates pedoturbation, and from work on Recent calcretes, Watts (1978) considered such brecciation as due to the growth of displacive calcite. The crystallaria in these Carboniferous calcretes all contain 'passive' void filling, drusy calcite and not the 'displacive' fibrous cements Watts describes. Perhaps the breccciation was due to the displacive growth of the crystic fabric matrix as Watts also suggests for Recent calcretes, but there is no direct evidence for this. Argillipedoturbation therefore again appears a reasonable possibility. Is there any other evidence that argillipedoturbation was active during pedogenesis? The well-developed prismatic structures are typical features of soils with a high shrink-swell potential, as are the pseudo-anticlinal (gilgai) structures, the platey structures, the increased brecciation up the profile, and the occurrence of slickensides (A1 Rawi et al., 1968; Buol et al., 1973; Bridges, 1978): Illuviation argillans have not been found in any of the profiles and this is also typical of soils where argillipedoturbation occurs but such argillans do occur in other, clay-poor palaeosols in the Llanelly Formation (Wright, 1981 a). Therefore, these palaeocalcretes originally had a clay content rich in swelling clays and possess a number of features indicative of argillipedoturbation. Soils charactetrised by a swelling clay content of more than 35% are called vertisols (Soil Survey Staff, 1975). The dominant process in such soils is the shrinking and swelling of the clays in response to seasonal variations in moisture. During the dry season, the clays contract and deep wide cracks form in the profile into which material often falls. Under conditions of increased moisture caused by rainfall or flooding, the clays expand and as a result of the increased volume (because of the new material added), stresses are set up which result in the buckling and shearing of the profile. This results in the formation of prismatic and wedge-shaped peds, gilgai and slickensides (Yaalon and Kalmar, 1978). The strong seasonal aridity also results in carbonate accumulation but this rarely reaches major concentrations (Allen, 1973, and references therein).The palaeosols described here probably represent calcretes (aridsols), which, because of the nature of the material in which they grew (a smectite-rich clay) possessed strong vertic characteristics. The third major process which operates in these palaeosols was dissolution. Carbonate-rich (calcic and petrocalcic) horizons usually develop at or just below the depth of seasonal wetting (Wieder and Yaalon, 1974, p. 118) but this level will vary with the vagaries of the soil-moisture budget and horizons near this level will

28 periodically undergo dissolution. This was clearly the case at the top of the limestone beds where many peds show solution-rounding and this may also have been the cause of the fenestral calcretes. CLAY D I S P L A C E M E N T OR R E P L A C E M E N T ?

The importance of displacive calcite growth in these palaeocalcretes is difficult to assess (see above), but the occurrence of clay particles concentrated in the outer parts of some nodules indicates that displacement of the clays occurred (Wieder and Yaalon, 1974, p. 118), but on the whole, the crystic fabrics in these palaeocalcretes contain very little clay. If the massive and blocky forms have formed by the coalescence of nodules followed by the filling of internodular areas, some clay should have been trapped within these horizons, but is conspicuously absent. The conclusion to draw is that the calcite has replaced the clay, and Hay and Reeder (1978~ p. 670) have described the calcite replacement of clays in calcretes from Olduvai Gorge, Tanzania. Exactly how calcite is able to replace clay is unknown. C O M P A R I S O N S WITH O T H E R P A L A E O C A L C R E T E S

As mentioned earlier, these palaeocalcretes have many similarities to ones widely described from the Devonian (Allen, 1974a, b; McPherson, 1979) and the PermoTriassic (Steel, 1974; Hubert, 1978). One significant difference is that in all these cases the underlying siltstones and mudstones often contain large numbers of crystallaria (e.g. see McPherson, 1979, fig. 6). These are conspicuously absent from the Llanelly Formation and one explanation for this might be that the constant shrinking and swelling of the clays was such that planes were not open long enough for crystallaria to form. The planes which remained open long enough for crystallaria to form were the ones in the clay-poor areas, such as in the insides of the nodules or the massive or blocky horizons. P A L A E O C L I M A T E A N D D E P O S I T I O N A L RATES

