大同盆地中侏罗世河流沉积体系及古河型演化

Fluvial Depositional Systems and River Pattern Evolution of Middle Jurassic Series, Datong Basin

通过对大同云冈出露的部分中侏罗统的沉积物特征、沉积构造特征、岩相和结构单元的研究 ,识别出该剖面是由下伏的曲流河沉积体系和上覆的辫状河沉积体系组成的。由此认为研究区在中侏罗世早期为曲流河沉积环境 ,此后开始转化为辫状河流沉积环境。河型的转化受制于流域基底差异抬升的影响 ,而流域气候由湿润向半干旱—干旱的转化也起了促进作用。

There are many outcrops of middle Jurassic Series in the Yungang area of Datong Basin. Some of them are the sections that were cut during road building.Others are the outcrops formed by natural erosion. However, the outcrops are the fine windows to study the depositional system of the Jurassic stratum. Chen et al (1962) only thought that the lower Jurassic Series was deposited by the fluvial river in plain and the upper Jurassic Series was deposited by the river in intermountain, but they did not study the fluvial sedimentary environment and fluvial channel patterns in detail. Few studies deal with the stratum in the study area . In this work we study the paleoflow directions, lithofacies and its dominant transforming directions and architectures element of the main sections. The sandstone of the outcrops has rich structures formed by ancient river flow. The average paleoflow direction is 187°. Eleven types of lithofacies are distinguished from the sediments at the cross section A-A′. They are Gm, Gt, Ge, Sm, Slt, Sp, Sl, Sst, Sr, Sh and Fsc. The predominant transforming directions of lithology calculated using Markov Chain method are alternative rhythm of coarse and middle sandstone, discontinuous normal rhythm of conglomerate and/or very coarse sandstone and mudstone, and continuous normal rhythm from the very sandstone to mudstone. Some unusual cross bedding such as the trough foreset, irregular replacement of foreset and backset, and shuttle shape cross bedding, appeared in some longitudinal sections along the ancient flow direction. Six architecture elements of fluvial depositional systems are distinguished in the study area. They are gravel bars and bedforms (GB), channels (CH), lateral accretion macroforms (LA), laminated silt and mud sheet (LS), overbank fines (OF) and coal accumulation (CA). The channel elements (CH) include braided (BCH), meandering (MCH) and abandoned channels (ACH). Minitype channel sandstone with parallel lateral accretion bedding locally appears at the lower part of the section A-A′. This indicates that the sandstone was deposited by remnant meandering channel. Braided channel sandstone appears fully at upper part of the section. It is obvious that the channel patterns transformed from meandering to braided rivers and the transformation occurred at middle Jurassic Series in the study area. The channel pattern transformation or the sedimentary environment change from meandering to braided was influenced by the increase of crustal uplift velocity in the upper reach and paleoclimate change from humid to arid semiarid.