水系与构造复合作用下的冲积扇沉积演化——以南天山山前黄水沟冲积扇为例

Sedimentary Evolution of Alluvial Fan Under Combined Action of Drainage System and Structure: A Case of Huangshuigou Alluvial Fan in Piedmont of Southern Tianshan Mountains

通过解剖南天山山前现代冲积扇的扇根—扇端水系演化特征、砂砾质沉积物变化与搬运距离关系、沉积构造特征、冲积扇表面坡降梯度以及构造活动对沉积物的控制作用,认为现今南天山山前发育多个由水系与构造复合作用下的新型冲积扇。冲积扇表面的水系自物源区至冲积平原,具有由山间河—单一辫状河道—辫状河道复合带—低弯度单一径流河道—季节性河流的演化特征。冲积扇表面坡降梯度的突然变大,引起河道类型、宽度以及砂砾质沉积物粒径的变化。同时,早期沉积的弱固结成岩冲积扇,在构造隆升作用下,形成与南天山物源区走向平行、延伸数十公里的北陡南缓的褶皱带,其如同山前次级台阶,增加了沉积区的地形坡度,弱固结成岩的砂砾岩成了现今沉积的次要物源。在活动水系与构造隆升褶皱带复合作用下,形成“品”字形多期砂砾质冲积扇。建立了半干旱—干旱气候条件下水系与构造复合作用下的冲积扇沉积演化模式,为前陆冲断带砂砾质大面积分布的成因机理研究提供了依据。

By dissecting the evolution characteristics of the proximal distal drainage systems of the modern alluvial fan in the piedmont ofthe southern Tianshan Mountains, the relationship between the sandy conglomeratic sediment variation and the transport distance, the struc tural characteristics, the slope gradient of fan surface, and the controlling effect of tectonic activities on sediments, it is considered thatthere are many new types of alluvial fans developed in the piedmont of the southern Tianshan Mountains, which are influenced by bothdrainage system and structure. The drainage system on the surface of the alluvial fans is distributed from provenance area to alluvial plain,which has the evolution features of intermountain river-single braided channel-composite braided channel zone-low curvature single run off channel-seasonal river. The sudden increase of the slope gradient on the alluvial fan surface caused the changes of river channel type,channel width and sediment grain size. Meanwhile, a "steep in the north and gentle in the south" fold belt which is parallel to the prove nance area in the southern Tianshan Mountains and extends tens of kilometers formed in the early deposited alluvial fan of weak consolida tion diagenesis under the action of tectonic uplift. Like the secondary step in the piedmont area, the topographic slope of the depositional ar ea increased and the sandy conglomerate with weak consolidation diagenesis provided the secondary source for the present deposition. Un der the combined action of active drainage system and fold belt resulted from tectonic uplift, multi stage sandy conglomeratic alluvial fanoccurred. Based on the above understanding, the paper establishes a new model of alluvial fan sedimentary evolution under the combinationof drainage system and tectonism in arid-semi arid climate, which provides references for the genetic mechanism of large scale distributionof sandy conglomeratic sediments in foreland thrust belt.