Tertiary lateritic weathering in Devon, England, and the Palaeogene continental environment of South West England

Tertiary lateritic weathering in Devon, England, and the Palaeogene continental environment of South West England K. P. Isaac ISAAC, K. P. 1983. Tertiary lateritic weathering in Devon, England, and the Palaeogene continental environment of South West England. Proc. Geol. Ass. 94, (2),105-114. The plateau deposits of East Devon contain a variety of residual deposits including non-indurated kaolinitic weathering profiles and silcretes. These reflect a complex pedological, diagenetic and geomorphological history which began with the emergence of the post-Chalk land surface at the end of the Cretaceous. During the Palaeocene, lateritic weathering of the Chalk led to the formation of kaolinitic residual flint gravels which are up to 10 m thick over much of the East Devon tableland. These deposits show a variety of deep weathering profile morphology, but well differentiated lateritic weathering profiles are preserved in irregular deep pockets on Chalk. Where resting directly on Albian sediments the residual gravels are thought to represent the response of earlier profiles developed on Chalk to continued pedogenesis and diagenesis once the Chalk had been completely removed. Locally, late silicification of the weathering profile formed silicretes. Cenomanian-Albian calcarenites and arenites have been decalcified and kaolinised to considerable depths beneath the gravels where a protective Chalk capping is absent. In areas adjacent to the Sticklepath-Lustleigh Fault Zone destruction of the Palaeocene weathering mantle was accomplished by late Middle Eocene times when new weathering profiles were established on newly exposed Upper Palaeozoic rocks. These younger profiles were subsequently eroded and redeposited in deep tectonic basins along major wrench fault zones. This phase of erosion and sedimentation was accompanied by a climatic change which is reflected by changes in profile morphology and clay mineralogy with time. The oldest Palaeocene profiles are mature, took> 107 years to form and developed in a tropical climate, whereas profiles dating from the Lower Eocene onwards are immature, took = 106 years to form and developed in a sub-tropical to temperate climate.

Department of Geology, University of Exeter, North Park Road, Exeter EX4 4QE

1. INTRODUCTION The plateau deposits of East Devon (Fig. 1) were first described by Woodward & Ussher (1911) who recognised that residual 'true Clay-with-Flints' was present in the lithologically complex stratigraphical unit they designated 'Clay-with-Flints and chert in part Eocene' on the Geological Survey one inch Sidmouth sheet (326 & 340). They assigned a Tertiary age because of the involvement of the 'Clay-with-Flints' in faulting. Subsequently, Waters (1960) and Hamblin (1973a, b) have studied these deposits and invoked an origin involving dissolution of Chalk beneath a Tertiary sedimentary cover. The author's interest was aroused by the dissimilarity of the East Devon plateau deposits to the well known Clay-with-Flints on Chalk in South East England (Loveday, 1962; Pepper, 1973). In particular, the clay mineralogy is unusual in that it consists almost exclusively of kaolinite. As work progressed it became apparent that the East Devon deposits have marked similarities with lower Tertiary deep weathering profiles developed on Upper Cretaceous sediments in France (Millot, 1970; Klein, 1971; Parron & Nahon, 1980). This paper is an extension to the author's previous work (1979, 1981) and describes the evidence that led to this conclusion and comments on

the significance of Tertiary residual deposits in general in South West England, particularly on their clay mineralogy.

2. GEOLOGICAL SETTING The high plateau of East Devon forms a deeply dissected tableland about 150 m about sea-level in the east but rising westwards to 300 m in the Haldon Hills (Fig. 1). The New Red Sandstone dips eastwards, resting unconformably on Upper Palaeozoic rocks deformed in the Hercynian orogeny. In the east, near Lyme Regis, the New Red Sandstone passes conformably upwards into Jurassic sediments. This succession is overstepped by a major unconformity upon which a generally thin Upper Cretaceous succession rests. In the Beer-Seaton district intra-Cretaceous (Smith, 1957; Drummond, 1970) and possibly intra-Tertiary folding has preserved up to 80 m of Lower and Middle Chalk (Rowe & Sherbourne, 1903). Elsewhere, the Upper Cretaceous consists of the Upper Albian to Cenomanian Upper Greensand composed of glauconitic arenites, calcareous sandstones and chert beds (Tresise, 1960, 1961) and a condensed Cenomanian carbonate sequence (Smith, 1961) preserved only between Sidmouth and Beer. Capping the Upper Cre-

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Fig. 1. Map showing the distribution of Upper Cretaceous sediments, Palaeocene residual deposits and the sedimentary Tertiary basins in Devon and East Cornwall. Principal localities mentioned in the text are indicated along with the outcrop of the South West England granites and Tertiary wrench fault zones (after Dearman, 1963; Freshney et al., 1982). Insets show position of South West England in Great Britain.

taceous are plateau deposits having a superficial resemblance to the well known 'Clay-with-Flints' of South East England and the Argille a Silex of France; it is these deposits which are initially the subject of this paper.

