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NON-LOCAL SPONGE CHERTS — DESCRIPTIONS AND SIGNIFICANCES H. B. M O T T R A M Sponges (Porifera) are simple soft-bodied animals that emerged s o m e 570 million years ago during the Cambrian period. M o s t sponges have a 'skeleton' which is either composed of a horny mesh, as in the case of the original bathroom sponge, or is composed of stiffeners called spicules. Spicules may be isolated or interlocking. T h e simplest spicules have a needle-like shape, more elaborate versions may have stubby branches at one end while other common variants have Y, X and even six-limbed shapes (see Fig. 1). In some sponges the spicules are formed f r o m calcium carbonate and in others f r o m silica. Silica is normally insoluble in water but the spicules of siliceous sponges are composed of a colloidal type of silica. It is thought that when ancient spicules became buried in sediments below the sea-bed, the colloidal silica dissolved and migrated to become concentrated at certain horizons. When the Sediment hardened into rocks the concentrations of silica crystallised to f o r m extremely tough layers and nodules of chert. These are referred to as sponge cherts to distinguish them from cherts which are believed to o w e their silica to other sources such as radiolaria. One of the purest varieties of chert is flint and this is certainly the c o m m o n e s t sponge chert found in Suffolk. However, other sponge cherts derived from further afield may be found in the County. Some occurrences are random but some are constant and these may be of significance. T h e I n g h a m Sand & Gravel Sponge chert occurs as nodules and slabs in the Carboniferous Limestone Series. Most of the chert bearing limestones are dark grey and they usually contain dark cherts; black, dark grey and occasionally reddish varieties. T h e intensity of pigmentation often decreases towards the periphery of a chert nodule or slab and mottling and banding are also c o m m o n . Pale cherts, light grey and even cream or white varieties, are less c o m m o n and tend to occur in limestones of light-grey colour. In fine grained limestones the chert is dense, rarely contains fossils and fractured surfaces are similar to those of flint but are generally not as conchoidal or as shiny. Chert found in fossiliferous limestones has a slightly grainy feel to fractured surfaces and c o m m o n l y contains fossil material partly replaced or entirely infilled with microscopic crystals of quartz. The c o m m o n e s t spicules in Carboniferous chert are minute six-limbed ones belonging to the genus Hyalostelia (Fig. le). Spicules of Geodites and Erythrospongia are also k n o w n . Other fossil material includes stem segments (ossicles) of sealillies (Crinoidea) and shell fragments and spines of productid brachiopods (Brachiopoda). T h e Ingham Sand and Gravel Formation of Suffolk is an early Pleistocene river deposit which has up to 10% of Carboniferous chert in the 16 to 32 mm size ränge. T h e Ingham Stratum has been traced upstream towards Kings Lynn (Fig. 2) and linked with the Baginton-Lillington Sand & Gravel of Leicestershire and Warwickshire (Rose, 1987). The river which deposited these sediments acquired vast quantities of material in the Midlands by reworking Triassic conglomerates which contain pebbles and cobbles of several different
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ages and rock types. Düring transportation to Suffolk many pebbles and cobbles would have been broken down and effectively lost from the gravel fraction but the extremely robust types would have remained in virtually the same relative proportions. One would therefore expect to find in the Ingham Sand and Gravel in Suffolk that the ratio of recycled Carboniferous chert to recycled Ordovician quartzite would be around 1:100 but in fact it is about 1:10. The relative increase in the Carboniferous chert component suggests that the river which deposited the Ingham Sand and Gravel had an additional supply of Carboniferous chert. This has been taken to support the likelihood of the river having had tributaries which drained the southern Pennines (Rose, 1987) where the tributaries would have obtained cherts from the Carboniferous Limestone Series of Derbyshire (Fig. 2). Kesgrave Sand & Gravel A second source of material orginates from the Lower Greensand (Lower Cretaceous). In the string of outcrops of Lower Greensand which runs from near York to the Isle of Wight, chertification did not occur except in limited developments near Faringdon and Shaftesbury. However, in the Weald region chert development was relatively good, particularly in the sandy beds of the northern rim of the Weald (Fig. 3). Here there are several types of cherty material but the durable types are usually yellow, buff or brown and somewhat translucent. Most chert nodules do not contain fossils, but where calcareous spicules were present they have often been dissolved leaving