Sequence Stratigraphy and Lithofacies Paleogeography of the Lower Jurassic in Southern Altyn Tagh, Western Qaidam Basin

Key words: 
• western Qaidam Basin /
• Xiaomeigou Formation /
• sequence stratigraphy /
• lithofacies paleogeography

Abstract: The Jurassic strata of southern Altyn tagh in the western Qaidam Basin have a similar sedimentary background as that in the northern Qaidam Basin, with well-developed shale beds and promising resource prospects. Based on the sedimentary characteristics of outcrop and borehole cores, sequence stratigraphic framework of the Lower Jurassic Xiaomeigou Formation of southern Altyn tagh was established, the lithofacies paleogeography of third-order sequence was reconstructed, and the sedimentary evolution characteristics were analyzed. The lithology of the Xiaomeigou Formation mainly consists of mudstone, shale, siltstone, sandstone, and conglomerate. Three depositional systems were classified, including braided fluvial, fan delta, and lake.The braided fluvial system is composed of river bed and floodplain facies; the fan delta system consists of fan deltaic plain and fan deltaic front facies; and the lacustrine system includes shore-shallow lacustrine, lake-bay, and semi-deep lake facies. The sequence boundaries, including the regional unconformities, fluvial incision surfaces, and abrupt change surfaces of both lithology and color were identified. According to these sequence boundaries, the Xiaomeigou Formation was subdivided into three third-order sequences. Based on the analysis of the sedimentary facies in both single and comparative profiles, the single factor analysis and multifactor comprehensive mapping method were used to reconstruct the lithofacies palaeogeography of each third-order sequence. The semi-deep lake and shore-shallow lacustrine facies mainly developed in the north, while the fan delta and the river developed in the south and northeast of the research area. From southeast to northwest, the terrain gradually decreases, and Altyn tagh had not yet risen in this period. The uplift of the Qaidam Basin in the south and the ancient Alabashitao mountain in the northeast are mainly provenance. The northeast region of Qingshuigou and Xiaoxigou is the depocenter in the prototype basin, which developed shale with large thicknesses, and is the favorable area for exploration of Jurassic shale gas.