3. METHODS This study is predominantly field based, but the following laboratory work has also been carried out. The sample localities are shown on Fig. 1. The -211m size

T ERTI AR Y W E ATH ERI N G AN D PALAEO GENE ENVI RONMENT I N SOUT H W ES T ENGLAN D

fraction of 125 samples has been ana lysed using standard X-ra y diffracti on methods and identification procedures in order to determine clay miner alogy. Twenty-thr ee selected sample s of kaolinitic residual gravel matrix from East Devon were analy sed on a Philips PW1220 X-ray spectrometer for maj or oxides and 20 trac e elements. Selected samples wer e examin ed using a Philips Scannin g Electron Micro scope SOlE with an att ached LINK Systems energy dispersi ve ana lyser for chemical analysis. Ele ven resin impregnated samples of weathering profile materials have been examined in thin section.

4. PALAEOCENE WEATHERING It has recently been suggested that th e plateau de-

posits of East Devon may represent a complex of Tertiary deep weathering profiles formed by deep weathering of the Chalk in a tropical climate (Isaac, 1981). The oldest and principal stratigraphic unit is called, informally, the Combpyne Soil (Isaac op. cit.) and where subst antial disturbance of the profile has occured the depo sits are called the Peak Hill Gra vels (Isaac , 1979).

(a) The Combpyne Soil The similarity between a section at Combp yne (National Grid Reference SY 300923 to 306923) (Fig. 1) and th e typical lateritic weathering profile (Fox , 1936; Woodhouse, 1940; Prescott & Pendl eton , 1952) led the author to conclude th at these residual deposits represent a tropical residual soil. For details of the pro file morphology and clay mineralogy the reader is referred to Isaac (1981). Both the clay mineralogy (Isaac op . cit. fig. 7) and textures observed in thin section (Isa ac op. cit. plate 1b and c) are typical of tropical residual soils. Two type s of texture are observed under the scanning electron microscope (SEM) : (1) Open, porous and randomly oriente d kaolinite texture . Books and plates are of a range of sizes generally less than 2 11m in diameter. Some are well formed and often curved in shape, others are indistinct having ragged and irregular shapes. This texture is typical of kaolinite formed in weathering environments (Keller, 1976, 1978) and ind icate s neoformation of kaolinite in a highly porous med ium . (2) Tightl y packed , strongl y oriented kaolinite texture with little interstitial pore space. Plate s are often oriented parallel to detrital quartz grain surfaces and can often be seen to have a 'flow' fabr ic spreading from the top of a detrital quartz gra in downwards. This texture suggests that infiltration from above was significant at least for the depo sition of some of the kaolinite . Both textures suggest that abunda nt space was present prior to kaolinisation in situ and infiltration of other kaolinite from a position higher in the profile .

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This implies that dissolution of CaC0 3 from the Chalk preceded the kaolinising process and probably proceeded to consider able depths in th e weathering profile. Although not genetically significant in itself, chemical analysis of 11 samples taken at intervals up the profile at the type locality indicates that these sediments have a geochemistry comparable with deep tropical residual weath ering profiles (compare Kronberg et al, 1979). With the exception of SiOz and Ah03 which form the only main components, quartz and kaolinite, and minor FeZ03 in the red earth , all th e major oxides are strongly depleted (Ta ble 1) over average crustal value s (Turekian, 1971). Of the tra ce elements analysed , only La and Ce show any significant enrichment (>10.0 x average crust), As, Y, Cs and Pb show slight enrichment (5.D-1O.0 x), Y, Mn and U are strongly depleted «0.01 x) and Cr, Mn , Zn , Ga, Rb, Sr, Y and Ba are slightly depleted (0.5-0.1 x ). Zr, and to a lesser extent Nb, alone show a systematic variation with depth in the profile with the highest concentrat ions in the red earth decre asing downwards into the pallid zone . The concentration of Zr in the red earth could reflect a relative concentration of detrital zircons toward s the top of the profile. La , Ce , Cs and Sr show a marked system atic variation with Al z03 and thus kaolinite content . Rare earth s are kno wn to be relatively inert in weath ering profiles (Piper , 1974) and Cs may exhibit some biological affinities in tropical soils (Kronberg et al., 1979). Kronberg et al. (op . cit.) concluded that, overall , the concentration of many trace elements dur ing weathering may by controlled by the domin ant clay minerals and in this case adsorption of La , Ce , Cs and Sr onto kaolin ite is en visaged.