The occurrence of calcretes in ancient sequences has often been used as a climatic indicator (e.g. Allen, 1974b; Hubert, 1978), and today such soils typically form in warm to hot, semi-arid or arid climates with a marked seasonal rainfall distribution (Goudie, 1973, pp. 96-111). A similar climate has been inferred for early Carboniferous times in South Wales (Wright, 1980). Calcrete palaeosols also provide information on the depositional rates during their formation (see Allen, 1974b; Leeder, 1975). From studies of Quaternary and Pleistocene calcrete profiles, it appears that most mature profiles, i.e., like those in the Tyle'r bont Pedocomplex and Cwm Dyar Pedoderm, need many thousands of years to form and only develop on geomorphic surfaces which experience prolonged

29 pedogenesis and little or no deposition (e.g. Gile, 1967, 1970; Gile and Hawley, 1966). The presence of a number of such profiles throughout the outcrop area (Figs. 1 and 3) in the Tyle'r bont Pedocomplex is clear evidence of very low rates of deposition over the area. The radiocarbon dating of Williams and Polach (1971) suggests that periods of pedogenesis lasting in the order of 10,000 yrs were required for each stage 4 unit to develop. Although these results are often quoted (e.g. Allen, 1974b; Leeder 1975; McPherson, 1979), these values may not be representative of all calcretes because Hay and Reeder (1978, p. 658) have described mature calcretes having formed in only a few thousand years in East Africa. They speculated that this rapid formation was caused by an abundant and readily available source of calcium carbonate from carbonatite ash. These Carboniferous calcretes formed in a carbonate terrain, on an exhumed carbonate shelf where calcium carbonate, derived from aeolian dust, rainfall and sheet wash would also have been in abundant supply. Therefore, these m/ature profiles indicate long periods of little or no net sediment input on the floodplains, but these periods were not necessarily as long as they would have been if the calcretes had formed in more 'continental' settings such as that described by Williams and Polach (1971). The calcretes in the Cwm Dyar Pedoderm also indicate a prolonged period of exposure but the variations in calcrete stages seen in the Llanelly Pedocomplex require more explanation. The different concentrations of plates and nodules in this unit indicate that deposition rates probably varied and that there were three periods of lower sediment accretion (Fig. 6): (1) at the base of the unit (platey horizon); (2) at the 2.7-m mark; and (3) at the 4-5-m level. At these levels, the calcretes are at a higher stage of development than those between or above. One other explanation might be that small changes in climate or soil moisture budget occurred, influencing the degree of nodule development. CONCLUSIONS (1) There are three main calcrete palaeosol units in the Llanelly Formation: the Tyle'r bont Pedocomplex, the Cwm Dyar Pedoderm and the Llanelly Pedocomplex. The two pedocomplexes consist of a number of separate palaeosol 'profiles' whereas the Cwm Dyar Pedoderm consists of a single 'profile'. (2) The two pedocomplexes were deposited on arid or semi-arid floodplains on which depositional rates were very low. The Cwm Dyar Pedoderm was deposited on a floodplain during a subaerial phase during a period of mainly peritidal deposition. (3) The mineralogy of the clays consists mainly of mixed-layer illite-smectites and allowing for illitisation during diagenesis the original clays probably had a high smectite content. (4) The profiles show evidence of carbonate accumulation, pedoturbation and dissolution. (5) The predominance of disorthic nodules in the clay horizons, the evidence of

30 extensive brecciation with the formation of small angular peds and numerous crystallaria, the strongly prismatic structure of some horizons and the occurrence of gilgai-like structures, all suggest that these palaeosols underwent significant pedoturbation and shrinkage during their formation. This process was probably caused by the shrink swell behaviour of the smectite-rich clays. Thus, these palaeosols had a strong vertic component during formation. The importance of the action of the displacive growth of calcite in this pedoturbation is unknown. (6) The carbonate nodules, beds and plates are composed of finely crystalline calcite spar. Much of this is a rhombic spar which is a typical form in the subaerial fresh-water environment. The carbonate may have been both replacive and displacive of the clay matrix and there is circumstantial evidence that the carbonate in part replaced clay. ACKNOWLEDGEMENTS

