Sedimentary particles range from the fine dust transported by high-altitude winds to gigantic erratic blocks moved by glaciers. Due to their highly diverse shapes, during granulometrie investigations the particles are conventionally considered as spheres of equivalent volume. This naturally will lead only to approximate results: it is especially the case for bioclastic sands with frequently curved particles which pass diagonally through square-mesh screens, and for the clay mineral platelets, the size of which is studied by sedimentation in reference to spherical grains (cf. Rivière 1977). The subdivision of sedimentary particles in three large categories, sand (2–0.063 mm), silt (0.063–0.004 mm), and clay (below 0.004 mm), leads to a general classification of sediments (Fig. 2.1). The term clay is used here without its mineralogical connotation. In the majority of cases, the size scales are subdivided into granulometric classes on a geometric scale. The one most widely used is that of Udden-Wentworth (US Standard) in which the upper limit of each principal class is double the limit of the preceding class, the basic diameter corresponding to 1 mm (Table 2.1). This type of progression results in a logarithmic graphic presentation. In day-to-day use the latter is often replaced by an arithmetic system of better readability, in which each main class limit is given by a full digit: the progression of Udden-Wentworth thus corresponds to the Φ (phi)-scale of Krumbein in which Φ = log 2 mm and Φ0 = 1 mm.
Weitere Kapitel dieses Buchs durch Wischen aufrufen
- Properties of Sedimentary Particles
Prof. Dr. Hervé Chamley
- Springer Berlin Heidelberg