Megaclasts and blocks: urgent questions of classification of large sedimentary particles

Traditional classifications used in sedimentology usually define sedimentary particles with the size of more than 1 m as blocks and/or boulders without further subdivision, which can be explained by the relatively rare occurrence of such geological objects. However, the latter are not only widely distributed locally, but also form the very character of some deposits. Particularly, this takes place in the Rufabgo River canyon in Mountainous Adygea. Moreover, an interest to tsunamites associated commonly with very large clasts, which has risen in the last years, stimulates thinking about the development and broad use of the detailed nomenclature of large sedimentary particles. The Blair-McPherson classification, which has gained international value, establishes the boundary between boulders and megaclasts by the particle size of 4096 mm, and megaclasts are further subdivided into four classes. Its evident advantage is detailed and unified approach. However, too large interval of the boulder fraction, which embraces clasts with the size from ~ 25 cm to ~ 4 m, should be noted. This makes certain difficulty for the practical use of this classification, and, particularly, when it is used for colluvial deposits of Mountainous Adygea. Enlargement of the interval of megaclasts and, subsequently, lowering the upper limit of the boulder fraction to 2048 mm, as this is suggested by the Blott-Pye classification, solves the noted problem only in part. Terminology, which is proposed for large clasts in the analysed Blair-McPherson classification, is also not very suitable. The only term to accept is "block", whereas the terms "slab", "monolith", and "megalith" face problems when applied to sedimentary particles. Particularly, archaeology treats megaliths as large stones used for construction by ancient civilizations, and the term "slab" is used actively and with different meaning for the purposes of lithospheric plate tectonics. Among urgent questions on the nomenclature of large sedimentary particles, we can indicate those relevant to the following issues: 1) the definition of the terms "glyba", "megaclast", and "block", 2) the establishment of the upper limit of the boulder fraction, and also 3) the choice of appropriate accuracy for subdivision of large clasts. It is proposed in this work to judge the terms "glyba" and "megaclast" as synonyms, and the term "block" as closely related to them. Distinction between glyba and block seems to be reasonable to make by the size of 1 m. Finally, the only smallest megaclasts deserve detailed subdivision because of their more frequent occurrence. All said above forms sufficient premises for new proposals on the nomenclature of large sedimentary particles. The natural monument "Maiden's Stone" in Mountainous Adygea, which is of colluvial origin, can serve for the purposes of discussion of classification of large sedimentary particles.

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