(b) The Peak Hill Gravels Where residual flint gravels rest directly on Upper Greensand the gravels tend to be poorly differentiated in term s of vertical zonation. Discr ete red, mottled and pallid zones do occur, but the variation is frequently horizontal and in places an intimate mixing of red and mottl ed, or red and pallid zone material is observed. A degree of heterogeneity with considerable lateral variation in profile morphology in lateritic weathering profiles is not uncomm on (Stephens, 1961) , but , her e, may reflect periglac ial processes operative during the Pleistocene. The miner alogy of the clay fraction is everywhere qu artz + kaolinite . The kaolinite is normally very disordered (Fig. 2) but a spectrum of kaolinites from highly disordered to well ordered varieties can be recogni sed . In some areas, for example just west of Sidmouth (Fig. 1) , the kaolinite diffraction patt ern is indistinct with a low, bro ad basal reflection ranging up to 7.5 A. Both elong ate and book-like clay particle s are observed under the SEM in this mat erial makin g

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TABLE 1. Whole rock major oxide and trace element analyses, determined by XRF, of 11 'samples from a lateritic weathering profile developed on chalk at Combpyne (Fig. 1). Depths, in metres, are measured from top of the section, a disused railway cutting from National grid reference SY 300923 to 306923 (see Isaac 1981). Means with standard deviations are also given. Red earth Depth (m) Si0 2 Ah0 3

rio,

Fe Z03

MgO CaO NazO K20 PzOs Loss V

Cr Mn Ni Cu Zn Ga As Rb Sr Y Zr Nb Cs Ba La Ce Pb Th U

1.0 76.68 13.66 0.89 4.83 0.38 0.15 0.29 1.09 0.14 4.72 79 87 158 25 6 51 10 9 77 65 59 493 20 10 270 62 77 27 16 4

1.5 73.14 13.71 0.93 5.67 0.64 0.19 0.91 1.58 0.12 2.13 78 107 154 29 10 65 12 12 99 63 62 485 18 12 322 63 92 26 16 4

2.0 70.96 14.92 0.91 5.95 0.28 0.14 0.50 0.87 0.13 2.13 86 107 60 25 10 46 14 8 75 56 54 490 19 11 206 66 98 28 20 4

Pallid zone

Mottled zone 2.5 70.45 16.37 0.78 5.32 0.24 0.22 0.03 0.86 0.13 0.72 96 105 71 41 11

54 13 11 72 89 55 390 18 9 208 109 142 45 24 3

3.0 68.77 17.32 1.03 5.51 0.19 0.15 0.15 0.76 0.30 6.64 114 117 27 63 48 134 13 6 54 174 95 444 21 11 249 152 211 55 27 3

4.0 71.13 16.94 0.79 4.36 0.13 0.17 0.18 0.61 0.18 6.69 96 95 39 58 32 97 11 5 44 126 75 294 18 7 244 161 158 54 24 2

the presence of a mixture of disordered kaolinite and meta-halloysite a possibility. The base of the residual flint gravels is marked by a black to chocolate coloured flint-free clay band about 0.1-0.3 m thick. Systematic sampling of this band shows that at the base the mineralogy consists of disordered kaolinite + illite-smectite mixed-layer mineral, about 60% expandable + minor quartz (Fig. 2). The proportion of kaolinite remains constant to the top of the band but the mixed-layer mineral changes gradually to a 90% expandable illite-smectite or pure montmorillonite. The boundary with the overlying flint gravels is marked by a sudden disappearance of the mixed-layer mineral and the incoming of the typical kaolinite + quartz assemblage. This smectite-rich layer cannot represent the basal horizon of the present

5.0 66.77 20.25 1.12 2.51 0.35 0.20 0.00 0.77 0.56 7.65 90 90 0 66 41 68 14 2 54 431 240 389 26 14 362 459 571 62 24 3

6.0 68.62 20.64 0.47 0.97 0.28 0.22 0.11 0.45 0.59 8.69 32 51 0 70 10 58 8 0 32 695 196 164 10 17

353 495 633 87 27 1

7.0 83.83 12.21 0.31 0.39 0.00 0.11 0.40 0.25 0.06 3.33 20 35 0 29 0 22 6 0 18 108 22 129 9 7 245 135 173 42 11 2

8.0 71.70 18.57 0.53 1.14 0.08 0.19 0.00 0.49 0.44 7.31 38 54 0 55 7 48 9 0 35 504 130 179 13 20 345 397 500 84 24 1

9.0 88.80 8.39 0.23 0.35 0.03 0.10 0.01 0.16 0.00 3.52 12 17 6 3 0 10 4 0 12 59 15 113 8 3 232 60 56 24 12 0

X

S.D.

