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A Tertiary basin at Dutson, near Launceston, Cornwall, England
A Tertiary basin at Dutson, near Launceston, Cornwall, England E. C. Freshney, R. A. Edwards, K. P. Isaac, G. Witte, G. C. Wilkinson, M. C. Boulter and J. A . Bain E . C. FRESHNEY, R . A . EDWA RDS , K . P. ISA AC, G . W ITTE , G o C. W ILKINSON , M . C. BOULTER & J. A . BAIN. 1982. A Tertiary basin at Dutson , near Lau ncesto n, Cornwall , England. Proc. Geol. A ss. 93 (4), 39 5-402. A pr eviously und escribed , partly fault-bounded ba sin contains Palaeogene fluvial and ?f1 uvio- lacustrine kaolinitic clays, Iignitic clays and clayey sands comparab le to the Bovey Fo rmation in nearby Tertiar y basins in SW E ngland. O ne sample fro m a 9.35 m bore hole yielded a good Upper E oce ne poll en and spore assemblage corresponding in age to part of the main ly concealed lower Bovey For mation in the Bovey Basin, to she lf and lower axial-tro ugh seque nces in the Petrockstow Basin , and possibly to part of t? e lower seq ue nce in the Stanley Banks Basin. The clay min eralogy of 28 samp les suggests de n vation from imm atu re tropical we athering profiles dev elop ed on Upp er Pal aeozo ic sediments. The sedime nts are more closely comparable with the Petro ckstow Basin deposits than with the known Bovey Basin seq uence. E. C. Freshney & R. A. Edwards, Institute of Geological Sciences, St Just, 30 Pennsylvania
R oad, Ex eter EX4 6BX. K. P. Isaac & G. Witte, Department of Geology, University of Ex eter, North Park R oad, Ex eter EX44QE. G. C. Wi lkinson* & M. C. Bou lter, North East Londo n Polytechnic, R omford R oad, London EI54LZ. J. A. Bain, Institute of Geological Sciences, 64-78 Gray' s Inn R oad, London WCIX 8NG. * Present address: Palaeoservices Ltd., Unit 15, Paramount Industrial Estate, Sa ndown Road, Warfo rd W0 24XA.
1. INTRODUCTION The presence of a de po sit of clays and lignitic clays at Du tson, 1.75 km NE of La unceston , Cornwall (Fig. 1), was first bro ught to the attention of one of the authors (ECF) by Mr. L. Stacey, who saw a working brickp it [344862*) in 1928 and noted the similarity of the sediments to those being wor ked in the Pet rockstow Basin, Nort h Devon. Th e deposit was worked by the North Cornw.all Clay Co mpany, and subsequen tly by 'a Plymouth company' to manufacture br icks for local use (Mr. E. J. Broad, personal comm unication). Two of the a utho rs, RAE and E CF , visited the pit in 1977 and made a preliminary assessment of the extent of the deposit . Subsequently, a shallow borehole was sunk next to the pit to a depth of 9 .35 m. In 1979/80 KPI and GW, working as part of the Exeter U niversity resea rch team revising the Institute of Geological Scie nces 1 : 50,000 T avistock Sheet (33 7), discove red exposures of possible Te rt iary sedi me nts in the bed of the R iver Ot tery which modified the bounda ries of th e deposit (Fig. 1). Miner alogical analysis of the sedime nts (KPI) and palynological exa minatio n of th e boreh ole samples (MCB and GCW) have enabled the likely env iro nments of deposition and the age of the de posit to be
* All grid refere nces are withi n Na tional Grid 100 km square SX.
est ablished. An appraisal of the ceramic properties of o ne borehole sample has also been made (J A B).
2. SETIlNG AND LITHOLOGICAL DESCRIPTION (a) Field relations
Th e D utson Basin lies in a major NW-SE-tre nding wre nch fault syste m which is one of a series of important fa ult zones in SW England alo ng which Te rtiary movem en t has bee n demonstrated (Dearma n, 1964). The western and eastern margins of the basin are bounded by app roximately NNW-SSE -tren ding highang le faults, which show a compon ent of dextral shear in the surro und ing Upper P alaeozoic rocks. The nor thern mar gin is very likely also fault- boun ded . Mapping of the extent of the basin was favoured during the wint er of 1979/80 , when flood co nditions stripped large areas of alluvial gra vel from the bed of the R iver Ottery expos ing possible Te rtiary sediments. T he extent of the dep osit was determined by hand augering on the flood plain of the rivers Ot tery and Tamar and by the constrai nts provided by exposures of Devo nian and Carbo nifero us rocks aro und (Fig. 1). At the western mar gin of the de pos it [3371 8660) are pale greyish-buff and wh ite plastic clays with variable amo unts of silt and sa nd , and sporad ic ora nge
395
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B ound ar y of alluvium E xposures of Te rtiary sedime nts beneath alluvium
Inset shows position of Dut s on Ba s in , 1.7 5 k m NE of L aun c e s t o n,C or nw all
Fig. 1. Geological map of the Dutson Basin and surrounding area.
