丹棱-思濛砾石层成因与时代

ORIGIN AND GEOLOGICAL AGE OF THE DANLING-SIMONG GRAVEL BED

根据组成丹棱-思氵蒙砾石层的不同岩性的砾石的统计分析,论证了该砾石层的物源区。应用粒组分析资料经数据处理取得的趋势分布所显示的砾石分选性和递变规律,并应用沃克划分砾岩成因类型的组构标志,判别砾石层成因。最后根据砾石的风化程度和川西前陆盆地第四纪演化特征,以及青衣江阶地位相所显示的第四纪沉积物序次,论述了丹棱-思氵蒙砾石层的形成时代。结论认为,丹棱-思氵蒙砾石层为冲积成因,物源区为青衣江流域,为古青衣江出山后在前陆盆地中充填的冲积扇,当时青衣江古河道由洪雅黄坪流经丹棱最后在思氵蒙汇入岷江。现代的丹棱-思氵蒙河是青衣江改道出平羌峡汇入大渡河后残留下来的断头河。砾石层的形成时代为早中更新世(Q12)。

To determine the origim and geological age of the Danling_Simong gravel bed,a series of investigation works,such as field geological and geomorphological reconnaisance,statistic analysis of gravels of vasious petrographic composition,measurments of the grain size and analysis through digital data processing of the trend distribution of different size fraction,and fabric analysis as well as terrace correlation of gravel deposit,have been carried out.The fan-shaped distribution of the gravel bed is delineated out through field geological and geomorpholgical reconnaisance.The source area of the gravels is determined through statistic analysis of gravels of vasious petrographic composition.The alluvial origin and the palaeo_current might be judged by the a(t)b(i) fabric and the dip direction of the a_b_plane of gravels.The relative geological age may be decided from comparison ofweathering degree and terrace correlation between different geological age.Finally the following conclusions are obtained.1)The gravel bed is of alluvial origin,its source area is the Qingyi river basin.2)The gravel bed was an alluvial fan,deposited by Palaeoqingyi river in the west Sichuan foreland,where the river emenged from the mountainous area and then flowed along the present Danling_Simong river and then joined the Min river.3)The present Danling_Simong river is only an abondened segment of the Palaeoqingyi river.4)The geological age of the gravel bed is early middle pleistocene.