洞庭盆地安乡凹陷下更新统层序地层及其对构造活动的响应

Lower pleistocene sequence stratigraphy in the Anxiang depression of Dongting basin and its response to structural movement

通过地表调查和钻孔资料,运用高分辨率层序地层学理论及方法,识别出了区域不整合面、岩性岩相突变界面、冲刷面、相序间断面、高频层序叠加样式转换面、湖泛面等几种层序界面,并按基准面旋回原理将第四纪洞庭盆地安乡凹陷早更新世华田组和汨罗组划分为2个长期、5个中期和25个短期旋回层序;短期基准面旋回细分为向上变深非对称型(A型)、向上变浅非对称型(B型)和对称型(C型)3种基本类型;整个下更新统反映出一个大的湖盆从发育到消亡,然后暴露风化剥蚀,以及之后的缓慢沉降到抬升过程。认为安乡凹陷早更新世基准面升降与沉积动力学(即沉积物的剥蚀、搬运和堆积过程)具有很好的吻合性,基准面旋回以自旋回为主,还受到了构造作用的影响;盆地沉积与构造分析表明,早更新世洞庭盆地安乡凹陷经历了伸展断陷-挤压抬升-弱挤压凹陷-挤压抬升的构造演化过程,其深层次的构造动力学机制可能与深部地幔上隆炙烤地壳、下地壳拆沉和物质逃逸有关。

Through the field survey and drilling data, as well as the application of the theory and method of high resolution sequence stratigraphy, several sequence boundaries were reorganized in Anxiang depression of Dongting basin, such as the regional unconformity surface, lithology and lithofaeies transition surface, erosion scouring structure, phase interval, high-frequency sequence superimposed transformation surface and flooding surface. Based on the principle of base-level cycle, we recognized two long-term, five middle-term and twenty five short-term eyelie sequences from Early pleistocene Huatian Formation and Miluo Formation in Anxiang depression of Dongting basin. Short-term eyelie sequences were detailedly divided into deepening upward nonsymmetrie type （type A）, shallowing upward nonsymmetrie type （type B）and symmetric type （type C）. The lower Pleistocene sequence was show up as presses of a big lake basin from development to die, then exposed to weathering and denudation, and then slowly settling to the lifting process. Researches also show that the rising and falling of base-level was consistent with the sedimentary dynamics （sediment erosion, transportation and accumulation proeess）,whieh was controlled by the spin cycle and tectonism in the early Pleistoeene.Ana-lyzing the tectonic sedimentary dynamics of the early Pleistocene basin in Anxiang depression,it can be considered that the tectonic evolution of the Quaternary Anxiang basin experienced extension fault depression, extrusion uplift, weak extrusion sag and extrusion uplift. The deep-level tectonic dynamics mechanism may be related to the lower crust baked by the deeper upper mantle, the lower crust delamination and material escape.