Reseach On The Chronology Of Xiashu Formation In Hefei

Xiashu Formation is a stratigraphic unit in Quaternary period, and widelydistributed in the middle and lower reaches of Yangtze River. This formation ismainlycomposed by sub-clay which dominated by brown yellow, gray yellow andyellowish brown color. Since Xiashu Formation records southward invasion processof East Asian monsoon, its chronological study is significant to clarify the temporalevolution of the East Asian monsoon pattern. However, due to the locations andmethods used by different scholars, the age results of Xiashu Formation show a largediscrepency. Therefore, a systematic chronological study of Xiashu Fromation isnecessary. Hefei is a city located at the central of Jianghuai hilly region, and widelydistributed with the deposition of Xiashu Fromation, but chronological andenvironmental research of the Xiashu Formation are rare so far.Based on the investigation of Hefei Xiashu Formation section, combined withdating study, such as AMS14C dating, optically stimulated luminescence(OSL) datingmethods, and grain size, magnetic susceptibility and oxide-geochemistry analysis, thispaper obtains the following preliminary conclusions:1、According to the field observation of Hefei Xiashu Formation drilling outcrop,it could identify that the whole Xiashu Formation deposit continuously; from thedepth of1.20m to35.10m, which is33.90m thick, composed mainly by brown clayand silty clay, and contain iron-rich manganese nodules and calcareous concretions.As the color and structural features are consistent with1:50000Hefei regional surveyreport of Xiashu Formation and the characteristics of the typical Nanjing Xiashu loess,this Hefei typical Xiashu Formation could be confirmed.2、 The grain size and Oxide component analysis shows that the XiashuFormation samples grain size are mainly composed by silt and clay, whichcharacteristics are highly consistent with Nanjing Xiashu loess similar. The Oxidecomponent are mainly composed by SiO2, Al2O3and Fe2O3; meanwhile the K2O,Na2O, CaO and MgO content is low. The average Oxide component content of theXiashu Formation samples are highly consistent with other aeolian deposition such as the typical Nanjing Xiashu loess, Luochuan loess. So this Xiashu Formation could beaeolian deposition.3、The Paleomagnetic test results show that the Xiashu Formation at depth of1.50m-35.00m recorded all normal polarity, so the Xiashu Formation age were notearlier than Brunhes-Matsuyama transition (780ka B.P.).4、According to AMS14C and optically stimulated luminescence dating results,the Xiashu Formation section of the Holocene stage is at the depth of0-1.20m. Theages of4samples from the upper section of1.20m-5.00m are between26.89-60.81ka B.P.; The ages of14samples from the section of5.00m-35.10m are between195.23-484.37ka B.P.. Thus the Hefei Xiashu Formation (1.20m-35.10m) begin todeposit from the mid of Middle Pleistocene (500ka B.P.) to the end of the latePleistocene (13ka B.P.). The deposition rate decreased from fast to slow.