Silica pseudomorphs from the Bembridge Limestone (Upper Eocene) of the Isle of Wight, Southern England and their palaeoclimatic significance

Palaeogeography, Palaeoclimatology, Palaeoecology, 69 (1989): 233-240 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

233

SILICA PSEUDOMORPHS FROM THE BEMBRIDGE LIMESTONE (UPPER EOCENE) OF THE ISLE OF WIGHT, SOUTHERN ENGLAND AND THEIR PALAEOCLIMATIC SIGNIFICANCE BRIAN DALEY Department of Geology, Portsmouth Polytechnic, Burnaby Building, Burnaby Road, Portsmouth P01 3QL (Great Britain) (Received January 26, 1988; revised and accepted August 8, 1988)

Abstract Daley, B., 1989. Silica pseudomorphs from the Bembridge Limestone (Upper Eocene) of the Isle of Wight, southern England, and their palaeoclimatic significance. Palaeogeogr., Palaeoclimatol., Palaeoecol., 69: 233- 240. Silica pseudomorphs from localised silcrete developments in the Bembridge Limestone (Late Eocene) of the Isle of Wight, England are described. These are thought to have formed after ]enticular gypsum, whilst the presence of length slow chalcedony confirms the former presence of evaporitic conditions. A pedogenic origin is suggested which, with the development of contemporaneous calcrete horizons, indicates that southern Britain was subject to climatically dry phases towards the end of the Eocene, and was not continually humid tropical to sub-tropical as has hitherto been inferred.

Introduction T h e B e m b r i d g e L i m e s t o n e of F o r b e s (1853) h a s r e c e n t l y b e e n f o r m a l l y defined as a format i o n w i t h i n the Solent G r o u p of I n s o l e a n d D a l e y (1985). Its c o r r e l a t i o n w i t h t h e G y p s e of the P a r i s B a s i n ( C u r r y et al., 1978) w a s established many years ago and has been r e c e n t l y c o n f i r m e d ( L i e n g j a r e n et al., 1980). R e c o g n i s e d since t h e e a r l y n i n e t e e n t h cent u r y as a f r e s h w a t e r limestone, t h e B e m b r i d g e L i m e s t o n e is well k n o w n for its f r e s h w a t e r a n d land gastropods. Some evidence suggests the periodic d e v e l o p m e n t of m o r e b r a c k i s h w a t e r s (See M u r r a y a n d W r i g h t , 1974, pp. 25-26), whilst, m o r e r e c e n t l y , the r e c o g n i t i o n t h a t c a l c r e t e s o c c u r w i t h i n t h e f o r m a t i o n implies c o n t e m p o r a n e o u s exposure.

lithified. S e t t i n g the few t h i n l i m e s t o n e s aside, t h e n u m e r o u s s a n d s a r e a l m o s t t o t a l l y uncem e n t e d . W h e r e l i t h i f i c a t i o n h a s o c c u r r e d , calc a r e o u s or f e r r u g i n o u s c e m e n t s a r e responsible. W i t h i n the limestones, d i a g e n e t i c silica is v e r y a t y p i c a l l y found, a l t h o u g h e a r l y a u t h o r s m e n t i o n e d its o c c u r r e n c e in t h e B e m b r i d g e L i m e s t o n e . F o r b e s (1856) r e f e r r e d to n o d u l a r " s i l e x " a n d chert, w h i l s t B r i s t o w (1862) mentioned a s u b s t a n c e r e s s e m b l i n g " m e n i l i t e " (a v a r i e t y of opal a c c o r d i n g to D a n a a n d Ford, 1958). N e i t h e r m e n t i o n e d the c h e r t s f o u n d in t h e B e m b r i d g e L i m e s t o n e at G u r n a r d (Nat i o n a l Grid R e f e r e n c e SZ 468954) w h i c h cont a i n t h e silica p s e u d o m o r p h s w h i c h f o r m the s u b j e c t of this paper.

