Catagenesis and burial metamorphism of terrigenous and volcanic sequences (section recovered by the En-Yakhinskaya parametric borehole SG-7)

Abstract

Secondary alterations of Cretaceous, Jurassic, and Triassic terrigenous complexes recovered by borehole SG-7 were studied from the depth of 3620 m to 6920 m (roof of basalts). Down to the depth of ∼6770 m, the section shows a gradual intensification of catagenetic alterations of sandy rocks: the formation of pressure dissolution textures and regeneration of clastic quartz. Intensification of the transformation of clay minerals is not observed in this mineral. Variations in the contents of illite, hydromicas, mixed-layer minerals, smectite, chlorite, and kaolinite at different depths of the recovered section are related to changes in the provenance during the accumulation of sedimentary complexes. The Middle Triassic coarse-grained sandy rocks (suprabasalt sequence) are more intensely transformed: they are marked by microstylolitic textures of pressure dissolution, recrystallization blastesis of clastic quartz grains, and newly formed zoisite. The composition of clay minerals is also characterized by variation: micas are represented by sericite with ΔD = 0; Fe-chlorite and kaolinite are noted. These features suggest the absence of linear positive correlation of T and P with the subsidence depth of sedimentary complexes. Intense heating (up to 200–300°C) of the Middle Triassic suprabasalt rocks is likely caused by trap activity (intense effusion of basalt lavas). Investigation of secondary minerals in basalts recovered by borehole SG-7 revealed that the grade of their transformation matches the medium-temperature subfacies of the greenschist facies.