怒江上江镇一级阶地上覆粗颗粒土成因探讨

The Cause of Formation of Coarse-grained Soil on the First Terrace of Nujiang River Shangjiang Town in Yunnan Province

晚近期怒江河谷演化中表生地质作用复杂,各种成因堆积往往交织在一起,研究其成因对山区城镇地质灾害危险性区划意义重大。在对怒江河谷云南段的灾害调查过程中发现,怒江右岸上江镇一级阶地上残存有大量的巨大漂砾及块碎石堆积,从颗粒成分、磨圆、结构等方面对堆积体成因进行分析,主要结论如下:(1)堆积体由漂砾、孤石、块石夹砂土及黏土组成,无分选、磨圆,超覆于一级阶地之上,非河流冲积成因,形成时代晚于一级阶地;(2)粗颗粒成分为灰色、深灰色变粒岩,与左岸陡立岸坡出露三叠系紫红色泥质灰岩岩性不同,为非近源高位滑坡或崩塌堆积;(3)据其成分沿怒江右岸一级支流丙贡河示踪调查后揭示,其堆积分布不仅限于一级阶地,而在丙贡河河道两侧也有相同类型堆积,在沟上游(怒江断裂上盘)岩体极其破碎,出山口保存有老泥石流堆积残体,据此确定上江镇一级阶地上覆粗颗粒土属全新世超大型黏性泥石流堆积;(4)目前沟上游物源丰富,入江口有河道被泥石流堆积挤占并形成险滩,未来形成大规模泥石流可能性依然存在,沟道两侧泥石流堆积区为泥石流威胁高风险区。

The epigenetic geological effect in the evolution of the Nujiang valley in the late and recent times is complex with various genetic accumulations intertwined.The study on their geneses is of great significance for the dangerous regionalization of geological disasters in mountain towns.During the disaster investigation of the Yunnan section of Nujiang valley,it was found that a large amount of huge boulder and rubble remained on the first-level terrace of Shangjiang Town on the right bank of the Nujiang River.The following results were got from the research.The accumulation body is composed of boulder,solitary rock,sandstone and clay with no rocks,and it is not sorted,rounded,and overlaid on the first-level terrace.The non-river alluvial cause is later than the first-level terrace.The coarse-grained composition is gray and dark gray meta-granite,with different lithology from the Triassic purple-red argillaceous limestone exposed on the steep slope of the left bank.It is a non-near-source high-level landslide or avalanche.According to its composition,a tracer survey along the Bingong River,a first tributary on the right bank of the Nujiang River,reveals that its accumulation distribution is not limited to the first-level terraces,but has the same type of stacking on both sides of the Bingong River channel.The rock mass in the upper reaches of the ditch(hanging wall of the Nujiang fault)is extremely fragmented,and old debris flow deposits are preserved at the exit.Based on this,it is determined that the coarse-grained soil overlaid on the first-level terrace of Shangjiang Town belongs to the Holocene super-large cohesive debris flow accumulation.At present,the source of the upper reaches of the ditch is rich.There are river channels at the mouth of the river that are crowded by debris flow and formed dangerous beaches.The possibility of large-scale debris flow in the future still exists.The debris flow accumulation areas on both sides of the channel are of high risk of debris flow threats.