Sedimentary Characteristics And Paleocliamate Evolution Of The Upper Permian And Middle Upper Triassic Strata In Ordos Basin,China

Ordos Basin,locating at the western margin of North China platform,developed continental sediments since the late Paleozoic.The Permian-Triassic strata developed during this period is extensive and relatively successive.Thus,it is a significant region for studying palaeoclimatic and paleoenvironmental evolution in the Permian-Triassic boundary.There is a set of red beds locating near the P/T boundary,due to the lack of fossils and markers,it is difficult to study.Therefore this paper takes the Late Permian to the Early-middle Triassic strata in the Ordos Basin as the main research objects.11 outcrop sections,367 well logs and 27 drill cores are used and multiple methods including microscope and SEM observation,major/trace element and TOC measure,X-ray and carbon/oxygen isotope analysis are applied to study the palaeoclimate and paleoenvironment and the strata sequences,sedimentary facies.This paper summarizes the sedimentary sequence of the Late Permian to the Early-middle Triassic strata,defines the palaeoclimatic and paleoenvironmental evolution rules,discussed the origin of the red beds and finally determines the distribution law of sedimentary facies and establishes the sedimentary facies model under different climate environment.4 super long term and 13 long term base level cycles(SLSC1~SLSC4)from the Late Permian to the Early-Middle Triassic in the Ordos Basin are identified combining the use of high-resolution sequence with outcrops,cores and logs.There are palaeoclimatic and paleoenvironmental changes between the Late Permian and the Early Triassic according to the geochemical analysis.The degree of oxidation in SLSC1 is relatively low with the warm and humid paleoclimate.Fuchsia sand mudstones in SLSC2 represent fluvial-delta facies in the shallow water,the oxidation is strong and the paleoclimate is hot and arid.The water level increases in SLSC3 with the strengthen element migration,weaken oxidation,lower temperature,the paleoclimate changed into sebhumid-semiarid.The water level and the lake area in SLSC4 reached to its maximum,the paleoclimate is warm and humid,the paleoenvironment is reductive relatively.The carbon isotope of lacustrine mudstonesin the continental Permian-Triassic boundary is correlated with the world-wide marine mudstone in the Permian-Triassic boundary characterized by a negative shift in the isotopic signature.This indicates that are also major geological events such as the mass extinction in the continental P/T boundary of the North China platform.Red bed,developed in the Early Triassic Liujiagou and Heshanggou Formations which are situated above the Permian-Triassic boundary,are products of arid and hot paleoclimate and strong oxidation environment.The red color is primarily due to the coloring agent,hematite.The formation of the purple red layer of Shiqianfeng Formation and Zhifang Formation is also developed under drought climate,but the drought degree is much less than that of Liujiagou and Heshanggou Formation.Therefore,it contains less hematite,and retains some organic matter and calcite.The occurrence of different colors of sedimentary rocks is mainly due to the different mineral composition,which from tectonic evolution and climate change.The sedimentary facies distribution and sedimentary facies model in the sequence stratigraphic framework are established under different climatic and environmental background.The sedimentary facies model under dry-hot climate is characterized by rever-delta-shallow lake sedimentary facies with small lake,shallow water,wide area river system and branched sandbody.However,under humid-warm climate the lake expanded greatly,with the deep lake and gravity flow faceies developed and the sandbody distributed contiguous with deep thickness and wide area.