Week-end Field Meeting: The Wealden Group in the Isle of Wight

17-19 June 1977

Report by the Directors: Brian Daley and D. J. Stewart

DALEY, B. & D. J. STEWART, 1979. Week-end Field meeting: The Wealden Group in the Isleof Wight. Proc. Geol. Ass., 90 (2),51-54. During the meeting, members studied the lithologiesof the Wealden Group on the south-western coast of the Isle of Wight. On the first field day,fluvial deposits in the Wessex Formation were examined from Brook Chine to east of GrangeChine. On the Sunday, members examined the uppermost part of the Wessex Formation and theremainder of the succession to the top of the overlying Vectis Formation.

B. Daley, Department of Geology, Portsmouth Polytechnic, Portsmouth.D. J. Stewart. Formerly of Portsmouth Polytechnic. Current address: Robertson Research Ltd.,Tyn-y-coed, Llanrhos, Llandudno, Gwynedd, North Wales.

On the evening of Friday, 17 June, fifteen membersassembled at the Bonchurch Manor Hotel, Bonchurch.The purpose of the meeting was to examine the sedi­ments, and, to a lesser extent, the palaeontology of theWealden strata exposed on the south-western coast ofthe Island, with a view to demonstrating the environ­ments under which they had accumulated.

In order to comply with the International Strati­graphic Guide (Hedberg, 1976), the stratigraphicalnomenclature of the Wealden strata on the Island hasbeen revised. The Wealden Group now comprises twoformations, the Wessex Formation (formerly the Weal­den Marls) and the younger Vectis Formation (formerlythe Wealden Shales; see Stewart 1978). The latter hasbeen further divided into the Cowleaze Chine Member,the Barnes High Sandstone Member and the ShepherdsChine Member, in ascending order.

The Wessex Formation was of fluvial origin. Sand­stones, a number of which comprised fining-upwardsunits, had mainly originated as channel deposits, whilstthe variegated muds with which they were interbeddedwere essentially the products of overbank deposition.The accumulation of the Vectis Formation was associ­ated with increasing salinity. The Cowleaze ChineMember accumulated in quiet, brackish lagoons awayfrom the influence of rivers, whilst the Barnes HighSandstone Member above, although of somewhatdebateable origin, was probably deposited as distribut­ary mouth bars where rivers entered the lagoonal area.The uppermost Shepherds Chine Member reflected areversion to quiet lagoonal conditions. Thin (average70-90 em thick) fining-upwards cycles characterised thismember and formed in response to alternating phases ofgreater or lesser river influence. Marine influences weremore apparent near the top of the member where Ostreaand other brackish water fossils heralded the subsequentLower Greensand transgression.

Brook Chine to Grange Chine, Saturday, 18 June

The party descended to the foreshore via Brook Chine(sz 385835) and walked eastwards past RoughlandsCliff, an area of broken ground with very low exposures.A first stop was made at a small promontory formed by athin sandstone. This exhibits a slight fining-upwards ofgrain size and has a thin intraformational conglomerateat its base. Amongst the variety of structures observed,were large and small scale cross stratification and verti­cally orientated 'escape' burrows. The sandstone is trun­cated towards the landward side of the promontory by aconcave-upwards erosion surface. Above the latter, the'hollow' filled with thin sands and muds is thought torepresent channel fill deposits of an abandoned meanderor oxbow lake.

To the east of the promontory, thin sands are inter­bedded with muds. The sands commonly display differ­ent types of ripple drift lamination. It was explained thatthese structures may have originated during the trans­portation of sand by floods into standing bodies of water,and that rapid decelleration of the currents had led torapid rates of deposition.

Members also showed some interest in the problemat­ical deformational structures within these sands. Theseappeared to be injection structures in which the bendingdownwards of adjacent laminations clearly indicated adownward rather than an upward movement. Previousobservations by the authors had also shown that somelateral post-depositional movement of sediment hadalso occurred. It was observed that some of these struc­tures passed up into large-scale cross-stratified sedimentwithout any apparent break, and that this indicated thatdeformation had been synsedimentary.