73.71 15.72 0.73 3.36 0.24 0.16 0.29 0.72 0.24 4.87 67 78 47 42 16 59 10 5 52 215 91 324 16 11 276 196 250 48 20 2

6.51 3.46 0.28 2.19 0.17 0.03 0.30 0.38 0.19 2.54 33 32 56 20 15 32 3 4 25 211 67 146 5 4 56 160 200 21

5 1

observed weathering profile because kaolinitic clays occur beneath in the altered Upper Greensand. It is possible, however, that these clays represent the basal horizon of the weathering profile before the intervening Chalk had been removed by dissolution. The fact that similar smectitic clays occur as thin deposits lining solution pipes in the Cenomanian Limestone beneath the flint gravels further east makes this hypothesis plausible. The lowest horizons in lateritic weathering profiles on a variety of lithologies are recorded as being characterised by montmorillonite and mixedlayer minerals (Loughnan, 1969; Millot, 1970). (c) The Tower Wood Gravel On the Haldon Hills just east of the Bovey Basin (Fig. 1), the kaolinitic nature of the plateau deposits has

TERTIARY WEATHERING AND PALAEOGENE ENVIRONMENT IN SOUTHWEST ENGLAND

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Fig. 2. Representative X-ray diffraction analyses (Co K ex radiation from 0-30°28) showing the typical clay mineralogy of the residual deposits. Palaeocene residual deposits are represented on the left and the Eocene-Oligocene residual deposits are on the right. (a) Typical Peak Hill Gravels matrix of disordered kaolinite and quartz probably with metahalloysite being present in some samples. (b) Typical Peak Hill Gravels matrix from red earth type lithology. (c) Disordered kaolinite + quartz assemblage from the red earth at Combpyne (see text). (d) Well ordered kaolinite + quartz assemblage from the pallid zone at Combpyne. (e) Clay matrix of horizon separating Peak Hill Gravels from Upper Greensand showing the typical kaolinite + illite + mixedlayer illite-montmorillonite + quartz assemblage. (f) Exceedingly fine grained disordered kaolinite separated from the deeply weathered Upper Greensand. The rather low, assymetrical and board reflections are a function of grain size disorder. (g) kaolinitic sediment of the Petrockstow-Dutson type Bovey Formation showing the typical disordered kaolinite + illite + mixed-layer clay + quartz assemblage. Again, the very broad reflections are a function of the very small particle size of the material. (h) Deeply weathered Upper Carboniferous shale from the Eocene-Oligocene weathering profiles associated with the Bovey Formation at Petrockstow. Disordered kaolinite, illite, and mixed layer clays dominate. (i) Slightly weathered Upper Carboniferous shale retention of some primary mineralogy. (j) Unweathered Upper Carboniferous shale dominated by chlorite and illite (2M) with quartz. There is an increase in kaolinite content from (j)-(h). All samples were air dryed -2/lm size fraction mounted by sedimentation on glass slides. Glycolation and heat treatments were carried out for identification purposes but are not shown here.

been known for sometime (Hamblin, 1973a, 1973b). Here, a residual flint gravel, the Tower Wood Gravel, rests on completely decalcified Upper Greensand and is overlain by fluvial gravel deposits. The Tower Wood

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Gravel is lithologically and clay mineralogically identical to the Peak Hill Gravels and is thought to be a continuation of the East Devon kaolinitic residual deposits (Isaac, 1981 p. 167). (d) Evidence for the Palaeocene age of the kaolinitic residual deposits The evidence for the lower Palaeogene age of the kaolinitic residual flint gravels of the East Devon tableland can be summarised as follows: (1) The residual gravels pass beneath, and are thus older than, the Upper Eocene-Lower Oligocene Bovey Formation (Edwards, 1976; Isaac, 1981 p. 166). (2) The residual flint gravels are cut by faults which are likely to be late Middle Eocene or early Upper Eocene in age (Isaac op. cit., p. 166). Thus these residual deposits are pre-Upper Eocene. It has also been argued (Isaac op. cit. p. 167) that these products of lateritic weathering correlate with the Interbasaltic Formation of Northern Ireland, which has a lateritic weathering origin (Eyles, 1952) and an age closely constrained by radiometric datings of igneous rocks associated with it. The age of the Interbasaltic formation is early Palaeocene (Danian) and it has been suggested (Isaac, 1981) that this is the age of the East Devon kaolinitic residual deposits. 5. DEEP WEATHERING OF CENOMANIAN AND ALBIAN SEDIMENTS West of Beer a condensed Cenomanian succession occurs, characterised by the Cenomanian Limestone (Smith, 1957), a series of thick bedded to rubbly calcarenites, limestones and nodular chalks up to 5 m thick. Beneath this is the Upper Greensand of Upper Albian age, but including the Cenomanian in the far west. The Upper Greensand consists of calcareous sandstones, arenites, massive, nodular to well bedded cherts and is locally glauconitic. Tresise (1960, 1961) divided the Upper Greensand into a normal facies in the east and the Blackdown facies which occurred west of a NE-SW trending line running just west of Sidmouth. The Blackdown facies in contrast to the normal facies is completely decalcified, and glauconite, common in the normal facies is absent, having been replaced by 'limonitic oxides' (Tresise, 1960). Tresise argued that the Blackdown facies originated by weathering of the normal facies deprived of a protective Chalk capping in comparatively recent times. However, about Sidmouth, in the transitional area between the normal and the Blackdown facies, the Upper Greensand beneath substantial outliers of Cenomanian Limestone has suffered partial decalcification to considerable depths. Beneath a mantle of up to 5 m of kaolinitic residual flint gravel, 5 m of Cenomanian Limestone occurs resting on deeply weathered Albian calcareous sand-