The Gurnard c h e r t s Silicification in the P a l a e o g e n e s t r a t a Little of the p r e d o m i n a n t l y clastic P a l a e o gene s u c c e s s i o n in t h e H a m p s h i r e B a s i n is 0031-0182/89/$03.50

The cherts at Gurnard comprise irregular, c o n c r e t i o n a r y m a s s e s w i t h i n the l o w e r p a r t of the B e m b r i d g e L i m e s t o n e . T h e y e x t e n d for

@ 1989 Elsevier Science Publishers B.V.

234 only a short distance parallel with the bedding and have a maximum vertical thickness of about 0.2 m. The concretions are markedly ferruginous and in places appear brecciated. No vertical trend or zonation is apparent (cf. Thiry and Millot, 1987). The textures of these ferruginous cherts are complex and reflect a high degree of diagenetic modification. Much of the chert is strongly limonitic, and although limonite is often opaque or near opaque in thin section, a clotted or earthy texture is apparent. Crystalline, sometimes spherulitic, aggregates of tiny lanceolate limonitic crystals occur throughout the cherts in amorphous patches or ill-defined, very contorted layers. Irregularly shaped, granular to larger remnants of limestone occur throughout the cherts. Many exhibit an internal intraclastic texture, although the extent to which this is a primary depositional feature is not clear. Altered shell debris and Chara oSgonia occur both within the limestone remnants and elsewhere within the ferruginous material. Both megaquartz and chalcedonic silica are readily recognisable in thin section. The former occurs in part as a space-filler in gastropod shells, the centres of Chara oSgonia and between intraclasts, and corresponds to the isometric quartz of Arbey (1980). Both equant and elongate silica crystals have replaced moluscan debris, with the elongate crystals tending to grow inwards from the shell margins. Neomorphic replacement is indicated by relict shell fabric 'ghosts'. Some of the silica crystals are isometric (equant) quartz. Some of the more elongate crystals may be palisadic quartz (long crystals lying in parallel with their neighbours), but many appear fibrous and have oblique extinction more typical of lutecite (see below). Elsewhere in the Bembridge Limestone, molluscs are almost invariably represented only by moulds or casts, the original aragonite shells having been dissolved away. There is, therefore, some indication that silicification took place early in the history of the sediment prior to the widespread solution of aragonite shell materials.

Chalcedonic silica occurs in irregular aggregates and in many of the pseudomorph crystals. The former comprise irregularly interlocking aggregates of spherulitic fibrous silica. Some of the spherulites have length slow fibres whilst others have length fast, indicating respectively the presence of quartzine and chalcedonite, first differentiated by MichelL~vy and Munier-Chalmas (1890, 1892). Aggregate fabrics resemble that illustrated by Arbey (1980, fig. 12). Spherulitic silica also forms an intercrystalline matrix between the aggregates of limonitic crystals referred to above. A third type of fibrous silica, lutecite, is found in many of the pseudomorph crystals and is described further below. The relative age of silicification and ferruginisation is not completely unambivalent since whilst, as described above, limonitic rhombs occur in a siliceous matrix, there are instances where the silica pseudomorphs are surrounded by ferruginous material. Where quartz fills voids, such as the centres of Chara oSgonia, it is unstained and there is no indication of pre-quartz ferruginous deposition on the void surfaces. Also, where original intraclastic carbonate fabrics are preserved, quartz has filled the interparticle voids, whilst the pervasive ferruginisation has 'digested' and defined the bounding surfaces of intraclasts.

Silica pseudomorphs The silica pseudomorphs present are considered both in terms of their pseudomorphic crystal form and mineralogy.

Crystal form The silica pseudomorphs, as seen in thin section, represent single and twinned crystals. The pseudomorphs either occur individually or within crystal aggregates. The single pseudomorphs appear mainly as very small (up to 0.75mm). sharply defined lanceolate crystals. Some of these are narrowly rhombic (diamond shaped) whilst others are

235 rhomboidal, with only opposite sides equal (Plate I, A, B). The sharp ends of these crystals encompass an average enfacial angle of 29 °. Whilst many of the pseudomorphs have straight edges, some have curved edges. Such curvature is best seen in a relatively small number of rhombic crystals with much larger enfacial angles (Plate I, C). No triangular cross sections like those apparent in rocks with well developed dolomite rhombs were observed. Although much less common than the pseudomorphs of individual crystals, there are present a number of distinctive pseudomorphs of twinned crystals of essentially similar size and composition, and which are characterised by a distinctive re-entrant angle (Plate I, D). From their morphology these are referred to in this paper as "arrow-head" twins. The crystal aggregates comprise concentrations of numerous very small crystals of similar habit to the individual crystals described above. Their form and distribution is similar to the limonitic crystals referred to earlier in this paper. Some aggregates comprise both tiny pseudomorphs and spherulites of silica.

