The Paleontology Phase Zones and Its Geological Significance on the Late Triassic Yanchang Stage Palaeo-lacustrine Ordos Basin

Key words: 
• Ordos Basin /
• the late Triassic lake /
• paleoecological environment /
• sedimentary facies /
• paleolake water depth

Abstract: Ordos Basin in the Late Triassic stage developed large-scale depression lake basin.The rise and fall of Lake water affected the area of the sedimentation, depth and depositional system distribution in the lake basin, thereby affecting the whole basin characteristics of the development of the source rock combination, reservoir and sealling rock in the Late Triassic formation. Through identification of outcrop, drilling rock cores palaeontological fossils, combined with paleontological assemblages of Ordos Basin in the Late Triassic lacustrine paleo-ecological environment were recovered. It is concluded that the Late-Triassic Ordos Basin was a shallow lake in which the depth of the water was less than 60 m, the salinity of the water was freshwater-brackish and the weather was warm and humid. From the shore to the lake centre it can be divided into four paleontology phase:river-marsh biofacies-assemblage zone in which the water depth was predicted within 1~2 m, shore-delta biofacies-assemblage zone in which the water depth was predicted within 3~15 m, shallow lake biofacies-assemblage zone in which the water depth was predicted within 15~35 m and semi-deep lake biofacies-assemblage zone in which the water depth was predicted between 35~60 m. These conclusions have important theoretical significance to understanding the characteristic of the Ordos lake basin in late Triassic Yanchang Stage. These facies division, for the restoration of the Ordos Basin during the Late Triassic lithofacies palaeogeography laid the foundation for the extension of the sedimentary basin boundaries, depositional systems and sedimentary facies division.