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stones with chert beds. Th e calcareo us sandstones have been decalcified to depths of 30 m. Glauconite and clays have been altered to goethite and kaolinite (Fig. 2), the goe thite imp arting a pale reddi sh brown colour to th e weathered sedime nts . At Dunscombe (SY160887), 2 km ea st of Sidmouth , 1.5 ~ of .the Cenoma nian Limestone is expos ed . Soluti on pipes about 1 m in diameter filled with black montmorillonitic clays and residual flint gravels protrude down into the Limeston e and in plac es pass through into decalcified sedime nts ben eath , giving rise to a hea vily clay infiltrated zone beneath th e pipe. Th e base of the Cenomanian Lime stone is very irregular and varies from being sharp to gra d a t i o ~ a l over about 1 m. Solution cavities at the boundary with th e Upper Greensand suggest that co ns o li d a~i o n .and collapse of the residuum occurred after ~ ecalc.lficatlOn. Glauconite is common in the Cenom am an Limestone as rounded grains but is replaced by irregular aggregat es of mixed kaolinite and goeth ite in the Upper Greensand beneath. Pore space in the Upper Greensand are lined with tangenti al kaolinite skins ; large solution cavities are filled with finely layered goethitekaolinite sediments containing ra re sand lenses. It appears therefore that alteration and decalcification of the Upper Greensand began before th e dissolution of younger calcareous rocks above was complete . ~he similarity of the alteration products to the clay fraction of the kaolinitic re sidual gravels above suggests that th e form ation of the residu al gravels matrix and the alt er at ion of the U pper Greensand were conte mporaneo us . 6. SILCRETES Although th e kaolin itic residu al deposits of E ast De von are largel y non-indurated , local silicification at vari ous levels in the weather ing profiles has led to th e form ation of silcretes, highl y siliceous terre strial sediments which are the products of surface or near-surface dia genesis . Silcretes are particularly abunda nt in the Sidm outh area (Fig. 1) and wer e recorded by Kerr (1955, fig. 1). More recently the aut~or has described the micromorphology and geoch emistr y of silcretes from the Sidmouth area (Isaac, 1983) and concluded th at they show features typical of both weathering and non-weath ering profile silcretes (Summerfield, 1979). Perhaps most significantly the Ea st De von silcre tes are titania-rich and contain ab und ant colloform textures, two features that are character istic of silcretes associated with kaolinitic weathering profiles worldwide . Th e association of the kaol initi c residu al deposits with , albeit local , developments of silcretes is a featur e typical of deeply weathered tropical to sub-tropical regions (e.g. Millot , 1970; Steph ens, 1971). The ~n­ viro nmental par ameters which control the formation and distribution of silcretes are poorly unde rstood , ho wever , so th at further discussion of the East De von silcre tes her e can make littl e contribution to the