spotting. Similar lithologies crop out in the river bed [33718660 to 33988666]. Two hundred metres farther west, Lower Carboniferous, dark grey, thickly bedded, silty cherts and black siliceous slate are exposed in the river banks [33608662], thus signifying that the western margin of the Tertiary deposit occurs somewhere between. Two metres of alluvial gravels overlie a 0.2 m thick iron- and manganese-cemented
hardpan, beneath which 1.5 m of Tertiary sediments are exposed in the river bank [34068667]. An observed thickness of 0.75 m of soft pale grey to brownish grey, plastic, slightly silty and sandy clay with lignitic wisps and lenses is overlain by 0.35 m of clay-rich grey to white sand. One further inaccessible outcrop of clay occurs some 50 m downstream [34328681]. No other exposures occur downstream
T ERTIARY BASIN AT DUTSON . COR NWA L L
until an exposure of Namurian sed iments [34328681]. The northern margin is defined by exposures of inverted, gre y, fine-grained , turbiditic sandstones and black micace ous shales [34498678 , 34588688 and 349 08667]. The disposition of exposures indicates a fault-bounded margin (trending WNW-ESE) lying just to the sout h of [34328681]. The sedi ments at the latt er locality comprise 2.0 m of inverted, grey, finegrained, turbiditic sandstones and fissile black shale overl ain by 2.5 m of alluvium. The upper 1.5 m of the sa ndsto ne and shale is deeply weathered and disturbed by cryoturbation. No changes in clay mineralogy were observed in passing from weathered to unweathered sed ime nts. No further exposure of Tertiary sediments occurs farther east in the River Ottery or in the River Tamar. Along the western bank of the River Tamar are Upper Devonian pale greyish-buff siltstones and dark grey silty slates containing small, lenticular limestone nodules and further numerous exposures of inverted Namurian turbiditic sandstones and fissile black shales. The ea stern margin of the Tertiary deposit must therefore lie to the west of the River Tamar, although recent ploughing has revealed brownish-orange to grey, silty, soft clay fragments in the so il only 50 m W of the river. Similar clay fragments associated with manganese-stained loosely te xtured sandy clay were obser ved in the bank of the River Tamar [34918636]. The clay fragments seen here and adj acent to the Tam ar are of weathered material derived from the Tertiary clay deposit farther west . Pale grey and orange plastic clay fragments were widely scatt ered in an area extend ing 400 m west and 200 m south of [34418636]. Augering (34698627] reve aled pale grey and buff sandy clays with abundant orange mottling beneath 0.35 m of soil. Augering re vealed similar reddish-brawn-weathered clay fragments in a brown earth soil [34638636 and 34588641]. Trenching [34828629 to 34718626J exposed up to 0.5 m of sticky, very sandy, white and pale buff, silty clay with abundant orange mottling. Near the southern margin of the basin, an easterly flowing stream has exposed discontinuous outcrops of Tertiary sediments [34438595 to 34938595]. The weathered pale grey to white plastic clays with a very variable silt and sand content are exposed beneath less th an 1 m of surface cover. A dense manganese- and iron-rich hardpan deposit up to 0 .3 m thi ck commonly overlies the Tertiary sediments. The most easterly exposure of the clay occurs in the river [34938596], while 60 m downstream [35018595J are Upper Devonian , dark gre y, fissile silty slates and limestone nod ules, thus delimiting the eas tern margin of the dep osit. The southern margin of the deposit, which is normally marked by a slight break in slope, is exposed in a series of NNW-SSW-trending drainage ditches [34088525 to 33848535]. The sequence observed in one of the trenches [33958626 to 33978637] is as
397
follows: at the southern end of the trench da rk grey, so ft, fissile shales and fine- grained sa ndstones are see n to be incre asingly weath ered to a so ft , incoherent, pale grey and ora nge , sa ndy clay with scattered grey sand sto ne frag me nts; further north are expos ures of pale grey and buff sandy plastic clay. Thi s sectio n of the southern mar gin is interpret ed as a stra tigrap hical boundary between the T ertiary and Up per Carboniferous.
(b) Dutson Brickpit borehole A Geo nor piston sampler was used to sink a shallow borehole (34448618] (Fig. 1) to a depth of 9. 35 m in 1977 . The Tertiary strata proved (Fig. 2) can be divided into three main units. (Colo ur not at ion is from the G eol ogical Society of America Ro ck Colour Chart). The topmost unit, estimated to be 2.30 m thick, was sampled between 1.65 and 2.05 m, where it was soft, light grey (N7) , very sandy clay cont aining sparsely scatt ered angular quartz grains up to 5 mm in diameter, and with patches and mottles of light olive brown (5Y 5/6), greyish-orange (10YR 7/4) and moderate yellowish-brown (10YR 5/4 ) sandy clay. This unit probably includes some mad e ground above 1.65 m. The underlying unit, penetrated between 2.30 and 5.90 m, comprises brown clay with a bed of black extremely lignitic clay. These beds are the probable equivale nts of those formerly extracted from the adjacent pit . The clays, sampled between 2.35-2.60 m, 3.25-3.45 rn, and 4.15-4.55 rn, vary in colo ur between dark yellow ish-brown (10YR 412), dusk y yellowishbrown (10Y R 2/2), moderate brown (5YR 3/4), pale brown (5YR 5/2), and greyish-brown (SYR 3/2); the y are sandy to slightly sandy, and generally contain scatt ered particles of lignite . The samples from between 2.35-2.60 m and 3.25-3.45 m show bands which dip between 35° and 40°. These steep dips are attributed to periglacial deformation comparable to that recorded from the Tertiary deposits of the Bovey Basin in south Devon by Dineley (1963). A bed of black, extremely lignitic clay, virtually extremely clayey lignite, was sampled between 3.45-3.65 m. Grey sandy clay and clayey sand were proved between 5.90 m and the bottom of the bor ehole at 9.3 5 m. The samples from between 5.95-6.35 m and 6.80-7 .15 m were firm light grey (N7) to medium light grey (N6), extremely sandy clay containin g wisps of brown clay that define a sub-horizontal banding. Minor rootl et s were seen in the sa mple from 5.956.35 m, which also contained scattered small fragme nts of black lignite . A sample fro m 8 .55-8.95 m was light grey (N7) to medium light grey (N6) extremely clayey fine-grained sand with po orl y defined bands picked out by wisps of brown clay; between 8.95 m-9.35 m similar but unb anded clayey sands occur.
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3. CLAY MINERALOGY DEPTH ( rn)
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Gr ey ex t reme ly clayey fine -grained sand wit h w isp y partings of bro w n c lay in top OAm T .D . 9 .35 m
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Fig. 2. Log of the Dutson Brickpit bor ehol e. The position of the borehole is given in Fig. 1.
(a) Clay mineralogy of the sediments Samples of a variety of lithologies were taken from field exposures and analysed using standard X-ray diffraction identification procedures (Brown & Brindley, 1980). The Dutson Basin sediments are characterised by clay mineral asse mblages dominated by kaolinite a nd illite . Quartz occurs in all samples a nd in addition irre gular chlo rite-smectite, irregular and possible regular illite-smectite mixed-layer minerals, and smectite are important components. The kaolinite is a disordered variety and from the strongly skewed (001 ) reflection , towards d-spacings up to 7.5 A, a significant proportion of the kaolinite particles are of the order of 70 nm in diameter (Trunz, 1976; Brindley, 1980). The illite reflection patterns indicate a 1M or 1~ polytype (Bailey, 1980), however this almost certainly represents a mixture of degraded detrital mica of extremely fine grain size and diagenetic illite . The smectite identified is a fully expandable montmorillonit e. The mixed layer minerals present are irregular chlorite-smectite and illite-smectite types which could have formed either from diagenetic alte ra tio n of smectite during deposition and burial, o r by breakdown during weat he ring and sedi men ta tion of metamorphic chlori te a nd illite derived from the Upper Palae ozoic ro cks surr ounding the basin. Sampl es from the Upper Carboniferous interbedded sha les exposed at the southern margin of the basin contain illite, chlorite and quartz, but show a crude increase in kaolinite and mixed-layer clay towar ds the first re cogni sable Tertiary sediments abo ve. The assemblage quartz, kaolinite, illite , mixed-layer minerals and smectite is characteristic of immature tropical and sub-tropical weathering profile environments (Millot, 1970 ,; Kronberg et al., 1979). The clay min er alogy of the sediments is best interpreted as being derived by erosion and sedimentation of weathering pro file materials developed on the Upper Palaeozoic rocks during the Tertiary. As the clay mineralogy of continental sedimentary environments is sensitive to climate (Singer, 1980), so me deductions can be made about the type of we athering mantle from which the sediments were derived . Kronberg et al. (1979) found that tropical weathering profiles from different re gions of Brazil could be arra nged in a sequence rel ating the intensity of weathering to time. Normally the first stages of weathering result in the formation of complex smectitic and mixed-layer clays. Continued weathering leads to kaolinite-quartz assemblages a nd, eventually, gibb site, goethite and hematite form . The predominance of kaolinite and quartz, bu t also with large propo rtions of illite, mixedlayer clays and smectite, implies th at onl y a yo uth ful stage of maturity had been reached by the weathering profiles before the y were eroded and the materials
399
TERTIARY BASIN AT DUTSON, CORNWALL
redeposited, Either the continental sedimentary environment was highly dynamic and the period of time in which the tropical weathering profile became established was short, of the order of 5 x 10 6 years (Kronberg et al., 1979), or the area had a climate lying on the fringe of the tropical weathering zone,
(b) Ceramic appraisal X-ray diffraction analysis of a composite sample of brown clay and lignitic clay from between 2.35 and 4.55 m in the borehole indicated the presence of kaolinite, vermiculite, montmorillonite and degraded mica, together with quartz. A rapid check for alkali content by X-ray fluorescence, indicated a K 2 0 content of 1.6%, signifying a (degraded) mica content of about 20%. The sample contained about 10% organic matter. A small hand-moulded test piece of the plastic clay, after oven-drying, was heated to the standard ignition temperature of 1050°C. The organic matter completely burned off, producing a total loss on ignition of 14.7% by weight for the carbonaceous constituent and clay hydroxyl-water, and the specimen fired to a white or slightly off-white colour. Heating beyond 1050°C indicated a vitrification temperature of about 1250°C, at which temperature the fired colour was very pale buff. Compared to the average ball clay, the Dutson clay is only moderately plastic and contains vermiculitic and smectitic phases in addition to the usual kaolinite/mica/quartz assemblage. Organic matter content is also high. It is, however, white or off-white firing and is self-fluxing, although full vitrification is at temperatures more akin to stoneware clays. Used as a ceramic clay in its own right it would provide strong hard ware at normal kiln temperatures but both drying and firing shrinkages are likely to be high.