I especially thank Robert Riding (Cardiff) for his supervision during my work at University College, Cardiff and Dr. M. Bridges (University College, Swansea) provided stimulating discussions on vertisols. I am grateful to Dr. G. Brown (Soil Survey, Rothamstead) for carrying out the clay-mineral analyses and to fan Henderson and Lawrence Badham (Cardiff) for preparing the thin sections. Paula Westall and Eira Parker skilfully typed the manuscript. 1 thank the referees for their helpful suggestions. REFERENCES AI Rawi, G.J., Sys, C. and Laruelle, J., 1968. Pedogenic evolution of the soils of the Mesopotamian flood plain. Pedologie, 18: 63-109. Allen, J.R.L., 1973. Compressional structures (patterned ground) in Devonian pedogenic limestones. Nature (London) Phys. Sci., 243: 84-86. Allen, J.R.L., 1974a. Sedimentology of the Old Red Sandstone (Siluro Devonian) in the Clee Hill area, Shropshire, England. Sediment. Geol., 12: 73-167. Allen, J.R.L., 1974b. Studies in fluviatile sedimentation: implications of pedogenic carbonate units, Lower Old Red Sandstone, Anglo-Welsh outcrop. Geol. J., 9: 181-208. Allen, J.R.L. and Williams, B.P.J., 1979. Interfluvial drainage on Siluro-Devonian alluvial plains in Wales and the Welsh Borders. J. Geol. Soc. London, 136: 361-366. Bal, L., 1975a. Carbonate in soils: a theoretical consideration on, and proposal for its fabric analysis. I. Crystic, calcic and fibrous plasmic fabric. Neth. J. Agric. Sci., 23: 18-35. Bal, L., 1975b. Carbonate in soil: a theoretical consideration on, and proposal for its fabric analysis. 2. Crystal tubes, inercalary crystals and K fabric. Neth. J. Agric. Sci., 23: 163-176, Birkeland, P.W., 1974. Pedology, Weathering and Geomorphological Research. Oxford University Press, New York, N.Y., 285 pp. Blokhuis, W.A., Pape, Th. and Slager, S., 1968. Morphology and distribution of pedogenic carbonate in some Vertisols of the Sudan. Geoderma, 2:173 200. Braunagel, L.H. and Stanley, K.O., 1977. Origin of variegated red beds in the Cathedral Bluffs Tongue of the Wasatch Formation (Eocene), Wyoming. J. Sediment. Petrol., 47:1201 1219.

31 Brewer, R., 1964. Fabric and Mineral Analysis of Soils. Wiley, New York, N.Y., 470 pp. Brewer, R., 1972. The basis of interpretation of soil micromorphological data. Geoderma, 8:81-94. Brewer, R., Cook, K.A.W. and Speight, J.G., 1970. Proposal for soil stratigraphic units in the Australian Stratigraphic Code. J. Geol. Soc. Aust., 17: 103-109. BridgeS, E.M., 1978. World Soils. Cambridge University Press, Cambridge, 128 pp. Buol, S.W., Hole, F.D. and McCraken, R.J., 1973. Soil Genesis and Classification. Iowa State University Press, Ames, Iowa, 360 pp. Chafetz, H.S. and Butler, J.C., 1980. Petrology of recent caliche pisolites, spherulites, and speleothem deposits from central Texas. Sedimentology, 27: 497-518. Fink, J., 1976. Mitteilungen der Kommission fur Quatar Forschung der Osterreichischen Akademie der Wissenschaften. 1: 65-71. Finkl, C.W., 1980. Stratigraphic principles and practices as related to soil mantles. Catena, 7: 169-194. Fitzpatrick, E.A., 1971. Pedology: a Systematic Approach to Soil Science. Oliver and Boyd, Edinburgh, 306 pp. Folk, R.L., 1959. Practical petrographic classification of limestones. Bull. Am. Assoc. Pet. Geol., 43: 1-38.

