Silica diagenesis of Palaeogene residual deposits in Devon, England

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Silica diagenesis of Palaeogene residual deposits in Devon, England K. P. Isaac Department of Geology, University of Exeter, North Park Road, Exeter EX4 4QE.

1. INTRODUCTION

3. SILCRETES

In the Sidmouth area of East Devon 'large blocks of siliceous breccia composed of chalk flints and united by a strong siliceous cement' (Woodward & Ussher, 1911) are abundant. These sediments have been described (Kerr, 1955; Isaac, 1979) as silcretes (Lamplugh, 1902), the products of surface or near-surface diagenesis in continental sedimentary environments. Although most common in the Sidmouth area, silcretes are also observed in association with plateau deposits over a wide area of the East Devon tableland. These plateau deposits consist of residual flint gravels with a matrix of residual detrital quartz grains and kaolinite (Isaac, 1981). It has been suggested by the author (Isaac, 1981, 1983) that these residual deposits formed by the lateritic weathering of the Upper Cretaceous Chalk and any later cover during the early Tertiary. Consideration of the field relations of the residual deposits imply a lower Palaeogene age (Isaac, 1983). Two types of siliceous indurated residual deposits can be recognised in East Devon: (1) residual quartz arenite silcretes and (2) conglomeratic silcretes. The silcretes are found lying within the Peak Hill Gravels (Isaac, 1979, 1981), at the boundary with overlying Pleistocene gravels ('head') or as completely exhumed blocks up to 3 m long lying on the plateau surface. Where exposed within the residual gravels they exhibit botryoidal surface textures and appear to have formed as discrete nodular concretions within the gravel. Due to widespread evidence of periglacial disturbance in the Pleistocene, it is difficult to assess whether or not some blocks are in situ. 2. METHODS Thirteen silcretes from East Devon were analysed on a Philips PW1220 X-ray spectrometer for major oxides and 19 trace elements. Technical problems required that SiOz was analysed for in some samples using atomic absorbtion spectroscopy. Selected samples were examined using a Philips Scanning Electron Microscope 501B with an attached LINK Systems energy dispersive analyser for chemical analysis. Thirtyone thin sections of silcretes from the Sidmouth area of East Devon have been examined.

(a) Residual quartz arenites These are the most widely distributed silcretes in East Devon and have an appearance similar to the 'sarsens' of southern England (Summerfield, 1979). They consist of quartz cemented sandstones, siltstones or indurated residual flint gravel with a quartz arenite matrix. The latter are the 'siliceous breccia' of Woodward & Ussher. In thin section a variety of textures are observed. The terminology employed in describing the textures is that of Summerfield (1979) who classified silcrete fabrics as GS-, or grain supported; F-, or floating; M-, or matrix dominated and C-, or conglomeratic fabrics. In the quartz arenite silcretes a mixture of GS- and F- fabrics is observed. Tightly interlocking euhedral to subhedral quartz grains formed by optically continuous quartz overgrowths on residual detrital grains form the GS-fabrics. Although not easily distinguished in thin section, the quartz overgrowths are readily identified under the SEM (Fig. 1a). Microcrystalline quartz associated with small TiO z crystallites less than 1 {tm in diameter are seen to replace the margins of detrital quartz grains producing indistinct grain boundaries in thin section and F-fabrics (Fig. 1b, c, d and 2a, b). Interstices between grains are filled with microcrystalline quartz but abundant pore space still remains (Fig. 1d). Small detrital grains appear to have been completely recrystallised to microcrystalline quartz in F-fabrics. Flint clasts have indistinct boundaries with the surrounding matrix and optically continuous quartz overgrowths on quartz grains impinge deep into the original area of the clast. Large grains (>30 {tm), and sometimes much smaller ones, have undergone dissolution. Evidence for dissolution is suggested by deeply embayed grain margins, which frequently grade into microcrystalline quartz. In thin section, colloform textures (Rogers, 1917) are seen to fill pores space in the F-fabric (Fig. 2e). These features, along with the geochemistry are discussed in the sections below. (b) Conglomeratic silcretes With the exception of rare flint conglomerates (Cfabric sensu Summerfield, 1979) the conglomeratic sil-

