Geochemical Characteristics and Environmental Implications of Trace Elements of Zhifang Formation in Ordos Basin

Key words: 
• paleosalinity /
• redox conditions /
• palaeo-depth /
• paleoclimate /
• trace element /
• Zhifang Formation /
• Ordos Basin

Abstract: The trace elements of sedimentary rocks have a high sensitivity on hydrological changes of sedimentary environment, so the trace elements can be used as an effective method on the study of paleoclimate and paleoenvironment during deposition period. The Zhifang Formation in Ordos Basin is a new series of strata of Triassic with oil & gas seeps, the ancient climate and ancient water environment have great significance in restoring the ancient sedimentary framework and geography of that period and hydrocarbon accumulation. In this paper, the trace element of the profile and 21 shale and siltstone core sample of the middle Triassic Zhifang Formation in Ordos Basin have been determined by means of ICP-MS, and trace elements of strontium (Sr), ba (Ba), thorium (Th), uranium (U), vanadium (V), nickel (Ni), cobalt (Co) were selected as the discriminant mark, and combining sedimentology and sporopollen analysis, the sedimentary environment and ancient climate characteristics of Ordos Basin were discussed. The results show that by Li, Sr, Ni, Ga content and the ratio of Sr/Ba indicating the sedimentary environment was continental fresh-brackish water. The ratio of U/Th is from 0.09 to 0.38, δU is from 0.43 to 1.07, the ratio of V/(V+Ni)is from 0.69 to 0.89, the ratio of V/Cr is from 0.87 to 5.57, the ratio of Ni/Co is from 1.21 to 2.93, which indicate the oxidation-weak reduction environment. Combining sedimentology mark and quantitative calculation of Co elements, the average water depth of Zhifang Formation is less than 30 m, and the large catchment lake has not yet formed. The ratio of Sr/Cu is from 0.85 to 16.7, most Sr/Cu ratio is less than 10, and combining with the sporopollen analysis, which show that the dry climate of Heshanggou Formation was translated into warm climate of Zhifang Formation.