宜昌地区砾石层的磁性特征与物源分析

分布于宜昌-宜都-鸦鹊岭-枝江一带的宜昌砾石层，是江汉平原边缘丘陵区厚度最大的第四纪沉积物，对地层划分、环境演化及长江形成等研究具有十分重要意义。通过对代表该砾石层完整沉积序列的善溪窑剖面（上段）、云池剖面（中段）与李家院剖面（下段）的磁学特征分析发现，从云池剖面距底部9m处向上，磁化率和磁化参数F_300mT等均明显高于其下部分，具体为：  1）下部磁化率平均值为28×10^-8 m³/kg，而上部磁化率平均达到17010^-8 m³/kg；   2）云池剖面距底部9m处以上主导砾石层剖面磁性特征的物质以低矫顽力的亚磁铁性矿物为主，而以下却含有更多的高矫顽力磁性矿物(如赤铁矿)；   3）善溪窑和云池砾石层的亚磁性物质颗粒要粗于李家院样品。初步分析认为，上述磁性差异主要是物源变化引起的，距云池剖面底部9m以下的砾石层物源主要为鄂西山地，从云池剖面9m开始有四川以西的物质进入江汉平原，表明此时三峡已经贯通。根据已有的测年资料，推测长江贯通三峡的时间大约为1.0～1.1MaB.P.。

MAGNETIC PROPERTIES OF THE GRAVEL LAYERS IN YICHANG ARAE AND THEIR PROVENANCE

Yichang gravel layers distributed in the Yichang Yidu Yaqueling Zhijiang zone are the thickest Quaternary sediments in the edge of the Jianghan Plain hilly regions，which are important to the research of stratigraphic division，environment evolution and formation of the Yangtze River. The Shanxiyao section is located at the side of the State Road 318(30°28′53.4″N，111°27′38.76″E)in Baiyang Town of Zhijiang City. The section is 19.5m thick and composed mainly of a gravel layer with a thin sand layer. The Yunchi section is located at the side of the State Road 318（30°28′52.32″N，111°27′30.12″E)in Xiaoting District of Yichang City. The section is 60m thick，with its lithologic features similar to those of the Shanxiyao section. The Lijiayuan section is located at Lijiayuan of Xiaoting District of Yichang City(30°28′22.86″N，111°27′11.58″E). The section is 21m thick，with small gravestones(generally 1～5cm). We collected 9 samples from the Shanxiyao section，11 from the Yunchi section，and 6 from the Lijiayuan section. We used Bartington MS2 to measure samples of low frequency(470Hz)magnetic susceptibility(X_lf)and high frequency(4700Hz)magnetic susceptibility(X_hf)，used DTECH AC demagnetization instrument and Minispin rotating magnetometer instrument to measure anhysteretic remanent magnetization(ARM)and isothermal remanent magnetism，then calculated anhysteretic magnetic susceptibility(XA_RM)，saturation isothermal remnant magnetization，and magnetization ratio(F_300mT). By analyzing the magnetic properties of the Shanxiyao section(upper)，the Yunchi section(middle)，and the Lijiayuan section(lower)，which represent the integrity of sedimentary sequence，we found that the values of magnetic susceptibility and magnetization ratio F_300mT above the 9m from bottom in the Yunchi section are obviously higher than those below： (1)The mean magnetic susceptibility values are 28×10^-8m³·kg^-1of the lower，while 170×10^-8m³·kg^-1 of the upper.(2)Low coercivity ferrimagnetic minerals dominate the magnetic signatures in the upper part while the lower part has more high coercivity magnetic minerals such as hematite.(3)Sub-magnetic material particles from the Shanxiyao and the Yunchi boreholes are larger than those from the Lijiayuan borehole. Preliminary analysis shows that these differences may have been caused by source change. The main source of the lower part might be from the Western Hubei  mountainous area while the upper part might be from the West Sichuan  Province，delivered to the Jianghan Plain at the time of about 1.1～1.0MaB.P.，based on dating analysis. Therefore it is suggested that the Yangtze River had cut through the Three Gorges at 1.1～1.0MaB.P. 