Non-local sponge cherts - descriptions and significances

Page 1

60

Suffolk Natural

History,

Vol. 31

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

Trans. Suffolk Nat. Soc. 31

(1995)


Turn static files into dynamic content formats.

Create a flipbook
Issuu converts static files into: digital portfolios, online yearbooks, online catalogs, digital photo albums and more. Sign up and create your flipbook.