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Fig. 1. SEM photographs. (a) Int erlocking euhedral optically continuous quartz overgrowths forming GS- fabric silcrete . Scale bar is 50 /lm. (b) Typical F- fab ric showing residual detrital quartz grains mantl ed and separated by microcrystalline quart z cement. Scale bar is 100 tar: (c) En larged view of (b) showing small (1-5/lm in diameter) euhedral to subhedral quartz and titania crysta llites between the residual detrital quartz grains. Scale bar is 20 /lm . (d) En larged view of (b) showing det ails of the inters titial cement in F- fabric. Note the abundant pore space still remaining after silicification. Scale bar is 10 /lm.

cretes consist of M- or F- fabric clasts in an M-, F- or GS- fabric matri x. Thus both clasts and matrix invariably consist of fine grained silica and are analogous to the porcellanitic silcretes of some authors (e.g. Smale, 1973). These silcretes have a more restricted occurrence than the quartz arenite silcretes but in the Sid-

mou th area become locally very abund ant. The bulk of the matrix in M-fabric silcrete (Fig. 2c, d) consists of crypto- or microcrystalline qu artz with abundant TiO z crystallites. The F- and GS- fabrics are micromorphologically the same as in the quartz are nites. Colloform textures are abund ant and

Fig. 2. Opti cal photomicrographs. (a) Typical F- fabric silcrete with large rounded to subrounded residual detrital quartz grains in a matrix of intimat ely mixed microcrystalline quart z and titania . Gr ain boundaries are often stro ngly ernbayed and diffuse suggesting that both dissoluti on and replacemen t of primary grains by matrix cement, respectively has occurred . Scale bar is 300 /lm , plane polarised light. (b) Same view as (a) , but with crossed polars, showing sub-isotropic natur e of the cement. (c) M- fabr ic silcrete. Large well-rounded detrital quart z float in a matrix of turbid, intimately mixed microcrystalline quart z and titania containing much smaller sub-rounded quartz grains. Scale bar is 1 mm, plane polarised light. (d) M- fabric with crossed polars showing the isotropic natur e of the cement which here makes up over 90% of the rock. Scale bar is 300 /lm . (e) Colloform texture infilling interstitial pore space in F- fabric silcrete. scale bar is 500 /lm, plane polarised light. (f) Colloform banding in M- fabric matrix of conglomeratic silcrete. Titania-rich clasts are visible at middle Jeft and top right. Scale bar is 1 mm, plan e polarised light. (g) Colloform texture in F- fabric silcrete. Fine silica-rich and titania-rich laminae altern ate and are offset by microfra cture s which may relate to desiccation of the original gel. Scale bar is 1 mm, plane polarised light. (h) Englarged port ion of (g) showing laminated texture and microfracture (top right). Scale bar is 200 usn, plane polar ised light.

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form laterally extensive (Fig. 2f, g) but disrupted parts of the matrix or occur as rounded to irregular shaped clasts. Cryptocrystalline silica and titania alternate in the finely banded colloform textures (Fig. 2h). The cryptocrystalline silica and titania in the colloform textures and the tur bid matrix occur as crystallites less than 1 uu: in diame ter. Under low magnification in thin sectio n this cement has very high relief and is almost isotropic under crossed polars (Fig. 2d). Because of the small grain size and the intimate nature of the silica-titania mixture, precise identification of the Ti0 2 phase has not been possible, but it almost certainly lies in the anatase-rutile series. (c) Geochem istry

TABLE 1. Majo r oxide and trace element analyses

sio, Al z03

rto,

Fe Z0 3 MgO CaO NazO KzO PzOs loss V Cr Mn Ni Cu Zn Ga As Rb Sr

I 96.20 0.11 2.06 0.39 0.00 0.11 0.30 0.00 0.00 0.75

2 96.32 0.02 1.17 0.13 0.00 0.10 0. 19 0.00 0.00 1.25 0 5 13 0 0 1 0 1

a

17 19 0 0 2

3 95.99 0.07 1.81 0.35 0.00 0.10 0.32 0.00 0.00 0. 87 0 16 16 0 0 1 0 1 0 18 15 618 36 436 < 10 < 11 6 <4 3

