Silt: 2-62 mu m, 9-4 phi

The particles in finer-grained detrital sediments are usually composed of q uartz. They fall into two size grades: sand (2 mm-62 mu m) and silt (62 mu m-2 mu m) These size gradings conceal important geological processes since there are geological controls on both the quartz sand and silt populations. Sand nature is largely controlled by geochemical reactions, for example in cooling granite from which it is eventually released by weathering action. The quartz is among the last part of the rock system to solidify and the e utectic-like reaction which occurs ensures that the quartz exists as small crystalline units, each suffering considerable cooling and recrystallisatio n stresses. Silt is broken quartz, and has traditionally been considered to lack a specific geological control. The situation is confused by the large range of materials that fall into the size category, but there do seem to be distinguishable modes and possibly comminution limits within the 60 mu m span. The controls operating in this size range are probably the critical concentration of 'Moss' defects in the quartz particles. Moss postulated th e formation of specific crystal defects in the quartz formed in granites. T hese affect sand formation and it is possible that they also control the mo de size of silt particles. Within the silt range there may be several usefu lly definable populations, as Moss proposed for sand. The Quaternary appear s to be a silt-rich period due to tectonic and glacial activity, but silt p roduction is apparent throughout the sedimentary record. Very long-term sil t producing processes are required. To produce silt in nature, on a large s cale, very energetic processes are required. Many processes that are believ ed to generate silt particles have been listed. However, large-scale produc tion is essentially due to glacial grinding, or to intense weathering proce sses in high, cold, tectonically active mountain regions. The region of Hig h Asia (of crustal overlap) is a major generator of silt particles. These f orm the productive alluvial soils of north India, most of the nation of Ban gladesh, the loess deposits along the Syr-darya and Amu-darya rivers in Cen tral Asia, and the great loess deposits of North China. Some claims have be en made for silt production in hot deserts. Large amounts of very fine aero solic dust are produced, but in terms of loess-sized particles this is a sm all-scale process and leads to modest, disputed deposits. The loess deposit s around the Sahara usually have smaller mode sizes (e.g., Nigeria) or larg er mode sizes (e.g., Libya, Tunisia) than the true Chinese mode at 25 mu m This paper aims to introduce the distinguishing properties of sand and silt , discuss in detail the possible processes of silt formation in relation to the evidence of what is produced, describe the major silt populations, and conclude on the most likely formation process(es). In particular three sil t ranges are discussed in relation to the R-size diagram: C, 2-5 mu m, D-1, 20-30 mu m, and D-2, similar to 60 mu m. These can represent Canadian quic kclay (QQ), Chinese loess and the ideal Loughborough Loess (LL) and North A frican loess (NAL). It is intended that this paper will put into context th e diverse work in the area, and in so doing shed light on what is an often overlooked subject. (C) 1998 Elsevier Science B.V. All rights reserved.