气候变化与粗砾冲积扇的改造过程的响应与其影响因素分析——以岱海湖园子沟-半滩子冲积扇为例

Reworking process of climatic change in the coarse-grained alluvial fan and its impact factors analysis:A case study of Yuanzigou and Bantanzi,Daihai Lake,North China

粗砾冲积扇作为我国砾岩油气藏的优势相带,深入分析其沉积特征、建立沉积模式及序列对油气勘探具有重要的指导作用。半干旱气候中,洪水容易对早期形成的粗砾冲积扇产生差异性改造。本次研究通过对卫星和无人机照片分析确定了内蒙古省乌兰察布市凉城县岱海湖西侧园子沟冲积扇为点物源型冲积扇,半滩子冲积扇为线物源冲积扇;针对岱海湖园子沟-半滩子地区出露全长约276 m的粗砾冲积扇剖面进行现场粒度筛析、剖面描述和沉积特征研究,刻画了受改造的粗砾冲积扇不同层位沉积物的颗粒大小和排列方式,并建立对应的沉积序列。园子沟点物源沉积据物源区较近,地形变化明显,形成厚层的粗砾-巨砾沉积;而半滩子线物源据物源区较远地形变化平缓,形成连续的粗砂-细砾沉积。通次本次针对季节性洪水对两种类型的扇体改造过程的研究,指出由于岱海湖在1500年以来气候发生3次明显的变化,因此存在不同类型的洪水改造冲积扇的过程,即早期的粗粒碎屑流改造过程和后期的细粒洪流(牵引流)改造过程。参考演化过程,本次研究从沉积物供给和可容纳空间角度对影响因素进行探讨,指出气候因素的为降水量变化导致可容纳空间的变化,与湿润气候造成的大量沉积物剥蚀与粗粒碎屑的供给。

As the conglomerate reservoir sever as a favorable belt,it is signification that we study its depositional characteristics,sedimentary pattern and vertical sequence.Especially for the amid and semi-arid climate,flooding are prone to rebuild the early alluvial fans.In this study we select Yuanzigou-and Bantanzi-alluvial fans as an example,which located at north of Yuanzigou-Bantanzi area,Daihai Lake with the distance of 1km.A NE-SE direction flowing braided channel and server meandering channels.In this study,we interpreted the profile in Yuanzigou alluvial fan(40.59°N,112.57°E)and Bantanzi Alluvial fan(40.61°N,112.58°E).All the profile extends 276 m,and its thickness range from 3 m to 5 m.All the sediment layers are covered by the unconsolidated sediment since the Holocene.With satellite and unmanned aerial vehicle photographs,we identified that Yuanzigou alluvial fan determined as a point-provenance-type alluvial fan,and Bantanzi alluvial fan is a liner-provenance-type alluvial fan.Analyzed by in-site grain size experiment,profile description and sedimentation characteristics in Yuanzigou and Bantanzi profile,we descripted the particle orientation,reconstructed coarse gravel alluvial fan pattern and built the corresponding sedimentary sequences.In this study,we found that Yuanzigou alluvial fan sediment point-provenance alluvial fan close to the provenance area,and its morphology show an obviously changes.Thus,this kind of alluvial fan developed a thick layer of coarse gravel-boulder deposits,while Bantanzi is completely opposite.After the study of reworking process in terms of seasonal flooding and combing the climatic data since 1500 years ago,we can get three different stages of flooding change sharply,that is the initial debris flow stage(SQⅠ);coarse-grain debris rebuilding stage(SQⅡ);fine-trunction/flooding rebuilding stage(SQⅢ).Thus,there are also three corresponding flooding reworking stage,namely,primary alluvial fan development stage,debris flow reworking stage,and fine traction flow reworking stage.Refer to above evolution process,we put out the climate impact factors based on the relationship between the accommodation area and sediment supply.The factor is the lake level and accommodation change caused by the rainfall,and a large amount of sediment erosion and coarse debris provided by the humid climate.