SEDIMENTARY CONSTRUCTION AND MECHANISM OF QIAOJIA BASIN IN JINSHA RIVER—EVIDENCE FROM DRILLING AND GEOPHYSICAL EXPLORATION

The Qiaojia Basin locates at the lower reaches of the Jinsha River and adjoins to the"Y-shape" converging point of Zemuhe, Daliangshan and Xiaojiang faults. It extends 13km with a width of 3~5km in surface, and the thickness of Qiaojia Basin is at least 733.6m, which is the deepest known intramountain basin in Southwestern China. The scale, sedimentary construction, and evolution mechanism of the Qiaojia Basin have been confusing for long time because of lack of field evidence. This paper reports 20 boreholes(the maximal depth is 748m) and a wide-area electromagnetic resistivity sounding profile across the Qiaojia Basin, and acquiring the following conclusions:(1) The maxim depth of the Qiaojia Basin is deeper than 733.6m according to borehole records and may be deeper than 900m inferred by geophysical profile result. (2) The Qiaojia Basin consists of six groups of alluvial sediments, groups ①, ② are modern in-laid terraces, while groups ③-⑥ are vertical accretion sediments in the buried Qiaojia Basin. (3) the Qiaojia Basin is a pull-apart basin between Zemuhe Fault and Xiaojiang Fault, and was born in 1.1Ma(Early Pleistocene). Qiaojia Basin stopped growth at about 34ka because of the final connectivity of these two faults. The sediment construction of Qiaojia Basin were dominated by alluvial deposits, intercalated with lacustrine and fluvial, and intersperse with flood fans. (4) No great quantity of landslide deposits was found in the Qiaojia Basin. Thus, the Qiaojia Gigantic paleo-landslide may be not existent.