GRUS WEATHERING MANTLES-PROBLEMS OF INTERPRETATION

Abstract

Grus is an ill-defined product of deep weathering of coarse-grained rocks whose relationships to other weathering changes remain unclear. This paper attempts to address this issue by reviewing a number of examples of coarse saprolites from a variety of climatic and topographic settings. Grus is the category of weathering mantle that possesses the following characteristics: sand+gravel 75-100%; silt+clay <25%; clay <10%; Chemical Index of Alteration (CIA) 60-70; Chemical Weathering Index (CWI) 15-20. The origin of grus is connected with weakening of rock fabric by development of microcracks, biotite expansion, and initial alteration of plagioclase. It may originate either beneath the surface or at greater depths within a weathering profile. The climatic approach to the formation of grus mantles offers limited explanation of field occurrences, as these materials are widespread across climatic zones, from the humid tropics to cool temperate areas, although rates of grusification are likely to be influenced by climatic parameters. By contrast, topographic and also petrographic factors appear to play key roles in the development of grus, which may be regarded as a response of weathering systems to rapid relief differentiation. Grus mantles are preferentially associated with moderate to high relief; hence, they are essentially azonal and their development is under way in many areas of the world.