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The cherts in the Hythe Beds (Lower Cretaceous) of South-east England
The cherts in the Hythe Beds (Lower Cretaceous) of South-east England FRANK A. MIDDLEMISS
MIDDLEMISS, F. A. 1978. The cherts in the Hythe Beds (lower Cretaceous) of South-east England. Proc. Geol. Ass., 89(2) pp. 283-298. The cherts in The Hythe Beds (Lower Greensand, Aptian) of the western part of the Weald, southern England, have been re-examined. The cherts vary among themselves. Some possess petrographic characters which show them to have been formed by silicification of calcareous rocks and, contrary to all previous authors, it is claimed that replacement cherts are present in these beds. Sponge spicules are invariably present and mostofthe silica in the cherts is thought to have been, at one stage ofits history, incorporated into the spicules and to have been subsequently redistributed diagenetically. Some ofthe cherts are sponge-spicule beds, as described by Hinde in the nineteenth century. The silica is thought to have been derived ultimately from tropical weathering of rocks exposed on the London land area. Depanment of Geology, Queen Mary College (University of London), Mile End Road, London, E1 4NS.
CONTENTS page
1. 2. 3. 4. 5.
INTRODUCTION . FIELD EVIDENCE . PETROGRAPHIC EVIDENCE ... DISCUSSION... CONCLUSIONS REFERENCES
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1. INTRODUCTION
The Hythe Beds (Aptian), a subdivision of the Lower Greensand formation, outcrop all round the Weald in southern England (Fig. 1). In the east of the Weald, approximately east of a line joining Westerham and Pulborough, they consist essentially of calcareous sands and sandy limestones, with some chert. West of this line they are almost entirely non-calcareous sands and sandstones with considerable local developments of lenticular chert. It has been generally accepted that the cherts in the eastern Hythe Beds owe their existence to replacement of limestone, or the calcareous matrix of sandstone, by silica. Humphries (1957, 299) believed the silica to be derived from sponge spicules or other siliceous bodies in the sands. Those in the western Hythe Beds have been discussed in some detail by four authors: Hinde, Hill, Smith and Humphries. Hinde (1886) was the first to propose a theory of their formation. As a sponge specialist he was attracted by the abundant sponge spicules which occur, sometimes in thin, closely packed spicule beds, and he regarded the sponge spicules as the source of the silica in the cherts. He noted the presence of porous, spicular cherty sandstones which usually occur in association with chert, although they also occur without such association, and regarded these two rock types as different stages in one process: the spicules were dissolved, leaving the 283
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Fig. 1. Locality map of the western Weald. Stipple: rocks younger than Lower Greensand. Blank: Lower Greensand. Vertical Lines: rocks older than Lower Greensand.
porous sandstone, and the resulting silica migrated in solution either to be precipitated immediately as a core of more solid chert or to be carried further by percolating water and precipitated in similar fashion elsewhere. Hill (1911), again, thought the silica had been supplied by sponge spicules and regarded the cherts as essentially spicule-beds impregnated by free precipitation of silica in the interstices of the rock. Hinde's ideas have received adverse criticism, first from Tarr (1926) who developed, mainly from his studies of the Mississippian Burlington Limestone in the U.S.A. (Tarr, 1917), a concept of primary deposition of silica to form chert, denying that sponge spicules played any essential part in the process. Tarr, in fact, made no direct reference to the Lower Greensand, but Smith (1950) believed that the Hythe Beds chert originated as silica gel directly and inorganically precipitated on the sea floor, in the manner advocated by Tarr, the purer chert representing a phase of maximum precipitation in the earlier and closing stages of which sand grains and silica gel were deposited simultaneously, giving cherty sandstone. The association of sponge spicules with the chert was explained by postulating that the sponges flourished in the silica-rich waters before the maximum of precipitation and also after precipitation had finished and the mass of gel was buried under a cover of sand but still available as a source of silica. The abundant moulds and casts of spicules in sandstones adjacent to the chert were due to migration of silica towards the buried mass long after deposition. Humphries (1957), as far as the Hythe Beds of the western Weald were concerned, strongly advocated the views of Tarr and put forward a theory essentially similar to Smith's, allowing no special importance to sponge spicules in the formation of the chert. He summarised the main criticisms of Hinde's ideas: (a) according to Hinde's theory, the true cherts should be spicule beds whereas Humphries believed that they often contaiQed few or no sponge spicules; (b) many of the cavities in the sandy cherts, regarded as moulds of sponge spicules by Hinde, were not of that origin; (c) where hollow moulds of sponge spicules did occur, their walls were formed of chert and the cast of the axial canal, frequently present traversing the centre of the mould cavity, was itself often made of chert, thus the chert had been already present before the spicule was dissolved and was not formed from the dissolved silica of the sponge spicule.
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Humphries' summary of evidence for the primary origin of the chert needs to be quoted in extenso: (a) chert is involve~ in cont~mporaneous slumping and therefore must be syngenetic; (b) chert transgressmg beddmg planes, and hence clearly secondary, is almost absent; (c) no cherts occur in the calcareous Hythe Beds of the south-east Weald, east of Pulborough in spite of the presence in them of siliceous sponge spicules; , (d) colloform structures, indicative of a colloidal origin, are commonly present in the true cherts; (e) sponge spicules are not always present in the cherts; (f) where empty moulds of spicules are present in the cherts they are often perfectly preserved in a siliceous matrix (see above); (g) the sand grains in the cherts are usually 'floating' (Le. not in contact with one another; Humphries shows this by a numerical analysis); this must be due to a primary matrix, which point (d) above shows to have been silica; (h) calcite and pseudomorphs after calcite are completely absent from the cherts, cherty sandstones and sandstones of the Hythe Beds in the western Weald, indicating that the formation of chert is not a result of the replacement of calcium carbonate by silica. Richardson (1947) described the formation of chert in the Bargate Beds, the subdivision of the Lower Greensand which overlies the Hythe Beds in the western Weald. The Bargate Beds include much calcareous material and, in contrast to Smith and Humphries, Richardson claimed a major role for silicification oflimestone in this case. He distinguished two secondary processes, decalcification and silicification, which had taken place sometimes separately and sometimes in association. Decalcification alone converted the limestone to loose ferruginous sand. Where silicification accompanied decalcification the two processes were 'almost simultaneous', but three stages could be recognised: (a) the calcareous matrix and the infillings of foraminiferal chambers began to be silicified, (b) the matrix and some of the organic fragments were silicified, (c) the organic debris was completely silicified. Richardson pointed out that sedimentary laminae, including those of cross bedding, could be seen to pass from the surrounding sand into and through the chert lenses, just as they did into and through the neighbouring calcareous lenses, showing that both had formed secondarily. Hill, Smith and Humphries all specifically rejected the possibility of the Hythe Beds chert in the western Weald being formed by replacement of limestone on the grounds of rarity or absence of pseudomorphs after calcite. Smith observed that silicified calcareous organisms were occasionally present in the chert, especially fragments of foraminifera, but claimed that they were rare. Humphries stated that calcite and pseudomorphs after calcite were completely absent and above all, like Hill, claimed that there was no evidence for the presence of limestone in association with the chert of the Hythe Beds in the western Weald, so that replacement mechanisms could not be invoked. It is upon this last argument that his dismissal of replacement really depends. If it could be demonstrated that these beds had formerly been more calcareous than they are now, many of his points in favour of primary origin of the chert could be used equally well in favour of a secondary origin. Evidence will be presented in the following pages for the view that there may be more than one mode of formation for these cherts, and that these modes may be complex, but that replacement of limestone has played a larger part than hitherto recognised.
