鄂尔多斯盆地晚古生代含煤地层层序地层与海侵成煤特点

层序划分的关键是层序界面的确定,无论是高级别的层序还是层序内部单元,都是以关键沉积或地层等时界面为边界的。由于海侵或者大规模水进水退导致的区域侵蚀面、事件性沉积在陆表海盆地充填中比较发育,因此,大面积水域扩张和退缩导致的大面积水流冲刷界面、沉积体系的大规模转换界面、盆地转折时期大面积的水流冲刷和河流复活形成的下切谷充填界面等都是层序划分的重要界面。煤层的形成是一个特殊的沉积事件,它可能是一个事件的结束,也可能是一个沉积事件的开始,因此也可能是划分层序的重要依据。将本溪组至下石盒子组划分出9个三级层序,4个二级层序。陆表海盆地三级层序为海侵体系域高位体系域二个单元组成;盆地转换期的沉积充填层序,以及陆相盆地三级层序均为低位、水侵和高位体系域三元结构层序。海侵成煤和事件成煤是鄂尔多斯盆地晚古生代煤聚积的基本特色。 

Sequence Stratigraphy of Late Paleozoic Coal-bearing Measures and the Transgressive Coal-formed Features in Ordos Basin

The identification of sequence boundaries is the key point for sequence stratigraphic classification. Both the higher-order sequences and the units within the sequences are bounded with the key sediments or isochronous surfaces. It is common that regional erosion and event sedimentation, resulting from transgression occurred in epicontinental sea basin infill. So, water flow washing surface originated from the overspreading and shrinking of water over a large area, the large scale switchover surfaces between depositional systems and the filling surface of incised valley caused by rejuvenated river and flow washing during basin inversion are the important surfaces in classifying sequences. The forming of a coal seam is a special event that could be the end or start of an event and then could become the basis for sequence classification. It is classified into nine third-order sequences and three second-order sequences from Benxi formation to Xiashihezi formation. The four-order sequence of epicontinental sea basin is composed of two units, that is, the Transgressive System Tract (TST) and Highstand System Tract (HST).Both the infill sequence during basin inversion and that of continental basin are composed of three parts: Low system tract, water invasion system tract and highstand system tract. Transgressive coal forming and coal forming under event are the essential features of Late Palaeozoic coal accumulation in Ordos basin.