Depositional controls on glaucony texture and composition, Upper Jurassic, West Siberian Basin

Abstract

This study examines textural inhomogeneity and variable chemical composition of Upper Jurassic glaucony in relation to small-scale synsedimentary and postsedimentary authigenic processes controlled by the palaeonvironmental and palaeogeographical context. Four glaucony types with complex textural and compositional features have been recognized in cores of the Georgiev Formation of the West Siberian Basin. Samples exclusively made of light green type 1 glaucony (K2O < 6·5%: the less mature type, richer in glauconite–smectite mixed layer) formed under dysoxic conditions in the deepest distal marine environments of the northern sectors of the West Siberian Basin. Dark green type 2 glaucony is the most mature (richest in glauconitic mica: K2O up to 8·5%), is sometimes associated with type 1 glaucony, and is typical of high bottom areas with a low sedimentation rate within the central sectors of the basin. Type 3 glaucony is formed by brown grains, poorer in K and Fe but richer in Al and Si than type 2 glaucony, and is only present in strongly condensed successions of the central-eastern sectors of the West Siberian Basin. Type 4 glaucony is much richer in Fe than any other type, shows fresh yellowish green cores slightly less mature than type 2 glaucony, and brown rims and cracks with composition similar to that of type 3 grains; it was formed in western sectors of the West Siberian Basin, close to Urals. Weathering under a subtropical to temperate climate, and erosion of badly drained peneplaned lowland areas around the basin, provided Al-rich terrigenous clays as substratum for glauconitization, which explains Al and Si enrichment in Siberian glaucony. Maturation from glauconite–smectite to glauconitic mica is monitored by a change from light to dark green colour related to decrease in Al, Si, Mg, Ca and Na, and to increase in K and Fe. Brown rims of type 4 glaucony, and brown type 3 grains formed after leaching of Fe and K from mature glauconite, with formation of clays and Fe oxyhydroxides as reaction products, as a result of free oxygen exposure related to a hydrodynamic regime and temporary sea-level fall. Glauconitization stopped and diagenetic pyrite formed due to basin deepening and burial under black shales during the latest Jurassic–earliest Cretaceous transgression. This study demonstrates that, due to the complex nature of glaucony, the authigenesis of glauconitic minerals in the rock record cannot be correctly understood if the palaeoenvironmental context and the palaeogeographical context of glaucony-bearing sediments are not considered.