these cherts with small elongate cavities which give them a distinctive appearance. Perhaps dissolution continued into any calcareous Sediment around some of the spicules as cavities may be up to 3mm in diameter, somewhat larger than the spicules themselves. Occasionally the calcareous spicules were replaced with silica and in these instances spicules of Geodites are most often reported. Geodites had three types of spicule, the most readily identifiable of which is a 1 to 2 mm long rod with stubby branches at one end (Fig. lc). Other fossils occasionally found include radiolaria, ostracods and foraminifers. The Kesgrave Sand and Gravel Group is another early Pleistocene river deposit (Rose et al„ 1978; Whiteman, 1992); it has been traced back through Essex and Oxfordshire (Fig. 3). In Suffolk and Essex, the regulär presence of Lower Greensand chert, about 0.8% in the 16 to 32 mm size ränge, (Hey, 1976; Whiteman, 1992) suggests that tributary rivers drained the Weald. This ties in well with the tributary-like branches of Kesgrave Sand and Gravel in southern Essex and northern Kent (Bridgland, 1986). It is worth noting that in comparison to the cherts in the Lower Greensand outcrops, those recovered from the Kesgrave Sand and Gravel have become paler in colour, being light grey to buff. Glacial Sand & Gravel A third type of chert contains the minute spicules of the Upper Jurassic sponge Rhaxella perforata. The spicules of this sponge are unusual in that they only occur as an ovoid form and although they merely appear as dots under a hand lens, a slight kidney-like profile may be observed under a powerful microscope (Fig. 1 f).
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COMPLED FROM SEVERAL SOURCES PRESENT DAY OUTCROPS OF;_ / fOiAM/BAGmON-lXJJNGTON SAND & GRAVEL :•:;: TR1ASSJC CONGLOMERATES (APPROX EXTENT) ^
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R. perforata seems to have flourished in shallow to moderately deep water when there was a soft but not muddy substrate available (Wilson, 1968; Wright, 1972). Evidence of Rhaxella can be found in several Corallian limestones and sandstones from Weymouth to Scarborough (Fig. 3). Although cherty material occurs at several locations the only area where resilient chert composed of Rhaxella spicules can be found is in a few moorland exposures around Helmsley. The chert here is white to light-grey (Wright, 1974, 1983 and pers. comm.). Near an exposed surface the spicules have often been weathered out giving the chert a closely spaced needle-punched appearance. The glacial sands and gravels of Anglian age in Suffolk contain Rhaxella chert, about 0.5% in the 16 to 32 mm size ränge, and this chert is also present in the Anglian tills. It has been stated that this chert is 'best matched' with that from the Helmsley area (Hey, 1976). The inference has been made that the Anglian glacial sediments in Suffolk were due to ice which had either passed directly Over North Yorkshire (Bridgland, 1986), or had amalgamated with other ice that had done so. The Rhaxella chert found in Suffolk is often black and therefore may not have come from the Helmsley area. It is possible that at least some of the Rhaxella chert found in Suffolk was derived from a source which no longer exists or has not yet been recognised. Support for the latter comes from the discovery of Corallian rocks in the bed of the North Sea adjacent to Yorkshire and the recovery of Rhaxella chert from Corallian sands in the Moray Firth (G. A. Blackbourn, pers. comm). Conclusions Although flint, mainly of local derivation, is the commonest type of sponge chert found in Suffolk, other types of sponge chert derived from rocks of more distant outcrop may also be found. Nodules or layers in the distant source rocks are rarely more than Im thick and some outcrops are also of limited lateral extent. It is immediately apparent that after reworking and mixing with other materials, these cherts will only be minor constituents in subsequent deposits as far away as Suffolk. However, even when scarce, some occurrences are constant rather than spasmodic. The indication of a persistent supply may be insufficient evidence on its own but when used alongside other 'teil tales' can be of significance in elucidating the origins of a Pleistocene Sediment. References Bridgland, D. R. (1986). The rudaceous components of the gravels of eastern Essex: their characteristics and provenance. Quat. studies, 2, 34. Hey, R. W. (1976). Provenance of far-travelled pebbles in the pre-Anglian Pleistocene of East Anglia. Proc. Geol. Ass. 87, 279. Rose, J. (1987). The status of the Wolstonian glaciation in the British Quaternary. Quat. newsl. 53, 1. Rose, J„ Allen, P. & Wymer, J. J. (1978). Weekend field meeting in south-east Suffolk. 15-17 October 1976. Proc. Geol. Ass. 89, 81. Whiteman, C. A. (1992). The palaeogeography and correlation of pre-Anglian glaciation terraces of the River Thames in Essex and the London Basin. Proc. Geol. Ass. 103, 37.
