Humid Lithogenesis

formation of sedimentary rocks on continents and in seas under conditions of a humid climate. Humid lithogenesis is contrasted with glacial, arid, and volcanogenic-sedimentary lithogeneses. The concept and the term “humid lithogenesis” were introduced by N. M. Strakhov (1956).

In the contemporary epoch regions of humid lithogenesis occupy a large portion of our land surface. The quantity of solar heat received in these regions is insufficient to evaporate all moisture falling as precipitation, and excess water is carried off by surface and ground runoff. The following types of genetically different groups of rock originate under humid lithogenesis: residual, or eluvial; terrigenous, sand-silt-clay; authigenic; and terrigenous-authenic. Eluvial rocks arise as a result of intensive chemical erosion. Their composition depends on the nature of the parent rock and the conditions of erosion; they may attain a thickness of 100 meters or more. Terrigenous rocks accumulate on transportation routes of sedimentary material and in the terminal body of water; their mineralogical and granular composition depends greatly on the tectonic process. In tectonically active regions there accumulate polymictic, coarse-grained sandy rocks and hydromicaceous-montmorillonitic clays. Monomineral rocks are most often formed under platform conditions. Authigenic rocks arise biogenically and chemogenically. They include iron and manganese ores, sediments containing phosphorus and siliceous sediments, limestones, and organogenic rocks (such as coal). Typically humid rocks include bauxites, iron and manganese ores, and coal. Phosphorites, siliceous sediments, limestones, and dolomites arise under both humid and arid conditions. In all types of authigenic rocks one finds a more or less significant admixture of terrigenous material (sand, silt, or clay).

When there is a large admixture, there arise mixed terrigenous-authigenic rocks (such as marl).