Upper Devonian environments and ostracods biofacies of the Kuznetsk Basin

Abstract

The volcanogenic-carbonate-terrigenous Upper Devonian complex of deposits of the margin of the Kuznetsk Basin has been studied. The results of the studying of sedimentological processes, the composition of sedimentary rocks, the specifics of the petrographic composition of impurity pyroclastic material, make it possible to reconstruct the sedimentation conditions here in the Late Devonian. The predominance as an impurity and sometimes as the main component in sedimentary rocks of andesitic fine pyroclastics with a lower value of acidic and basic volcanic components has been established. From the Late Givetian — Early Frasnian to Famenmian, an uneven increase in the content of acid pyroclastics occurs in the rocks. The studied sections relate to the vast shelf and continental slope of the Siberian paleocontinent. The western direction of the paleoslope is determined. The studied sediments belong to the slope and basin type, they are represented by fine-grained tephroid and volcanomictic sandstones, fine-detrite limestones, olistostromes and slope landslide breccias, turbidites and rarer turbidite canalites, peculiar “conglomerate limestones” (paleoseismites). The composition of pyroclastics and volcanoclastics in sedimentary rocks indicates the manifestation of andesitic volcanism synchronously with sedimentation and the existence of paleogeodynamic conditions corresponding to the active continental margin possibly of the Andean type in the Late Devonian in the studied area. Based on the data obtained as a result of quantitative calculation of the generic composition of the ostracod associations from the studied sections of the Middle (upper part) and Upper Devonian within three structural-facies subdistricts of the margin of the Kuznetsk Basin, the analysis of ostracods was carried out. The features of the biofacial distribution of ostracods allowed us to identify 10 associations and establish their matching with certain parts of the basin from the Late Givetian to the Late Famennian time. The influence of volcanism on the composition of ostracod associations is traced, the appearance of pyroclastics in rocks reduces the taxonomic diversity of ostracod associations and quantitative composition. Variations in the taxonomic composition of ostracod associations correlate well with trends in T-R cycles, the genus is Bairdia is dominant during transgression.