4. PALYNOLOGY Palynological analysis of samples from the borehole core was carried out but only one sample, from 2.352.60 m, provided a good pollen and spore assemblage (Table 1). Pollen and spore analysis of sample 868 has TABLE 1. Palynological yield of core samples from the Dutson Brickpit borehole. Sample No. Depth (m) *868 2.35-2.60 **869 3.25-3.45 ***870 4.15-4.55 ***871 5.95-6.35 ***872 8.95-9,35 * indicates a good pollen and spore assemblage. ** indicates a poor pollen and spore assemblage. *** indicates a barren sample (no pollen and spore assemblage).
given percentage frequency counts of an assemblage containing 26 form-genera; 68% of the specimens are fern spores and 15% conifer pollen, an unusual characteristic for Tertiary assemblages from the western British Isles (Wilkinson & Boulter, 1980). Most of the 26 form-genera range through the European Tertiary, but Nudopollis, Plicatopollis and some of the spores are particularly characteristic of the Eocene. Boulter & Hubbard (in press) have analysed percentage frequency results of pollen and spore studies from TABLE 2. The percentage frequency of the 26 form-genera which occur in Dutson sample 868. Classified according to the three groups of principal components thought to have affinity with Eocene plant communities. Taxa of Group I are thought to be associated with Angiosperm Forest, those of Group II with Fern and Conifer Forest, and those of Group III with Paratropical Forest (Boulter & Hubbard, in press). The Dutson sample shows greatest (42%) representation of Group II by statistical analysis. More simply, 83% of the specimens counted are from ferns and conifers, % % group freq. sum 26 GROUP I-ANGIOSPERM POLLEN Tricolporopollenites 15 Momipites 2 Periporopollenites 2 Erecipites 3 GROUP II-FERN SPORES Polypodiaceaesporites Reticulosporis Camarozonosporites Cicatricosisporites
42 4 1 25 1
GROUP III-POLLEN FROM EXTINCT ANGIOSPERMS Toroisporis 19 Plicatopollis 3 Nudopollis 5 Tiliaepollenites 1 OTHER POLLEN AND SPORE TAXA Polypodiidites Deltoidospora Microfoveolatisporis Cedripites Pityosporites Cycadopites Monocolpopollenites Graminidites Tricolpopollenites Engelhardtioipollenites Myricipites Subtriporopollenites Gothanipollis Porocolpopollenites
1 12 5 2 13 1 4 1 9 5 2 1 1 1
32
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E. C. FRESHNEY ET AL.
three IGS boreholes in the Hampshire Basin (Edwards, 1978; Edwards & Freshney, 1978; Freshney 1978), which represent most of the British Isles Eocene sequence. This was done by a method of principal components analysis and identified three groups of pollen and spore taxa, which may represent distinct plant communities in the Hampshire Eocene: Angiosperm Forest, Fern and Conifer Forest, and Paratropical Forest. The percentage frequency data from the Dutson Basin sample have been incorporated into this principal components analysis, giving a matrix of 85 taxa within 175 samples. The results are set out in Table 2. The taxa allocated to the three groups account for 65% of the specimens counted, of which those in group II (Fern and Conifer Forest) form the largest part. The other 35% of specimens counted are of taxa thought to represent the effects of transport and facies (Boulter & Hubbard, in press). This domination by Group I! taxa suggests that conifers and ferns were an important part of the Dutson Tertiary vegetation. Though any comparison with modern ecosystems must remain very tentative, an environment with poor soils and no tree canopy is a possible interpretation. Comparison with Wolfe's (1979) climatic data suggests that the mean annual temperature was 15°C and the mean annual range of temperature was 16.5°C. The multivariate statistical analysis of 174 samples from the IGS boreholes in the Hampshire Basin has given a stratigraphical control to the occurrence of Groups I, I! and II!. The Dutson Basin sample is equivalent to a sample which occurs at 78 m in the Ramnor Borehole, in the middle of the Barton Beds (Upper Eocene). This dating is made with 95% confidence for all principal components that have been calculated by this statistical method.
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Fig. 3. Map of part of Southwest England showing the principal Tertiary basins and their relationships to the wrench fault zones. That part of the wrench fault zone south of Dutson is from the revision of the Institute of Geological Sciences 1: 50,000 sheet 337 (Tavistock and Launceston) currently in progress.