Silica mineralogy The silica present in the pseudomorphs comprises quartz and/or chalcedonic silica. The latter, in part spherulitic, comprises both length slow and length fast fibrous aggregates (Plate I, E). Some fibres clearly show the oblique extinction of lutecite, whilst a grid-like crosshatching, visible particularly in some of the twinned crystals (Plate I, D), is reminiscent of lutecite textures described elsewhere (cf. King and Merriam, 1969; Folk and Pittman, 1971). Both isometric quartz and elongate crystals resembling palisadic quartz occur in some of the pseudomorphs. The former is best developed towards the centre of the pseudomorphs and may be bordered by the more elongate crystals or fibrous chalcedony (Plate I, F). The elongate quartz crystals often extend from opposite margins of the pseudomorphs and

meet to form a slightly irregular median line extending along the long dimension of the latter (Plate I, A, G, H). These crystals are not perpendicular to the pseudomorph margins but apparently to the overall pseudomorph elongation. Although resembling palisadic quartz, the elongate crystals have a weakly fibrous texture and a characteristic oblique extinction (Plate I, G, H). They are, therefore, actually lutecite into which palisadic quartz may grade imperceptibly (cf. Hutchins, 1962; Arbey, 1"980). A number of the pseudomorphs show a fourfold zonation in cross-polarised light (Plate I, B, H). Zonal boundaries may be ill-defined, but where this is so (Plate I, H) diametrically opposite quadrants contain a majority of crystals which go into optical extinction simultaneously whilst those in adjacent quadrants extinguish in different positions. Some pre-silicification "ghost" fabrics are present in some of the pseudomorphs. Perhaps the most obvious of these features is a fine parallel lineation which extends across the short dimension of some of the pseudomorphs (Plate I, C) and which cuts across quartz crystals of different optical orientation or occurs parallel to the orientation of the palisadic crystals. A faint zonation parallel to the pseudomorph margins is also apparent in a number of the pseudomorphs.

Inferences from the crystal form o f the pseudomorphs The limited morphological variation of the pseudomorphs, even including the twinned crystals, suggests that the pseudomorphs represent the former presence of only one mineral species. Euhedral crystals such as those represented by the pseudomorphs described here are indicative of diagenetic mineral modification or growth. Rhomboidal cross sections are well developed in dolomitised rocks. However, the lanceolate shape of the overwhelming majority of pseudomorphs mitigates against the replacement of dolomite or other carbonate rhombs. Triangular cross sections are common where

236

PLATEI

237 the l a t t e r are p r e s e n t but a b s e n t in the p s e u d o m o r p h s described here, whilst the c h a r a c t e r i s t i c e l o n g a t i o n and a c u t e enfacial angle at t h e i r s h a r p t e r m i n a t i o n would be impossible to derive from a n y section cut t h r o u g h c a r b o n a t e rhombs. By c o n t r a s t , the shape of the p s e u d o m o r p h s c o m p a r e s well with t h a t of p s e u d o m o r p h s a f t e r gypsum described by West (1964, plate 36, figs. 1 and 6; 1973; fig. 1), and Arbey (1980, fig. 23) and with gypsum crystals figured by L a c r o i x (1897, plate 8, fig. 10). T h e l a n c e o l a t e cross sections, b o t h in this and these o t h e r papers r e p r e s e n t sections t h r o u g h gypsum crystals of l e n t i c u l a r h a b i t such as those described by M e r r i t t (1935) and p a r t i c u l a r l y M a s s o n (1955, fig. 4), as well as sand crystals (cf. desert roses) in the a u t h o r ' s possession. It m a y be t h a t the b o u n d i n g edges of the more s y m m e t r i c a l l y d i a m o n d shaped (rhombic) pseud o m o r p h s from the B e m b r i d g e L i m e s t o n e repr e s e n t 111 (the u n i t pyramid) faces of c r y s t a l s m a r k e d l y e l o n g a t e d along the y crystallographic axis. H o w e v e r , the t e n d e n c y t o w a r d s c u r v e d faces, seen also in West (1964), m a y i n d i c a t e a c o m p a r i s o n w i t h the doubly c o n v e x discoidal crystals described by M a s s o n (1955) as lying in a plane a p p r o x i m a t i n g to 102. To r e l a t e the l a n c e o l a t e cross sections of the B e m b r i d g e L i m e s t o n e p s e u d o m o r p h s to e i t h e r of these possible o r i e n t a t i o n s would be supported by an i n t e r p r e t a t i o n of the " g h o s t " l i n e a t i o n across t h e i r s h o r t dimensions as

