A Preliminary Study On The Upper Reaches Of Minjiang River Terrace

River geomorphology has been one of the focus and hot spots for geoscientists toresearch. As a basic form of river geomorphology, the study of terrace can recur theriver geomorphology evolution history, and also plays a dominate role in gatheringinformation related to tectonic movement and climate change.The Minjiang river system of Minshan mountain tectonic belt，located in eastmargin of the Tibetan plateau has been developing since the late Cenozoic.Both thesedimentary records of river basin and the typical topographical features areprofoundly revealing the structural lifting in east margin of the Tibetan plateau.Theeast margin of the Tibetan plateau distributes a series of basin and canyon, which areinduced by the cised and head ward erosion through the Minjiang river. Theinformation about climate change extracted from terrace sedimentary can reflect thechange rules of the ancient climate, and also offer us abundant information about theterrace responded from tectonic and climate change. At the same time, the large-scalegeological events of landside triggering river blocking occur in the upper Minjiangriver induced the large-scale and long-term barrier lakes so that it is difficult tomeasure yet. The existence of these lakes also have dramatical impacts on rivergeomorphology and river system evolution in the upper Minjiang river.In this paper, three typical areas are chosen to study and organize the relevantfield investigation: Maoxian basin、Diexi ancient barrier lake and Zhangla basin. Bymeasuring terrace section、gathering revelant testing samples to analyze、collecting thetime series of uplift movement in Tibetan plateau and neotectonic movement in theupper Minjiang river area, and the same time comparing the basic devision standardsince the quaternary ice age. With the preliminary study of terrace revolution affectedby tectonic、climate and other factors in the upper Minjiang, some achievements canbe reached as follows:(1) There is a significant difference in the terraces sedimentation featuresbetween the area of Diexi、Maoxian and the area of Zhangla basin、the upper Minjiang river. The terrace developed at Dexi and Maoxian has obvious barrier lakesedimentary background, but the terrace at Zhangla basin is characteristiced bydeveloping normal fluvial facies sedimentary.(2)The ancient barrier lake in Maoxian form in about100.5Ka. BP and break in62Ka BP. And the average accumulation rate is about0.79mm/a; After one-time dambreaking, it receives the river downward erosion again, and finally formed today’striple terrace.The development age of third terrace is around39.9Ka. BP and secondterrace is around27.8Ka. BP; The formation time of Diexi ancient barrier lake is atabout30.8~22ka. BP and its average accumulation rate is about11.5mm/a. Theancient barrier lake starts to break at15.2~10ka BP,and the fifth terrace is based onLacustrine sediment.The pebble bed of fluvial facies may stand for five dam breakingof ancient barrier lake.While regional tectonicelevation led Zhangla basin to bedowncutted by30~40m in35.5~22.5Ka.BP and average incised rate of about1.1mm/a.The second terrace of the basin development age is about19.2~18.6Ka. BP.(3)The formation of terrace is affected mainly by three factors:tectonic, climate,base level erosion etc. But the main controlling factors of the different river terrace isnot the same. The terrace sedimentary in Zhangla basin recorded the ancient climateappeared the last ice age, At about35~20Ka.BP climate is cold, in the20~10Ka.BPperiod, the climate gradually changed from cold to warm, generally in the2.8ka.BP,the climate is once again a relatively cold period.which has small interglacial climatefluctuations, but the basic characteristics of the terrace and distribution form reflectsthe fact that the tectonic movement is the main controlling factors instead of theclimate.(4)There are numerous ancient landsides trace from Diexi to Wenchuan area,among which the large-scale landsides occurred in Diexi,Maoxian,Koushan etc. Thelandsides blocked river triggered by earthquake is also the controlling factor to affectthe evolution of river system. The formation of terrace in Maoxian and Diexi withinthe study area is closely related to induced by barrier lakes which made the base levelerosion raising suddenly and remained this status for a long period. Finally formingaobvious “crack” and leading to the river incised rate within the barrier lakes see adramatic change.(5)As for the research of terrace incised rate: if the distinction of incised ratebetween different terraces driving from the same river but different levels is slightlyaffected by ancient climate and base level erosion, thus it could more accurately reflect the regional tectonic movement history, but should be cautious enough toreplace tectonic uplift rate with incised rate. Focusing on the contrast erosion research,it is necessary to take the dominate controlling factors triggered by large geologicaldisaster into consideration.