Chapter 7 - Loess

Abstract

Loess is a deposit of wind-blown silt that blankets large areas of the continents. It is often light brown in color, consisting predominantly of quartz, feldspars, micas, and calcium carbonate. Geographically, loess is extensive in the North American Great Plains, south-central Europe, Ukraine, central Asia, China, and Argentina. In North America, loess was derived from both glacial and nonglacial sources but is commonly related to formerly extensive outwash deposits from the Laurentide ice sheet and to the floodplains of large, braided rivers; sections along the Mississippi River valley may reach 20m in thickness. The thickest loess deposits are found in the unglaciated regions of Alaska and in Nebraska. In Europe, loess deposits are common in areas that lie between the former Alpine and the Scandinavian ice sheets and in regions to the east associated with major river systems. Some of the thickest loess sections are related to formerly extensive braided rivers at a time when vegetation cover was greatly reduced (primarily grassland). Although rarely continuous, some sites have loess and paleosol sequences that extend back into the Early Pleistocene (particularly in the middle and lower Danube River Basin. Elsewhere, loess is related to desert conditions, especially the extensive deserts of central Asia where weathered material, transported from adjacent mountain ranges, has accumulated. These vast arid areas, and the adjacent Yellow River system, are the source regions for the extensive Chinese loess deposits. In north-central China (southeast of the Gobi Desert), loess deposits are extremely thick, up to 300m in places, and completely cover the underlying topography, forming a loess plateau that extends over an area of ~500,000km2. It is there that the most comprehensive paleoclimatic studies of loess deposits have been carried out.