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2. FIELD EVIDENCE
In fact there are, even at present, several occurrences of limestone in the western Hythe Beds. Humphries himself describes some (not, indeed, in direct association with chert) between Fittleworth and Petersfield (Humphries 1953; 1964). Hayward (1932) describes lenticular calcareous bands near Abinger, much resembling the finer types of the Bargate Beds and containing chalcedony which could be replacive. At Tilburstow Hill is a limestone bed, no longer exposed but described by Hinde (1886), which contains sponge spicules replaced by calcite as in the 'calcareous' facies of the eastern Weald; this limestone is also referred to by Whitaker & Jukes-Brown (1894) and Cox (1915) and one of Hinde's thin sections of it is preserved in the British Museum (Natural History) (S 960). In the Bramshott area there is much limestone (Knowles & Middlemiss, 1958,213), associated with chert and in some cases showing the same silicification phenomena as Richardson described. These Bramshott localities, classified as Hythe Beds by Knowles & Middlemiss, were mapped as Bargate Beds by the Geological Survey, for reasons not made clear (Thurrell, Worssam & Edmonds, 1968, 68); the distinction is irrelevant in the present context as the sediments themselves are indistinguishable from the Hythe Beds and palaeontological evidence of age is lacking. In addition, throughout the western Weald the Hythe Beds contain much of the kind of evidence for decalcification which Richardson cited in the Bargate Beds. This is best shown by the phenomena of 'ovoid ferruginous cavities' and 'ferruginous sand lenses' (Knowles & Middlemiss, 1958). The former are ovoid cavities in sandstone beds, usually 0·3 to 1·0 m in length and up to O' 3 m in height and lined with deeply ferruginous sand, sometimes arranged in lines parallel to the bedding. The latter are ovoid lenticles of deeply ferruginous sand, of the same size and shape as the 'ovoid cavities' and showing the same arrangement, but set in unconsolidated sands. These appear to represent stages in the destruction by decalcification of small calcareous doggers. As the dogger becomes decalcified it turns into an ovoid mass of loose highly ferruginous sand with the liberation of ferric iron from the iron-bearing calcite. In unconsolidated sands this remains as a 'ferruginous lens' but in a sandstone the difference in resistance to weathering between the decalcified lens and the surrounding stone results in the sand being cleared out to leave a sand-lined 'ovoid ferruginous cavity'. The link in this sequence is supplied by some exposures near Bramshott, especially just west ofthe church (Nat. Grid Ref. SU 840329), where ferruginous lenses can be seen which still retain in the centre a remnant of highly calcareous stone, exactly comparable with that of the fully calcareous doggers of the same neighbourhood; the decakified material is silty sand typical of the surrounding beds but is unusually ferruginous, while the calcareous stone at the centre shows some incipient silicification. An earlier stage is seen at Sheet (SU 763244) in beds approximately equivalent to those at Bramshott, where small rounded calcareous doggers are set in ruddy sands; these also show incipient silicification. The association of ferruginous staining with decalcification is well-known in the calcareous Hythe Beds of the eastern Weald and in the Bargate Beds. The iron-bearing nature of much of the calcite in these rocks is demonstrated by staining techniques. Thurrell & others (1968, 83), with reference to the ferruginous sand lenses of Cold Ash Hill, Bramshott (SU 849327), said that 'it seems unwise to assume a single process of development'. Two other processes which may be considered, as alternatives to decalcification, are decomposition of pyrite and upward or downward percolation of iron-bearing ground-water blocked by an impervious medium. Pyrite certainly occurs in the Hythe Beds both as nodules (Smith, 1957) and as fine granules (microframboids) scattered in the cherts, but never on a large enough scale to give rise to the
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phenomena under discussion. The second process has taken place in the Folkstone Beds of the Lower Greensand, where iron staining is sometimes concentrated along the base of a clay seam. An example in which chert has possibly acted as the impervious medium is at Woolmer Lodge Lane, Bramshott (SU 841336), where the sand for about 5 cm beneath a band of solid chert is deeply ferruginous. This process, however, is unlikely to give rise to discrete ferruginous sand lenses or ovoid ferruginous cavities. The amount of ferric iron required to give a deeply ferruginous stain to the sand is quite small; analysis of deep rusty-coloured decalcified Bargate Beds from Richardson's type locality (Stock Farm, Churt, SU 877382) gave a total iron content of 3·29 per cent. This field evidence shows that the Hythe Beds of the western Weald (a) are not entirely non-ealcareous at present and (b) have been more calcareous in the past, thus there was formerly calcareous material available for silicification. The main evidence that such silicification has in fact taken place comes, however, from study of the cherts in thin section. 3. PETROGRAPHIC EVIDENCE (a) Characteristics of the replacement cherts The type of chert which is produced by silicification of sandy bioclastic limestones and calcareous sandstones like those ofthe Hythe and Bargate Beds is clearly shown by examples from Richardson's type locality of Stock Farm and many similar exposures of the Bargate Beds to the north of Haslemere. These have, in thin section, the following characters: (i) The general matrix is microcrystalline quartz (Folk & Weaver, 1952) in which the clastic quartz grains are usually 'floating'. The abundance of clastic quartz grains is extremely variable. The quartz grains may show corroded margins, as do those in the limestones (Plate 1, A & B). Progressive silicification can be shown in many examples of mixed calcareous-cherty rocks. Series of sections can easily be cut from one 20-em block of rock which is chert at one end and limestone at the other. These show the usual order of silicification described by both Raisin (1903) and Richardson: firstly sporadic replacement of the matrix calcite by microquartz; secondly complete replacement of the matrix calcite and of some organic fragments; thirdly complete replacement of the organic fragments. The last to be silicified are calcareous fragments showing clear organic microstructure, especially those of echinoid plates and spines and of bryozoa. Colloform structures are abundant in the matrix. Humphries (1957,306,308) treats these as diagnostic of primarily deposited silica, but here they are in undoubted replacement cherts. Their presence was recorded by Richardson (1947,168). Raisin (1903), in her paper on cherts in the Upper Jurassic Portland Beds, recorded colloform microquartz (although the term 'colloform' had not then been invented (Rogers, 1917» growing into calcareous ooliths during silicification of limestone (Raisin, 1903, plate 14, fig. 1). In the same way, in the Bargate Beds, colloform structures can be seen bulging into calcite matrix crystals, into patches of limonite and into aggregates of fan-acicular chalcedony. The colloform structures can sometimes be seen to be based upon pieces of silicified skeletal material, and to be apparently growing out from these into the pre-existing matrix. (ii) The calcareous rocks are bioclastic, containing abundant fragments of echinoid plates and spines, Bryozoa and foraminifera, of shells of brachiopods, bivalve molluscs and ostracods and spicules of siliceous sponges. All this material is silicified in the cherts. Silicification involves recrystallisation which often obscures or obliterates the outlines and the microtextures of the organic fragments. Because of this, the echinoid fragments provide the most useful evidence as
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(a) they are abundant and .(b) their ste.reo.m structure gives them a highly dis~i?~t~ve microtexture which often survIves recrystallIsatlOn (Plate 1 A & B; Plate 2 A-e). The sIlIcIfIed skeletal fragments in the cherts are normally, to a varying extent, impregnated with limonite. In some cases the skeletal fragments appear to have been completely limonitised and have then been able to resist silicification. Much more commonly the fragments are silicified and are represented by areas of especially fine-grained microquartz, which appear in oblique light as formless, wispy cream- or rust-coloured patches, Le. they are composed of microcrystalline silica impregnated with fine amorphous limonite (Plate 1 C & D, Plate 2 A-e). The sIs:eletal material is represented by fine-grained microquartz while the pores of the stereom are often filled with slightly coarser microquartz. In some cases the amorphous limonitic material is concentrated in the pores, in others the sites of both pores and skeletal material are equally impregnated. In other cases again the stereom pores are infilled with glauconite. Detection of the echinoderm origin of these formless, limonite-impregnated fragments in the cherts is difficult, requiring a combination of transmitted and oblique light and frequent comparison of plane-polarised light with cross-polarised light under a good quality microscope (Plate 2 A-e). In the calcareous rock skeletal fragments are often found to be 'surrounded by secondary overgrowths in the form of scalenohedra or acute rhombohedra of calcite. In the cherts these are not uncommon, with the acute-angled crystal forms perfectly pseudomorphed in silica, or more rarely in limonite or limonite-impregnated silica. Examples of these were figured by Richardson (1947, fig. 11, ii-vi). The bryozoan fragments are also easily recognisable but are much less common than echinoderm fragments. Similar remarks apply to them, except that they are more frequently heavily limonitised in the cherts (Plate 1 C & D). (b) The cherts of the Hythe Beds The cherts of the Hythe Beds in the western Weald are more varied than those of the Bargate Beds but fall into two main types, which grade into each other by way of intermediate examples. These can for convenience be called (i) replacement cherts and (ii) spicule-be,d cherts. These divisions cut across the classifications based upon density of clastic quartz grains which previous authors have used. Hinde (1886) and Smith (1950) distinguished cherty sandstones from cherts; Humphries (1957) recognised true chert, sandy chert and cherty sandstone. These are real distinctions, of course, but 'true chert' may be either of the types given above, or an intermediate stage, and can pass into sandy chert and cherty sandstone by increase in clastic grains.
Plate 1. A & B. Silicified fragment of echinoid test in a replacement chert. The fragment is pseudomorphed in very fine-grained microquartz. A: plane polarised light; the stereom is clearly visible. B: crossed pol. Hythe Beds, Weysprings, Haslemere. Corroded grains are visible. C & D. Typical replacement chert with a bryozoan fragment. Hythe Beds, Highfield, Thursley. C: plane polarised light. D: crossed pol. Light-brown limonitised areas in C, representing silicified organic fragments, can he seen to correspond to areas of fine-grained microquartz in D, as in Plate 2, A--e. Plate 2 A, B & C. Replacement chert. Hythe Beds, Hyde Hill, Churt. A: Plane polarised light. B: Crossed pol. C: Oblique reflected light. Limonitised silicified echinoid material at E is detectable only under oblique light. Areas of light-brown limonitised material in A and C correspond to areas of very fine-grained microquartz seen in B. Sponge spicules are also visible. D. A spicule-bed cherty sandstone, showing rim-lined spicule moulds. Hythe Beds, Combe Hill, Rogate. Crossed pol.
