Study Carbonate Content And Paleoclimatic Of Late Miocene Strata Jianzha Basin In Qinghai Province

Jianzha Basin is located in the northeastern Tibetan Plateau and situated in a climatically sensitive area — at the transition zone between the monsoon region to the east and the arid region to the west;its location,together with its thick,continuous sequence of Cenozoic sediments,makes it well be suited to monitoring changes in Late Cenozoic climate.it also provide clues and evidence for exploring relation between the uplift of the Tibet Plateau,the change of polar ice volume and the evolution of East Asian monsoon and Asian inland aridification.In this paper,the main object of the study is Jiarang sectionsedimentary sequence of11.80~5.80 Ma.Jiarang section is on the left bank of the Jiarang river,near Jiarang village Maketang town,it is located west of the Jianzha Qinghai Province,section bottom coordinates and elevation is 35°57?43.1?N,101°58?24.1?E,2200 m.Based on field observations,the 361-m-thick sediments sequences of the Jiarang section mainly consist of clastic material of fluviolacustrine and eolian origin,with loess-like silt or clay comprising about two-thirds of the entire section.The sample of this experiment was sampled at 20 cm intervals,A total of 1781 samples were collected.The magnetostratigraphic results show that the sediments have recorded a continuous geomagnetic polarity sequence from C5 r.3r to C3 r,spanning the interval from 11.80~5.80 Ma in the Late Miocene.Based on the field observation and the study of sedimentary facies,by the experiments analysis of the carbonate minerals in red clay.The main results of this study as follows:(1)Stratigraphic correlation between Jianzha Basin and surrounding Basin and lithology and stratigraphic observation of Jiarang section,the exposed section investigated in the Jianzha Basin can be divided into the Late Miocene Xiadongshan and Jiarang formations in this text.Jiarang section is mainly composed of aeolian red clay and intercalated fluviolacustrine deposits in the Jianzha Basin.Based on field lithologic characteristics observation and grain size analysis,the section is divided into four sedimentary stages,from the bottom to the top,the red clay is accumulated,the shallow lacustrine facies and the aeolian red clay alternately deposits,the aeolian red clay accumulates with multiple alluvial and flood layers,the fluvial facies deposits.(2)Mainly composed of calcite of the carbonate minerals in sedimentary section in the Jianzha Basin,the content of calcite is similar to that of secondary calcite in the red clay,its Content change can directly reflect the change of regional climate and environment.The lower calcite content indicates that the weathering leaching and soil forming effect are strengthened,increased precipitation,the climate becomes warm and humid,the East Asian summer monsoon strengthen;on the contrary,the climate is cold and cold,the East Asian winter monsoon strengthen.By the experiments analysis of the carbonate minerals in red clay,the content of calcite and the low frequency magnetic susceptibility change were analyzed to reveal the paleoclimatic information from the deposits during 11.80-5.8 Ma,which the paleoclimate change can be divided into four stages.Stage ?: 11.80~9.80 Ma,dry and cold climate;Stage ?: 9.80~8.57 Ma,warm and humid climate;Stage ?: 8.57~6.15 Ma,warm and humid climate and Stage IV: 6.15~5.80 Ma,dry and cold climate.(3)Dolomite is mainly carried from the source area in the Jiarang section,the change of its content reflects the change of environment in the source area.The content of dolomite increased significantly in the 10~8 Ma,possibly due to the rapid uplift of the Tibetan Plateau,the weathering and denudation of the surround mountains of the basin,and the increase of near-source substances.The content of calcite and the magnetic susceptibility change dramatically before ~8.57 Ma reflect intensified precipitation which resulted from the growth of the NETP.From ~8.57 to ~7.21 Ma,the East Asian summer monsoon was impacted by global cooling and ice build-up in the Northern Hemisphere in addition to the uplift of the Tibetan Plateau in the Late Miocene.From ~10 Ma to ~8 Ma,the content of dolomite increases significantly,which probably caused by the rapid uplifting and deforming of the Tibetan Plateau in this stage.