palaeoenv ironm ental discussion at the end of the paper. 7. EOCENE-OLIGOCENE WEATHERING The Tertiar y de posits of De von can be divided into two groups: (1) residual deposits and (~) th~ Bovey Format ion ; kaolinitic sediments deposited 10 deep fault cont rolled basins and formed largely of ero de d residu al depo sits. Two types of residu al deposits can be reco gnised: (1) kaolinit ic residual dep osits formed from Upper Cre taceous sedi men ts and described above , and (2) kaolini tic residu al deposits formed from Upper Pal aeozoic met asediments. Th e later situation is observed in the Petrockstow Basin (B ristow, 1968; Freshney, 1970) . (a) Eocene-Oligocene weathering profiles The transition from Upper Carboniferous interbe dded sandstones and shales to Oligocene Bovey Formation sediments has been described by Bristow (1968) from bo reh oles in th e relati vely thin shelf successio ns near the margins of th e basin , and the autho r has examined similar bor ehole mat erial. Un weathered bedrock , usually dark grey fissile shales with interbedded greenish grey turbiditic sandsto nes, passes gra dua lly into pale grey to cream c~l­ oured to white , soft sandy clays and clayey sands ~n which bedding, sedimentary structures and tect omc cleavage are still preserved. The minerology of the bedrock consists of qu artz + illite + chlor ite ± albite . Changes obs erved with increasing alte ra tion are the incom ing of disordered kaolinite , the loss of ch l ? r~ te , the form ation of irregular mixed-layer clays (illitesmectite , chlorite-s me ctite) (Fig. 2) , and the growth of irregular swirling masses of hematite. The first stag~s of visible alteration occur at a depth of about 20 m In the weathe ring profile , but mineralogical cha nges attributable to weathering are observed to greate r depths. Only at the top of the profile are bedding and cleavage difficult to identify . The geo chemical changes accompanyi.ng the deep weathering have been documented by Bristow (1968). The age of .the Bovey Formation overl ying th ese residual depo sits is Lower Oligocene although th e gre ater part of the Bovey Formation at Petrockstow IS known to be U pp er Eo cene (Freshney et al, 1979). Th is type of resid ua l deposit is onl y clearl y obs erve d in situ at Pet rockstow . (b) The Bovey formation Bristow (1968) concluded th at the sediment s of the Petrockstow Basin were deri ved from deepl y weathered Upper Palaeozoic strata and ~ h a t a ~ea t he ri ?-g mantle similar to the residual deposits see n In association with the Bovey Formation at Pet rockstow was

T ERTI ARY W E ATH ERI N G A ND P ALA EOGEN E EN V I RON M E NT IN SOU T H W E ST E N GL A ND

once wide spread in South West Engl and . The recent discov ery of additional basins, one east of Lundy at Stanley Banks (Fletcher, 1975) on the SticklepathLustleigh Fault zone (Dearman , 1963) and one at Dutson near Launceston, Cornwall (Freshney et al, 1982) confirms this supposition: both the Stanley Banks and Dutson basins are sedimentologically and lithologically most similar to the Petrockstow Basin. The clay mineralogy of the Dutson Basin is characterised by the assemblage disordered kaolinite + illite + mixed-layer clays (Fig . 2) ± verm iculite ± smectite and suggests derivation from immature subtropical weathering profiles (Freshney et al, op. cit.) . The Bovey Basin, in the south , apa rt from being by far the largest of the Bovey Formation basins, is different from the other basins in several respects, but most significant to this discussion is the clay mineralogy. Both well-ordered and disordered kaolinites are present with some lithological units consisting entirely of well-ordered kaolinite (Vincent , 1978, p. 28). The proportion of illite is often markedly less and mixedlayer clays are rarer or absent . Several authors (Bristow, 1968; Edwards, 1976) have noted that the Bove y Formation of the Bove y Basin was of mixed origin with sediment input coming from both deeply weathered Upper Palaeozoic strata and from hydro thermal kaolin deposits on Dartmoor. This gives rise to the disordered and well-ordered varieties resp ectively. It is unlikely that materials from residu al deposits on the Upper Cr et aceous, only 25 km to the ea st , ever contributed significantly to the known Bo vey formation as this is singularly flint-fre e. However , in view of the widespread occurrence of well-ordered kaolinite either on its own or as a mixture in the East Devon residual deposits, a weathering profile origin for the Bo vey Formation well-ordered kaolinites should be considered. Deep chemical weathering , attributed to Tertiary tropical or sub-tropical processes , has been described from the northern margins of Dartmoor (Fookes et al., 1971; Dearman & Fookes, 1972). On the Dartmoor Granite itself, Linton (1955) envis aged deep Tertiary weathering playing a significant role in the formation of tors . Recent authors (Dearman & Baynes, 1978; Dearman et al., 1976) have concluded that the effects of Tertiary chemical weathering and Pleistocene mechanical weathering are both pr esent. The discovery of wide spread gibbsite in the weathered Dartmoor Gr anite (Green & Eden , 1971) is significant because this is a mineral typical of well drained , tropical weather ing profiles although it need not be diagnostic of such con sider ations. Support for a widespread deep weathering of the granite during Tertiary times comes from studies of the commerci al kaolin deposits of South West England (Bristo w, 1969). Some aut ho rs (Exley, 1959) favour an entirely hydrothermal origin for the se deposits whilst others (Sheppa rd, 1977) favour an entirely supergene