PROe. GEOL. ASS., VOL. 89 (1978) PLATE 1
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PROC. GEOL. ASS., VOL. 89-(1978) PLATE 2
PROC. GEOL. ASS., VOL. 89 (1978) PLATE 3
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PROC. GEOL. ASS., VOL. 89 (1978) PLATE 4
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(i) Replacement cherts. These have precisely the same hand-specimen and petrographic characteristics as those of the Bargate Beds. As replacement cherts have not previously been described from the western Hythe Beds a schedule is given here of the specimens used in preparing this paper: SU 841336 Woolmer Lodge Lane, Bramshott. In strata mapped as near the top ofthe Hythe Beds by Knowles & Middlemiss (1958), as near the base of the Bargate Beds by the Geological Survey. SU 895378 Highfield, Thursley. Near the top of the Hythe Beds (Plate 1 C & D). SU 884379 Hyde Hill, Churt. Near the top of the Hythe Beds (Plate 2 A-e). SU 878329 Critchmere, Haslemere. Groups 3 and 5 of Knowles & Middlemiss, 1958,209-10. Middle and lower parts of the Hythe Beds. Group 5 is strongly ferruginous in the field. SU 908323 Haste Hill, Haslemere. Middle of the Hythe Beds (Plate 3 B & C). SU 889333 Weysprings, Haslemere. Low in the Hythe Beds (Plate 1 A & B). Fittleworth, precise locality unknown. Hythe Beds. This type of chert therefore occurs all through the Hythe Beds except in the extreme basal part. (ii) Spicule-bed cherts. This is the type of chert accurately described by Hinde (1886) and Hill (1911). Both characterise their 'true cherts' as packed with sponge spicules, largely to the exclusion of clastic quartz grains and of other organic fragments, and as passing into sandy chert and cherty sandstone by increasing admixture of clastic quartz with the spicules. Both note also that in the 'true chert' the outlines of the spicules tend to be obscured by recrystallisation into a mass of chalcedony and microquartz but that a clue to the spicular origin of the mass usually remains in the form of casts of the axial canals. This type of chert is common in the western Hythe Beds. The essential character is the dominance of sponge spicules and rarity or absence of other organic fragments. The abundance of clastic quartz is very variable; only rarely is the chert so free of clastic quartz as to be a 'true chert' in Hinde's sense. Some noteworthy examples used in the present work came from: Combe Hill, Rogate (Plate 2 D) and Rad Lane, Abinger. Highly spicular cherty sandstones which appear to have been originally kaolinitic sandstones poor in calcareous debris but with the addition of great quantities of sponge spicules. The microquartz is of the 'rim-lining' type, apparently seeded on sponge spicules which are now represented mainly by empty moulds, or more rarely on clastic quartz grains. That some calcareous material was formerly present in rocks of this type is suggested by the common occurrence of corroded clastic quartz grains and the rare occurrence of silicified scalenohedral overgrowths. Plate 3. A. Spicule-bed chert, with spicules in various states of preservation. Hythe Beds, Inval, Haslemere. Plane polarised light. B & C. Tetraxon sponge spicule in a replacement chert. The outline of the spicule and of the axial canal is picked out by rim-lining chalcedony, while the body of the spicule is represented by fan-acicular chalcedony. Hythe Beds, Haste Hill, Haslemere. B: plane polarised light. C: Crossed pol. Plate 4. A. Spicule-bed chert, with spicules in various states of preservation. Hythe Beds, Inval, Haslemere. Plane polarised light. B & C. A spicule-bed chert, with sponge spicules in various orientations and states of preservation. Hythe Beds, Rutton Hill, Hindhead. B: plane polarised light. C: Crossed pol. 4
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Brook Turning, Portsmouth Road, Hindhead. This is similar to (a) but contains more shell debris and is more heavily limonitised. The formerly calcareous fragments (ostracod and lamellibranch shell, foraminifera) have been so strongly limonitised as to withstand silicification. Even the scalenohedral overgrowths are limonitised, not silicified. The microquartz seems all to be recrystallised sponge spicules. Inval, Haslemere (Plates 3 A, 4 A). Cherts from this locality vary in the amount of clastic quartz present, from 'sandy chert' to 'true chert', but are full of completely recrystaUised sponge spicules, many of them clearly shown by their axial canals which are infilled with limonite. They show no sign of formerly calcareous organic debris. Colloform structures are frequently present. Sailors' Road, Thursley. Essentially highly spiculiferous sandstone with patchy chertification. The microquartz is largely of the 'rim-lining' type, the spicules upon which the growth of secondary silica was seeded being now represented by empty moulds, but recrystallised spicules are also present. Some calcareous skeletal material was formerly present in this rock as there are spaces present from which scalenohedral overgrowths protrude, the overgrowths being both silicified and limonitised. There is not now any sign of calcareous skeletal fragments, those formerly present being now represented by 'spaces with scalenohedra'. Rutton Hill, Hindhead (Plate 4 B & C). A good example of cored chert, like those described by Hinde and Humphries, in which 'solid' or 'pure' chert, with few, small and scattered quartz grains, grades peripherally into cherty sandstone. The cherty sandstone contains abundant recrystallised sponge spicules, induding some good examples of spinulate form. In the 'pure' chert spicules are equally abundant but more difficult to see. In this case, for some reason, recrystallization of the spicules has involved loss of the axial canal; this is so even in the cherty sandstone, where the shapes of spicules are relatively easy to see. In the true chert the infilled axial canals, which demonstrate the abundance of spicules in the true chert from Inval (above), are absent and thus the rock at first sight appears a structureless mass of microquartz, although on detailed study the characteristic alignments of the microquartz crystallites marking the shapes of spicules, can be detected. Silicified shell material is present but very rare. Hinde's thin section of 'chert with spicules' from Petworth (presumably Hythe Beds), figured by Hinde 1886, plate 45, fig. 17 (British Museum (N.H.), no. S 962). This specimen illustrates very well both Hinde's and Hill's descriptions of 'true chert'. Clastic quartz grains are small and scattered, but the rock is highly spicular. On the other hand there is plenty of evidence of echinoderm material (some recognisable only by oblique light) which in this case appears to be limonitised rather than silicified. Hill's specimen from Raike's Lane, near Gomshall, upon which all his ideas of the genesis of the Hythe Beds chert seem to have been based (Hill, 1911, Plate 13, fig. 2), was clearly of this spicule-bed type. 4. DISCUSSION (a) Arguments against the presence of replacement cherts The main reasons given by previous authors for dismissing the possibility of replacement chert in the western Hythe Beds may now be considered.
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(i) Absence ofcalcareous material to be replaced means that replacement is impossible. This has been discussed above. (ii) Floating sand grains imply a siliceous primary matrix. They obviously imply the presence of a primary matrix to keep the sand grains apart. They make no implication as to the nature of that matrix. To deduce from the floating nature of the grains that the primary matrix was of primarily deposited silica is to make a prior assumption regarding point (i). The primary matrix could equally well have been mainly calcareous, or mainly composed of silica in the form of sponge spicules. (iii) Colloform structures show that the primary matrix was of primary silica. Colloform structures are very clearly present where a calcareous matrix is in process of being silicified, as in the Bargate Beds at Stock Farm and the rocks from Woolmer Lodge Lane, Bramshott. The replacing microquartz advances on curving 'colloform' fronts into the matrix calcite as well as into cavities. (iv) Cherts are involved in contemporaneous slumping and therefore must have been present at a pre-diagenetic stage of the rock. Humphries (1957, 300) based this point upon a beautiful example at Combe Hill, Rogate (SU 801258), where a mass of sandy, spicular chert (spicule-bed chert-see Plate 2 D) is buckled parallel to slumped foreset beds of large-scale cross bedding in which it is included. Clearly the material which is now chert was present in the beds before diagenesis. This does not mean that the material was then chert or even silica gel. It was material which has since been transformed into chert. (v) Chert veins transgressing bedding planes are unknown. This seems to be true, although small chert veins traversing individual beds at right-angles to the bedding planes occur. (vi) Calcite and pseudomorphs after calcite are rare (Smith) or absent (Humphries). In the case of some of the cherts, especially those regarded here as due to replacement, this is quite untrue. They are full of organic fragments, originally of calcium carbonate, now pseudomorphed in silica. Humphries (1957, 303), although denying the presence of pseudomorphs after calcite, mentions the presence of organic fragments in one of his cherts, saying 'they appear to be mainly bryozoans'. Silicified bryozoans are certainly present in the replacement cherts (Plate 1 C & D) but in my experience pieces of echinoid test are much the commonest of the recognisable fragments. The presence of silicified shell material in the cherts does not in itself necessarily prove that the rock is a silicified limestone. Tarr's (1917) description of the cherts of the Burlington Limestone makes a convincing case for regarding these as primary precipitates yet they contain silicified, formerly calcareous, organic fragments which Tarr considers were washed into the soft silica gel soon after it was deposited. The fragments are discrete, clearly delimited and are distinct from the matrix-'Silicified forms are always chalcedony or quartz (more frequently the latter) ... The surrounding crypto-erystalline material of the chert or flint is entirely different' (Tarr, 1926, 39). In contrast to this, the silicified fragments in the Hythe cherts (i) are abundant and varied: (ii) usually grade into the surrounding matrix; fragments of silicified echinoid test showing stereom have no distinct boundary but are composed of finegrained microquartz which merges into the surrounding matrix microquartz exactly as in the limestones the recrystallising fragments of stereom grade into the surrounding sparry calcite matrix: (iii) often have silicified scalenohedral overgrowths which must have been originally scalenohedral calcite: (iv) are usually accompanied by corroded clastic quartz grains which must