Members observed that these thin sands thickenedeastwards to form the Sudmoor Point Sandstone, afining-upwards, fluvial sandstone, averaging some 6 m

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52 BRIAN DALEY AND D. 1. STEWART

thick and reminiscent of the Old Red Sandstone cyclesdescribed by J. R. L. Allen (1964,1965). The directorsexplained that, since the sandstone was exposed for wellover 1·5 km along the coast, it provided a rare oppor­tunity to study lateral variation in a fluvial sandstone.The Sudmoor Point Sandstone comprises five laterallyadjacent but distinct sand bodies, separated from eachother by erosion surfaces. Apart from one body, thoughtto represent an abandoned channel, each was con­sidered the product of an individual point bar. The ero­sion surfaces represented point bar intersections.

Members noted that within each sand body individualbeds were inclined at an angle of up to 6° to the boundingsurfaces. These inclined beds had accumulated on thesloping surfaces of point bars. Vertical sequences withinsandstone bodies differed in detail laterally, althoughthey all exhibited a sharp base and fining-upwardssequence.

Members examined the vertical profile in some detailwhere a sand body reached shore level near ChiltonChine (sz 409829). Here, the basal beds comprise struc­tureless and trough cross-bedded conglomerates above asharp erosion surface. Pebbles mostly consist ofintraformational sandstone, mudstone and worn cal­careous nodules all of which were derived from associ­ated overbank deposits. The calcareous nodules prob­ably originated as calcretes on well drained areas of theflood plain.

Thin muds interbedded with the gravels are indicativeof periodic variation in the competence of the river. Thisphenomenon might have originated under conditions ofseasonal variation in rainfall with the muds accumulat­ing at low water stages when ponds developed behindgravel bars.

More or less conglomeratic sands were examinedabove the conglomerates; these are mainly cross-beddedbelow and become flat-bedded above. Mud-flake con­glomerates at intervals attest to the contemporaneouserosion of mud layers, whilst occasionally these alsocontain small armoured mudballs. The upper part oftheSudmoor Point Sandstone at this locality comprisedalternations of sandstone and mudstone, which mayhave formed the upper parts of point bars (ct. Singh,1972) or even levees (Fisk, 1944).

After lunch, the party walked eastwards from ChiltonChine, passing sandstones interbedded with variegatedmuds, and which had probably originated as the depositsof flood waters that had invaded overbank areas. Thethicker Brighstone Sandstone, which descends the cliffabout half way between Chilton Chine and GrangeChine (sz 420817), was seen to be similar lithologicallyto the Sudmoor Point Sandstone.

Eastwards from Grange Chine, the Grange ChineBlack Band (one of a number of beds in the WessexFormation rich in plant debris) descends the cliff. It wasobserved that below this horizon, varicoloured muds

and silty fine sands dipped more steeply to the east thanthe major bedding surfaces, indicating that it was cross­stratified on a giant scale.

Members noted that the colouration of these bedsshowed some degree of vertical striping, but that boththe colouration and stripiness ceased abruptly at thebase of a zone of reduction immediately underlying theBlack Band. It was suggested that this proved that thestripes predated the formation of the Black Band andthat they had perhaps originated as a result of groundwater movement penecontemporaneous with sedi­mentation.

The Grange Chine Black Band contains trunks andbranches of wood and has angular mud clasts at the base.There seems little doubt that it was deposited from floodwaters. It yields vertebrate material including Iguano­don, fish, crocodile fragments, unionids and plant conesare also present.

