Making the material: The formation of silt sized primary mineral particles for loess deposits

Abstract

In 1911 E.E. Free wrote... “it may be well to point out that there are really two problems of the loess—a problem of the origin of material and a problem of deposition”. The problem of the ‘origin of material’ has been relatively neglected, in comparison to problems of deposition. of the formation of deposits, and latterly in comparison to studies of loess stratigraphy. If we are to understand the true nature of loess, this fascinating material, we need to be aware of all the significant events in the processing of the particles which form a deposit. A large amount of silt-sized primary mineral material has to be produced to make a loess deposit. Loess is a Quaternary phenomenon and we see the entire process being completed in Quaternary times. Some powerful Quaternary forces are able to produce very large amounts of granular material. ‘Glacial grinding’, vigorously supported by Smalley (1966a), provides an obvious source for some loess material, but the vast deposits of North China and Central Asia have no obvious glacial connection. However they are adjacent to ‘High Asia’, a region of vast tectonic activity and an enormous release of terrestrial energy in the last several million years. ‘High Asia’ is a large producer of loess silt, much of which finds its way south to make the Ganges delta but large amounts are moved north and east to make huge loess deposits. Africa presents a problem. Recent work (e.g. Coudé-Gaussen, 1987) suggests that there is a region of ‘peri-Saharan’ loess but earlier observations (e.g. Penck) indicate quite clearly that the Sahara region is free of loess. Rathjens (1928) in his study of Tripolitania indicated the presence of loess-like deposits and described their nature, but he did point out that 90% of the granular material was greater than 100 pm in diameter—which would remove it from a strict loess definition. On the Soil Taxonomy map of Africa there is a belt of soil D3 which could include considerable loess deposits; it stretches from the Matmata ‘loess’ in Tunisia, across northern Libya to the western part of the Egyptian coastal region. If D3 contains loess this points to a genuine desert loess particle producing mechanism operating in the Sahara.