第四纪洞庭盆地安乡凹陷及西缘构造-沉积特征与环境演化

通过地表观察和钻孔资料,对洞庭盆地安乡凹陷及其西缘第四纪构造沉积特征和环境演化进行了研究,为江汉—洞庭盆地第四纪地质研究补充了新的资料。凹陷总体呈南北向,周边为正断裂。凹陷内第四系厚一般为100-220 m,最厚达300 m,自下而上依次为早更新世华田组、汨罗组,中更新世洞庭湖组,晚更新世坡头组和全新世湖冲积。第四系以砾石层、砂层为主,次为（含）粉砂质黏土、黏土,岩性、岩相横向变化大。安乡凹陷西缘（即太阳山隆起东缘）,呈自西向东缓倾的丘岗地貌。区内主要发育中更新世白沙井组,其中南部下部以砂、砾石层为主,上部为黏土;北部以粉砂质黏土沉积为主,下部可发育砂层。根据地貌、沉积及控凹断裂特征,重塑安乡凹陷及其西缘第四纪构造活动与环境演化过程：早更新世—中更新世早期,凹陷西边的北北东向周家店断裂伸展活动,安乡凹陷不均匀沉降,总体具河流和过流性湖泊环境并接受沅水沉积;同期凹陷西缘构造抬升,处于剥蚀的山地环境。中更新世中期断陷活动向西扩展,凹陷区为过流性湖泊环境;凹陷西缘地区转为河流（南部）和湖泊（北部）环境并接受沉积。中更新世晚期安乡凹陷及其西缘整体抬升并遭受剥蚀,凹陷西缘同时具有自西向东的掀斜。晚更新世安乡凹陷拗陷沉降,具河流和湖泊环境;同期凹陷西缘遭受剥蚀。晚更新世末受区域海平面下降影响,安乡凹陷遭受剥蚀。全新世安乡凹陷拗陷沉降,具泛滥平原之河流、湖泊环境。

Quaternary Tectonic-sedimentary Characteristics and Environmental Evolution of Anxiang Sag in Dongting Basin and Its West Periphery

Detailed geologic mapping and bore data was conducted to study on the Quaternary tectonic and sedimentary characteristics and environmental evolution of Anxiang Sag and its west periphery of Dongting Basin,which provided new materials for Quaternary geology of Jianghan-Dongting Basin.The sag trends north-south direction and is surrounded by normal faults.The thickness of Quaternary deposits in the sag is commonly 100-220 m with maximun thickness of 300 m,and from lower to upper,the Quaternary strata is composed of Early Pleistocene Huatian Formation and Miluo Formation,Middle Pleistocene Dongtinghu Formation,Late Pleistocene Potou Formation and Holocene lacustrine-alluvial deposits.Quaternary deposits in the sag are mainly gravel and sand,and secondly silt-bearing clay,silty clay and clay,while there exists distinct lateral lithological and lithofacies variation.In west periphery of Anxiang Sag,namely east margin of Taiyangshan uplift,there exists low hills with the earth＇s surface inclining east clearly,and there occurs Middle Pleistocene Baishajing Formation which is composed of sands and gravels in the lower and clay in the upper of the south,and mainly silty-clay in the north.Based on geomorphologic,sedimentary and sag-controling fault characteristics,evolution of the Quaternary tectonic and environment of the study area is defined.During early Middle Pleistocene,NNE-trending Zhoujiadian fault at the west boundary of Anxiang Sag extended and the sag subsided unevenly with deposits were formed in rivers and river like flowing lakes;in the same time the west periphery of Anxiang Sag rose and was denudate.During Middle Pleistocene faulted subsidence expanded westward,when the environment of the sag liked flowing lake,and the environment of southern and northern segment of the west periphery was river and lake,respectively.The sag and its west periphery rose as a whole during late Middle Pleistocene,when there occurred eastward tilt in west periphery.During Late Pleistocene Anxiang Sag subsided for depression and the environment was of river and lake;in the same time the west periphery was denuded.At the end of Late Pleistoncene regional sea-level dropped,which caused Anxiang Sag denudated. The sag subsided and the environments were river and lake in flood plain during Holocene.