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Wilson, R. C. L. (1968). Carbonate facies variations within the Osmington O o l i t e Series in Southern E n g l a n d . Palaeogeog. Palaeoclim. and Palaeoecol. 4, 89. Wright, J. K. (1972). T h e stratigraphy of the Yorkshire Corallian. Proc. Yorks. Geol. Soc. 39, 236. Wright, J. K. (1974). Field meetings. The Corallian of the vale of Pickering, 14 to 16 September 1973. Proc. Yorks. Geol. Soc. 40, 208. Wright, J. K. (1983). The Lower Oxfordian (Upper Jurassic) of North Yorkshire. Proc. Yorks. Geol. Soc. 44, 249. H. B. Mottram, 66 Glastonbury Close, Ipswich IP2 9 E E
A hole new
world?
Over the winter of 1994/1995 I visited many churchyards in East Suffolk. The main aim of these Visits was to record woodlice and harvestmen, but spiders were also collected. On these trips I have started lifting drain and man-hole Covers. An important practice is to work out the prevailing wind direction, before lifting the Covers, and to stand down wind to avoid the worst of the smells. A strong stomach is also useful. In less than half the churchyards visited could the man-hole Covers be lifted, with some churchyards lacking man-holes and a sewer system altogether. O n c e the man-hole cover has been raised it is laid upside down and its underside is examined first; then the rest of the hole is investigated. S o m e common species of spider are often present, such as Amaurobius ferox and Tegenaria duellica. Occasionally more interesting species are found, which have included Lessertia dentichelis, Nesticus cellulanus and Meta bourneti. All three species are new vice-county records for East Suffolk, with the records M. bourneti being the 13th and 14th for Britain (nine records mentioned in 'Roberts', with the SRS (Spider Recording Scheme) in possession of a further three records). There is an old, pre-1950s, record of M. bourneti f r o m Gedding, West Suffolk. M. bourneti was found in two covered man-holes, in the grounds of Pettistree churchyard ( T M 2 9 5 4 ) on February 4th, 1995, when five specimens were recorded, four female and a inedium-sized juvenile. A second record of this species comes from another larger sewer in the grounds of Earl Soham churchyard ( T M 2 3 6 6 3 2 ) , when five specimens were seen including three females, a sub-adult male and a medium-sized juvenile, on February 21st, 1995. Another new species for East Suffolk, Cicurina cicur, was collected from Elmsett churchyard ( T M 0 5 4 7 ) on January 2nd, 1995. W h e n it was found on the underside of a small drain cover. N o n e of these spiders are considered to be rare, but the habitat they live in is seldom investigated. So, the next time you are out looking at wildlife, you now have another dimension to investigate! Roberts, J. R. (1993). The Spiders of Great Britain and Ireland, Jon D a w s
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Harley Books.