4 0.31 0.04 0.37 0.30

0.005 0.22

0.39 6 6

Representative analyses of the silcretes are shown in Table 1. Chemica lly the silcretes are simple, consisting 2 of abo ut 96% Si0 2 and a mean value of 1.81% Ti0 2. a Fe203 is present in small amo unts but overa ll the 3 remaining major oxides are strongly depleted relative 0 a to average crusta l values (Turekian, 1971). Of the 33 9 6 trace elements analysed Zr, Nb, Ba, and U show 20 8 10 values around average crust (0.5-5 .0 x average crust) . y 549 626 132 It is thought that Nb is contained in the Ti0 2 phase Zr 41 23 7 and Ba and U in zircons which are observed as round - Nb 651 324 105 ed grains in thin section . The remaining elements Ba 20 17 (Y , Mn, Ni, Cu , Zn, Ga , As , Rb, Sr, La and Ce) are La 31 0 dep leted «0.05 x ) and generally below detection Ce 7 5 2 limit. The conglomeratic silcretes have higher Ti0 2 Pb 8 4 contents than the quartz aren ite silcretes but both Th 4 4 2 types are rich in titania according to Summerfield 's U (1979) > 1% criteria. Severa l authors have noted the (1) a representative conglomeratic silcrete , high abundance of Ti0 2 in silcretes elsewhere and (2) a representative F- and GS- fabric quartz arenite, and silcretes are charac terised by TiO-z/AI20 3 ratios (3) a mean ana lysis for 13 silcretes of various textural type markedly different from all other sedimentary rocks from Sidmouth area; (Cressman, 1962). In terms of major oxide geochemis- (4) are standard deviations. try these 13 samples from East Devon have very SiO z was determined using A .A.S. techniques and all other similar chemical compositions to analyses of silcretes elements were determined by X .R .F . The values of La and from elsewhere (Kerr, 1955; Frankel, 1952; Watts, Ce given in (1) and (2) are rat her anomalous in that all other 1975, 1977; Summerfield, 1979). Few trace element samples gave values of ppm. The figures given for La and analyses have been reported in the literature, howev- Ce in (3) a re lowest limits of detection as is that for Th . er, Hutto n et al. (1972) have recorded Zr values, fabric conglomeratic silcretes suggests that the two are similar to those presented in Table I, from silcretes in genetically and temporally related. Sout hern Australia. The sequence of events observed is similar to sequences observed in kaolinitic and siliceous residual deposits elsewhere in the world. Kaolinisation accompa 4. DI SCUSSION nied by dissolution of quartz followed by deposition of The textures of the conglomeratic F- and M-fabric quartz during silicification has been reported by silcretes suggests severa l phases of silicification, subae- numerous authors (e.g. Millot , 1970; Stephens, ]971; rial erosion and deposi tion followed by re-silicification Parron et al. 1976). The genet ic relationship between suggesting a near surface origin . The conglomeratic the two events is not simple and some authors prefer silcretes are envisaged as forming as crusts at or just an autochthonous origin for the silica (Bruckner , ]966; below the surface . The quartz arenite silcretes prob - Parron et al. 1976) whilst others prefer a source outably formed at a greater depth in the profile and show side the silcrete profile (Step hens,]964; Watts, 1975, little evidence of subaerial exposure. The similarity of 1978). In all the thin sections of silcrete examined there the tita nia-rich microcrystalline quartz matrix from Ffabric quartz are nites to the bulk of the M- and F- was clear evidence of high silica and titania mobility .

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Furthermore , the almost total repl acement of kaolinite during silicification indicate s high AI mobilit y. This is in contras t to the earlier ph ase of kaolinisation during deep weathering when silica and titania were mobile but AI was immobile . Aluminium mobility suggests a pH of less than 4 (We y & Siffert, 1961) and , assuming th at Ti forms Ti(OH)4 when released (Watts , 1977) , Ti rem ains soluble at a pH of less than 5. Th e solubility of silica is low at a pH less than 10 and rises sharply above that value (Wey & Siffert, 1961) . Silica sols , howev er , are very stable at low pH (Kitchener, 1971). Silica in suspension as a sol would be stable and at a pH between 2 and 5 the electric potential on Ti0 2 and Si0 2 particle surfaces would be oppo site , allowing an intimate mixture of Ti02 and Si02 to form. Watt s (1977) envi saged colloform textures in some Au stralian silcretes forming in this way. Summerfield (1979) classified silcretes in southern Africa, on the basis of field relationships, micromorphol ogy and geochemistr y, into those associated with weathering profile s (wea thering profile silcretes) and tho se which are not (non-weathe ring profile silcrete s). Weathering profile silcretes, he concluded, were generally characterised by F- and M- fabri cs, abundant authigenic glaebules (see also Williams on , 1957; Watt s, 1978) , abundant colloform textures and a high Ti0 2 content (> 1 %). Non-weathering profile silcrete , on the other hand , is char acterised by F- fabr ics with