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have been corroded in a calcareous environment (Dapples, 1967, 118). These last two features are again exactly as seen in the unsilicified limestones and must mean that these particular cherts have passed through a calcareous stage. (b) Sponge spicules and the cherts Hinde and Hill both believed that sponge spicules were the main source of the silica for the cherts. Later authors, especially Richardson (1947) and Humphries (1957) have quoted the authority of Tarr (1917; 1926) in refuting this. Richardson even implied that rarr had reexamined Hinde's thin sections of Lower Greensand material and found spicules to be absent or unimportant. This is totally untrue and appears to have been based on a misleading passage in Twenhofel (1932, 540). Nowhere does Tarr say that he has seen Hinde's original materials. He claims (Tarr, 1926, 8-9) to have studied thin sections of Carboniferous cherts similar to those used by Hinde and also to have seen a slide of Upper Greensand from Godstone, Surrey, but does not say that it was one of Hinde's. As a result of his studies Tarr did reject the importance of spicules in chert formation generally but he nowhere mentions the Lower Greensand and shows no sign of having been aware of its existence. A number of Hinde's original thin sections are kept in the British Museum (Natural History), where I have examined them. There is no doubt of the abundance of sponge spicules in them. Hinde was the leading authority on sponges of his time; to suggest as Humphries does (1957, 309) that he might not have known spicules when he saw them, or imagined that he saw spicules which were not there, is unconvincing. Sponge spicules have been present in every thin section of chert from the Hythe Beds that I have ever examined: they are often abundant and may make up the bulk of the rock. They are preserved in various ways: (i) Commonly the original opal is replaced by a microquartz mosaic, due probably to recrystallisation. The axial canal mayor may not be marked out by a differentinfill such as limonite or glauconite. (ii) Frequently a thin coating of fine fibrous chalcedonic crystals is seeded on to the periphery of the spicule and more rarely on to the walls of the axial canal. This effect is here called 'rim-lining'. Subsequently the spicule itself may be dissolved leaving a rim-lined mould (Plate 2 D). Rim-lined cavities not showing recognisable shapes are also common; they seem to represent moulds of broken fragments of spicules. Further, the empty mould may subsequently become infilled either with relatively coarse quartz mosaic or with fanacicular chalcedony (Plate 3 B & C). (iii) More rarely a spicule may be obliterated by being completely replaced by colloform growths of microquartz. One of Hinde's thin sections in the British Museum, actually from the eastern Weald (S 958, Hythe Beds, Sevenoaks, Kent), shows this. In this case the axial canals, infilled with limonite, mark the former presence of the spicules, but it is conceivable that this type of replacement could have occurred in some cherts where the axial canals did not happen to be infilled with distinctive material; an originally highly spiculiferous rock could then appear to be completely lacking in spicules. (c) Source of the silica The constant presence and usual abundance of siliceous sponge spicules in the cherts under discussion makes it a reasonable assumption that they have played a large part in the formation
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of the cherts. The processes by which the silica has been redistributed are complex, judging by the variety of replacement phenomena present, and any simple hypothesis such as that of Hinde must be misleading. The spicules themselves show evidence that they have in many cases been dissolved out, and their silica transported away, and then replaced by silica from elsewhere, perhaps more than once. In the case of the spicule-bed type of cherts, Hill was probably right in thinking that cementation was mainly due to redistribution of the silica originally present in opaline form in the spicules. In the case of the replacement cherts, my own conclusion is that the abundance of spicules was underestimated by Richardson, Smith and Humphries and that they probably would have been adequate to provide for the silicification of the calcareous matter; this conclusion is similar to that of Wilson (1966) for the Portland Beds. To claim that sponge spicules were the main contributors of silica to the cherts is, of course, to beg the question of the source of the silica. My postulation is that most of the silica present in the cherts has passed through a stage of being the skeletal silica of sponges but, as both Smith and Humphries have pointed out, the sponges would not have been there in Aptian times unless an abundance of silica had been available. There are no sponges and no cherts in the contemporaneous Lower Greensand of the Isle of Wight. I have elsewhere (Middlemiss, 1976) discussed the provenance of silica in the Lower Greensand. The absence of sponges and of cherts from the Lower Greensand of the Isle of Wight is one of several differences from that of the Weald which suggest deposition in a separate basin of sedimentation, into which silica (other than clastic quartz) was not being transported. In the Weald, cherts in the Hythe Beds are particularly characteristic of the northern and western outcrops. In the southern outcrops sponge spicules are present in a calcareous matrix but there are no cherts: to Humphries this is one reason for his rejection of the importance of spicules in chert formation. The spicules are either calcitised or replaced by microquartz and in the latter case are accompanied by signs of small-scale silicification of the calcareous matrix. These same phenomena are commonly seen in the calcareous Hythe Beds ofthe north-eastern Weald but there the process goes further, with the production of cherts. In other words, the absence of cherts from the southern outcrop is probably due to the chertification process being less well developed there and this in turn is probably due to sponge spicules being less abundant. The evidence in the Weald, then, is that in Aptian times silica was being supplied to the basin of sedimentation from the north and north-west and that relatively little was getting through to the south-east Weald. This suggests that its source was the London land area, where a tropical weathering regime was probably in force. Most of the silica was probably utilised directly from the sea water by the sponges, but some may have been deposited in amorphous form among the mainly bioclastic sediment, somewhat as Butler (1975) has described precipitation of amorphous silica from silica-bearing water simultaneously with predominantly calcareous precipitation. In either case the resulting opaline silica in the sediments would have been unstable and mobile during diagenesis would have undergone total transformation by recrystallisation and migration, especially in the presence of abundant calcareous material (Ernst & Calvert, 1969, and references therein). 5. CONCLUSIONS The cherts of the Hythe Beds in the western Weald are of two main types: replacement cherts, which result from silicification of sandy limestone and calcareous sandstone and are characterised by abundance of silicified bioclastic debris, and spicule-bed cherts, originally highly
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spicular sandstones which have been impregnated by silica. There is no clear-cut division between these types; spicule-bed cherts which contain a proportion of formerly calcareous shell-debris grade into replacement cherts. In both types sponge spicules are abundant, although they are more densley packed, to the partial or total exclusion of calcareous shell debris, in the spicule-bed type. The presence of the spicules has played a major part in the formation of the cherts in that all, or most, of the silica has formerly been incorporated in the spicules at one stage, before being diagenetically redistributed. REFERENCES BUTLER, J. C. 1975. Tea kettle carbonates. J. Sed. Pet., 45, 891-3. COX, A. H. 1915. Report of an excursion to Tilburstow Hill and Nutfield. Proc. Geol. Ass., 26, 324--{j. DAPPLES, E. C. 1967. Diagenesis of sandstones. In Larsen, G. & G. V. Chilingar, ed. Diagenesis in Sediments. Developments in Sedimentology, 8, Amsterdam. ERNST, W. G. & S. E. CALVERT. 1969. An experimental study of the recystallisation of porcellanite and its bearings on the origin of some bedded cherts. Amer. J. Sci., Schairer Vol. 267·A, 114-33. FOLK, R. L. & C. E. WEAVER. 1952. A study of the texture and composition of chert. Amer. J. Sci., 250, 498-510. HAYWARD, H. A. 1932. The Geology of the Lower Greensand in the Dorking-Leith Hill district. Proc. Geol. Ass., 43, 1-31. HILL, W. 1911. Flint and chert. Proc. Geol. Ass., 22, 61-94. HINDE, G. J. 1886. On beds of sponge remains in the Lower and Upper Greensand of the south of England. Phil. Trans. R. Soc., 176 (for 1885),403-53. HUMPHRIES, D. W. 1953. The lithology and conditions of deposition of the Lower Greensand of the south-west Weald. Unpublished Ph.D thesis, University of London. HUMPHRIES, D. W. 1957. Chert: its age and origin in the Hythe Beds of the western Weald. Proc. Geol. Ass., 67,296-313. HUMPHRIES, D. W. 1964. The stratigraphy of the Lower Greensand in the south-west Weald. Proc. Geol. Ass., 75,39-59. KNOWLES, L. & F. A. MIDDLEMISS. 1958. The Lower Greensand in the Hindhead area of Surrey and Hampshire. Proc. Geol. Ass., 69, 205-38. MIDDLEMISS, F. A. 1975. Studies in the sedi-
mentation of the Lower Greensand of the Weald, 1875-1975: a review and commentary. Proc. Geol. Ass., 86, 457-73. RAISIN, C. 1903. The formation of chert and its microstructures in some Jurassic strata. Proc. Geol. Ass., 18,71-82. RICHARDSON, J. A. 1947. Chert formation in the Bargate Beds of the Churt neighbourhood, Surrey. Proc. Geol. Ass., 58, 161-77. ROGERS, A. F. 1917. A review of the Amorphous Minerals. J. Geol., 25, 515--41. SMITH, W. E. 1950. The Origin of Chert and Flint. Unpublished M.Sc. dissertation, University of London. SMITH, W. E. 1957. Pyrite nodules in the Hythe Beds of the Tilburstow Hill area, Surrey. Proc. Geol. Ass., 68, 45-52. TARR, W. A. 1917. The origin of the chert in the Burlington Limestone. Amer. J. Sci., 44, 409-52. TARR, W. A. 1926. The Origin of Flint and Chert. Univ. of Missouri Studies, 1, 1-54. THURRELL, R. G., B. C. WORSSAM & E. A. EDMONDS. 1968. Geology of the Country around Haslemere. Mem. Geol. SUTV. U.K. TWENHOFEL, W. H.1932. Treatise on Sedimentation. 2nd ed. WHITAKER, W. & A. J. JUKES-BROWNE. 1894. On deep borings at CuIford and Winkfield, with notes on those at Ware and Cheshunt. Q. JI Geol. Soc. Lond., 50, 488-514. WILSON, R. C. L. 1966. Silica diagenesis in Upper Jurassic limestones of Southern England.J. Sed. Pet., 36, 1036--49. Received 28 January 1977 Revised version received 2 August 1977
MIDDLEMISS, F. A. 1978. The cherts in the Hythe Beds (lower Cretaceous) of South-east England. Proc. Geol. Ass., 89(2) pp. 283-298. The cherts in The Hythe Beds (Lower Greensand, Aptian) of the western part of the Weald, southern England, have been re-examined. The cherts vary among themselves. Some possess petrographic characters which show them to have been formed by silicification of calcareous rocks and, contrary to all previous authors, it is claimed that replacement cherts are present in these beds. Sponge spicules are invariably present and mostofthe silica in the cherts is thought to have been, at one stage ofits history, incorporated into the spicules and to have been subsequently redistributed diagenetically. Some ofthe cherts are sponge-spicule beds, as described by Hinde in the nineteenth century. The silica is thought to have been derived ultimately from tropical weathering of rocks exposed on the London land area. Depanment of Geology, Queen Mary College (University of London), Mile End Road, London, E1 4NS.
