DISCUSSION ON TERTIARY LAKE EVOLUTION OF BIYANG FAULT-DEPRESSION IN NANXIANG BASIN

Based on theory and method of sequence stratigraphy and synthetically using geological data and seismic profiles of Biyang fault depressed lacustrine basin,tectonic sequences have been identified and classified to probe into basin forming and evolution history of Biyang fault depression by analysis of regional tectonic background and taking structural unconformity or structural stress transformation interface as sequence boundary,and seismic sequences have been demarcated and classified by studying reflection characteristics and recognizing reflection surfaces on seismic profiles of Biyang fault depression.In view of the above,the chorostratigraphic framework model has been established to reveal sediment filling history of Biyang fault depressed lacustrine basin.Based on terrigenous sequence unit classificaton and Similar Condensed Section identification in study area,muddy rock samples were systematically collected from Similar Condensed Section in different sedimentary sequences of Biyang Palaeogene fault depression.The water soluble ion distribution on Palaeogens sedimentary section was determined by X ray fluorescent spectrum analyses to study palaeo lake type evolution for Biyang Palaeogene fault depressed lacustrine basin.The result shows that sediment filling history of Biyang fault depression can be classified into initial rift,main rift and downwarp filling stages which have in turn formed three tectonic sequences.The second stage includes:intense,stable and dormant rift filling substage which sedimentary rate decreases progressively.Biyang Palaeogene fault depressed lacustrine basin experiences four evolutionary phases:foothill floodng depression→sulfate coastal/shallow lake→normal lake→carbonate or sulfate coastal/shallow lake,in pace with this,the evolutionary process of sedimentary wedge in southern margin which belongs to steep slope zone is alluvial fan→steep slope fan delta→gentle slope fan delta→alluvial fan,and in northern margin which belongs to gentle slope zone is alluvial fan→delta→alluvial fan.In the wake of increasing of sulfate mineral in sediment,Biyang fault depressed lacustrine basin would finally go to death.