Folk, R.L., 1971. Caliche nodule composed of calcite rhombs. In: O.P. Bricker (Editors), Carbonate Cements. John Hopkins Univ., Studies in Geology, 19: 167-168. Folk,R.L., 1974. The natural history of crystalline calcium carbonate: effect of magnesian content and salinity. J. Sediment. Petrol., 44: 40-53. Folk, R.L., 1976. Reddening of desert sands, Simpson Desert, Northern Territory, Australia. J. Sediment. Petrol., 46: 604-615. Folk, R.L. and Land, L.S., 1975. M g / C a ratio and salinity: two controls over crystallization of dolomite. Bull. Am. Assoc. Pet. Geol., 59: 60-68. Freyet, P., 1971. Pal6osols residuels et pal6osols alluviaux hydromorphes associ~s aux depots fluviatiles dans le Cr6tace Supbrieur et l'Eocene basal du Languedoc. Rev. Geogr. Phys. Geol. Dyn., 13: 245-268. Gile, L.H., 1967. Soils on an ancient basin floor near Las Cruces, New Mexico. Soil. Sci., 103: 265-278. Gile, L.H., 1970. Soils of the Rio Grande Valley Border in southern New Mexico. Proc. Soil Sci. Soc. Am., 34: 466-472. Gile, L.H. and Hawley, J.H., 1966. Periodic sedimentation and soil formation on an alluvial-fan piedmont in southern New Mexico. Proc. Soil Sci. Soc. Am., 30: 261-268. Gile, L.H., Peterson, F.F. and Grossman, R.B., 1965. The K horizon: a master soil horizon of carbonate accumulation in desert soils. Soil Sci., 99: 74-82. Gile, L.H., Peterson, F.F. and Grossman, R.B., 1966. Morphological and genetic sequences of carbonate accumulation in desert soils. Soil Sci., 100: 347-360. Gill, W.D., Khalaf, F.I. and Massoud, M.S., 1977. Clay minerals as an index of the degree of metamorphism of the carbonate and terrigenous rocks of the South Wales Coal Field. Sedimentology, 24: 675-691. Goudie, A.S., 1973. Duricrusts in Tropical and Subtropical Landscapes. Clarendon Press, Oxford, 174 pp. Hay, R.L. and Reeder, R.J., 1978. Calcretes of Olduvai Gorge and the Ndolanya Beds of northern Tanzania. Sedimentology, 25: 649-673. Hole, F.D., 1961. A classification of pedoturbations and some other processes and factors of soil formation in relation to isotropism and anisotropism. Soil Sci., 91: 375-377. Hubert, J.F., 1978. Palaeosol caliche in the New Haven Arkose, Nework Group, Connecticut. Palaeogeogr., Palaeoclimatol., Palaeoecol., 24:151 - 168. Institute of Geological Sciences, 1976. Annual Report for 1975. Institute of Geological Sciences, London~ 116 pp. Irion, L. and Muller, G., 1968. Mineralogy, petrology and chemical composition of some calcareous tufa

32

from the Schwabische Alb, Germany. In: G. Muller and G.M. Friedman (Editors), Recent Development in Carbonate Sedimentology in Central Europe. Springer, Berlin, pp. 157 171. Jongerius, A., 1970. Some morphological aspects of regrouping phenomena in Dutch Soils. Geoderma, 4: 311-331. Klappa, C.F., 1978. Morphology, Composition and Genesis of Quaternary Calcretes from the Western Mediterranean: a Petrographic Approach. Ph.D. Thesis, Univ. of Liverpool. Klappa, C.F., 1980. Brecciation textures and teepee structures in Quaternary calcrete (caliche) profiles from Eastern Spain: the plant factor in their formation. Geol. J., 15: 81-89. Kubiena, W.L., 1970. Micromorphological features of soil geography. Rutgers Univ. Press, New Brunswick, N.J., 245 pp. Lattman, L.H. and Simonberg, E.M., 1971. Case hardening of carbonate alluvium and colluvium, Spring Mountains, Nevada. J. Sediment. Petrol., 41 : 274-281. Leeder, M.R., 1975. Pedogenic carbonates and flood plain sediment accretion rates: a quantitative model for alluvial arid-zone lithofacies. Geol. Mag., 112:257 270. Leeder, M.R., 1976. Palaeogeographic significance of pedogenic carbonates in the topmost Old Red Sandstone of the Scottish Border Basin. Geol. J., 11:21 28. McBride, E.F., 1974. Significance of colour in red, green, purple, olive, brown and grey beds of the Difunta Group, north eastern Mexico. J. Sediment. Petrol., 44: 760-773. McPherson, J.G., 1979. Calcrete (caliche) palaeosols in fluvial red beds of the Aztec Sihstone (Upper Devonian), southern Victoria Land, Antarctica. Sediment. Geol., 22:267 -285. Netterberg, F., 1967. Some roadmaking properties of South African calcretes. Proc. 4th Reg. Conf. Africa, Soil Mech. Found. Eng., Cape Town, 1:77 81. Perkins, R.D., 1968. Primary rhombic calcite in sedimentary carbonates. J. Sediment. Petrol., 38: 1371-1373. Reeves, Jr. C.C., 1970. Origin, classification and geologic history of caliche on the southern High Plains. Texas, and eastern New Mexico. J. Geol., 78: 352-362. Robert, M.. Tessier, D., lsambert, M. and Baize, D., 1974. Evolution of glauconite and illites. Contribution to the problem of smectites in soils. Trans. 10th Int. Congr. Soil Sci., Moscow, 7 : 9 7 106. Semeniuk, V., 1971. Subaerial leaching in the limestones of the Bowan Park Group (Ordovician) of central western New South Wales. J. Sediment. Petrol., 41:939 950. Soil Survey Staff, 1975. Soil Taxonomy. Agric. Handbook 436, USDA, Washington. Somerville, I.D., 1979. A cyclicity in the early Brigantian (D2) limestones east of the Clwydian Range, North Wales and its use in correlation. Geol. J.. 14: 69-86. Staub, J.R. and Cohen, A.D., 1978. Kaolinite-enrichment beneath coals: a modern analog, Snuggedy Swamp, South Carolina, J. Sediment. Petrol., 48: 203-210. Steel. R.J., 1974 Cornstone (fossil caliche): its origin, stratigraphic and sedimentological importance in the New Red Sandstone, W. Scotland. J. Geol., 82: 351-369. Usdowski, H.E., 1963. Der Rogensten des norddeutschen unteren Bunt sandsteins, ein Kalkoolith des marien Faziesbereichs. Fortschr. Geol. Rheinland Westfalen, 10: 337-342. Van Houten, F.B.. 1973. Origin of redbeds, a review: 1961-1972. Annu. Rev. Earth Planet. Sci., I: 39-61. Velde, B., 1977. Clay and Clay Minerals in Natural and Synthetic Systems. Elsevier, Amsterdam, 218 pp. Walkden. G.M., 1972. The mineralogy and origin of interbedded clay wayboards in the Carboniferous Limestone of Derbyshire Dome. Geol. J., 8: 143-159. Walker, T.R., 1967. Formation of redbeds in modern and ancient deserts. Geol. Soc. Am. Bull.. 78: 353-368. Ward, W.C., 1975. Petrology and diagenesis of carbonate eolianites of north eastern Yucatan Peninsula, Mexico. Am. Assoc. Pet. Geol., Stud. in Geol., 2: 500-571. Watts, N.L., 1977a. Pseudoanticlines and other structures in some calcretes of Botswana and South Africa. Earth Surf. Processes, 2: 63-74.