As evening approached, the party took advantage ofthe falling tide to return to the foreshore west of ChiltonChine to examine some recently discovered dinosaurfootprints on exposed surfaces in the upper part of theSudmoor Point Sandstone. Here, the party was met byDr. Allan Insole of the Museum of Isle of Wight Geol­ogy at Sandown, who explained that the prints had beendiscovered in March, 1977 by Mr. William Blows. Therewere earlier records of dinosaur footprints includingsome from this locality (Beckles, 1862). Thirty-nineprints had been identified, thirty-seven in separatetrackways, of which the longest had nine prints. Twodifferent animals are represented. Thirty-five printswere made by the theropod Megalosaurus. Each oftheseis approximately 30 cm long, and comprises a three-toedprint, with sometimes the faint indication of a fourthposterior vestigial toe.

The four other prints are much larger, the best pre­served being about 60 em long. These are also three toedprints, but with narrower angles between the toes, and,on the best print, distinct indications of a claw. Dr. Insoleexplained that the animal was definitely bipedal but thatit was not clear, as yet, which dinosaur produced them.Iguanodon seemed to be ruled out by the presence of aclaw. He suggested that they may have been producedby a large theropod similar to the American formAntrodemus, although no bones of a large carnivore ofthis size had been recorded on the Island.

Cowleaze Chine, Sunday, 19 June.

The party descended to the beach via Cowleaze Chine(sz 444801) and walked westwards past the cliff formedby the Barnes High Sandstone Member to where the topof the Wessex Formation is exposed.

Here, members were able to examine a variety ofsedimentary structures in the interbedded sandstonesand muds which form the top three metres of the Wessex

THE WEALDEN GROUP IN THE ISLE OF WIGHT 53

Formation. These include small-scale cross lamination,climbing ripple lamination, flat bedding, small scalechannels, mudcracks and contemporaneous microfaults.Members noted that the beds at this level were oftenconsiderably deformed as a result of water escape soonafter deposition. Interest also centred on the trace fossilspresent, including V-burrows but, more particularly,beautifully preserved specimens of Ophiomorpha. Thedirectors explained that, whilst the environmental sig­nificance of the latter was still a matter of some discus­sion (Stewart, 1978), it was widely believed to be amarine indicator. Hence, by the time these beds hadaccumulated, the environment of deposition was becom­ing more saline.

A small amount of time was spent on looking at thenature of the boundary between the Wessex and over­lying Vectis Formations. The boundary is associatedwith an upward colour change from red to grey; at Cow­leaze Chine it is marked by a thin lag of organic debrisand further west by a thin intraformational conglomer­ate.

The party then looked briefly at the overlying Cow­leaze Chine Member, particularly the 'White Rock'. Asmall channel filled with plant debris and intraforma­tional clasts was noted within this unit just to the west ofCowleaze Chine. Elsewhere the top of the 'White Rock'was seen to be penetrated by rootlets.

The Barnes High Sandstone Member was nextexamined. This forms the feature of Barnes High andcomes down to shore level to the west of ShepherdsChine (sz 446798). It comprises a limonite-stainedsandstone, 6 m thick, which rests conformably onthe underlying strata. The directors pointed out thatthis sandstone differs markedly from those from theWessex Formation seen on the previous day. It com­prises a coarsening-upwards sequence reflected byan upward change in the proportions of sand andmud. The sands themselves are generally medium tofine grained, and show a slight upwards increase ingrain size.

The decreasing presence of muds upwards is related toa vertical change in lithology from lenticular beddingtowards the base, through wavy bedding to large scalecross-bedding in the upper part of the member. Withinthe wavy bedded part, mud layers become more widelyspaced upwards, whilst in the higher cross-bedded part,wavy mud flasers, at first common, generally die outupwards. All these lithologies have transitional verticalrelationships and a certain amount of interbedding ofdifferent types occurs. The top of the member comprises

an intraformational conglomerate containing mud clastsand bivalves.

Many sedimentary structures were seen in the BarnesHigh Sandstone, including a variety of ripple types (cur­rent, oscillation, wave-current, etc.), small tool marks(including brush, groove and prod marks), flasers andorganic traces such as gastropod trails.

