四川泸定昔格达组的堰塞湖成因及其意义

文章通过对泸定大渡河上松林-大板厂地段T6阶地基座古堰塞坝崩塌-块体流堆积的识别,以及与其上游邻近地段甘海子T₆阶地基座昔格达组崩塌、块体流堆积至湖相沉积序列的对比和成因联系分析,阐明了泸定昔格达组的堰塞湖成因。该区在晚新生代时,由于古大渡河岸坡的不稳定性,发生大型崩塌-块体流,堵塞河道,形成古堰塞坝和古堰塞湖,并在湖盆中充填了以海子坪昔格达组上部为代表的湖相沉积和少量的低能河流相沉积;同时,指出在对昔格达组大区域对比和综合研究中,特别是在分析它的地质意义时,必须考虑这种复杂性和非等时性。古堰塞坝和古堰塞湖应对地质灾害监测和水利工程建设有很好的启示。

LANDSLIDE BLOCKED LAKE:ORIGIN OF THE XIGEDA FORMATION IN LUDING,SICHUAN AND ITS SIGNIFICANCE

Through the identification of landslide-mass flow deposits from a paleo-landslide blocked dam, which constitute the base of T6 terrace by the Dadu River in the Shangsonglin-Dabanchang section, Luding, Sichuan and their correlation with a depositional sequence from mass flow to lake sediments, which constitute the base of nearby T6 terrace of the upper reaches at Ganhaizi, it is concluded that the Xigeda Formation in Luding is originated from the deposits in paleo-landslide blocked lake. The landslide-mass flow deposits consist of a set of matrix supported or grain supported coarse breccias and blocks, which were transported from proximal bedrock outcrops on the sides of the river valley. In the Late Cenozoic, a large scale landslide-mass flowblocked the river valley due to the destabilization of slope of valley sides, resulting in a paleo-landslide blocked dam and a paleo-landslide blocked lake in Luding along the Dadu River, and a series of lacustrine pelitic sediments and minor low-energy river sand-granule sediments were deposited in the lake, forming the Xigeda Formation.This landslide-mass flow in Luding is a great landslide block event of Dadu River from late Neogene to early Quaternary. We may regard the landslide-blocked-lake stage of the Dadu River as a special period of evolution of the river system. Therefore, the evolution of the Dadu River is divided into 4 stages, i.e. Paleo-Dadu, landslide-blocked-lake, Neo-Dadu, and Modern-Dadu. The Paleo-Dadu River before the landslide-blocked-lake stage in Luding Area, was an erosion-type river in a mountainous area along the east margin of Xizang Plateau rather than a meandering stream on a low relief land-surface, suggesting that the eastern Xizang Plateau was lifted before the landslide-block-lake stage of the Dadu River. Thus, the great landslide block event happened at a time when the plateau had been uplifted and neotectonic activity was strengthenen, and when the climate was rapidly changing from warm-wet to cold-dry.The complexity and non-contemporaneity of deposition of the Xigeda Formation thus has special geological significance and must be considered in its regional correlations and synthetical study. A better understanding of paleo-blocked dam and paleo-landslide blocked lake would have important significance in monitoring geological hazards and in water-conservancy projects.