CONTENTS page
1. 2. 3. 4. 5.
INTRODUCTION . FIELD EVIDENCE . PETROGRAPHIC EVIDENCE ... DISCUSSION... CONCLUSIONS REFERENCES
283 286 287 294 297 298
1. INTRODUCTION
The Hythe Beds (Aptian), a subdivision of the Lower Greensand formation, outcrop all round the Weald in southern England (Fig. 1). In the east of the Weald, approximately east of a line joining Westerham and Pulborough, they consist essentially of calcareous sands and sandy limestones, with some chert. West of this line they are almost entirely non-calcareous sands and sandstones with considerable local developments of lenticular chert. It has been generally accepted that the cherts in the eastern Hythe Beds owe their existence to replacement of limestone, or the calcareous matrix of sandstone, by silica. Humphries (1957, 299) believed the silica to be derived from sponge spicules or other siliceous bodies in the sands. Those in the western Hythe Beds have been discussed in some detail by four authors: Hinde, Hill, Smith and Humphries. Hinde (1886) was the first to propose a theory of their formation. As a sponge specialist he was attracted by the abundant sponge spicules which occur, sometimes in thin, closely packed spicule beds, and he regarded the sponge spicules as the source of the silica in the cherts. He noted the presence of porous, spicular cherty sandstones which usually occur in association with chert, although they also occur without such association, and regarded these two rock types as different stages in one process: the spicules were dissolved, leaving the 283
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Fig. 1. Locality map of the western Weald. Stipple: rocks younger than Lower Greensand. Blank: Lower Greensand. Vertical Lines: rocks older than Lower Greensand.
porous sandstone, and the resulting silica migrated in solution either to be precipitated immediately as a core of more solid chert or to be carried further by percolating water and precipitated in similar fashion elsewhere. Hill (1911), again, thought the silica had been supplied by sponge spicules and regarded the cherts as essentially spicule-beds impregnated by free precipitation of silica in the interstices of the rock. Hinde's ideas have received adverse criticism, first from Tarr (1926) who developed, mainly from his studies of the Mississippian Burlington Limestone in the U.S.A. (Tarr, 1917), a concept of primary deposition of silica to form chert, denying that sponge spicules played any essential part in the process. Tarr, in fact, made no direct reference to the Lower Greensand, but Smith (1950) believed that the Hythe Beds chert originated as silica gel directly and inorganically precipitated on the sea floor, in the manner advocated by Tarr, the purer chert representing a phase of maximum precipitation in the earlier and closing stages of which sand grains and silica gel were deposited simultaneously, giving cherty sandstone. The association of sponge spicules with the chert was explained by postulating that the sponges flourished in the silica-rich waters before the maximum of precipitation and also after precipitation had finished and the mass of gel was buried under a cover of sand but still available as a source of silica. The abundant moulds and casts of spicules in sandstones adjacent to the chert were due to migration of silica towards the buried mass long after deposition. Humphries (1957), as far as the Hythe Beds of the western Weald were concerned, strongly advocated the views of Tarr and put forward a theory essentially similar to Smith's, allowing no special importance to sponge spicules in the formation of the chert. He summarised the main criticisms of Hinde's ideas: (a) according to Hinde's theory, the true cherts should be spicule beds whereas Humphries believed that they often contaiQed few or no sponge spicules; (b) many of the cavities in the sandy cherts, regarded as moulds of sponge spicules by Hinde, were not of that origin; (c) where hollow moulds of sponge spicules did occur, their walls were formed of chert and the cast of the axial canal, frequently present traversing the centre of the mould cavity, was itself often made of chert, thus the chert had been already present before the spicule was dissolved and was not formed from the dissolved silica of the sponge spicule.
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Humphries' summary of evidence for the primary origin of the chert needs to be quoted in extenso: (a) chert is involve~ in cont~mporaneous slumping and therefore must be syngenetic; (b) chert transgressmg beddmg planes, and hence clearly secondary, is almost absent; (c) no cherts occur in the calcareous Hythe Beds of the south-east Weald, east of Pulborough in spite of the presence in them of siliceous sponge spicules; , (d) colloform structures, indicative of a colloidal origin, are commonly present in the true cherts; (e) sponge spicules are not always present in the cherts; (f) where empty moulds of spicules are present in the cherts they are often perfectly preserved in a siliceous matrix (see above); (g) the sand grains in the cherts are usually 'floating' (Le. not in contact with one another; Humphries shows this by a numerical analysis); this must be due to a primary matrix, which point (d) above shows to have been silica; (h) calcite and pseudomorphs after calcite are completely absent from the cherts, cherty sandstones and sandstones of the Hythe Beds in the western Weald, indicating that the formation of chert is not a result of the replacement of calcium carbonate by silica. Richardson (1947) described the formation of chert in the Bargate Beds, the subdivision of the Lower Greensand which overlies the Hythe Beds in the western Weald. The Bargate Beds include much calcareous material and, in contrast to Smith and Humphries, Richardson claimed a major role for silicification oflimestone in this case. He distinguished two secondary processes, decalcification and silicification, which had taken place sometimes separately and sometimes in association. Decalcification alone converted the limestone to loose ferruginous sand. Where silicification accompanied decalcification the two processes were 'almost simultaneous', but three stages could be recognised: (a) the calcareous matrix and the infillings of foraminiferal chambers began to be silicified, (b) the matrix and some of the organic fragments were silicified, (c) the organic debris was completely silicified. Richardson pointed out that sedimentary laminae, including those of cross bedding, could be seen to pass from the surrounding sand into and through the chert lenses, just as they did into and through the neighbouring calcareous lenses, showing that both had formed secondarily. Hill, Smith and Humphries all specifically rejected the possibility of the Hythe Beds chert in the western Weald being formed by replacement of limestone on the grounds of rarity or absence of pseudomorphs after calcite. Smith observed that silicified calcareous organisms were occasionally present in the chert, especially fragments of foraminifera, but claimed that they were rare. Humphries stated that calcite and pseudomorphs after calcite were completely absent and above all, like Hill, claimed that there was no evidence for the presence of limestone in association with the chert of the Hythe Beds in the western Weald, so that replacement mechanisms could not be invoked. It is upon this last argument that his dismissal of replacement really depends. If it could be demonstrated that these beds had formerly been more calcareous than they are now, many of his points in favour of primary origin of the chert could be used equally well in favour of a secondary origin. Evidence will be presented in the following pages for the view that there may be more than one mode of formation for these cherts, and that these modes may be complex, but that replacement of limestone has played a larger part than hitherto recognised.