r e p r e s e n t i n g the f o r m e r p r e s e n c e of the domin a n t gypsum c l e a v a g e parallel to 010. Asymmetrical " f l a t t e n e d " r h o m b i c sections m a y be of sections parallel to 010. T h e y can be c o m p a r e d with crystal forms figured by M a s s o n (1955, fig. 4c), w h e r e the s h o r t d i m e n s i o n is given as 102. It is however, felt t h a t in the p r e s e n t case the s h o r t dimension is best c o n s i d e r e d as r e p r e s e n t i n g 103 (cf. D a n a and Ford, 1958, p. 758). The c u r v e d o u t l i n e of some of the p s e u d o m o r p h s also supports a gypsum parentage, since this has been r e c o r d e d in p r e s e n t day gypsum (Masson, 1955) and gypsum pseud o m o r p h s from the fossil r e c o r d (West, 1964). The form of the a r r o w head twin pseudom o r p h s supports a gypsum p r e c u r s o r . R o t a t i o n a b o u t a twin plane e x t e n d i n g b e t w e e n the obtuse angles of a " f l a t t e n e d " r h o m b o i d a l form would p r o d u c e such twins. F u r t h e r m o r e , alt h o u g h on a m u c h smaller scale, these twins b e a r a s t r o n g r e s e m b l a n c e to a t w i n n e d gypsum crystal figured by B e r r y et al. (1983, fig. 13-7) w h e r e the twin plane is 101. H o w e v e r , a s s u m i n g a u n i t p y r a m i d Tll, 101 would p r o d u c e a twin r e s e m b l i n g the "swallow-tail" variety. To p r o d u c e the " a r r o w - h e a d " twins of the G u r n a r d Cherts, a twin plane approximating to 201 seems more a p p r o p r i a t e .

Significance of the mineralogy T h e r e is little d o u b t t h a t the silica p r e s e n t o r i g i n a t e d by the n e o m o r p h i c r e p l a c e m e n t of

PLATEI Pseudomorphs in thin section photographed in cross-polarised light. The vertical and horizontal edges of the photographs are parallel to the N S, E-W polarisation directions respectively. A. Narrow rhombic, lanceolate pseudomorph, showing palisadic crystal development perpendicular to pseudomorph elongation ( x 100). B. Rhomboidal pseudomorph, comprising chalcedonic silica and showing marked crystal zonation ( x 100). C. Rhomboidal pseudomorph with curved crystal faces. Centre comprises isometric quartz with remnant finely parallel ghost fabric. Chalcedonic silica is developed at the margin of the pseudomorph ( x 80). D. Arrowhead twin pseudomorph, comprising lutecite with characteristic cross hatching ( × 80). E. Pseudomorph comprising partially spherulitic chalcedonic silica ( x 125). F. Pseudomorph with central development of isometric quartz and fine development of palisadic chalcedony at the margin (× 8O). G. Pseudomorph illustrating median line parallel to long dimension. Palisadic crystals are well developed towards the top, with some showing oblique extinction ( x 100). H. Lanceolate pseudomorph showing four-fold zonation. Optical orientation is broadly similar in opposite quadrants ( x 125).

