The Origin of English Fuller's Earths

It is suggested that the source materials of the English Aptian fuller's earths were soils of the Anglo-Belgian Land and that montmorillonite is the characteristic clay mineral of marine deposits formed when old land surfaces of low relief are transgressed. When the origin of English fuller's earths was discussed in the 1930's, Grim (1933, 1935) and Newton (1934) engaged in a polemic. Grim maintained that the Jurassic fuller's earth of Bath contained glass shards--relict structures which proved the igneous origin of the clay--as in bentonite, but this sample was found to be of doubtful provenance. Two samples collected by Newton from the Bath deposits and sent to Grim were either too calcareous or too ferruginous to permit the identification of shards. Although Kerr (1932) had earlier recorded abundant relict structures in Woburn fuUer's earth, Newton (1937) maintained that the fragments under discussion were either sponge spicules or crystals of authigenic anorthoclase, but not shards. Admittedly, a volcanic source of the raw material for the Aptian fuller's earth, especially, has its attractions. They are believed, though not proved, to be contemporaneous and since it is known that volcanic ashes can cover vast areas, one need not be surprised at fuller's earths being deposited at places as far apart as Maidstone, Redhill, Woburn, Clophill and Langton (Lincs.). Moreover, montmorillonitic rocks have been formed from volcanic ash in many other places including Nagyt6t6ny, Hungary (Vitalis, 1936), Texas (Broughton, 1932), Ventura, California (Kerr, 1931), Porter's Creek Missouri (Allen, 1934), New Zealand (Fife, 1934) and Le Locle, Switzerland (Hofmann, 1958). In other places they have been formed from volcanic glasses, such as liparite in Japan (Okazawa, 1928) or from trachytes and andesites or their tufts, for example in Styria, Carinthia and Ponza (Neuwirth, 1953) and in North Africa (Martin Vivaldi, Cano-Ruiz and Fontbote, 1956). In addition, MacEwan (1956) found a specimen o f ' W a l k e r ' s Ear th ' of the Silurian system, which also has a volcanic origin (Murchison, 1839), to contain a 2:1 illite-montmorillonite interstratified clay mineral. 282 ORIGIN OF ENGLISH FULLER'S EARTHS 283 There is, however, conclusive evidence that part at least of the fuller's earth is not derived from volcanic ash. Thus the non-clay minerals of Redhill fuller's earth are of two suites: one is authigenic (sphene, apatite, anorthoclase) and the other is detrital (epidote, tourmaline and staurolite). Davies (1915-1918) also found stauroFIG. 1--Aptian Palaeography--after Kirkaldy (1947), Wills (1951) and Wood (1956). lite, garnet and tourmaline in Woburn fuller's earth while Wood (1956) showed that when the authigenic minerals are excluded the detritals of the Redhill fuller's earth are identical with those of the normal Sandgate beds and that they are mainly derived from the London Platform. 284 R~ H. S. ROBERTSON A clue to the probable origin of the Aptian fuller's earth is furnished by a study of the palaeogeographical map of the area (Fig 1). In the Upper Aptian the sea broke through to the north over the London-Belgian Land and the commercial fuller's earths occur only a few miles from the Aptiart coastline. The source of the Sandgate beds, in which the deposits occur, is stated by Kirkaldy and others to be the London Platform, whereas in the later Folkstone Sands an Armorican source became more important. The lenticular fuller's earth deposits are interpolated with abrupt transitions in coarse sediments and the conditions of deposition reoccurred from time to time. The picture which comes to mind is of the end of the London Platform worn down to swampy lowlands, where montmorillonite can be formed. The area had been dry land throughout the whole of the Jurassic period and must have been deeply weathered, yet it was all to disappear in a short period when the Aptian sea invaded it. In consequence vast accumulations of weathered materials and soil clays were brought into the sea. In the underlying Wealden beds, only occasionally broken into by the sea, montmorillonite is not a dominant clay mineral, Thus, Butterworth and Honeyborne (1952) found that the clay minerals of the clays of the Hastings beds consisted of almost equal parts of illite, kaolinite and a chlorite-like mineral. The appearance of montmorillonite coincides with the breakthrough of seawater. The evaporation in Aptian times was high and calcium carbonate was precipitated along with the fuller's earth and, more markedly, with the associated coarse glauconitic sands. Montmorillonite is characteristic not only of the Aptian marine incursion and the submergence of the Anglo-Belgian Land, but of the great Cenomanian transgression. At this time further vast quantities of soil from a deeply-weathered worn-down landscape came into a calcareous sea and when the lands were maximally drowned the chalk became much purer. In the intervening Albian period the clay minerals of the Gault were doubtless mainly derived from Jurassic clays, particularly the Kimmeridgian which at that time was being eroded by the sea. Since montmorillonite is characteristic of the Cretaceous rocks over such a long period it is almost certain that it could not be formed from volcanic ash. The question now arises whether the montmorillonite came into the sea unchanged from the soil or whether the ORIGIN OF ENGLISH FULLER'S EARTHS 285 soi l p r o v i d e d oxides and hydrox ides wh ich r e c o m b i n e d in the seawa t e r to f o r m the clay minera ls . Since the v a r i a t i o n in ion ic c o m pos i t ion o f the A p t i a n fu l ler ' s ear ths is so smal l (Tab le 1) it seems p r o b a b l e to the a u t h o r tha t they were f o r m e d in the sea u n d e r s imi lar condi t ions . Soil clays p r o d u c e d by p r o l o n e d wea the r ing o f a va r i e ty o f rocks u n d e r di f ferent cond i t i ons o f slope, d ra inage and vege t a t i ona l c o v e r w o u l d u n d o u b t e d l y have s h o w n cons ide rab le var ie ty . TABLE 1--Ionic composition of montmorillonite in English fuller's earths, on the basis O10(OH)2.