TERMINOLOGY AND THE CLASSIFICATION OF FINE GRAINED SEDIMENTARY ROCKS – is there a difference between a claystone, a mudstone and a shale?

Fine grained sedimentary rocks, both clastic and carbonate, are believed to be the most abundant rock type on the Earth‟s surface (Picard, 1971; Blatt, 1982). Fine grained rocks appear to constitute somewhere in the region of 70% (Holmes, 1937) and 80% (Clarke, 1924) of all the sediment ever produced. In sedimentology the size grade scale most commonly used is that which was introduced by Udden in 1898, modified by Wentworth in 1922 and then converted into a logarithmic form by Krumbien in 1934. For a single particle, the sand-silt boundary, which is the distinction between coarser grained materials and those which are fine grained, is placed at 1/16 mm (≈63 microns or +4 Φ). For a given rock the proportion of material containing particles less than 63 microns in size is normally greater than 50% to satisfy the classification criteria of being fine grained. Despite their abundance, fine-grained sedimentary rocks, to date, have not been satisfactorily classified. Perhaps this is because they are not well exposed in the field compared to their coarser grained, more weather resistant counterparts, the sandstones (Pettijohn, 1975; Spears, 1980); perhaps it is due to their fine grained nature that makes their study more testing (Krumbien, 1947; Potter et al., 1980; MacQuaker & Adams, 2003); or perhaps, as noted by Rodgers (1950) the problem of classifying fine grained sedimentary rocks, simply, but inevitably entails the problem of terminology. It is the usage of imprecisely defined terms that has often led to confusion, and frustration, when trying to describe and discuss fine grained sediments. This report will identify where confusion has arisen, and determines the type of terminology and classification that will be used in future works.