238 an earlier mineral rather than by cavity filling. There is no indication of drusy texture nor incremental growth inwards from a cavity margin. The randomly complex chalcedonic textures of some of the pseudomorphs suggest in situ replacement as does the control of growth direction of the palisadic crystals by what appears to be cleavage in the original mineral. For many years, quartzine and lutecite have been found in association with evaporite deposits or minerals (Munier-Chalmas, 1890; Cayeux, 1929; West, 1964, 1973; West et al., 1968; Arbey, 1980). Moreover, Folk and Pittman (1971) concluded that these length slow forms of chalcedony occur almost exclusively in association with sulphates and evaporites, whilst Arbey (1980) found that palisadic quartz reflects the presence of sulphate ions. A mixture of length slow and length fast chalcedony such as that which occurs in the Bembridge Limestone does not detract from an evaporite origin and indeed represents the more usual occurrence in chalcedony bearing evaporites. The origin of the zoned pseudomorphs is not entirely clear. The optical orientation of the neomorphic palisadic crystals, however, has clearly been influenced by some internal variation within the original parent crystals, most probably related to twinning. The significance of the tiny limonitic crystals is not as yet apparent. Since they are morphologically similar to the silicified crystals, it may be that they too are pseudomorphs after gypsum. Discussion and conclusions

Early diagenetic silcretes have been identified from the Palaeogene succession of the Paris Basin by such workers as Thiry et al. (1983) and Thiry and Millot (1987).There seems little doubt that the Gurnard cherts are also of early diagenetic origin since the silica has neomorphically replaced both aragonitic shells and small, readily dissolvable gypsum crystals. The Gurnard cherts lack the sequential vertical profile variation found in some of the

silcretes described by the above authors. Since calcretes are found within the Bembridge Limestone, thereby demonstrating contemporary pedogenesis, it seems likely that these early cherts had an analogous origin. Although not laterally continuous, they may therefore be called silcretes (sensu Dury and Habermann, 1980). Leeder (1982, p. 289) refers to the development of chalcedonic lenses at a mature stage of calcrete formation and it may be that the Gurnard cherts originated in this way. Calcretes are largely confined to areas with less than 500 mm of rainfall per annum (Ollier, 1984) and mainly form from ground water drawn up to the surface by evaporation (Salomons et al., 1978). Their development may take place geologically relatively quickly; a few hundreds or thousands of years is sufficient (Ollier, 1984). Nevertheless, they provide evidence that dry climatic conditions existed from time to time and clear possibilities that evaporite minerals may have developed. West (1973) referred to the many criteria which may be used to indicate the former presence of evaporites and stressed the importance of using at least two to confirm such a diagnosis. For the Gurnard cherts these are the pseudomorphs and the presence of length slow chalcedony. There seems little doubt that the pseudomorph crystals and the very distinctive twins are after lenticular gypsum. Its presence is quite compatible with caliche formation since it is associated with carbonates in chestnut soils and in semi-desert soils (sierozem) where lime and gypsum develop close to the surface (Ollier, 1984, p. 156). Indeed, the smallness of the Bemnbridge Limestone pseudomorphs supports the latter (cf. Masson, 1955; Kerr and Thomson, 1963). The common association of length slow chalcedony with evaporite deposits has already been referred to. Folk and Pittman (1971) also quoted a number of examples where length slow chalcedony was found in semi-arid soils characterised by high alkalinity and caliche development. In some cases, such occurrences

