The biostratigraphy and sedimentary environments of the Lower Cretaceous section, Malaya Kheta structural-facies region, West Siberia

The biostratigraphic study of the Low Cretaceous sediments of the Middle Messoyakha swell (Malaya Kheta region) distinguished 12 biostratons based on biofossil data and thus confirmed Boreal Berriassian, Valanginian, and Lower Hauterivian strata. The genetic types of the sediments were determined by lithofacies and biofacies analyses. In the deposit, the Bazhenovo and Kulomzino horizons (Berriassian to basal Valanginian) consist of relatively deep-water marine, shallow-water marine, and coastal-continental facies that displace one another along the strike. The Tara and Ust’-Balyk horizons (middle Lower Valanginian to Lower Hauterivian) are composed of shallow-water marine facies. Paleontological data suggest a low-hilled island covered with conifer-gingko forests with fern understory in the place of the Middle Messoyakha swell in the Berriassian Age.