A STUDY ON MAGNETOSTRATIGRAPHY AND CYCLIC SEDIMENTOLOGY OF SEVERAL SECTIONS IN THE NIHEWAN BASIN, NORTH CHINA: SIGNIFICANCE ON AGE DETERMINATION OF STONE ARTIFACT LAYER

In this article, three sections including Haojiatai (HJT), Dongutuo (DGT) and Xiaochangliang (XCL) of the Nihewan Basin (40.2^oN, 114.6^oE) have been studied with magnetostratigraphy and cyclic sedimentology methods. HJT section (122.3m thick) is located in the center area of the paleo-lake. The stone artifact layer in DGT (43.7m thick) and XCL (63.2m thick) section reside in the sediments of about 42m and 54m below the top of sections, respectively. Lacustrine sediments of three sections are overlain by Late Pleistocene Loess about 10m thick. 1728 parallel samples were taken for granularity analysis and mass magnetic susceptibility measurement, respectively. 729 oriented paleomagnetic samples were collected from HJT and DGT sections. The main results as follows: (1) Changes of the characteristic remanent magnetization (ChRM) with depth show that Brunhes normal chron, Matuyama reversed chron and Jaramillo normal subchron were recorded clearly in sediments of HJT section. The B/M boundary was found at depth of 29 m of the section. The top and bottom limits of Jaramillo subchron are at depths of 54m and 63 m, respectively. The normal magnetized samples at depth of 74-75m could be Cobb Mountain polarity excursion. The bottom age of HJT section is about 1.78 Ma. Jaramillo subchron was recorded at depth of 35-38m in DGT section, and at depth of 38-43m in XCL section (reference to Cheng guolinang data); (2) The magnetic susceptibility shows a positive correlation with the fraction of grain size of 50-100mm and 100-250mm, and a negative correlation with the content of fine suspending sediments of 0.2 mm-7.5 mm diameters; (3) In HJT section, two large sedimentary cycles and 19 high frequency sedimentary cycles can be recognized from the lacustrine sediments. Each cycle is constituted by one lacustrine regressive system and lacustrine transgressive system, which means a rising-falling cycle of lake level and climate cooling down - warming up cycle; (4) The stone artifact layers in DGT and XCL sections are located at the sixth high frequency sedimentary cycle, before the Jaramillo subchron. Magnetostratigraphic and sedimentary cycles correlation of three sections indicate that the age of the stone artifact layers is about 1.2 Ma BP.