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2. FIELD EVIDENCE
In fact there are, even at present, several occurrences of limestone in the western Hythe Beds. Humphries himself describes some (not, indeed, in direct association with chert) between Fittleworth and Petersfield (Humphries 1953; 1964). Hayward (1932) describes lenticular calcareous bands near Abinger, much resembling the finer types of the Bargate Beds and containing chalcedony which could be replacive. At Tilburstow Hill is a limestone bed, no longer exposed but described by Hinde (1886), which contains sponge spicules replaced by calcite as in the 'calcareous' facies of the eastern Weald; this limestone is also referred to by Whitaker & Jukes-Brown (1894) and Cox (1915) and one of Hinde's thin sections of it is preserved in the British Museum (Natural History) (S 960). In the Bramshott area there is much limestone (Knowles & Middlemiss, 1958,213), associated with chert and in some cases showing the same silicification phenomena as Richardson described. These Bramshott localities, classified as Hythe Beds by Knowles & Middlemiss, were mapped as Bargate Beds by the Geological Survey, for reasons not made clear (Thurrell, Worssam & Edmonds, 1968, 68); the distinction is irrelevant in the present context as the sediments themselves are indistinguishable from the Hythe Beds and palaeontological evidence of age is lacking. In addition, throughout the western Weald the Hythe Beds contain much of the kind of evidence for decalcification which Richardson cited in the Bargate Beds. This is best shown by the phenomena of 'ovoid ferruginous cavities' and 'ferruginous sand lenses' (Knowles & Middlemiss, 1958). The former are ovoid cavities in sandstone beds, usually 0·3 to 1·0 m in length and up to O' 3 m in height and lined with deeply ferruginous sand, sometimes arranged in lines parallel to the bedding. The latter are ovoid lenticles of deeply ferruginous sand, of the same size and shape as the 'ovoid cavities' and showing the same arrangement, but set in unconsolidated sands. These appear to represent stages in the destruction by decalcification of small calcareous doggers. As the dogger becomes decalcified it turns into an ovoid mass of loose highly ferruginous sand with the liberation of ferric iron from the iron-bearing calcite. In unconsolidated sands this remains as a 'ferruginous lens' but in a sandstone the difference in resistance to weathering between the decalcified lens and the surrounding stone results in the sand being cleared out to leave a sand-lined 'ovoid ferruginous cavity'. The link in this sequence is supplied by some exposures near Bramshott, especially just west ofthe church (Nat. Grid Ref. SU 840329), where ferruginous lenses can be seen which still retain in the centre a remnant of highly calcareous stone, exactly comparable with that of the fully calcareous doggers of the same neighbourhood; the decakified material is silty sand typical of the surrounding beds but is unusually ferruginous, while the calcareous stone at the centre shows some incipient silicification. An earlier stage is seen at Sheet (SU 763244) in beds approximately equivalent to those at Bramshott, where small rounded calcareous doggers are set in ruddy sands; these also show incipient silicification. The association of ferruginous staining with decalcification is well-known in the calcareous Hythe Beds of the eastern Weald and in the Bargate Beds. The iron-bearing nature of much of the calcite in these rocks is demonstrated by staining techniques. Thurrell & others (1968, 83), with reference to the ferruginous sand lenses of Cold Ash Hill, Bramshott (SU 849327), said that 'it seems unwise to assume a single process of development'. Two other processes which may be considered, as alternatives to decalcification, are decomposition of pyrite and upward or downward percolation of iron-bearing ground-water blocked by an impervious medium. Pyrite certainly occurs in the Hythe Beds both as nodules (Smith, 1957) and as fine granules (microframboids) scattered in the cherts, but never on a large enough scale to give rise to the
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phenomena under discussion. The second process has taken place in the Folkstone Beds of the Lower Greensand, where iron staining is sometimes concentrated along the base of a clay seam. An example in which chert has possibly acted as the impervious medium is at Woolmer Lodge Lane, Bramshott (SU 841336), where the sand for about 5 cm beneath a band of solid chert is deeply ferruginous. This process, however, is unlikely to give rise to discrete ferruginous sand lenses or ovoid ferruginous cavities. The amount of ferric iron required to give a deeply ferruginous stain to the sand is quite small; analysis of deep rusty-coloured decalcified Bargate Beds from Richardson's type locality (Stock Farm, Churt, SU 877382) gave a total iron content of 3·29 per cent. This field evidence shows that the Hythe Beds of the western Weald (a) are not entirely non-ealcareous at present and (b) have been more calcareous in the past, thus there was formerly calcareous material available for silicification. The main evidence that such silicification has in fact taken place comes, however, from study of the cherts in thin section. 3. PETROGRAPHIC EVIDENCE (a) Characteristics of the replacement cherts The type of chert which is produced by silicification of sandy bioclastic limestones and calcareous sandstones like those ofthe Hythe and Bargate Beds is clearly shown by examples from Richardson's type locality of Stock Farm and many similar exposures of the Bargate Beds to the north of Haslemere. These have, in thin section, the following characters: (i) The general matrix is microcrystalline quartz (Folk & Weaver, 1952) in which the clastic quartz grains are usually 'floating'. The abundance of clastic quartz grains is extremely variable. The quartz grains may show corroded margins, as do those in the limestones (Plate 1, A & B). Progressive silicification can be shown in many examples of mixed calcareous-cherty rocks. Series of sections can easily be cut from one 20-em block of rock which is chert at one end and limestone at the other. These show the usual order of silicification described by both Raisin (1903) and Richardson: firstly sporadic replacement of the matrix calcite by microquartz; secondly complete replacement of the matrix calcite and of some organic fragments; thirdly complete replacement of the organic fragments. The last to be silicified are calcareous fragments showing clear organic microstructure, especially those of echinoid plates and spines and of bryozoa. Colloform structures are abundant in the matrix. Humphries (1957,306,308) treats these as diagnostic of primarily deposited silica, but here they are in undoubted replacement cherts. Their presence was recorded by Richardson (1947,168). Raisin (1903), in her paper on cherts in the Upper Jurassic Portland Beds, recorded colloform microquartz (although the term 'colloform' had not then been invented (Rogers, 1917» growing into calcareous ooliths during silicification of limestone (Raisin, 1903, plate 14, fig. 1). In the same way, in the Bargate Beds, colloform structures can be seen bulging into calcite matrix crystals, into patches of limonite and into aggregates of fan-acicular chalcedony. The colloform structures can sometimes be seen to be based upon pieces of silicified skeletal material, and to be apparently growing out from these into the pre-existing matrix. (ii) The calcareous rocks are bioclastic, containing abundant fragments of echinoid plates and spines, Bryozoa and foraminifera, of shells of brachiopods, bivalve molluscs and ostracods and spicules of siliceous sponges. All this material is silicified in the cherts. Silicification involves recrystallisation which often obscures or obliterates the outlines and the microtextures of the organic fragments. Because of this, the echinoid fragments provide the most useful evidence as
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(a) they are abundant and .(b) their ste.reo.m structure gives them a highly dis~i?~t~ve microtexture which often survIves recrystallIsatlOn (Plate 1 A & B; Plate 2 A-e). The sIlIcIfIed skeletal fragments in the cherts are normally, to a varying extent, impregnated with limonite. In some cases the skeletal fragments appear to have been completely limonitised and have then been able to resist silicification. Much more commonly the fragments are silicified and are represented by areas of especially fine-grained microquartz, which appear in oblique light as formless, wispy cream- or rust-coloured patches, Le. they are composed of microcrystalline silica impregnated with fine amorphous limonite (Plate 1 C & D, Plate 2 A-e). The sIs:eletal material is represented by fine-grained microquartz while the pores of the stereom are often filled with slightly coarser microquartz. In some cases the amorphous limonitic material is concentrated in the pores, in others the sites of both pores and skeletal material are equally impregnated. In other cases again the stereom pores are infilled with glauconite. Detection of the echinoderm origin of these formless, limonite-impregnated fragments in the cherts is difficult, requiring a combination of transmitted and oblique light and frequent comparison of plane-polarised light with cross-polarised light under a good quality microscope (Plate 2 A-e). In the calcareous rock skeletal fragments are often found to be 'surrounded by secondary overgrowths in the form of scalenohedra or acute rhombohedra of calcite. In the cherts these are not uncommon, with the acute-angled crystal forms perfectly pseudomorphed in silica, or more rarely in limonite or limonite-impregnated silica. Examples of these were figured by Richardson (1947, fig. 11, ii-vi). The bryozoan fragments are also easily recognisable but are much less common than echinoderm fragments. Similar remarks apply to them, except that they are more frequently heavily limonitised in the cherts (Plate 1 C & D). (b) The cherts of the Hythe Beds The cherts of the Hythe Beds in the western Weald are more varied than those of the Bargate Beds but fall into two main types, which grade into each other by way of intermediate examples. These can for convenience be called (i) replacement cherts and (ii) spicule-be,d cherts. These divisions cut across the classifications based upon density of clastic quartz grains which previous authors have used. Hinde (1886) and Smith (1950) distinguished cherty sandstones from cherts; Humphries (1957) recognised true chert, sandy chert and cherty sandstone. These are real distinctions, of course, but 'true chert' may be either of the types given above, or an intermediate stage, and can pass into sandy chert and cherty sandstone by increase in clastic grains.
Plate 1. A & B. Silicified fragment of echinoid test in a replacement chert. The fragment is pseudomorphed in very fine-grained microquartz. A: plane polarised light; the stereom is clearly visible. B: crossed pol. Hythe Beds, Weysprings, Haslemere. Corroded grains are visible. C & D. Typical replacement chert with a bryozoan fragment. Hythe Beds, Highfield, Thursley. C: plane polarised light. D: crossed pol. Light-brown limonitised areas in C, representing silicified organic fragments, can he seen to correspond to areas of fine-grained microquartz in D, as in Plate 2, A--e. Plate 2 A, B & C. Replacement chert. Hythe Beds, Hyde Hill, Churt. A: Plane polarised light. B: Crossed pol. C: Oblique reflected light. Limonitised silicified echinoid material at E is detectable only under oblique light. Areas of light-brown limonitised material in A and C correspond to areas of very fine-grained microquartz seen in B. Sponge spicules are also visible. D. A spicule-bed cherty sandstone, showing rim-lined spicule moulds. Hythe Beds, Combe Hill, Rogate. Crossed pol.
