Framework Of Cenozoic In The Qaidam Basin

Qaidam Basin,is located at the northeastern margin of the Tibetan Plateau,has deposited huge thickness of Cenozoic strata and recorded abundant geological information about the regional tectonic evolution and climate changes.The sedimentary stratigraphy studies in the basin has great significance for investigating the uplift of the Tibetan Plateau and global climate changes.At present,magnetic stratigraphy method is a primary approach in the chronostratigraphy studies of outcrops sedimented in the margin of Qaidam Basin.Unfortunately,the results of these studies are still divided.The controversies of Cenozoic strata,perplexing the studies about the evolution process of lacustrine basin links to regional tectonic evolution and global climate changes.Therefore,the detailed chronostratigraphy study of Qaidam Basin,establish,improve and unify the chronological framework of the Cenozoic in the basin is imminent.Based on the method of international geochronology construction,this paper establishes a work program suitable for Cenozoic geochronology framework of Qaidam Basin:(1)Based on the previous research results,the sedimentary centers of each layer in the basin are determined.The natural gamma curve data of lacustrine strata near the sedimentary center are selected to carry out cyclostratigraphy research,and determined the sedimentary time limit of stratum and the sedimentary cycle of the orbital parameters;(2)Combined with the isotopic ages of the boreholes in the basin,the astronomical tuning points are determined.The astronomical periodic signals identified from the natural gamma curves of each exploration well are filtered and spliced to form a 405 kyr long eccentricity curve.(3)According to the obtained tuning points,the long eccentricity curve is tuned to the la2004 target astronomical curve,and the stratigraphic chronological scale of the basin is establishedBased on the above research scheme,this paper integrates the previous research results and the internal data of Qinghai Oilfield Research Institute to complete and obtain a series of achievements and understanding.Firstly,the superimposed map of residual thickness and sedimentary facies of Cenozoic strata in Qaidam Basin is drawn.Based on the lake sedimentary evolution and distribution,exploratory wells were selected in the lake phase area close to the sedimentary centre of the lake.Gamma curves of the Qigequan Formation and the Shizigou Formation in the Taishen1,the Shangyoushashan Formation in the Jianshi2,the Xiayoushashan Formation and the Shangganchaigou Formation in the Kai2 well,the Upper Xiaganchaigou Formation in the Yanan1,the Lower Xiaganchaigou Formation in the Zhatan 5 well,and the Lulehe Formation were analyzed by Cyclostratigraphy methods.The depositional cycle thickness corresponding to the depositional time limit and the orbital parameters of long eccentricity and short eccentricity is determined.Based on the comparative analysis of Qigequan formation of well Qijia1 and Taishen1 in Sanhu depression and the isotopic ages of surrounding boreholes,the tuning points of the two exploration wells are determined.Then,the astronomical periodic signals identified from the natural gamma ray curves of each well are filtered and spliced to form 405 kyr long eccentricity curve of Cenozoic in Qaidam Basin;According to the tuning point,the long eccentricity curve is tuned to the la2004 target astronomical curve,and the stratigraphic chronology framework of Qaidam Basin is established.The results show that the age of the bottom boundary of the strata of the Qigequan Formation,the Shizigou Formation,the Shangyoushashan Formation,the Xiayoushashan Formation,the Shangganchaigou Formation,the Upper Section of the Xiaganchaigou Formation,the Lower Section of the Xiaganchaigou Formation and the Lulehe Formation respectively are4.98 Ma,8.63 Ma,11.92 Ma,19.04 Ma,35.09 Ma,40.57 Ma,44.27 Ma and earlier than49.37 Ma.The geomagnetic polarity bands corresponding to the bottom boundary of each stratum were determined against GPTS2012 to provide a reference for the subsequent magnetic stratigraphy and to achieve an accurate correspondence between absolute age and stratigraphic record.