239

were associated with the presence or former presence of gypsum crystals. A post-depositional, pedogenic origin for both the gypsum and the length slow chalcedony is quite compatible with their occurrence within a host rock whose fauna clearly indicates that the primary sediments accumulated in a freshwater lake, and may explain other cases where gypsum pseudomorphs are associated with freshwater fossils (West, 1961; Daley, 1967). The recognition of pedogenic evaporite mineralisation within the Bembridge Limestone sheds f u r th er light on an apparent palaeogeographical anomaly pointed out previously by Daley (1971). This was t ha t whilst palaeobotanical studies of the Palaeogene of S o u t h e r n England had consistently led to the conclusion that humid tropical to sub-tropical climates had prevailed during this period of time (Reid and Chandler, 1926, 1933; Pallot, 1961), the gypsum deposits of the Gypse, 2 3 ° of latitude f u rt h e r south, surely indicated the former existence of arid or at least semi-arid conditions (Green, 1961). Daley (1971) suggested t hat the most likely explanation for this juxtaposition was th at the gypsum and the humid floras were not in reality contemporaneous, but were the product of dry and pluvial periods respectively, albeit preserved within the considerably greater time span of a single chronostratigraphic correlation unit (Stage). With the recognition of evaporite minerals in the Bembridge Limestone, together with caliches in this and the underlying formation, it is now clear th a t towards the end of the Eocene there were at least some dry climatic periods alternating with those of a more humid n a t u re in the British area. Perhaps this recognition goes some way to explaining the anomalous dry climate plants Banksia and Ephedra identified in the palynological investigations by Machin (1971). W h e t h e r conditions were ever truly arid is not clear, but certainly there were times of mean annual moisture deficiency. Nor can the extent to which seasonality played a part be resolved. It may in fact be t h at pedogenic gypsum was a dry season phenomenon produced when soil water salinity

temporarily exceeded a critical value. With Britain at a palaeolatitude of around 40 '~ during early T e r t i a r y times, seasonal changes would certainly have occurred, and at this latitude dry summers which characterise the M e d i t e r r a n e a n at the present time would have seemed likely. Since, as mentioned earlier, calcretes (and presumably gypsiferous soils too) may take only a few hundreds or thousands of years to develop (Ollier, 1984), the drier periods represented in these late Eocene sediments may nevertheless be atypical of the local Palaeogene climate. It should not, however, be forgotten t hat the stratigraphical record is subject to a variety of biases. Inevitably the remains of plants growing during humid climatic phases are relatively readily transported by flowing water to depositional sites, whilst of those representing dry periods no trace may survive. The possibility therefore persists t hat the Palaeogene climate of Sout hern Britain may very well be more complex than has hi t hert o been recognised, whilst the apparent palaeoclimatological dissonance of the coeval successions in the Paris and Hampshire basins becomes more readily explanable.

Acknowledgements My thanks go to Drs G. M. Power, C. R. Rowley, M. J. Ryan and I. M. West for useful discussions and to Mr D. N. Weights for his expert help with the photomicrographs. I would particularly like to thank Dr Power for the interest he has shown in this work and for his very helpful and constructive criticism of the manuscript.

References Arbey, F., 1980. Les formes de la silice et l'identification des 6vaporites dans les formations silicifi6es. Bull. Cent. Rech. Explor. Prod. Elf-Aquitaine, 4(1): 309 365. Berry, L. C., Mason, B. and Dietrich, R. V., 1983. Mineralogy, Freeman, San Francisco, Calif, 300 pp. Bristow, H. W., 1862. The geology of the Isle of Wight. Mem. Geol. Surv U.K., 138 pp. Cayeux, L., 1929. Les Roches S6dimentaires de France Roches Siliceuses Imprimerie Nationale, Paris, 696 pp.