5. DISCUSSION The formation of basins containing terrestrial sediments preserved within local sags in Palaeocene lavas, or within fault zones, is an important and widespread event in the Eocene-Oligocene evolution of the western British Isles. Discoveries within the past 10 years of offshore Palaeogene basins such as that in Cardigan Bay near Mochras (Woodland, 1971), near Lundy Island in the Bristol Channel (Fletcher, 1975), and near Canna, Sea of Hebrides (Smythe & Kenolty, 1975; Evans et al., 1979) have shown that such basins are more widespread and have a greater chance of preservation in the geological record than was previously thought. In SW England and its offshore area (Fig. 3), the formation of basins containing fluviolacustrine sediments was dominated by the NW-SE Sticklepath fault zone along which an estimated maximum 1100 m of sediments accumulated in the Bovey Basin, 661 m in the Petrockstow Basin, and at least 340 m in the Stanley Banks Basin east of Lundy
Island. Outside the Sticklepath Fault zone, until the discovery of the Dutson Basin, the only Palaeogene continental deposits known (with the exception of the older (Palaeocene-Eocene) gravels of Haldon, Orleigh and Aller type) were the poorly documented small deposits at St Agnes and Crousa Down in Cornwall, both of which are apparently not associated with faults. The Dutson Basin formed along a NW-SE wrench fault zone some 45 km west of the Bovey Basin, thus demonstrating that formation of Boveytype basins was not restricted to the Sticklepath zone, or governed by some special character of that zone, but could also take place along other wrench fault zones in SW England. Possibly many such basins have been lost by erosion. Using pollen and spores, the basins along the Sticklepath fault zone have been given ages from ?late Oligocene to Eocene. The topmost 34 m of the Stanley Banks basin has been sampled and dated as middle
TERTIARY BASIN AT DUTSON, CORNWALL
Oligocene by Boulter & Craig (1979), but late to middle Oligocene by Dr C. Turner (in Fletcher, 1975); however, the basin is thought to be at least 340 m deep (Brooks & James, 1975) and thus an Eocene age for the unsampled sediments in the lowermost parts of the basin cannot be excluded. The Petrockstow Basin sediments are 'mainly Eocene with some Oligocene' (Freshney et aI., 1979, p. 21), The topmost 300 m of the Bovey Formation in a borehole in the Bovey Basin has been dated as early to middle Oligocene, with rare pollen grains at 297 m depth suggesting the probability of Eocene sediments at greater depth (Wilkinson et al., 1980, p. 72). Since the estimated depth of the Bovey Basin is possibly of the order of 1100 m (Vincent, 1974), the lowermost 700-800 m of the Bovey Formation is probably Eocene. The dated part of the Dutson sequence is Upper Eocene, and thus corresponds in age to part of the (mainly) concealed lower Bovey Formation in the Bovey Basin, to parts of the outcropping shelf and axial trough sequences in the Petrockstow Basin, and possibly to part of the lower (undated) sequence in the Stanley Banks Basin. Lithologically and clay mineralogically the sampled Dutson sediments are more closely comparable with the Petrockstow Basin deposits than with the known part of the Bovey Basin succession, possibly because sequences of similar age crop out in the Petrockstow Basin but are generally concealed by younger strata in the Bovey Basin. The limitations of comparing the very short sequence proved in the Dutson Borehole with other Bovey Formation sequences are obvious, but there is present in the Dutson borehole a finingupward sequence, capped by a lignite, which is similar to fining-upward sequences recorded from the Petrockstow Basin by Freshney et al. (1979) and regarded by them as fluvial in origin. Such fining-upwards sequences are not a distinctive feature of the known Bovey Basin sequence (Edwards, 1976). Clay mineralogically, the sediments of the Dutson Basin show close affinities with those of the Petrock-
401
stow Basin, which Bristow (1968) demonstrated were derived from deeply weathered Upper Palaeozoic sediments. Although tentative, the conclusion drawn from the palynological data indicating poor soils and no tree canopy supports the clay mineralogical data, which suggest derivation of the sediments from rather immature tropical weathering profiles. The estimates of palaeotemperature also indicate that conditions were not entirely pertinent to tropical weathering. Thus, during the Eocene, we envisage the Cornubian peninsula with rivers flowing north and south from a central upland spine. Along the Sticklepath line, a major river flowing NW filled the Petrockstow and Stanley Banks Basins with sediment on its way to the Celtic Sea, while a SE-flowing river debouching into a sea in the English Channel provided the sediment that filled the Bovey Basin. At the same time, it is likely that another river flowing NW or SE in the Launceston area deposited its sediment in the Dutson Basin; such a river is unlikely to have been a tributary of the Petrockstow or Bovey rivers in view of the close control that the NW-SE fault trend would have exerted on the course of the river. Whether the Dutson river flowed NW or SE is uncertain; the present disposition of the topography of the area, although an uncertain guide, suggests that it is more likely that the river flowed south to a sea somewhere in the English Channel.
ACKNOWLEDGEMENTS We are most grateful to Mr E. J. Broad of Lower Dutson Farm, Launceston, for allowing access to his land and for giving permission to sink the Dutson Brickpit Borehole. We thank D. Bray, M. C. George, D. J. Morgan and F. H. Stacey for technical assistance and Mrs D. E. Milton and Mrs G. Wright for typing the various drafts. The paper is published with the approval of the Director, Institute of Geological Sciences (N.E.R.C).
References BAILEY, S. W. 1980. Structures of layer silicates. In: (Brindley, G. W. & Brown, G.; Eds.) Crystal Structures of Clay Minerals and their X-ray Identification. Min. Soc. G.B. London, 1-123. BOULTER, M. C. & D. L. GRAIG, 1979. A middle Oligocene pollen and spore assemblage from the Bristol Channel. Rev. Palaeobotan. Palynol., 28, 259-72. - - & R. N. L. B. HUBBARD in press. Objective palaeontological and biostratigraphical interpretation of Tertiary palynological data by multivariate statistical analysis. Palynology.
BRINDLEY, G. W. 1980. Order-disorder in Clay Mineral Structures. In: (Brindley, G, W. & Brown, G.; Eds.) Crystal Structures of Clay Minerals and their X-ray Identification. Min. Soc. G.B., London, 125-96.
BRISTOW, C. M. 1968. The Derivation of the Tertiary Sediments in the Petrockstow Basin, North Devon. Proc. Ussher Soc., 2, 29-35. BROOKS, M. & D. G. JAMES, 1975. The geological results of seismic refraction surveys in the Bristol Channel, 19701973. J!. Oeol. Soc. Lond., 131, 163-82.
BROWN, G. & G. W. BRINDLEY, 1980. X-ray Diffraction Procedures for Clay Mineral Identification, In: (Brindley, G. W. & Brown, G., Eds.) Crystal Structures of Clay Minerals and their X-ray ldentification. Min. Soc. G.B. London, 305-62, DEARMAN, W. K 1964. Wrench-faulting in Cornwall and South Devon. Proc. Oeol. Ass., 74, 265-87. DINELEY, D. L. 1963. Contortions in the Bovey Beds (Oligocene) S.W. England. Biu!. Peryglac" 12, 150-60.
