SHORT COMMUNICATION

Phosphatic concretions in the Wealden of South-East England

Kevin Taylor

TAYLOR, K. 1991. Phosphate concretions in the Wealden of South-East England. Proc.Geol. Ass., 102(1), 67-70. Phosphatic concretions are reported from several locations in theWealden sediments of South-east England. The occurrence of such concretions in the Wealdensediments has not previously been recognised and in some cases in the past they may have beenmistaken for siderite concretions. The predominant mineral in these concretions, as determinedby X-ray diffraction analysis, is francolite, a hydroxy-carbonate apatite. Both primaryconcretions from the Weald Clay and reworked examples in the Hastings Beds have beenrecognised.

Postgraduate Research Institute for Sedimentology, University of Reading, Whiteknights,Reading, RG62AB

1. INTRODUCTION Standard marine stages

Fig. 1. Stratigraphy of the Wealden sediments in south-eastEngland.

Until now phosphatic sediments have not beenrecognised in the Early Cretaceous Wealden sedi­ments of south-east England (Fig. 1). This com­munication reports the occurrences and mineralogy ofphosphatic clasts and concretions, some of which mayhave been misidentified previously as siderite. It ishoped that this communication will stimulate thesearch for further examples and an awareness thatfield observations often need to be complemented byanalytical information to determine the mineralogy.

2. REWORKED PHOSPHATIC CONCRETIONSIN THE HASTINGS BEDS

The upper part of the Lower Tunbridge Wells Sandsand the whole of the Lower Grinstead Clay areexposed in Philpots Quarry, West Hoathly (TQ355322, Fig. 2). The section has been described byAllen (1975, 1976, 1990, Fig. 2), in which he noted abed ('g') of rolled and weathered "limoniticironstone" in the Ardingly Sandstone Member.Mineralogical work presented here shows that manyof these clasts are phosphatic.

The horizon lies about 2 m below the top of theLower Tunbridge Wells Sand (see Allen, 1990, fig. 2,for full sedimentological log). It is traceable along thepresent (1990) quarry face, a distance of approxim­ately 50 metres. Overlying a medium-grainedsandstone with an irregular erosional contact, it isfollowed by a thin grey clay and then fine-grainedsandstone. The thickness of the bed varies from 3 to7 ern and it changes in character from one end of thequarry to the other (NNW-SSE). At the southern endit consists of large flattened brown to dark brownlimonitic clasts up to 10 cm across. Many show browncentres when sectioned, but some have light greycores. The outer zones are concentrically bandedlighter and darker brown. Towards the northern end

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of the quarry the clasts become much smaller(maximum 2 cm) and only rarely have grey centres .Throughout the section the clasts are embedded in amatrix of brown, goethite-rich medium sand . Frag­ments of wood , bone, teeth, scales and rare quartzpebbles (up to 1 cm in diameter) have been found(Allen, 1976 and pers. comm., 1989).

A typical large clast is shown in Fig. 3, along withthe mineral ogical data (determined from X-raydiffraction analyses) for three points A -C within it(Table 1). The domin ant mineral in the grey centre isfrancolite (hydroxy-carbonate apatite) , but side rite,

TABLE 1. Mineralogy for zones A , Band C in Fig. 3(Wt%).

Zone Francolite Siderite Quartz Clays Go eth ite

Fig. 3. Hor izont al section through a clast from the reworkedbed at Philpots Quarr y.

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PHOSPHATIC CONCRET IO NS IN THE WEALD E N Of SOUT H -EAST E NG LA N D

TABLE 2. Exposures in the Weald Clay at which phosphatic concretions have been found(BGS bed numbers as in ThurreJl, Worssam & Edmonds, 1968).

69

Locality

A-Smokejacks Brickwork s, Ockl ey,Surrey. [TQ 115372)

B-Breare Green Brickworks, SouthHolmwood, Sussex . [TQ 185421]

C-Auclaye Brickpit , Ca pel, Surrey.[TQ 170388J

Stratigraphic details

Exposes clay beneath the Alfold Sand (BGS bedno . 5c) in Upper Weald Clay .Expo ses clay from above the Sayerscroft Sand(BGS bed no . 9b) in Upper Weald Clay.Exposes clay between BG S bed nos. 3a and 3c inLower Weald Clay.

TABLE 3. Mineralogy of phosphoate concretions from the WealdClay (Wt%). Localities as in Table

2.

and do not form continuous beds. At other localitiesthey have so far only been found as loose blocks in thebase of the quarry. Further examples from in situ areneeded to determine the exact sedimentologicalrelation ship of the concretions.

