辽西兴城地区上古生界沉积环境识别

Recognition on Sedimentary Environment of Upper Paleozoic in Xingcheng,Western Liaoning

以粒度分析结果为依据,结合地层的序列、结构、构造及岩性等方面的特征,在辽西兴城地区的上古生界中识别出四种沉积环境,每种环境均有独特的粒度特征:1曲流河环境:所在层位为本溪组与石盒子组中下部,沉积物平均粒径多集中在3附近,分选度较好,频率曲线多为微正偏的正态分布,概率累积曲线存在差异,河道为典型两段式,决口扇为低悬两段式;2三角洲平原环境,所在层位为太原组与山西组,其中平原沼泽沉积物粒度较细,集中在3.2之间,但分支河道粒度相对较粗,与曲流河类似,三角洲平原沉积物分选好,频率曲线正偏、负偏均存在,概率累积曲线存在差异,分支河道为低悬两段式而平原沼泽则为一段式,反映垂向加积特征;3辫状河环境,对应石盒子组上部层位,沉积物平均粒度2.5左右,分选中等至较好,频率曲线多为微正偏态,概率累积曲线介于典型两段式与高悬两段式之间;4冲积扇环境,所在层位为蛤蟆山组,平均粒径集中在2左右,分选中等,频率曲线为正偏态至很正偏态,表明沉积物以粗组分为主,概率累积曲线均为高悬两段式,代表很高的水动力条件。从水动力条件及沉积环境的演化趋势来看,本地区晚石炭世至中二叠世中期处于温暖湿润的濒海盆地环境,中二叠世后期开始,随着地壳抬升,本地区不断向内陆环境变化,至晚二叠世末期,已经进入干旱炎热的内陆盆地环境。

According to the result of grain-size analysis which combine with the characteristics of stratigraphical sequence,texture,structure and lithological properties,there are four kinds of sedimentary environments in the Carboniferous to the Permian could be discerned in Xingcheng,Western Liaoning. Each kind of sedimentary environments have distinct characteristics of grain-size. First one is meandering river environment whose layers are in Benxi Formation and middle and lower Shihezi Formation,the average grain-sizes of sediments are mainly 3  and sediments have a good gradation. Their frequency curves are slight skewed positive distribution. There are two kinds of grain-size probability cumulative curves. The one of distributary channel is typical bi-segment pattern and another of crevasse splay is＂low suspension＂bi-segment pattern. Second one is delta plain environment. It is identified in Taiyuan Formation and Shanxi Formation. The sediments of swamy have better gradation whose average grain-size is about 3. 2  and their grain-sizes are more fine than distributary channel＇s which is similar to meandering river. The frequency curves of sediments of delta plain reveal both positive and negative skewness. The grain-size probability cumulative curves of distributary channel manifest as ＂low-suspension ＂bi-segment pattern but ones of swamp are one-segment pattern which reflect vertical-accretion process. Third one is braided river environment. It is distinguished in upper Shihezi Formation and its average grain-size is near 2.5 . The gradation is middle to better and the frequency curve of it is slight skewed positive distribution. The type of grain-size probability cumulative curves is between typical bi-segment pattern and ＂high suspension＂bi-segment pattern. Fourth one is alluvial fan environment in Hamashan Formation. It has middle gradation and its average grain-size is about 2 . According to common and extreme skewed positive distribution and of its frequency curves,it shows that most of sediments are composed of coarse component. ＂High suspension＂bi-segment pattern of probability cumulative curves show that the hydrodynamic condition of the time was strong.Considering paleohydrodynamic condition and evolution of sedimentary environment,this area was a basin in closeshore environment which was warm and moist during Late Carboniferous to the middle of Middle Permian. With the Earth＇s crust uplift,the environment changed in direction of inland environment progressively from late Middle Permian. Up to the end of Late Permian,the environment had turned into inland which was dry and torrid.