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EDWARDS, R. A. 1976. Tertiary Sediments and Structure of the Bovey Basin, South Devon. Proc. Geol. Ass., 87, 1-26. - - 1978. Bunker's Hill Borehole. In: I.G.S. Boreholes 1977. Rep. Inst. Geol. Sci. No. 78/21, 6-7. - - & E. C. FRESHNEY. 1978. Shamblehurst Borehole. In: I.G.S. Boreholes 1977. Rep. Inst. Geol. Sci. No. 78/21, 4-5. - - & - - in press, The Tertiary Sedimentary Rocks. In (E. M. Durrance & D. J. C. Laming; eds.) The Geology of Devon. Exeter Univ. Press. EVANS, D., G. C. WILKINSON, & D. L. CRAIG. 1979. The Tertiary sediments of the Canna Basin, Sea of the Hebrides. Scott. J. Geol., 15, 329-32. FLETCHER, B. N. 1975. A New Tertiary basin east of Lundy Island. J. Geol. Soc. Land., 131, 223-25. FRESHNEY, E. C. 1970. Cyclical sedimentation in the Petrockstow Basin, Proc. Ussher Soc., 2, 179-89. - - 1978. Ramnor Inclosure Borehole. In 1.G.S. Boreholes 1977. Rep. Inst. Geol. Sci. No. 78/21. 5-6. - - K. E. BEER, & J. E. WRIGHT, 1979. Geology of the country around Chulmleigh. Mem. geol. Surv. G.B. KRONBERG, B. 1., W. S. FYFE, O. H. LEONARDS, J. R. SANTOS, &. A. M. SANTOS. 1979. The Chemistry of some Brazilian Soils: Element Mobility During Intense Weathering. Chern. Geol., 24, 211-29.
MILLOT, G. 1970. The Geology of Clays, Springer Verlag, Berlin. SINGER, A. 1980. The palaeoclimatic interpretation of clay minerals in soils and weathering profiles. Earth Sci. Rev., 15,303-26. SMYTHE, D. K. & N. KENOLTY. 1975. Tertiary sediments in the Sea of the Hebrides, JI. geol. Soc. Lond., 131, 227-33. TRUNZ, V. 1976. The Influence of Crystallite Size on the Apparent Basal Spacings of Kaolinite. Clays and Clay Minerals, 24, 84-7. VINCENT, A. 1974. Sedimentary environments of the Bovey Basin. Unpubl. M. Phil. thesis, University of Surrey, WILKINSON, G. C. & BOULTER, M. C. 1980. Oligocene pollen and spores from the western part of the British Isles. Palaeontographica B, 175, 27-83. - , R. A. B. BAZLEY, & M. C. BOULTER. 1980. The geology and palynology of the Oligocene Lough Neagh Clays, Northern Ireland. JI. geol. Soc. Lond., 137, 65-75. WOLFE, J. A. 1979. Temperature parameters of humid to mesic forests of eastern Asia. U.S. Geol. Surv. Prof. Pap., 1106, 1-37. WOODLAND, A. W. (ed.) 1971. The Llanbedr (Mochras Farm) Borehole. Rep. Inst. geol. Sci. London. 71/18. 115 pp.
R oad, Ex eter EX4 6BX. K. P. Isaac & G. Witte, Department of Geology, University of Ex eter, North Park R oad, Ex eter EX44QE. G. C. Wi lkinson* & M. C. Bou lter, North East Londo n Polytechnic, R omford R oad, London EI54LZ. J. A. Bain, Institute of Geological Sciences, 64-78 Gray' s Inn R oad, London WCIX 8NG. * Present address: Palaeoservices Ltd., Unit 15, Paramount Industrial Estate, Sa ndown Road, Warfo rd W0 24XA.
1. INTRODUCTION The presence of a de po sit of clays and lignitic clays at Du tson, 1.75 km NE of La unceston , Cornwall (Fig. 1), was first bro ught to the attention of one of the authors (ECF) by Mr. L. Stacey, who saw a working brickp it [344862*) in 1928 and noted the similarity of the sediments to those being wor ked in the Pet rockstow Basin, Nort h Devon. Th e deposit was worked by the North Cornw.all Clay Co mpany, and subsequen tly by 'a Plymouth company' to manufacture br icks for local use (Mr. E. J. Broad, personal comm unication). Two of the a utho rs, RAE and E CF , visited the pit in 1977 and made a preliminary assessment of the extent of the deposit . Subsequently, a shallow borehole was sunk next to the pit to a depth of 9 .35 m. In 1979/80 KPI and GW, working as part of the Exeter U niversity resea rch team revising the Institute of Geological Scie nces 1 : 50,000 T avistock Sheet (33 7), discove red exposures of possible Te rt iary sedi me nts in the bed of the R iver Ot tery which modified the bounda ries of th e deposit (Fig. 1). Miner alogical analysis of the sedime nts (KPI) and palynological exa minatio n of th e boreh ole samples (MCB and GCW) have enabled the likely env iro nments of deposition and the age of the de posit to be
* All grid refere nces are withi n Na tional Grid 100 km square SX.
est ablished. An appraisal of the ceramic properties of o ne borehole sample has also been made (J A B).
2. SETIlNG AND LITHOLOGICAL DESCRIPTION (a) Field relations
Th e D utson Basin lies in a major NW-SE-tre nding wre nch fault syste m which is one of a series of important fa ult zones in SW England alo ng which Te rtiary movem en t has bee n demonstrated (Dearma n, 1964). The western and eastern margins of the basin are bounded by app roximately NNW-SSE -tren ding highang le faults, which show a compon ent of dextral shear in the surro und ing Upper P alaeozoic rocks. The nor thern mar gin is very likely also fault- boun ded . Mapping of the extent of the basin was favoured during the wint er of 1979/80 , when flood co nditions stripped large areas of alluvial gra vel from the bed of the R iver Ottery expos ing possible Te rtiary sediments. T he extent of the dep osit was determined by hand augering on the flood plain of the rivers Ot tery and Tamar and by the constrai nts provided by exposures of Devo nian and Carbo nifero us rocks aro und (Fig. 1). At the western mar gin of the de pos it [3371 8660) are pale greyish-buff and wh ite plastic clays with variable amo unts of silt and sa nd , and sporad ic ora nge
395
396
E. C. FRESHNEY ET AL.
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) I
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,
86 I I
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Alluvium
o
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T ertiary Crackington Formation,sandstones and shales (U p p er C ar bo nif er ous) Cherts ( L o w er Ca r b onif er ous) Slat es ( Upp er De v o nian) Fault Bound ar y
- r r :«:>
•
B ound ar y of alluvium E xposures of Te rtiary sedime nts beneath alluvium
Inset shows position of Dut s on Ba s in , 1.7 5 k m NE of L aun c e s t o n,C or nw all
Fig. 1. Geological map of the Dutson Basin and surrounding area.