PROe. GEOL. ASS., VOL. 89 (1978) PLATE 1
289
290
PROC. GEOL. ASS., VOL. 89-(1978) PLATE 2
PROC. GEOL. ASS., VOL. 89 (1978) PLATE 3
291
292
PROC. GEOL. ASS., VOL. 89 (1978) PLATE 4
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(i) Replacement cherts. These have precisely the same hand-specimen and petrographic characteristics as those of the Bargate Beds. As replacement cherts have not previously been described from the western Hythe Beds a schedule is given here of the specimens used in preparing this paper: SU 841336 Woolmer Lodge Lane, Bramshott. In strata mapped as near the top ofthe Hythe Beds by Knowles & Middlemiss (1958), as near the base of the Bargate Beds by the Geological Survey. SU 895378 Highfield, Thursley. Near the top of the Hythe Beds (Plate 1 C & D). SU 884379 Hyde Hill, Churt. Near the top of the Hythe Beds (Plate 2 A-e). SU 878329 Critchmere, Haslemere. Groups 3 and 5 of Knowles & Middlemiss, 1958,209-10. Middle and lower parts of the Hythe Beds. Group 5 is strongly ferruginous in the field. SU 908323 Haste Hill, Haslemere. Middle of the Hythe Beds (Plate 3 B & C). SU 889333 Weysprings, Haslemere. Low in the Hythe Beds (Plate 1 A & B). Fittleworth, precise locality unknown. Hythe Beds. This type of chert therefore occurs all through the Hythe Beds except in the extreme basal part. (ii) Spicule-bed cherts. This is the type of chert accurately described by Hinde (1886) and Hill (1911). Both characterise their 'true cherts' as packed with sponge spicules, largely to the exclusion of clastic quartz grains and of other organic fragments, and as passing into sandy chert and cherty sandstone by increasing admixture of clastic quartz with the spicules. Both note also that in the 'true chert' the outlines of the spicules tend to be obscured by recrystallisation into a mass of chalcedony and microquartz but that a clue to the spicular origin of the mass usually remains in the form of casts of the axial canals. This type of chert is common in the western Hythe Beds. The essential character is the dominance of sponge spicules and rarity or absence of other organic fragments. The abundance of clastic quartz is very variable; only rarely is the chert so free of clastic quartz as to be a 'true chert' in Hinde's sense. Some noteworthy examples used in the present work came from: Combe Hill, Rogate (Plate 2 D) and Rad Lane, Abinger. Highly spicular cherty sandstones which appear to have been originally kaolinitic sandstones poor in calcareous debris but with the addition of great quantities of sponge spicules. The microquartz is of the 'rim-lining' type, apparently seeded on sponge spicules which are now represented mainly by empty moulds, or more rarely on clastic quartz grains. That some calcareous material was formerly present in rocks of this type is suggested by the common occurrence of corroded clastic quartz grains and the rare occurrence of silicified scalenohedral overgrowths. Plate 3. A. Spicule-bed chert, with spicules in various states of preservation. Hythe Beds, Inval, Haslemere. Plane polarised light. B & C. Tetraxon sponge spicule in a replacement chert. The outline of the spicule and of the axial canal is picked out by rim-lining chalcedony, while the body of the spicule is represented by fan-acicular chalcedony. Hythe Beds, Haste Hill, Haslemere. B: plane polarised light. C: Crossed pol. Plate 4. A. Spicule-bed chert, with spicules in various states of preservation. Hythe Beds, Inval, Haslemere. Plane polarised light. B & C. A spicule-bed chert, with sponge spicules in various orientations and states of preservation. Hythe Beds, Rutton Hill, Hindhead. B: plane polarised light. C: Crossed pol. 4
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Brook Turning, Portsmouth Road, Hindhead. This is similar to (a) but contains more shell debris and is more heavily limonitised. The formerly calcareous fragments (ostracod and lamellibranch shell, foraminifera) have been so strongly limonitised as to withstand silicification. Even the scalenohedral overgrowths are limonitised, not silicified. The microquartz seems all to be recrystallised sponge spicules. Inval, Haslemere (Plates 3 A, 4 A). Cherts from this locality vary in the amount of clastic quartz present, from 'sandy chert' to 'true chert', but are full of completely recrystaUised sponge spicules, many of them clearly shown by their axial canals which are infilled with limonite. They show no sign of formerly calcareous organic debris. Colloform structures are frequently present. Sailors' Road, Thursley. Essentially highly spiculiferous sandstone with patchy chertification. The microquartz is largely of the 'rim-lining' type, the spicules upon which the growth of secondary silica was seeded being now represented by empty moulds, but recrystallised spicules are also present. Some calcareous skeletal material was formerly present in this rock as there are spaces present from which scalenohedral overgrowths protrude, the overgrowths being both silicified and limonitised. There is not now any sign of calcareous skeletal fragments, those formerly present being now represented by 'spaces with scalenohedra'. Rutton Hill, Hindhead (Plate 4 B & C). A good example of cored chert, like those described by Hinde and Humphries, in which 'solid' or 'pure' chert, with few, small and scattered quartz grains, grades peripherally into cherty sandstone. The cherty sandstone contains abundant recrystallised sponge spicules, induding some good examples of spinulate form. In the 'pure' chert spicules are equally abundant but more difficult to see. In this case, for some reason, recrystallization of the spicules has involved loss of the axial canal; this is so even in the cherty sandstone, where the shapes of spicules are relatively easy to see. In the true chert the infilled axial canals, which demonstrate the abundance of spicules in the true chert from Inval (above), are absent and thus the rock at first sight appears a structureless mass of microquartz, although on detailed study the characteristic alignments of the microquartz crystallites marking the shapes of spicules, can be detected. Silicified shell material is present but very rare. Hinde's thin section of 'chert with spicules' from Petworth (presumably Hythe Beds), figured by Hinde 1886, plate 45, fig. 17 (British Museum (N.H.), no. S 962). This specimen illustrates very well both Hinde's and Hill's descriptions of 'true chert'. Clastic quartz grains are small and scattered, but the rock is highly spicular. On the other hand there is plenty of evidence of echinoderm material (some recognisable only by oblique light) which in this case appears to be limonitised rather than silicified. Hill's specimen from Raike's Lane, near Gomshall, upon which all his ideas of the genesis of the Hythe Beds chert seem to have been based (Hill, 1911, Plate 13, fig. 2), was clearly of this spicule-bed type. 4. DISCUSSION (a) Arguments against the presence of replacement cherts The main reasons given by previous authors for dismissing the possibility of replacement chert in the western Hythe Beds may now be considered.
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(i) Absence ofcalcareous material to be replaced means that replacement is impossible. This has been discussed above. (ii) Floating sand grains imply a siliceous primary matrix. They obviously imply the presence of a primary matrix to keep the sand grains apart. They make no implication as to the nature of that matrix. To deduce from the floating nature of the grains that the primary matrix was of primarily deposited silica is to make a prior assumption regarding point (i). The primary matrix could equally well have been mainly calcareous, or mainly composed of silica in the form of sponge spicules. (iii) Colloform structures show that the primary matrix was of primary silica. Colloform structures are very clearly present where a calcareous matrix is in process of being silicified, as in the Bargate Beds at Stock Farm and the rocks from Woolmer Lodge Lane, Bramshott. The replacing microquartz advances on curving 'colloform' fronts into the matrix calcite as well as into cavities. (iv) Cherts are involved in contemporaneous slumping and therefore must have been present at a pre-diagenetic stage of the rock. Humphries (1957, 300) based this point upon a beautiful example at Combe Hill, Rogate (SU 801258), where a mass of sandy, spicular chert (spicule-bed chert-see Plate 2 D) is buckled parallel to slumped foreset beds of large-scale cross bedding in which it is included. Clearly the material which is now chert was present in the beds before diagenesis. This does not mean that the material was then chert or even silica gel. It was material which has since been transformed into chert. (v) Chert veins transgressing bedding planes are unknown. This seems to be true, although small chert veins traversing individual beds at right-angles to the bedding planes occur. (vi) Calcite and pseudomorphs after calcite are rare (Smith) or absent (Humphries). In the case of some of the cherts, especially those regarded here as due to replacement, this is quite untrue. They are full of organic fragments, originally of calcium carbonate, now pseudomorphed in silica. Humphries (1957, 303), although denying the presence of pseudomorphs after calcite, mentions the presence of organic fragments in one of his cherts, saying 'they appear to be mainly bryozoans'. Silicified bryozoans are certainly present in the replacement cherts (Plate 1 C & D) but in my experience pieces of echinoid test are much the commonest of the recognisable fragments. The presence of silicified shell material in the cherts does not in itself necessarily prove that the rock is a silicified limestone. Tarr's (1917) description of the cherts of the Burlington Limestone makes a convincing case for regarding these as primary precipitates yet they contain silicified, formerly calcareous, organic fragments which Tarr considers were washed into the soft silica gel soon after it was deposited. The fragments are discrete, clearly delimited and are distinct from the matrix-'Silicified forms are always chalcedony or quartz (more frequently the latter) ... The surrounding crypto-erystalline material of the chert or flint is entirely different' (Tarr, 1926, 39). In contrast to this, the silicified fragments in the Hythe cherts (i) are abundant and varied: (ii) usually grade into the surrounding matrix; fragments of silicified echinoid test showing stereom have no distinct boundary but are composed of finegrained microquartz which merges into the surrounding matrix microquartz exactly as in the limestones the recrystallising fragments of stereom grade into the surrounding sparry calcite matrix: (iii) often have silicified scalenohedral overgrowths which must have been originally scalenohedral calcite: (iv) are usually accompanied by corroded clastic quartz grains which must