The environment of deposition was a continuallychanging one with a complex alternation of erosion anddeposition. Many of the features of the member re­sembled intertidal deposits (ct. Van Straaten, 1961;Reineck and Singh, 1974), although this origin had beenrejected since, according to Klein's (1971, 1972) modelfor palaeotidal regimes, too large a range of tide wouldhave been required to generate the Barnes HighMember. The most likely origin was thought to be a rivermouth bar (possibly in association with tidal activity).The vertical sequence of the member resulted from theadvance of the bar into a lagoon, whilst the break at thetop, together with the intraformational clasts and bettersorting at this level, suggested reworking as abandon­ment led to a reestablishment of lagoonal conditionssomewhat resembling those under which the CowleazeChine Formation had accumulated.

The remainder of the field meeting was spent in look­ing at the Shepherds Chine Member. The rhythmicnature of this essentially muddy sequence was clearlyapparent to members of the party. The directorsexplained that this reflected the presence of thin (aver­age 70-90 em), fining-upwards cycles. The cycles gener­ally have sharp bases, sometimes with large scours, andgrade upwards from sandy muds to muds. Characteristicstructures include lenticular bedding and parallel lami­nation. Gutter casts (ct. Whittaker, 1973) are oftenapparent, particularly towards the bottom of individualcycles. The origin of the cycles was briefly discussed. Thedirectors suggested that although the environment ofdeposition was essentially lagoonal, greater river influ­ence had led to periods of greater water flow when thelarge scours and smaller gutter scours were formed.

There was little time to see the quasimarine (brackishbeds) with Ostrea and other molluscs which occur in theuppermost part of the Shepherds Chine Member,although the party noted the useful marker horizon, aclay ironstone band with Diplocraterion some 11 m fromthe top.

This concluded the meeting and members climbed tothe top of the cliff over the tumbled Atherfield Clay.Here a vote of thanks was made to the directors by Mr.A. M. Evans and to Dr. A. Insole.

54 BRIAN DALEY AND D. 1. STEWART

References

ALLEN, J. R. L. 1964. Studies in fluviatile sedimentation: sixcyclothems from the Lower Old Red Sandstone, Anglo­Welsh Basin. Sedimentology, 3, 163-98.

ALLEN, J. R. L. 1965. Fining-upwards cycles in alluvial suc­cessions. Geol. J., 4, 229-46.

BECKLES, S. H. 1862. On some natural casts of reptilianfootprints in the Wealden Beds of the Isle of Wight and ofSwanage. Quart. Jl geol. Soc. Lond., 18, 443-7.

FISK, H. N. 1944. Geological investigation ofthe alluvial valleyof the lower Missippi River. Mississippi River Commission,Vicksberg, Miss.

HEDBERG, H. D. 1976. International Stratigraphic Guide.Wiley, N. York.

KLEIN, G. de V. 1971. A sedimentary model for determiningpaleotidal range. Bull. geol. Soc. Am., 82, 2585-92.

KLEIN, G. de V. 1972. Sedimentary model for determiningpaleotidal range: reply. Bull. geol. Soc. Am., 83, 539-46.

REINECK, H-E., and I. B. SINGH, 1973. Depositional sedi­mentary environments. Springer-Verlag, Berlin.

SINGH, I. B. 1972. On the bedding in the natural levee andpoint bar deposits of the Gornti River, Uttar Pradesh, India.Sediment Geol., 7,309-17.

STEWART, D. J. 1978. Ophiomorpha-a marine indicator?Proc. Geol. Ass., 89, 33-41.

VAN STRAATEN, L. M. J. U. 1961. Sedimentation in tidalflats areas. J. Alberta Soc. Petrol. Geol., 9, 203-26.

WHlTAKER, J. H. McD. 1973. 'Gutter casts', a new name forscour and fill structures: with examples from the Llan­dovarian of Ringerike and Malmoya, Southern Norway.Norsk geol. Tidsskr., 53, 403-17.