Research Of The Characteristics Of Quaternary Sediments And Sedimentary Environments In Ming-Shan Area, Sichuan

Studying area in southwest of Sichuan Basin is a intermediate landform between Chengdu Basin and Longmenshan Nappe Tectonic。Quaternary sediments of this region mainly distribut in the north-west of the XiongPo anticline ,and some cover on coast of Qingyijian and Dan-ling in south .1. Characteristics of Quaternary sedimentsAccording to fieldstudy,the division of Quaternary strata and the age-dating, we divide it into six lithostratigraphic units, which respectively form theⅠ-ⅥTerraces. The ESR of the sedments is 0.92～79.1 Million years ,which is formed by gravel layer,sands and clay .It distribut as the tongue-shaped or fan-shaped.The altitude of Gravel layer in north is over 700m,which is a part of the"gravel layers of the Ming-Qiong Region". It belong to an early-Pleistocene to Holocene.In south regions, Sediments have reached 500 ~ 600m above sea-level and belong to mid-Pleistocene-Holocene.Quaternary sediments of this area come from complex rock of north-west of Longmenshan Nappe Tectonic.2. Study of depositional-environment of Quaternary sedimentsFractional analysisthe and characteristic parameter display that it had a good classification. Frequency curve show it's a multispike. Using the scanning electron microscope to observe sands (1～2φ),which shap as subangular to subrounded. On the surface of the sand often leave some traces by alleviation and carrying, or directional-Corrosion pits by chemical denudation.They show that the sediments transport by rivers. With good-sorted,well-rounded and dualistic structure of the gravel layer,we consider that the main part of sedments caused by fluviatile deposit type.3. Research of the Quaternary paleoclimateThe characteristics of clay and pollen can be used to study the ancient climate. Geochemical characteristic of clay reflect that SiO2 have the highest content(70.94%), the average value of Fe3+/Fe2+ is greater than one. I/S mixed-layer minerals and Illite account the largest proportion. Both aspects reflect that clay form in a cold and wet climatic condition.In this paper, sampling is collected from different stratigraphic position and sent to analyze. We can find that the Palynological assemblages by woody plants, herbs and Spora pteridophytes in mid-Pleistocene have a obvious difference from the results. They have a similar content in early of mid-Pleistocene; but during this Times the content of Spora Pteridophytes increase and herbaceous plants decreased; The woody plants increase in late Pleistocene.There are a small difference between the woody plants, herbaceous and Spora pteridophytes and on the other hand a small number of tropical ferns appeare in Holocene.We can deduce that climate after Middle Pleistocene have udergone warm- subhumid,warm and humid,hot-humid. Overall, Quaternary have a warm and humid environment in this area,but exist a cold climate stage in Mid-Pleistocene.