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.
Changes in color of Upper Jurassic glauconite of the Georgiev Formation, in the West Siberian Basin, in Russia, are related to changes in physicochemical conditions that caused glauconite maturation and alteration, driven by regional paleoenvironmental evolution. Maturation produces dark green (bluish) glauconite formed from Fe-rich smectite by increasing the content of Fe 2+ together with K. Alteration produces brown rims and cracks that are enriched in Al and depleted in Fe and K with respect to the glauconite cores. The change from a yellowish green to dark green color in progressively more mature glauconite is explained by light absorption induced by enrichment in octahedrally coordinated Fe 2+ relative to the total Fe, associated with the progressive decrease in the proportion of Fe 3+ -rich smectite interleaved with glauconite. The brown color in alteration rims is due essentially to light scattered by nanometric inclusions of Fe oxyhydroxides. These, together with residual Al-rich glauconite and a subordinate Fe-rich smectite, constitute reaction products formed by leaching of K and Fe 2+ , and by the oxidation of yellowish green glauconite cores. Berthierine formed later in the brown rims on Al-rich glauconite, and pyrite formed as a result of drowning of the platform during the latest Jurassic -earliest Cretaceous and sedimentation of black shales under increasingly reducing conditions. Keywords SoMMAiRELes changements en couleur de la glauconite de la Formation de Georgiev, d'âge jurassique supérieur, dans le bassin Sibérien Occidental, en Russie, sont liés aux changements des conditions physicochimiques qui ont provoqué la maturation et l'altération de la glauconite, conditions régies par l'évolution du milieu paléo-environnemental. La maturation a produit une couleur vert foncé (bleuâtre) de la glauconite formée aux dépens de la smectite ferrifère par augmentation de la teneur en Fe 2+ et en potassium. L'altération a produit une bordure brune et des craquelures qui sont enrichies en Al et appauvries en Fe et K par rapport aux noyaux de glauconite. Le changement de couleur, de vert jaunâtre à vert foncé dans la glauconite progressivement plus mature, serait dû à l'absorption de la lumière induite par enrichissement du Fe 2+ en coordinence octaédrique par rapport au fer total, ainsi que la diminution progressive de la proportion de smectite riche en Fe 3+ en intercalation avec la glauconite. La couleur brune des lisérés d'altération serait due surtout à la dispersion de la lumière par des inclusions nanométriques d'oxyhydroxides de fer. Ces particules, ainsi que la glauconite résiduelle riche en Al et une proportion subordonnée de smectite riche en fer, constituent les produits de réaction formés par lessivage de K et Fe 2+ , et par l'oxydation des noyaux de glauconite vert jaunâtre. La berthierine s'est formée plus tard sur les bordures brunes des grains de glauconite alumineuse, et la pyrite s'est formée lors d'une submersion de la platteforme à la fin du Jurassique et au début du Crétacé, e...
In this study, we propose a new classification of rocks of the Bazhenov Formation based on the proportions of four principal components (siliceous, clay, and carbonate minerals and organic matter (kerogen)) of mostly biochemogenic and, to a lesser extent, allothigenic origin. The classification is based on the results of mineralogical and chemical analyses of more than 400 core samples from 15 wells drilled within the Bazhenov Formation, West Siberian petroleum basin. Four major classes of rocks, divided into 16 subclasses, have been recognized. The terms mixtite and kerogen-rich rock are introduced. Mixtites (biochemogenic mixtites) are defined as a class of rocks containing less than 50% of each component, including kerogen. It was shown that the most common rocks of the Bazhenov Formation are siliceous-argillaceous, kerogen-siliceous, and kerogen-argillaceous-siliceous mixtites and kerogen silicites, which together account for ~65% of all samples analyzed. The proposed approach can be used to study organic-rich black shales in different sedimentary basins worldwide.
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