spotting. Similar lithologies crop out in the river bed [33718660 to 33988666]. Two hundred metres farther west, Lower Carboniferous, dark grey, thickly bedded, silty cherts and black siliceous slate are exposed in the river banks [33608662], thus signifying that the western margin of the Tertiary deposit occurs somewhere between. Two metres of alluvial gravels overlie a 0.2 m thick iron- and manganese-cemented
hardpan, beneath which 1.5 m of Tertiary sediments are exposed in the river bank [34068667]. An observed thickness of 0.75 m of soft pale grey to brownish grey, plastic, slightly silty and sandy clay with lignitic wisps and lenses is overlain by 0.35 m of clay-rich grey to white sand. One further inaccessible outcrop of clay occurs some 50 m downstream [34328681]. No other exposures occur downstream
T ERTIARY BASIN AT DUTSON . COR NWA L L
until an exposure of Namurian sed iments [34328681]. The northern margin is defined by exposures of inverted, gre y, fine-grained , turbiditic sandstones and black micace ous shales [34498678 , 34588688 and 349 08667]. The disposition of exposures indicates a fault-bounded margin (trending WNW-ESE) lying just to the sout h of [34328681]. The sedi ments at the latt er locality comprise 2.0 m of inverted, grey, finegrained, turbiditic sandstones and fissile black shale overl ain by 2.5 m of alluvium. The upper 1.5 m of the sa ndsto ne and shale is deeply weathered and disturbed by cryoturbation. No changes in clay mineralogy were observed in passing from weathered to unweathered sed ime nts. No further exposure of Tertiary sediments occurs farther east in the River Ottery or in the River Tamar. Along the western bank of the River Tamar are Upper Devonian pale greyish-buff siltstones and dark grey silty slates containing small, lenticular limestone nodules and further numerous exposures of inverted Namurian turbiditic sandstones and fissile black shales. The ea stern margin of the Tertiary deposit must therefore lie to the west of the River Tamar, although recent ploughing has revealed brownish-orange to grey, silty, soft clay fragments in the so il only 50 m W of the river. Similar clay fragments associated with manganese-stained loosely te xtured sandy clay were obser ved in the bank of the River Tamar [34918636]. The clay fragments seen here and adj acent to the Tam ar are of weathered material derived from the Tertiary clay deposit farther west . Pale grey and orange plastic clay fragments were widely scatt ered in an area extend ing 400 m west and 200 m south of [34418636]. Augering (34698627] reve aled pale grey and buff sandy clays with abundant orange mottling beneath 0.35 m of soil. Augering re vealed similar reddish-brawn-weathered clay fragments in a brown earth soil [34638636 and 34588641]. Trenching [34828629 to 34718626J exposed up to 0.5 m of sticky, very sandy, white and pale buff, silty clay with abundant orange mottling. Near the southern margin of the basin, an easterly flowing stream has exposed discontinuous outcrops of Tertiary sediments [34438595 to 34938595]. The weathered pale grey to white plastic clays with a very variable silt and sand content are exposed beneath less th an 1 m of surface cover. A dense manganese- and iron-rich hardpan deposit up to 0 .3 m thi ck commonly overlies the Tertiary sediments. The most easterly exposure of the clay occurs in the river [34938596], while 60 m downstream [35018595J are Upper Devonian , dark gre y, fissile silty slates and limestone nod ules, thus delimiting the eas tern margin of the dep osit. The southern margin of the deposit, which is normally marked by a slight break in slope, is exposed in a series of NNW-SSW-trending drainage ditches [34088525 to 33848535]. The sequence observed in one of the trenches [33958626 to 33978637] is as
397
follows: at the southern end of the trench da rk grey, so ft, fissile shales and fine- grained sa ndstones are see n to be incre asingly weath ered to a so ft , incoherent, pale grey and ora nge , sa ndy clay with scattered grey sand sto ne frag me nts; further north are expos ures of pale grey and buff sandy plastic clay. Thi s sectio n of the southern mar gin is interpret ed as a stra tigrap hical boundary between the T ertiary and Up per Carboniferous.
(b) Dutson Brickpit borehole A Geo nor piston sampler was used to sink a shallow borehole (34448618] (Fig. 1) to a depth of 9. 35 m in 1977 . The Tertiary strata proved (Fig. 2) can be divided into three main units. (Colo ur not at ion is from the G eol ogical Society of America Ro ck Colour Chart). The topmost unit, estimated to be 2.30 m thick, was sampled between 1.65 and 2.05 m, where it was soft, light grey (N7) , very sandy clay cont aining sparsely scatt ered angular quartz grains up to 5 mm in diameter, and with patches and mottles of light olive brown (5Y 5/6), greyish-orange (10YR 7/4) and moderate yellowish-brown (10YR 5/4 ) sandy clay. This unit probably includes some mad e ground above 1.65 m. The underlying unit, penetrated between 2.30 and 5.90 m, comprises brown clay with a bed of black extremely lignitic clay. These beds are the probable equivale nts of those formerly extracted from the adjacent pit . The clays, sampled between 2.35-2.60 m, 3.25-3.45 rn, and 4.15-4.55 rn, vary in colo ur between dark yellow ish-brown (10YR 412), dusk y yellowishbrown (10Y R 2/2), moderate brown (5YR 3/4), pale brown (5YR 5/2), and greyish-brown (SYR 3/2); the y are sandy to slightly sandy, and generally contain scatt ered particles of lignite . The samples from between 2.35-2.60 m and 3.25-3.45 m show bands which dip between 35° and 40°. These steep dips are attributed to periglacial deformation comparable to that recorded from the Tertiary deposits of the Bovey Basin in south Devon by Dineley (1963). A bed of black, extremely lignitic clay, virtually extremely clayey lignite, was sampled between 3.45-3.65 m. Grey sandy clay and clayey sand were proved between 5.90 m and the bottom of the bor ehole at 9.3 5 m. The samples from between 5.95-6.35 m and 6.80-7 .15 m were firm light grey (N7) to medium light grey (N6), extremely sandy clay containin g wisps of brown clay that define a sub-horizontal banding. Minor rootl et s were seen in the sa mple from 5.956.35 m, which also contained scattered small fragme nts of black lignite . A sample fro m 8 .55-8.95 m was light grey (N7) to medium light grey (N6) extremely clayey fine-grained sand with po orl y defined bands picked out by wisps of brown clay; between 8.95 m-9.35 m similar but unb anded clayey sands occur.
398
E . C. FRESHNEY ET AL.
3. CLAY MINERALOGY DEPTH ( rn)
O -.----.-----------~
G r e y very sandy c lay
Br own slightly sandy t o 3
4
mod er a t el y san dy clay w it h lignite f r agments , b and in g dips 35 -40° (- Bl a c k extremely lignitic c lay Br o w n moderately sandy to slightly sandy clay with lignite fragments
5
C o r e lost - probably brown clay 6
Gr e y extremely sandy clay with wispy partings of brown clay
7
8
9
Gr ey ex t reme ly clayey fine -grained sand wit h w isp y partings of bro w n c lay in top OAm T .D . 9 .35 m
~
NO T SAMP L ED
Fig. 2. Log of the Dutson Brickpit bor ehol e. The position of the borehole is given in Fig. 1.