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or weathering or igin. Bristow (1977) summarised the evidence for both ori gins and suggested that a two phased evo lution seemed most likely with an early hydrothermal 'softening-up' phase, invo lving the argillic altera tion of the granite , follo wed by a later supergene ph ase of kaolini sation . Sheppard (1977) studied the oxygen isotope geochemistry of the kaolinised granites and the Tertiar y ball clays of th e Bovey Basin and concluded that the latter could only have been deri ved from weathered granite. It seems, therefore , that erosion of deeply weathered granite and deposition of the products in the Bovey Basin is not only po ssible but quite probable given the sp atial proximity of the potential source area to the Bovey Basin. In conclusion , it is apparent that the EoceneOligocene kaolinitic sediments of the Bovey Formation were derived exclusively from the erosion of a mantle of deep weathering profiles developed on Upper Pala eozo ic metasediments and, in the case of the Bovey Basin , weathered granite. As there is no evidence to suggest that the residual deposits developed on Upper Cretaceous sediments contributed to these accumulations, the bulk of these flint-be aring deposits must have been rem oved by erosion before the ons et of Bovey Formation sedimentation in the Eocene .

(c) Denudation of the Palaeocene weathering profiles In addition to the residual flint gravels, the plateau deposits of E ast De von include the Buller's Hill Gravel (H amblin, 1973) and the Aller Gr avel (E dwards, 1973) both representing fluvial sedimentary environments. The Buller's Hill Gr avel was formed by the eros ion and redeposition of the Tower Wood Gravel with the addition of material from deeply weathered Upper Palaeozoic sediments. The Aller Gr avel is very similar , cons isting of up to 20 m of rounded flint gravels and sands with subordinate bodies of white to pale grey silt and sandy clay, Upper Greensand chert and Upper Palaeozoic lithologies (Edwards, 1973). Although Hamblin (1974) and Edwards (1973) differed in their interpretations of the Buller's H ill and Aller Gravels, it is clear that they are spatially and temporally closely rel ated. In solution cavities in D evoni an limestone exposed in the Newton Abbot by-pass, Aller Gravel overlies Buller's Hill Gravel which in turn overlies Tower Wood Gravel , resting on Upper Greensand (Brunsden et al, 1976). All four unit s are overlain by the Bovey Form ation . Thus th e To wer Wood Gravel and its eastern equivalents ar e the oldest Tertiary litho str at igraphical unit ob serv ed in Devon. Fa sham (1981) carried out a gravimetric survey of the Bovey Basin and predicted a maximum depth for the basin of 1245 m . An Oligocene age for the higher part of the Bove y Formation has been known for some time (Chandler, 1964). However , as many authors have

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noted (Chandler, 1964; Edwards, 1976; Wilkinson & Boulter, 1980), the great thickness of sediments beneath the dated sections in the Bovey Basin implies the presence of a substantial Eocene succession, a hypothesis confirmed in the Petrockstow Basin (Freshney et at, 1979). It is believed that the fluvial flint-rich gravels represent a phase of erosion and destruction of the flint-rich Palaeocene weathering profiles prior to Bovey Formation sedimentation. This must have occurred before the Upper Eocene.

8. THE SIGNIFICANCE OF CLAY MINERALOGY Geochemical and mineralogical studies of residual tropical soils (e.g. Signiholfi et al., 1971; Kronberg et al., 1979) shows that clay mineralogy is the most sensitive indicator as to the age and intensity of weathering in a given weathering profile. The youngest, most immature soils are characterised by mineral assemblages containing quartz + illite + mixed-layer clays; mature soils contain exclusively quartz + kaolinite; and senile, the oldest soils, contain the assemblage quartz + kaolinite + gibbsite ± goethite ± hematite. The sensitivity of weathering profile clay mineralogy to palaeoclimate is also well known (Loughnan, 1969; Millot, 1970). In the absence of any other palaeoclimatic indicators the sensitivity of clay minerals to climate does provide significant, albeit tentative (Singer, 1980), constraints to the palaeoclimatic interpretation of the deposits. The main mineral assemblages can be recognised in the Tertiary residual deposits in Devon: (1) (ordered and disordered quartz + kaolinite varieties)±meta-halloysite±goethite in the Palaeocene residual deposits of East Devon and (2) quartz + kaolinite (disordered) + illite + mixed-layer clays in the weathering profiles associated with the Bovey Formation. The Tertiary sediments of the Petrockstow and Dutson Basins are typified by the assemblage quartz + kaolinite (disordered) + illite + mixed-layer clays (Bristow, 1968; Freshney et al., 1982) whereas the Bovey Basin is dominated by quartz + kaolinite (disordered and ordered varieties) + illite ± mixed-layer clays. From geochemical evidence, Kronberg et al. (1979) calculated that the time involved in producing mature tropical residual soils in Brazil was about 107 years. In the Palaeogene in SW England we are dealing with appropriate time periods. The destruction of the bulk of the Palaeocene weathering mantle was accomplished by Middle to Upper Eocene times giving a maximum of 1.5 to 2.0 x 107 years for deep weathering. Deep weathering profiles on Upper Palaeozoic rocks could only have developed once the earlier weathering profiles formed from Upper Cretaceous sediments had been removed. It is unlikely, therefore, that new weathering profiles would have developed beyond a relatively immature stage before they them-

