Home – Home – Jornals – Russian Geology and Geophysics 2008 number 10

Humid lithogenesis is characterized by the accumulation and postsedimentary transformation of eluvial products within terrigenous and terrigenous-carbonate (with calcite) formations containing mainly dioctahedral clay minerals. Platform fresh-water sediments of early-catagenesis zone accumulate 1 : 1 and 2 : 1 varieties, with a predominance of expanding layers in the latter. In sand-silty rocks authigenic kaolinite also appears. Trioctahedral chlorite of generation I is developed from the top of this zone in both clayey and sand-silty marine deposits. Also, mixed-layer minerals are synthesized: montmorillonite-glauconite at the top of the zone and montmorillonite-hydromica at the bottom.
Arid lithogenesis is controlled by the accumulation of sediments of terrigenous-carbonate (with dolomite), carbonate, and halogen formations. In contrast to the terrigenous and aclimatic volcanogenic formations, these ones are dominated by trioctahedral clay minerals: chlorite, sepiolite, and mixed-layer chlorite-based minerals. In early-catagenesis zone, sepiolite, palygorskite (di-analog of sepiolite), and halloysite quickly transform into talc- and chlorite-saponite and kaolinite, respectively. In hypergenesis zone climate is the main factor, whereas in early-catagenesis zone, especially in arid environments, the chemical composition of the medium plays a significant role. In late-catagenesis zone, degradation products of mica and montmorillonite transform first into varieties with less than 40% and somewhat more than 40% expanding layers, respectively, whereas trioctahedral minerals preserve up to 50% such layers. This process is accompanied by the recrystallization of both allothigenic and authigenic minerals with the transition of scaly montmorillonite into bladed hydromica, with the authigenic minerals of halogen-formation sediments characterized by the most perfect crystal habits and regular structures. In metagenesis zone, di-minerals of orogenic-area sediments transform into sericite, and trioctahedral minerals, into more magnesian chlorite of generation II. This indicates that in late-catagenesis and metagenesis zones, the PT-conditions of the environment are of crucial importance.