Abstract:

The co-development of alluvial fans and aeolian deposits in arid regions is a common phenomenon. While wind reworking and redeposition on fan surfaces are widespread,research on their interactive sedimentation and detailed internal architecture characterization is relatively scarce. Based on modern remote sensing images and field outcrops,this study conducted detailed observations and dissection of the lithofacies,lithofacies associations,and planar distribution within the middle part of the modern Dunhuang alluvial fan. The study area primarily develops 9 lithofacies and 5 lithofacies associations,identifying two main sedimentary types: braided channel deposits and aeolian deposits. Among these,the large-scale cross-bedded sandstone lithofacies(St)and parallel-bedded sandstone lithofacies(Sp)represent typical aeolian deposits. The braided channels mainly develop 5 lithofacies associations: cross-bedded conglomerate facies-small-scale cross-bedded sandstone facies(Gt-Scs),clast-supported conglomerate facies-small-scale cross-bedded sandstone facies(Gcm-Scs),clast-supported conglomerate facies-massive sandstone facies(Gcm-Sm),lateral accretionary thin-bedded conglomerate facies-parallel-bedded sandstone facies(Gla-Sp),and clast-supported conglomerate facies-sandy conglomerate facies/pebbly sandstone facies(Gcm-Gs/Sg). Horizontally,the braided channels of the alluvial fan develop adjacent to the aeolian dunes. Vertically,aeolian sandstones and braided channel sandy conglomerates are interbedded,reflecting the alternating dominance of these two geological agents during different periods. Based on this,a depositional model controlled by cycles of dominant and subordinate agents on the lateral margins of the middle fan under an arid to semi-arid climatic background was established,characterized by “aeolian processes advance when fluvial activity retreats” and “fluvial processes advance when aeolian activity retreats”. During arid periods with prevailing monsoons,wind was the dominant geological agent,depositing thick large-scale cross-bedded and parallel-bedded sandstones. During wet periods with abundant rainfall and glacial meltwater,fluvial processes dominated the fan surface,resulting in braided channel features. This study enriches the depositional architectural models of alluvial fans and provides theoretical support for the genetic identification and hydrocarbon exploration of sandy conglomerate reservoirs within alluvial fans.

Key words: alluvial fan, braided channel, aeolian deposits, depositional architecture, interactions, Dunhuang

摘要：

干旱—半干旱地区的冲积扇与风成沉积相伴发育是一种普遍现象,冲积扇扇体表面时常受到风的改造作用而发生再沉积,但目前两者的交互作用及其沉积构型研究尚显不足。为深入理解干旱—半干旱地区冲积扇中风成—水成交互沉积过程,本研究基于现代遥感影像和野外露头资料,对现代敦煌冲积扇扇中亚相开展岩相、岩相组合和平面展布的详细观察与解剖。结果表明: 研究区主要发育9种岩相、5种岩相组合,识别出辫状水道沉积和风成沉积2种沉积类型; 其中,大型交错层理砂岩岩相(St)和平行层理砂岩岩相(Sp),代表了典型的风成沉积; 5种岩相组合包括交错层理砾岩—小型交错层理砂岩岩相(Gt-Scs)、多级颗粒支撑砾岩—小型交错层理砂岩岩相(Gcm-Scs)、多级颗粒支撑砾岩—团块砂岩岩相(Gcm-Sm)、侧积薄层砾岩—平行层理砂岩岩相(Gla-Sp)、多级颗粒支撑砾岩—砂质砾岩/含砾砂岩岩相(Gcm-Gs/Sg),代表了辫状水道沉积。平面上,冲积扇辫状水道与风成沉积毗邻发育; 垂向上,风成沉积砂岩和辫状水道砂砾岩呈互层发育,表明2种地质营力在不同时期分别占主导地位。基于此,建立了干旱—半干旱气候背景下,冲积扇扇中侧缘主次营力旋回控制的“风进水退、水进风退”的沉积模式,在季候风盛行的干旱时期,风为主导地质营力,发育厚层大型交错层理砂岩和平行层理砂岩; 转为降雨量和冰川融水充沛的丰水期,扇面流水作用主导时,表现为辫状水道砂岩发育。本研究丰富了冲积扇的沉积构型模式,对冲积扇砂砾储集层的成因识别和油气勘探开发提供了重要理论依据。

关键词: 冲积扇, 辫状水道, 风成沉积, 沉积构型, 交互作用, 敦煌

CLC Number: 

• P512.2