selves were eroded and deposited in the deep tectonic basins. Thus the clay mineralogy of the two types of residual deposits could represent the length of the period of weathering: the Palaeocene residual deposits are mature and took at least 107 years to form whilst the Eocene-Oligocene residual deposits are immature and took less than 107 years to form, and probably nearer 106 years. Another factor to be considered is that a climatic change occurred between the formation of the mature East Devon profiles and the immature Petrockstow type profiles. A major climatic cooling at the end of the Eocene has been suggested by several authors using different lines of evidence (Savin, 1977; Buchardt, 1978; Wolfe, 1978). Collinson et al. (1981) noted that evidence from palynological studies of the French Palaeogene is difficult to interpret in terms of a single event and indicates cooling earlier in the Eocene. They presented palynological data from the Hampshire Basin which suggests gradual cooling commencing in the latest early Eocene and continuing throughout the Eocene. They concluded that climatic cooling took place over 15 X 106 years, a period which corresponds to the phase of immature weathering profile formation and sedimentation in tectonic basins.

9. CONCLUSIONS The residual deposits of the Dartmoor area record the climatic and denudation history of SW England in the Palaeogene. The East Devon residual deposits represent the establishment of a relatively stable land surface under an essentially tropical climate. This occurred with the emergence of the Chalk at the end of the Cretaceous and although the thickness of Chalk dissolved is unknown, it is likely to have been substantial. Tertiary residual deposits in East Devon represent a pedogenic or lithostratigraphical unit dating from the early Palaeogene. The deposits are dominantly kaolinitic non-indurated, lateritic weathering profiles, but include a variety of silcretes, or siliceous indurated residual deposits, widely developed but most abundant in the Sidmouth area. A long and complex history of pedogenesis and diagenesis is observed, following a pattern typical of other deeply weathered regions of the world. This begins with decalcification followed by kaolinisatiorr accompanied by dissolution of residual detrital quartz and ends with localised silicification of weathering profile materials at a variety of depths in the profile. This complex of residual deposits rests on deeply altered Upper Greensand of Upper Albian age, which underwent complete decalcification once any protective capping of Chalk had been removed. Consideration of the field relations of the residual kaolinitic flint gravels and associated silcretes demonstrates that these deposits are older than the Upper Eocene-Oligocene Bovey formation and probably date from the early Palaeocene. Intra-Eocene tectonism played a key role in the destruction of the residual

TERTIARY WEATHER ING AND PALAEOG ENE ENVI RONMENT IN SOUTHW EST ENGLAN D

m antle formed by d eep we a t he ring o f th e U pp e r C re tac eous an d culminated in th e in itiation of downwarpin g to form the deep tectonic b asin s alo ng t he wren ch fa ult zo nes. O n ly in East D e von di d th e Palaeocene re sidual d eposit s su rvive extensivel y. D eep weath erin g of the fr e shl y exp os e d U p pe r P al a e ozoic m et asediments during th e Eoce ne provid ed the so urce material for th e ka ol initic sediments of the Bovey form ation. Climatic co oling during the Eocene coupled wit h a m ore d yn amic se di menta ry envir onment is reflected in th e im mat ure n a ture o f th e clay m in eral ogy in th e latter we athering profiles. In co n t rast , the older resid ual deposi ts are mineralogicall y m ature to se nile , su ggesti ng a prolonged p e riod of deep weathering which in t he Dartmoor area may h ave begun before the P al aeocene .

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ACKNOWLEDGEMENTS The author wish es to thank Messrs. D . R . Br ay , C. J . Brice , M . C. Geo rge, J. Jones , J . R. M erefield , A . J . Palmer a n d G . Sp encer for technical assis ta nc e and Mr. C. M . Brist ow , Dr. R . A . Edwards, Dr. E. C. F re sh ne y, Profe sso r J . W. Murray a n d D r . M . A. Summerfield for much useful di scuss ion an d encouragement . Dr. K. Co e, Mr. J . R . M erefield , Profe ssor J . W. Murray a nd Dr . E . B . Selwood kindly read and cri tic ise d e a rlie r ve rs io ns o f th e manuscript and I a m indebted to Miss L. M . Costello fo r translating th e va rio us French papers referred to . I ac k n owledge E . C. C. B all Cl ay s Ltd. for a llo wi ng me acc ess to borehole m ateri al from the Petrocksiow Basin . I thank Mrs. V . Ellis for typing the final manuscript.

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