Terminology and the Classification of Fine Grained
SEDIMENTARY ROCKS – is there a difference between a claystone, a mudstone and a shale? Summary Fine grained sedimentary rocks, both clastic and carbonate, are believed to be the most abundant rock type on the Earth " s surface 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 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. In the literature there is a plethora of names for fine grained sedimentary materials – Definition and usage of the word 'shale' The term shale has been around for a few centuries. The first recorded use of the written word shale appears in Hooson " s " The Miners Dictionary " , published in 1747; within which the term shale was meant as an indurated, laminated, clayey rock. Tourtelot (1960) gives a good summary on the origin and use of the world shale, and by outlining its historical use as i) a general term for describing " laminated clayey rock " ; ii) as a formation name, e.g. the " Pierre Shale " ; and iii) it " s widespread use as a bucket term for the entire class of fine grained rocks (whether they are laminated …
A Classification of Sedimentary Rocks
A simple field and laboratory classification of sedimentary rocks is proposed. It is based fundamentally on composition and texture, with the primary divisions determined by the mode of origin of the constituents. Tt is tripartite in nature, consisting of (1) a dominantly fragmental division, including conglomerates, sandstones, and shales; (2) a division represented by rocks which are partly fragmental and partly precipitated, including ironstones, silicastones, coal, limestones, and dolostones; and (3) a division of salinastones, which are dominantly precipitated but may possibly be fragmental. The several new terms in the classification were first proposed in a paper read before the Chicago meeting of the Geological Society of America in December, 1946 (Shrock, p. 1231).
“Algal Dust” and the Finer-grained Varieties of Carboniferous Limestone
Certain of the Lower Carboniferous algae are preserved as moulds in a fine-grained calcareous matrix precipitated by their own life processes. The cell walls of the alga are not preserved. This observation applies to the genera Ortonella, Mitcheldeania, Bevocastria, and Girvanella. In the case of Mitcheldeania the filaments are so close together and so parallel that an impression of a definite wall may be obtained, but occasional irregularities and a comparison of the microstructure with that of other forms leaves no doubt that this too is a precipitate.