Francolite Quartz Clays

quartz and minor amounts of clay minerals (mainlykaolinite) are also present. In the brown outer zonesfrancolite is dominant, goethite (FeOOH) is alsopresent but siderite is absent. Thin sections of theclasts show that they are extremely fine-grained.Under electron microscopy the francolite takes theform of very small crystals (less than 10 ~m),approximately the same size as the clay grains. Nostructure is visible in the grey cores whilst in the outerzones only darker and lighter bands relat ing to lesserand greater amounts of goethite respectively arevisible. The goethite , which gives the browncolouration to the outer zones, can be attributed tooxidation of siderite originally present in the clast.This suggests exposure of the clasts to an oxygenatedenvironment prior to being tran sported and thendeposited at their present site.

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3. PRIMARY PHOSPHATE CONCRETIONS INTHE WEALD CLAY

Primary concretions of grey francolite have beenfound in the stratigraphically higher Weald Clay.Some of these were at first taken to be of sideritecomposit ion being very similar in appearance to thesiderite concretions that are abundant in theWealden of the Weald. However , the phosphaticconcretions generally have a pale yellow-brown colourin contrast to the grey colour of the sideriteconcentrations. To date such concretions have beenfound at three localities in the Weald Clay:Smokej acks Brickworks, Ockley ; Breare GreenBrickworks, South Holmwood; and Auclaye Brickpit ,Capel (Fig. 2; see Table 2 for full grid references andstratigraphical details of these localities) . Theirmineralogical composition (Table 3) is very similar tothose from Philpots, except that siderite is absent.Well preserved insects in them (Jarzembowski, 1988)suggest that the francolite precipitated very early inthe sediment prior to significant bur ial and compac­tion. Like that at Philpots, the phosphate is extremel yfine-grained with very little detail being discernibleunder the electron microscope. Only at one locality(Auclaye Brickpit) can the concretions be found insitu, where they take the form of isolated concretions

4. DISCUSSION

The similarity between the primary francoliteconcretions in the Weald Clay and the reworkedexamples at Philpots Quarry suggest that the latterformed during early diagene sis and were laterexposed , weathered and then tran sported with otherdebris to the site of deposition. Reworked sideriteconcretions, similarly formed during early diagenesis(Allen , 1990), occur in many of the Wealden pebblebeds (e .g. the Cliff End Pebble-Bed at Brede)indicating recurrent exposure and reworking . Thetime-gap represented by the Philpot s horizon isuncertain . The contact with the underlying sandstoneis erosional, being discordant to the laminationunderneath and truncating the tops of upstandingLycopodites. Th e same horizon can be seen atPembury, 20 km to the east (TQ 414615), where theerod ed Lycopodites and overlying shale are present .but phosphatic clasts are absent. A fairly widespreadevent is therefore recorded . At both localities theabsence of a well developed weathering profile wouldseem to suggest only brief sub-aerial exposure andrapid deposition , although this may have beenremoved by erosion.

Phosphate concretions are well-known from strat­igraphically higher beds in south-east England . Thesetake the form of primary and rework ed concretions in

70 KEVIN TAYLOR

the Gault Clay and Lower Greensand (Ruffell, 1990).These contrast with the non-marine Wealden ex­amples , however, in that they are clearly of marineorigin. It is probable that there are other examples ofphosphatic sediments, previously unrecognised, in theWealden sediments, of both primary and reworkednature . More examples are needed in order tounderstand better their origin and significance. Inparticular, exposures which show the exact sedimen­tological relations between primary phosphate concre­tions and the surrounding clays need to be recognised

and studied. Other occurrences of phosphate mayhave been mistaken in the field for siderite.

ACKNOWLEDGEMENTS

Perce Allen and Ed Jarzembowski are thanked fordiscussion and advice, and Alan Cross is thanked fordrafting assistance. This work was carried out whilst inreceipt of a NERC studentship which is gratefullyacknowledged . Reading University PRIS ContributionNumber 100.

References

ALLEN, P. 1975. Wealden of the Weald: a new model.Proc. Geol. Ass., 86, 389-438.

-- 1976. Wealden of the Weald: a discussion. Proc. Geol.Ass., 87,433-42.

ALLEN, P. 1990. Wealden research-ways ahead. Proc.Geol. Ass., 100,529-64.

JARZEMBOWSKI, E. A. 1988. A new aeshnid dragonflyfrom the Lower Cretaceous of South-east England.

Palaeontology, 31, 763-76.RUFFELL, A . H. 1990. The mineralogy and petrography of

the Sulphur Band phosphates (Aptian-Albian), atFolkestone, Kent. Proc. Geol. Ass., 101,79-84.

THURRELL, R. G., B. C. WORSSAM & E . A.EDMONDS. 1968. Geology of the Country aroundHaslemere. Mem. geol. Surv. U.K.