33 Watts, N.L., 1977b. A Comparative Study of some Quaternary, Permo-Triassic and Siluro-Devonian Calcretes. Ph.D. Thesis, Univ. Reading, Reading. Watts, N.L., 1978. Displacive calcite; evidence from Recent and ancient calcretes. Geology, 6: 699-703. Watts, N.L., 1980. Quaternary pedogenic calcretes from the Khalahari (southern Africa): mineralogy, genesis and diagenesis. Sedimentology, 27: 661-686. West, I.M., 1973. Carbonate cementation of some Pleistocene temperate marine sediments. Sedimentology, 20: 229-249. Wieder, M. and Yaalon, D.H., 1974. Effect of matrix composition on carbonate nodule crystallization. Geoderma, 11 : 95-121. Williams, G.E. and Polach, H.A., 1971. Radiocarbon dating of arid-zone calcareous palaeosols. Geol. Soc. Am. Bull., 82: 3069-3086. Wright, V.P., 1980. Climatic fluctuation in the Lower Carboniferous. Naturwissenschaften, 67: 252-253. Wright, V.P., 1981a. The Stratigraphy and Sedimentology of the Llanelly Formation between Penderyn and Blorenge, South Wales. Unpubl. Ph.D. Thesis, Univ. of Wales, 408 pp. Wright, V.P., 1981b. A subaerial stromatolite from the Lower Carboniferous of South Wales. Geol. Mag., 118: 97-100. Wright, V.P., 1981c. Algal aragonite-encrusted pisoids from a Lower Carboniferous schizohaline lagoon. J. Sediment. Petrol., 51: 479-489. Wright, V.P., in press. Field Meeting: Calcrete-bearing palaeosols in the Lower Carboniferous of South Wales. Welsh Soils Discussion Group, Report No. 22. Wright, V.P., Raven, M. and Burchette, T.P., 1981. Field Guide to the Lower Carboniferous Rocks near Abergavenny. Dept. of Geology, Univ. Coll. Cardiff, 76 pp. Yaalon, D.H. and Kalmar, D., 1978. Dynamics of cracking and swelling clay soils: displacement of skeletal grains, optimum depth of slickensides, and rate of intra-pedonic turbation. Earth Surf. Processes, 3: 31-42.