FIRST FIND OF LOWER JURASSIC AMMONITES (DACTYLIOCERAS) IN WEST SIBERIA

Abstract

Lower Jurassic sediments are widespread in West Siberia, where they rest upon compositionally variable Triassic–Precambrian strata. Their stratigraphy is based on the concept of the gradual cyclic (reciprocal) advancement of marine sedimentation settings from north to south and the substantial influence of eustatic sealevel changes in the paleobasin. Three sublatitudinal facies zones with facies–genetic differences in Lower Jurassic sedimentary sequences are defined in West Siberia. The northern zone is dominated by marine sediments; the middle (transitional) zone, by coastal-marine facies with admixture of deltaic and continental sediments; and the southern zone, by a prevalence of continental facies.