Sedimentary Evolution And Geological Background Of The Triassic, NW Junggar

The thesis investigated5outcrop profiles on the northwestern margin of the Junggar basin (NW Junggar), they are Shendigou-Dazhuluogou, Huayuangou, Heiyoushangou, Shuikugou and Tuzigou. On basis of the deep investigation and collection of the previous results, the NW Junggar Triassic stratigraphic framework has been built up. Then the sedimentary system are identified, and facies are subdivided. The characteristics of the microfacies are described as well. According to the analysis of the paleocurrent, geochemistry and heavy mineral, the Triassic tectonic background, the types and combination of source rocks and the provenance of the basin are predicted. Combing with the results of pollen、carbon isotopes and oxcygen isotopes analysis, the age and paleoclimate of the NW Junggar Triassic are discussed. The achieved results and understandings are as follows:1. According to the research of the NW Junggar Triassc outcrops, cores, seismic and logging data, the boundary of strata are identified. Then the regional stratigraphic framework of the5profiles are built up, and strata are subdivided. The stratigraphic framework is partly confirmed by using the element stratigraphic method.2. It is suggested that alluvial fan-fan delta, river-delta and lacustrine systems are developed in the NW Junggar Triassic. Subfacies are subdivided into several microfacies. During the early period of Triassic, alluvial fan and fan delta facies are mainly developed, then braided rivers and meandering rivers are alternately distributed. Huge lacustrine transgression occured during the early stage of Late Triassic. The water regressed and formed braided rivers until the medium stage of Late Triassic.3. Through the measurement of wedge-shaped cross-beddings, planar cross-beddings, trough cross-beddings and conglomerate occurrance, according to the paleocurrent rose diagrams, it is seen that the main type of the paleocurrent are is with single-peak and low variation. The secondary type is with single-peak and high variation characteristics or inclined two-peaks. The paleocurrent are stable, the current are mainly from the Front Zaire, which direction is mainly from the northwest to the southeast.4. Trace element and rare earth element (REE) analysis show that the light rare earth elements (LREE) are rich and the heavy rare earth elements (HREE) are flaten, with the right-inclined distribution patten according to the REE distribution curve. HREE obviously fractionate, also with the characteristics of slight negative anomaly of Eu and the negative anomaly of Ce or the disappearance of Ce. Several provenance determination graphs show that high field-strength element (HFSE) of the large ion lithophile element (LILE) are enriched. The elastics are from the felsic volcano rocks and transit to the source from stable environment, which suffered high degree weathering effect. The source of the elastics are mainly from the upper crust.5.18heavy minerals are identified in the Middle to Upper Triassic samples, grains are mainly angular, subangular and sunbround. Four heavy mineral combinations are recognized. The sediments are probably mixed by the near and the further source elastics, the provenance is Qier Mountain-Halaalat Mountain area which belongs to the Front Zaire. The combination of source rocks is the medium to acid magmatite, sedimentary rocks and metamorphic rocks of the Lower Carboniferous Tailegula formation, and the medium to acid magmatite occupy the largest proportion.6. Pollen, carbon isotope and oxygen isotope indicate that it was a continental environment with fresh water during the Triassic period. From the Early to Late Triassic, it was also a process of lacustrine transgressio to lacustrine regression. The climate changed from arid to semi-arid, then to humid.