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 (K 2 O < 6AE5%: 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: K 2 O up to 8AE5%), 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 1 Deceased. 1365 palaeoenvironmental context and the palaeogeographical context of glauconybearing sediments are not considered.

Color, Mineralogy and Composition of Upper Jurassic West Siberian Glauconite: Useful Indicators of Paleoenvironment

“…(Eder et al 2007). Independently of its green or brown color, which has never been systematically evaluated together with associated chemical variations, the origin of Al-rich glauconite is debated, and the physicochemical changes that have led to such Al enrichment have been interpreted as related to synsedimentary (Berg-Madsen 1983) or postsedimentary processes, the latter corresponding to weathering (Odin & Fullagar 1988), diagenesis (Longuépée & Cousineau 2006, and references therein) or anchimetamorphism (Guimaraes et al 2000).…”

“…Textural and compositional observations of many grains of glaucony actually reveal that their mineralogy is very complex in detail, and that a variety of mineral phases occur within each glaucony grain, in addition to glauconite (Lamboy 1976, Odin 1988, Martín-Algarra & Sánchez-Navas 2000, Eder et al 2007. Such heterogeneity certainly influences the color of glaucony grains, and is related to a complex pattern of sedimentological and chemical evolution before burial.…”

Glauconitic laminated crusts from hydrothermal alteration of Jurassic pillow-lavas (Betic Cordillera, S Spain): a microbial influence case

“…The role of microbes in the genesis of glauconite has been pointed out previously in sedimentary environments (Geptner and Ivanovskaya, 2000;Zanin et al, 2004Zanin et al, , 2006Eder et al, 2007;Baldermann et al, 2013) and hydrothermal environments (Tazaki and Fyfe, 1992;Geptner et al, 2008). Tazaki and Fyfe (1992) reported celadonite and glauconite related to filamentous bacteria in the hydrothermal alteration of volcanic sandstones in deep-sea sediments from IzuBonin fore-arc (west Pacific Ocean).…”

“…ly expandable smectitic component (e.g., Odin and Matter, 1981;Amorosi, 1995;Eder et al, 2007;Sánchez-Navas et al, 2008). In such contexts, glauconite is an authigenic mineral developed as pelletal facies (mud coprolites, foraminiferal tests and mineral debris), and less frequently as films growing in holes, cracks and borings on hardgrounds and omission surfaces that may be associated with phosphates (e.g., Odin and Letolle, 1981;Odin and Matter, 1981;Fischer, 1990;Jiménez-Millán et al, 1998;Wigley and Compton, 2007).…”

“…Porrenga, 1967;Odin and Letolle, 1980;Odin and Matter, 1981;Bornhold and Giresse, 1985;Amorosi, 1995;Jiménez-Millán et al, 1998;Eder et al, 2007;Sánchez-Navas et al, 2008;Baldermann et al, 2013). In both cases -volcanic-hydrothermal and sedimentary-diagenetic-glauconite, a Fe-and K-rich mica structure mineral (Odin and Matter, 1981), is mainly recorded as peloidal grains.…”