Research On Tectono-Sedimentary Characteristic Of Marine Strata From Caboniferous To Middle Triassic In Southern Qilian Area And Northern Belt Of West Qinling Mountains

Southern Qilian area is located in the southern part of Qilian fold belt, the northern parts connect to the Middle Qilian block, southeast is linked together with the West Qinling. It is limited between Muli falt-the south margin of Lajishan falt and the north margin of Zongwulong mountain. The northern belt of West Qinling Mountains area is located in the east of Xiahe county in Gansu province, west of the Fengxian county in Shaanxi province, the noth side from the line of Wushan-Tianshui and the south side from the line of Lintan-Minxian-Dangchang-Fengxian county. No matter from which angle between the division of tectonic units both in the Early Paleozoic and today, South Qilian area and the northern belt of West Qinling Mountains area belonged to two separate tectonic units. They are separated by Shangdan ancient suture zone. South Qilian area is a part of Qilian Caledonian fold and fault zone while the norh zone of West Qinling area is a part of the West Qinling hercynian-indosinian fold and fault zone. However, the strata form Carboniferous to middle Triassic are all marine deposits. Whether they are the products of the same sedimentary tectonic environment, and the correlation of the stratigraphic sequence, lithofacies features and geochemical characteristics between both two areas are the main research contents.The research results show that the late Caledonian movement at the end of the Silurian, made the south Qilian area linked with middle Qilian-west Gansu block, the Qaidam massif and Alxa Block form a northern unified continent, and in the Carboniferous-Middle Triassic epoch, the south Qilian area should be a continental margin of the northern continent. In the Middle to the late Silurian, the closing of Qilian ocean made the north zone of the West Qinling collage from the Yangtze plate to the north to the North China plate, forming a continental margin of a unified northern continent with the south Qilian area.During the period of Carboniferous, the gravity flow deposits and the odd intermediate volcanic rock and sediments developed in deep water carbonate facies showed up in the southern margin of the northern belt of West Qinling Mountains. The combination of these sediments proved that there had been a submarine canyon west east wide-narrow and deep-shallow existing in Carboniferous in the area of Hezuo-Dangchang. Bounded by the trench, the components of the north and south belt could both be found in the gravity flow deposits. Meanwhile the sediments’ characters in the northern belt of West Qinling Mountains were quite different from the sediments in the south, indicating there were great differences between the sedimentary facies of the northern and southern belt. However, the same characters of sedimentary structures, lithofacies, geochemistry and debris components of Carboniferous-Middle Triassic in the northern belt of West Qinling Mountains and the southern region of Qilian declared that marine facies sediments of the two areas in the same period could be comparable. From tectonic background analysis, in the Carboniferous-Triassic period, the northern belt of West Qinling Mountains was connected to the south Qilian area, forming acontinental margin of a unified northern continent.