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uncommon GS- and C- fabrics, chalcedonic overgrowths and optically co ntinuo us quartz overgrowths . The silcretes described here are different in severa l respe cts from Cenozoic silcre tes ('sa rsens' or 'sarse n stones' ) described from else where in southern England (Kerr , 1955; Summerfield, 1979; Summerfield & Whalley, 1980; Summ erfield & Goudie , 1981). Th ese other silcretes are char acteri sed by abunda nt GS- fab rics and optically continuous quartz overgrowths . No colloform textures have been recorded and they are also typically Ti0 2 poor. Summerfield (1979) concluded that silcretes from southern England were most similar to non-weathering profile silcre tes. The silcretes described in th is paper have cha racte ristics of both weat he ring and no n-weathe ring profile silcretes , accord ing to Summerfield's crite ria . They are clearl y associated with the weathering profiles however, suggesting that firstly the silcretes of E ast Devon represent a complex diagenetic histor y, perhaps involving several phases of silcretiza tion at different levels in the weatherin g pr ofile , and secondly that any distinction between weathering or non -weathering profile silcrete can only be made sat isfactorily on the ba sis of field relations. ACKNOWLEDGEMENTS The chemical analyses and thin sections were made by the Department of Geol ogy, University of Exeter.

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PARRO;;;, c., NAHON , D., FRITZ, B., PAQ UET , H. & MILLOT , G . 1976. Desilicification et qua rtzification par alteration des Gres Albians du Ga rd. Modeles geocherniques de genese des dalles qu artz itiques et silcretes. Sci. cea. Bull., 29, 273-81. ROGERS , A . F. 1917. A Review of the A morp hous Minerals. J. Geol., 25, 515-41. SMALE , D. 1973. Silcretes an d associa ted silica diagenesis in southern Africa and A ustralia . 1. Sedim. Petrol. 43, 107789. STEPHENS , C. G . 1964. Silcretes of centr al Au stralia. Na ture Lond., 203, 1407. - - 1971. Later ite and silcrete in Au stralia: A study of the Genetic Relationship of Laterite and Silcrete and their Compa nion Materials, and their Collective Significance in the Formation of the Weathered Mantle Soils, Relief and Drainage of the Australian Contin ent. Geoderma, 5, 5-52 . SU MME RFIELD . M. A . 1979. O rigin and palaeoenvironmental interpret ation of sarsens. Nature, Lo nd., 281, 1379. - - & WHALLEY. W. D . 1980. Petro graphic investigation of sarse ns (Ce nozoic Silcretes) from Southern England . Geol . Mijnb ou w, 59, 145-53. - - & GOUDIE , A. S. 1981. The sarsens of southern En gland : their palaeoen vironm en tal interpr etati on with refere nce to other silcretes. Inst. Br. Geog r. Spec. Publ., 11, 71-100.

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TUREKIAN, K. K. 1971. McGraw-Hill Encyclopedia of Science and Technology, 2nd Ed. 4, 719 pp. WAITS, S. H. 1975. An Unusual Occurrence of Silcrete in Adamellite-Implications for Duricrust Genesis. Search, 6, 434-35. - - 1977. Major element geochemistry of silcrete from a portion of inland Australia. Geochim. et Cosmochim. Acta, 41,1164-7. - - 1978. A Petrographic Study of Silcrete from Inland Australia. Journ. Sedim. Petrol., 48, 987-94.

WEY, R. & SIFFERT, B. 1961. Reactions de la silice monomoleculaire en solution avec les ions AP+ et Mg2+. Genese et synthese des argiles. Coil. Intern. C.N.R.S., 105,11-23. WILLIAMSON, W. O. 1957. Silicified sedimentary rocks in Australia. Am. J. Sci., 255, 23--42. WOODWARD, H. B. & USSHER, W. A. E. 1911. The Geology of the Country near Sidmouth and Lyme Regis. Mem. Geol. Surv. G.B.