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have been corroded in a calcareous environment (Dapples, 1967, 118). These last two features are again exactly as seen in the unsilicified limestones and must mean that these particular cherts have passed through a calcareous stage. (b) Sponge spicules and the cherts Hinde and Hill both believed that sponge spicules were the main source of the silica for the cherts. Later authors, especially Richardson (1947) and Humphries (1957) have quoted the authority of Tarr (1917; 1926) in refuting this. Richardson even implied that rarr had reexamined Hinde's thin sections of Lower Greensand material and found spicules to be absent or unimportant. This is totally untrue and appears to have been based on a misleading passage in Twenhofel (1932, 540). Nowhere does Tarr say that he has seen Hinde's original materials. He claims (Tarr, 1926, 8-9) to have studied thin sections of Carboniferous cherts similar to those used by Hinde and also to have seen a slide of Upper Greensand from Godstone, Surrey, but does not say that it was one of Hinde's. As a result of his studies Tarr did reject the importance of spicules in chert formation generally but he nowhere mentions the Lower Greensand and shows no sign of having been aware of its existence. A number of Hinde's original thin sections are kept in the British Museum (Natural History), where I have examined them. There is no doubt of the abundance of sponge spicules in them. Hinde was the leading authority on sponges of his time; to suggest as Humphries does (1957, 309) that he might not have known spicules when he saw them, or imagined that he saw spicules which were not there, is unconvincing. Sponge spicules have been present in every thin section of chert from the Hythe Beds that I have ever examined: they are often abundant and may make up the bulk of the rock. They are preserved in various ways: (i) Commonly the original opal is replaced by a microquartz mosaic, due probably to recrystallisation. The axial canal mayor may not be marked out by a differentinfill such as limonite or glauconite. (ii) Frequently a thin coating of fine fibrous chalcedonic crystals is seeded on to the periphery of the spicule and more rarely on to the walls of the axial canal. This effect is here called 'rim-lining'. Subsequently the spicule itself may be dissolved leaving a rim-lined mould (Plate 2 D). Rim-lined cavities not showing recognisable shapes are also common; they seem to represent moulds of broken fragments of spicules. Further, the empty mould may subsequently become infilled either with relatively coarse quartz mosaic or with fanacicular chalcedony (Plate 3 B & C). (iii) More rarely a spicule may be obliterated by being completely replaced by colloform growths of microquartz. One of Hinde's thin sections in the British Museum, actually from the eastern Weald (S 958, Hythe Beds, Sevenoaks, Kent), shows this. In this case the axial canals, infilled with limonite, mark the former presence of the spicules, but it is conceivable that this type of replacement could have occurred in some cherts where the axial canals did not happen to be infilled with distinctive material; an originally highly spiculiferous rock could then appear to be completely lacking in spicules. (c) Source of the silica The constant presence and usual abundance of siliceous sponge spicules in the cherts under discussion makes it a reasonable assumption that they have played a large part in the formation
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of the cherts. The processes by which the silica has been redistributed are complex, judging by the variety of replacement phenomena present, and any simple hypothesis such as that of Hinde must be misleading. The spicules themselves show evidence that they have in many cases been dissolved out, and their silica transported away, and then replaced by silica from elsewhere, perhaps more than once. In the case of the spicule-bed type of cherts, Hill was probably right in thinking that cementation was mainly due to redistribution of the silica originally present in opaline form in the spicules. In the case of the replacement cherts, my own conclusion is that the abundance of spicules was underestimated by Richardson, Smith and Humphries and that they probably would have been adequate to provide for the silicification of the calcareous matter; this conclusion is similar to that of Wilson (1966) for the Portland Beds. To claim that sponge spicules were the main contributors of silica to the cherts is, of course, to beg the question of the source of the silica. My postulation is that most of the silica present in the cherts has passed through a stage of being the skeletal silica of sponges but, as both Smith and Humphries have pointed out, the sponges would not have been there in Aptian times unless an abundance of silica had been available. There are no sponges and no cherts in the contemporaneous Lower Greensand of the Isle of Wight. I have elsewhere (Middlemiss, 1976) discussed the provenance of silica in the Lower Greensand. The absence of sponges and of cherts from the Lower Greensand of the Isle of Wight is one of several differences from that of the Weald which suggest deposition in a separate basin of sedimentation, into which silica (other than clastic quartz) was not being transported. In the Weald, cherts in the Hythe Beds are particularly characteristic of the northern and western outcrops. In the southern outcrops sponge spicules are present in a calcareous matrix but there are no cherts: to Humphries this is one reason for his rejection of the importance of spicules in chert formation. The spicules are either calcitised or replaced by microquartz and in the latter case are accompanied by signs of small-scale silicification of the calcareous matrix. These same phenomena are commonly seen in the calcareous Hythe Beds ofthe north-eastern Weald but there the process goes further, with the production of cherts. In other words, the absence of cherts from the southern outcrop is probably due to the chertification process being less well developed there and this in turn is probably due to sponge spicules being less abundant. The evidence in the Weald, then, is that in Aptian times silica was being supplied to the basin of sedimentation from the north and north-west and that relatively little was getting through to the south-east Weald. This suggests that its source was the London land area, where a tropical weathering regime was probably in force. Most of the silica was probably utilised directly from the sea water by the sponges, but some may have been deposited in amorphous form among the mainly bioclastic sediment, somewhat as Butler (1975) has described precipitation of amorphous silica from silica-bearing water simultaneously with predominantly calcareous precipitation. In either case the resulting opaline silica in the sediments would have been unstable and mobile during diagenesis would have undergone total transformation by recrystallisation and migration, especially in the presence of abundant calcareous material (Ernst & Calvert, 1969, and references therein). 5. CONCLUSIONS The cherts of the Hythe Beds in the western Weald are of two main types: replacement cherts, which result from silicification of sandy limestone and calcareous sandstone and are characterised by abundance of silicified bioclastic debris, and spicule-bed cherts, originally highly
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spicular sandstones which have been impregnated by silica. There is no clear-cut division between these types; spicule-bed cherts which contain a proportion of formerly calcareous shell-debris grade into replacement cherts. In both types sponge spicules are abundant, although they are more densley packed, to the partial or total exclusion of calcareous shell debris, in the spicule-bed type. The presence of the spicules has played a major part in the formation of the cherts in that all, or most, of the silica has formerly been incorporated in the spicules at one stage, before being diagenetically redistributed. REFERENCES BUTLER, J. C. 1975. Tea kettle carbonates. J. Sed. Pet., 45, 891-3. COX, A. H. 1915. Report of an excursion to Tilburstow Hill and Nutfield. Proc. Geol. Ass., 26, 324--{j. DAPPLES, E. C. 1967. Diagenesis of sandstones. In Larsen, G. & G. V. Chilingar, ed. Diagenesis in Sediments. Developments in Sedimentology, 8, Amsterdam. ERNST, W. G. & S. E. CALVERT. 1969. An experimental study of the recystallisation of porcellanite and its bearings on the origin of some bedded cherts. Amer. J. Sci., Schairer Vol. 267·A, 114-33. FOLK, R. L. & C. E. WEAVER. 1952. A study of the texture and composition of chert. Amer. J. Sci., 250, 498-510. HAYWARD, H. A. 1932. The Geology of the Lower Greensand in the Dorking-Leith Hill district. Proc. Geol. Ass., 43, 1-31. HILL, W. 1911. Flint and chert. Proc. Geol. Ass., 22, 61-94. HINDE, G. J. 1886. On beds of sponge remains in the Lower and Upper Greensand of the south of England. Phil. Trans. R. Soc., 176 (for 1885),403-53. HUMPHRIES, D. W. 1953. The lithology and conditions of deposition of the Lower Greensand of the south-west Weald. Unpublished Ph.D thesis, University of London. HUMPHRIES, D. W. 1957. Chert: its age and origin in the Hythe Beds of the western Weald. Proc. Geol. Ass., 67,296-313. HUMPHRIES, D. W. 1964. The stratigraphy of the Lower Greensand in the south-west Weald. Proc. Geol. Ass., 75,39-59. KNOWLES, L. & F. A. MIDDLEMISS. 1958. The Lower Greensand in the Hindhead area of Surrey and Hampshire. Proc. Geol. Ass., 69, 205-38. MIDDLEMISS, F. A. 1975. Studies in the sedi-
mentation of the Lower Greensand of the Weald, 1875-1975: a review and commentary. Proc. Geol. Ass., 86, 457-73. RAISIN, C. 1903. The formation of chert and its microstructures in some Jurassic strata. Proc. Geol. Ass., 18,71-82. RICHARDSON, J. A. 1947. Chert formation in the Bargate Beds of the Churt neighbourhood, Surrey. Proc. Geol. Ass., 58, 161-77. ROGERS, A. F. 1917. A review of the Amorphous Minerals. J. Geol., 25, 515--41. SMITH, W. E. 1950. The Origin of Chert and Flint. Unpublished M.Sc. dissertation, University of London. SMITH, W. E. 1957. Pyrite nodules in the Hythe Beds of the Tilburstow Hill area, Surrey. Proc. Geol. Ass., 68, 45-52. TARR, W. A. 1917. The origin of the chert in the Burlington Limestone. Amer. J. Sci., 44, 409-52. TARR, W. A. 1926. The Origin of Flint and Chert. Univ. of Missouri Studies, 1, 1-54. THURRELL, R. G., B. C. WORSSAM & E. A. EDMONDS. 1968. Geology of the Country around Haslemere. Mem. Geol. SUTV. U.K. TWENHOFEL, W. H.1932. Treatise on Sedimentation. 2nd ed. WHITAKER, W. & A. J. JUKES-BROWNE. 1894. On deep borings at CuIford and Winkfield, with notes on those at Ware and Cheshunt. Q. JI Geol. Soc. Lond., 50, 488-514. WILSON, R. C. L. 1966. Silica diagenesis in Upper Jurassic limestones of Southern England.J. Sed. Pet., 36, 1036--49. Received 28 January 1977 Revised version received 2 August 1977