Место издания: Florence , Год издания: 2004

Upper Jurassic glauconite, which is widespread in the condensed Georgiev horizon of the West Siberia Geosyneclise (WSG), was investigated under SEM and HRSEM. Low magnification SEM study (x 150-200) has revealed that glauconite grains from the central part of WSG have smooth surfaces, whereas those of its western and northern parts have rough surfaces. HRSEM study reveals that glauconite grains of central part of WSG are characterized by a lamellar-petalous ultramicrostructure, that is absent in grains of the northern WSG; under high magnification these latter grains show globular, biomorphic forms that were previously interpreted (Zanin et. al. in print) as capsules of cyanobacterial (?) threads and cocco-like forms. Glauconite grains from the western and northern-western regions of the WSG reveal flaky structures, and a transition from flaky forms to lamellar ones. According to Odin & Matter (1981), there are three types of microstructures in the glauconite of different maturity: 1) globules and caterpillar structures are characteristic for nascent (K2O=2-4 %) and slightly evolved glauconite (K2O=4-6 %); 2) boxwork and rosette structures are typical of evolved glauconite (K2O=6-8 %); and 3) lamellar structures are indicative of highly evolved glauconite (K2O>8 %). In the WSG glauconites we find a similar situation: a) glauconites with globular biomorphic forms contain reduced K2O content (5,6-6,3 %) and can be regarded as nascent; glauconites with lamellar-flaky ultramicrostructure and higher K2O content (7,31-7,42 %) are considered to be evolved glauconites; finally glauconites with lamellar microstructure and the highest K2O content (7,59-8,1 %) are highly evolved. The latter is confined to the central part of Georgiev sea basin, where sedimentation rate was lowest (according to thickness of Georgiev formation) and glauconite was longtime preserved from burial at sea bottom. Evolved glauconite grains observed in the western part of WSG were formed under higher sedimentation rates but in more oxidizing environments. Nascent glauconite was formed in the northern part of WSG, where the sedimentation rate was more than twice higher than in the central part of WSG and where the sea was deepest. This research was made in the framework of the INTAS grant YSF 2002-108