240 Curry, D. et al., 1978. A correlation of Tertiary rocks in the British Isles. Geol. Soc. Lond. Spec. Rep., 12, 72 pp. Daley, B., 1967. Pseudomorphs after gypsum from the Bembridge Marls. Proc. Geol. Assoc., 78(2): 319-324. Daley, B., 1971. Some problems concerning the Early Tertiary climate of southern Britain. Palaeogeogr., Palaeoclimatol., Palaeoecol., 11: 177-190. Dana, E. S. and Ford, W. E., 1958. A Textbook of Mineralogy, 4th Edition, Wiley, New York, N.Y., 4th ed., 851 pp. Dury, G. H. and Habermann, G. M., 1980. Australian silcretes and their northern hemisphere correlatives. In: T. Langford-Smith (Editor), Silcrete in ,~ustralia. Univ. New England, Armidale, pp. 223 260. Folk, R. L. and Pittman, J. S., 1971. Length-slow chalcedony: a new testament for vanished evaporites. J. Sediment. Petrol., 41(4): 1045-1058. Forbes, E., 1853. On the fluvio-marine tertiaries of the Isle of Wight. Q. J. Geol. Soc. Lond., 9:259 270. Forbes, E., 1856. On the Tertiary fluvio-marine Formation of the Isle of Wight. Mem. Geol. Surv. UK., 162 pp. Green, R., 1961. Palaeoclimatic significance of evaporites. In: A. E. M. Nairn (Editor), Descriptive Palaeoclimatology, Wiley Interscience, New York, N.Y., pp. 61-88. Hutchins, P. F., 1962. Authigenic minerals in Carboniferous sediments from Central Vest Spitzbergen. Geol. Mag., 99:63 68. Insole, A. and Daley, B., 1985. A revision of the lithostratigraphical nomenclature of the Late Eocene and Early Oligocene strata of the Hampshire Basin, Southern England. Tertiary Res., 7(3): 67 100. Kerr, S. D. and Thomson, A., 1963. Origin of nodular and bedded anhydrite in Permian shelf sediments, Texas and New Mexico. Bull. Am. Assoc. Pet. Geol., 47: 1726 1732. King, R. J. and Merriam, D. F., 1969. Origin of the ~'Welded Chert", Morrison Formation (Jurassic), Colorado. Bull. Geol. Soc. Am., 80:1141 1148. Lacroix, A., 1897. Le gypse de Paris et les min~raux qui l'accompagnent. Nouv. Arch. Mus. Hist. Nat., (3)9: 201-296. Leeder, M. R., 1982. Sedimentology. Allen and Unwin, London, 344 pp. Liengjaren, M., Costa, L. and Downie, C., 1980. Dinoflagel-

late cysts from the Upper Eocene-Lower Oligocene of the Isle of Wight. Palaeontology, 23:475 499. Machin, J., 1971. Plant microfossils from Tertiary deposits of the Isle of Wight. New Phytol., 70: 851-872. Masson, P. H., 1955. An occurrence of gypsum in southwest Texas. J. Sediment. Petrol., 25(1): 72 77. Merritt, C. A., 1935. Gypsum crystals from Afalfa County, Oklahoma. Am. Mineral., 20: 674. Michel-Levy, A. and Munier-Chalmas, E., 1980. Sur de nouvelles formes de silice cristallis~e. C.R. Acad. Sci. Paris, 110:649 652. Michel-Levy, A. and Munier-Chalmas, E., 1982. M~moires sur diverses formes affect~es par le r~seau ~l~mentaire du quartz. Bull. Soc. Fr. Min~r., 15: 159-190. Murray. J. W. and Wright, C. A., 1974. Palaeogene Foraminiferida and palaeoecology, Hampshire and Paris Basins and the English Channel. Spec. Pap. Palaeontol., 14:1 129. Ollier, C., 1984. Weathering. Longman, London, 270pp. Pallot, J. M., 1961. Plant microfossils from the Oligocene of the Isle of Wight. Thesis. Univ. London (unpublished). Reid, E. M. and Chandler, M. E. J., 1926. The Bembridge Flora - - Catalogue of Cainozoic Plants in the Department of Geology, 1. Br. Mus. Nat. Hist., London, 206 pp. Reid, E. M. and Chandler, M. E. J., 1933. The flora of the London Clay. Br. Mus. Nat. Hist., London, 561 pp. Salomons, W., Goudie, A. and Mook, W. G., 1978. Isotopic composition of calcrete deposits from Europe, Africa and India. Earth Surf. Proc., 3: 43-58. Thiry, M. et al., 1983. Les p~riodes de silicification au Cenozoique dans le Bassin de Paris. Bull. Soc. G~ol. Fr., 25(1): 31-40. Thiry, M. and Millot G., 1987. Mineralogical forms of silica and their sequence of formation in silcretes. J. Sediment. Petrol., 57(2): 343-352. West, I. M., 1961. Lower Purbeck Beds of Swindon Facies in Dorset. Nature, 190: 526. West, I. M., 1964. Evaporite diagenesis in the Lower Purbeck Beds of Dorset. Proc. Yorks. Geol. Soc., 34(3): 315 330. West, I. M., 1973. Vanished evaporites - - significance of strontium minerals. J. Sediment. Petrol., 43:278 279. West, I. M., Brandon, A. and Smith, A., 1968. A tidal flat evaporite facies in the Visean of Ireland. J. Sediment. Petrol., 38:1079 1093.