(a) Clay mineralogy of the sediments Samples of a variety of lithologies were taken from field exposures and analysed using standard X-ray diffraction identification procedures (Brown & Brindley, 1980). The Dutson Basin sediments are characterised by clay mineral asse mblages dominated by kaolinite a nd illite . Quartz occurs in all samples a nd in addition irre gular chlo rite-smectite, irregular and possible regular illite-smectite mixed-layer minerals, and smectite are important components. The kaolinite is a disordered variety and from the strongly skewed (001 ) reflection , towards d-spacings up to 7.5 A, a significant proportion of the kaolinite particles are of the order of 70 nm in diameter (Trunz, 1976; Brindley, 1980). The illite reflection patterns indicate a 1M or 1~ polytype (Bailey, 1980), however this almost certainly represents a mixture of degraded detrital mica of extremely fine grain size and diagenetic illite . The smectite identified is a fully expandable montmorillonit e. The mixed layer minerals present are irregular chlorite-smectite and illite-smectite types which could have formed either from diagenetic alte ra tio n of smectite during deposition and burial, o r by breakdown during weat he ring and sedi men ta tion of metamorphic chlori te a nd illite derived from the Upper Palae ozoic ro cks surr ounding the basin. Sampl es from the Upper Carboniferous interbedded sha les exposed at the southern margin of the basin contain illite, chlorite and quartz, but show a crude increase in kaolinite and mixed-layer clay towar ds the first re cogni sable Tertiary sediments abo ve. The assemblage quartz, kaolinite, illite , mixed-layer minerals and smectite is characteristic of immature tropical and sub-tropical weathering profile environments (Millot, 1970 ,; Kronberg et al., 1979). The clay min er alogy of the sediments is best interpreted as being derived by erosion and sedimentation of weathering pro file materials developed on the Upper Palaeozoic rocks during the Tertiary. As the clay mineralogy of continental sedimentary environments is sensitive to climate (Singer, 1980), so me deductions can be made about the type of we athering mantle from which the sediments were derived . Kronberg et al. (1979) found that tropical weathering profiles from different re gions of Brazil could be arra nged in a sequence rel ating the intensity of weathering to time. Normally the first stages of weathering result in the formation of complex smectitic and mixed-layer clays. Continued weathering leads to kaolinite-quartz assemblages a nd, eventually, gibb site, goethite and hematite form . The predominance of kaolinite and quartz, bu t also with large propo rtions of illite, mixedlayer clays and smectite, implies th at onl y a yo uth ful stage of maturity had been reached by the weathering profiles before the y were eroded and the materials
399
TERTIARY BASIN AT DUTSON, CORNWALL
redeposited, Either the continental sedimentary environment was highly dynamic and the period of time in which the tropical weathering profile became established was short, of the order of 5 x 10 6 years (Kronberg et al., 1979), or the area had a climate lying on the fringe of the tropical weathering zone,
(b) Ceramic appraisal X-ray diffraction analysis of a composite sample of brown clay and lignitic clay from between 2.35 and 4.55 m in the borehole indicated the presence of kaolinite, vermiculite, montmorillonite and degraded mica, together with quartz. A rapid check for alkali content by X-ray fluorescence, indicated a K 2 0 content of 1.6%, signifying a (degraded) mica content of about 20%. The sample contained about 10% organic matter. A small hand-moulded test piece of the plastic clay, after oven-drying, was heated to the standard ignition temperature of 1050°C. The organic matter completely burned off, producing a total loss on ignition of 14.7% by weight for the carbonaceous constituent and clay hydroxyl-water, and the specimen fired to a white or slightly off-white colour. Heating beyond 1050°C indicated a vitrification temperature of about 1250°C, at which temperature the fired colour was very pale buff. Compared to the average ball clay, the Dutson clay is only moderately plastic and contains vermiculitic and smectitic phases in addition to the usual kaolinite/mica/quartz assemblage. Organic matter content is also high. It is, however, white or off-white firing and is self-fluxing, although full vitrification is at temperatures more akin to stoneware clays. Used as a ceramic clay in its own right it would provide strong hard ware at normal kiln temperatures but both drying and firing shrinkages are likely to be high.
4. PALYNOLOGY Palynological analysis of samples from the borehole core was carried out but only one sample, from 2.352.60 m, provided a good pollen and spore assemblage (Table 1). Pollen and spore analysis of sample 868 has TABLE 1. Palynological yield of core samples from the Dutson Brickpit borehole. Sample No. Depth (m) *868 2.35-2.60 **869 3.25-3.45 ***870 4.15-4.55 ***871 5.95-6.35 ***872 8.95-9,35 * indicates a good pollen and spore assemblage. ** indicates a poor pollen and spore assemblage. *** indicates a barren sample (no pollen and spore assemblage).
given percentage frequency counts of an assemblage containing 26 form-genera; 68% of the specimens are fern spores and 15% conifer pollen, an unusual characteristic for Tertiary assemblages from the western British Isles (Wilkinson & Boulter, 1980). Most of the 26 form-genera range through the European Tertiary, but Nudopollis, Plicatopollis and some of the spores are particularly characteristic of the Eocene. Boulter & Hubbard (in press) have analysed percentage frequency results of pollen and spore studies from TABLE 2. The percentage frequency of the 26 form-genera which occur in Dutson sample 868. Classified according to the three groups of principal components thought to have affinity with Eocene plant communities. Taxa of Group I are thought to be associated with Angiosperm Forest, those of Group II with Fern and Conifer Forest, and those of Group III with Paratropical Forest (Boulter & Hubbard, in press). The Dutson sample shows greatest (42%) representation of Group II by statistical analysis. More simply, 83% of the specimens counted are from ferns and conifers, % % group freq. sum 26 GROUP I-ANGIOSPERM POLLEN Tricolporopollenites 15 Momipites 2 Periporopollenites 2 Erecipites 3 GROUP II-FERN SPORES Polypodiaceaesporites Reticulosporis Camarozonosporites Cicatricosisporites
42 4 1 25 1
GROUP III-POLLEN FROM EXTINCT ANGIOSPERMS Toroisporis 19 Plicatopollis 3 Nudopollis 5 Tiliaepollenites 1 OTHER POLLEN AND SPORE TAXA Polypodiidites Deltoidospora Microfoveolatisporis Cedripites Pityosporites Cycadopites Monocolpopollenites Graminidites Tricolpopollenites Engelhardtioipollenites Myricipites Subtriporopollenites Gothanipollis Porocolpopollenites
1 12 5 2 13 1 4 1 9 5 2 1 1 1
32
400
E. C. FRESHNEY ET AL.
three IGS boreholes in the Hampshire Basin (Edwards, 1978; Edwards & Freshney, 1978; Freshney 1978), which represent most of the British Isles Eocene sequence. This was done by a method of principal components analysis and identified three groups of pollen and spore taxa, which may represent distinct plant communities in the Hampshire Eocene: Angiosperm Forest, Fern and Conifer Forest, and Paratropical Forest. The percentage frequency data from the Dutson Basin sample have been incorporated into this principal components analysis, giving a matrix of 85 taxa within 175 samples. The results are set out in Table 2. The taxa allocated to the three groups account for 65% of the specimens counted, of which those in group II (Fern and Conifer Forest) form the largest part. The other 35% of specimens counted are of taxa thought to represent the effects of transport and facies (Boulter & Hubbard, in press). This domination by Group I! taxa suggests that conifers and ferns were an important part of the Dutson Tertiary vegetation. Though any comparison with modern ecosystems must remain very tentative, an environment with poor soils and no tree canopy is a possible interpretation. Comparison with Wolfe's (1979) climatic data suggests that the mean annual temperature was 15°C and the mean annual range of temperature was 16.5°C. The multivariate statistical analysis of 174 samples from the IGS boreholes in the Hampshire Basin has given a stratigraphical control to the occurrence of Groups I, I! and II!. The Dutson Basin sample is equivalent to a sample which occurs at 78 m in the Ramnor Borehole, in the middle of the Barton Beds (Upper Eocene). This dating is made with 95% confidence for all principal components that have been calculated by this statistical method.
