Study Of Jurassic Lithofacies And Paleogeography In North Of Qaidam Basin

The study of Mesozoic lithofacies and palaeogeography in north of Qaidam Basin is very little. The intense and frequent tectonic activity after Mesozoic and the varying degrees of metamorphism make the recovery of Jurassic lithofacies and palaeogeography extremely difficult in the study area, so the study of Jurassic lithofacies and paleogeography is still lower in North of Qaidam.Basin. In this paper, under the guidance of plate tectonic theory, using the method of combining basin and range, through the outcrop, combining with drilling and seismic datas, a systematic research about strata, tectonic, sedimentary facies and the distribution of sedimentary system are carried out. Then combined with previous researches, the lithofacies and palaeogeography maps of early, middle and late Jurassic in Qaidam blocks are established.The tectonic evolution of North Qaidam Basin controlled the north-south zonation of Jurassic sedimentary and the migration of deposit center moved from west to east;The extension of Altyn Tagh fault during early Jurassic and the tectonic framework formed by the Qilian orogenic belt collision during Indo-end controlled the Early Jurassic sediments; the NW long axis of the sedimentary basin was controlled by the two mountain ranges. In Middle Jurassic, the extension of Altyn Tagh fault stopped gradually, but the relax extension increased gradually after Qilian orogenic activity, and the deposit center migrated toward east. Starting from the Late Jurassic, the regional extension weakened gradually, but under the overlying sediments, the structural environment that had a continuous decline and deposition still preserved in the North of Qaidam Basin, and the type of the basin changed from faulted to depressed.On the base of complex structure, Qaidam basin is a Mesozoic basin formed by Early, Middle Jurassic rifting basin and Late Jurassic depressed basin. Five sedimentary facies are developed in North Qaidam basin during Jurassic, they're fan delta, lakes, fluvial facies, the alluvial fan and swamp, according to the sectional assembling rules portfolio or sequence characteristics of these lithofacies units, two main sedimentary systems that are alluvial fan-fluvial-swamp deposition system and fan delta-fluvial swamp-shallow lacustrine depositional system can be recovered. The formation of lower Jurassic is mainly distributed in Cold Lake tectonic belt, Quint depression, Nanbaxian areas and so on, which are the western part of the North Qaidam Basin,and with a NW banded distribution; the formation of middle Jurassic almost covered the entire western part of the North Qaidam Basin and Delingha area, with a widely distribution; the distribution range of upper Jurassic is relatively narrow, and mainly developed in the east of the South magpie fault. Among them, lacustrine and swamp facies were mainly developed during early- middle Jurassic, occasionally developed alluvial fan facies; shore-shallow lacustrine facies, lake delta facies and fluvial facies were developed during late Jurassic.