\ Sta nl e y Banks Basin
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Fig. 3. Map of part of Southwest England showing the principal Tertiary basins and their relationships to the wrench fault zones. That part of the wrench fault zone south of Dutson is from the revision of the Institute of Geological Sciences 1: 50,000 sheet 337 (Tavistock and Launceston) currently in progress.
5. DISCUSSION The formation of basins containing terrestrial sediments preserved within local sags in Palaeocene lavas, or within fault zones, is an important and widespread event in the Eocene-Oligocene evolution of the western British Isles. Discoveries within the past 10 years of offshore Palaeogene basins such as that in Cardigan Bay near Mochras (Woodland, 1971), near Lundy Island in the Bristol Channel (Fletcher, 1975), and near Canna, Sea of Hebrides (Smythe & Kenolty, 1975; Evans et al., 1979) have shown that such basins are more widespread and have a greater chance of preservation in the geological record than was previously thought. In SW England and its offshore area (Fig. 3), the formation of basins containing fluviolacustrine sediments was dominated by the NW-SE Sticklepath fault zone along which an estimated maximum 1100 m of sediments accumulated in the Bovey Basin, 661 m in the Petrockstow Basin, and at least 340 m in the Stanley Banks Basin east of Lundy
Island. Outside the Sticklepath Fault zone, until the discovery of the Dutson Basin, the only Palaeogene continental deposits known (with the exception of the older (Palaeocene-Eocene) gravels of Haldon, Orleigh and Aller type) were the poorly documented small deposits at St Agnes and Crousa Down in Cornwall, both of which are apparently not associated with faults. The Dutson Basin formed along a NW-SE wrench fault zone some 45 km west of the Bovey Basin, thus demonstrating that formation of Boveytype basins was not restricted to the Sticklepath zone, or governed by some special character of that zone, but could also take place along other wrench fault zones in SW England. Possibly many such basins have been lost by erosion. Using pollen and spores, the basins along the Sticklepath fault zone have been given ages from ?late Oligocene to Eocene. The topmost 34 m of the Stanley Banks basin has been sampled and dated as middle
TERTIARY BASIN AT DUTSON, CORNWALL
Oligocene by Boulter & Craig (1979), but late to middle Oligocene by Dr C. Turner (in Fletcher, 1975); however, the basin is thought to be at least 340 m deep (Brooks & James, 1975) and thus an Eocene age for the unsampled sediments in the lowermost parts of the basin cannot be excluded. The Petrockstow Basin sediments are 'mainly Eocene with some Oligocene' (Freshney et aI., 1979, p. 21), The topmost 300 m of the Bovey Formation in a borehole in the Bovey Basin has been dated as early to middle Oligocene, with rare pollen grains at 297 m depth suggesting the probability of Eocene sediments at greater depth (Wilkinson et al., 1980, p. 72). Since the estimated depth of the Bovey Basin is possibly of the order of 1100 m (Vincent, 1974), the lowermost 700-800 m of the Bovey Formation is probably Eocene. The dated part of the Dutson sequence is Upper Eocene, and thus corresponds in age to part of the (mainly) concealed lower Bovey Formation in the Bovey Basin, to parts of the outcropping shelf and axial trough sequences in the Petrockstow Basin, and possibly to part of the lower (undated) sequence in the Stanley Banks Basin. Lithologically and clay mineralogically the sampled Dutson sediments are more closely comparable with the Petrockstow Basin deposits than with the known part of the Bovey Basin succession, possibly because sequences of similar age crop out in the Petrockstow Basin but are generally concealed by younger strata in the Bovey Basin. The limitations of comparing the very short sequence proved in the Dutson Borehole with other Bovey Formation sequences are obvious, but there is present in the Dutson borehole a finingupward sequence, capped by a lignite, which is similar to fining-upward sequences recorded from the Petrockstow Basin by Freshney et al. (1979) and regarded by them as fluvial in origin. Such fining-upwards sequences are not a distinctive feature of the known Bovey Basin sequence (Edwards, 1976). Clay mineralogically, the sediments of the Dutson Basin show close affinities with those of the Petrock-
401
stow Basin, which Bristow (1968) demonstrated were derived from deeply weathered Upper Palaeozoic sediments. Although tentative, the conclusion drawn from the palynological data indicating poor soils and no tree canopy supports the clay mineralogical data, which suggest derivation of the sediments from rather immature tropical weathering profiles. The estimates of palaeotemperature also indicate that conditions were not entirely pertinent to tropical weathering. Thus, during the Eocene, we envisage the Cornubian peninsula with rivers flowing north and south from a central upland spine. Along the Sticklepath line, a major river flowing NW filled the Petrockstow and Stanley Banks Basins with sediment on its way to the Celtic Sea, while a SE-flowing river debouching into a sea in the English Channel provided the sediment that filled the Bovey Basin. At the same time, it is likely that another river flowing NW or SE in the Launceston area deposited its sediment in the Dutson Basin; such a river is unlikely to have been a tributary of the Petrockstow or Bovey rivers in view of the close control that the NW-SE fault trend would have exerted on the course of the river. Whether the Dutson river flowed NW or SE is uncertain; the present disposition of the topography of the area, although an uncertain guide, suggests that it is more likely that the river flowed south to a sea somewhere in the English Channel.
ACKNOWLEDGEMENTS We are most grateful to Mr E. J. Broad of Lower Dutson Farm, Launceston, for allowing access to his land and for giving permission to sink the Dutson Brickpit Borehole. We thank D. Bray, M. C. George, D. J. Morgan and F. H. Stacey for technical assistance and Mrs D. E. Milton and Mrs G. Wright for typing the various drafts. The paper is published with the approval of the Director, Institute of Geological Sciences (N.E.R.C).
References BAILEY, S. W. 1980. Structures of layer silicates. In: (Brindley, G. W. & Brown, G.; Eds.) Crystal Structures of Clay Minerals and their X-ray Identification. Min. Soc. G.B. London, 1-123. BOULTER, M. C. & D. L. GRAIG, 1979. A middle Oligocene pollen and spore assemblage from the Bristol Channel. Rev. Palaeobotan. Palynol., 28, 259-72. - - & R. N. L. B. HUBBARD in press. Objective palaeontological and biostratigraphical interpretation of Tertiary palynological data by multivariate statistical analysis. Palynology.
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