鄂尔多斯盆地东缘晚古生代含煤岩系沉积体系和聚煤作用的时空演化

THE TIME AND SPATIAL EVOLUTION OF SEDIMENTARY SYSTEMS AND COAL-ACCUMULATING CENTRE IN LATE PALAEOZOIC COAL-BEARING MEASURES ON THE EASTERN MARGIN OF ORDOS BASIN

鄂尔多斯盆地东缘晚古生代含煤岩系包括本溪组、晋祠组、太原组和山西组,依据海相层的发育和沉积物粒度变化,自下而上可划分为10个区域上稳定的旋回。这些旋回属它旋回成因,主要受控于区域性海平面的变化。第Ⅰ～Ⅲ旋回为本溪组和晋祠组,其沉积时开阔的海域位于研究区的北、中部,海水由东或北东方向侵入。研究区南部存在的古中条隆起使这两组的地层厚度自北向南减小,古地形向北倾。第Ⅳ～Ⅷ为海侵—海退旋回(太原组和山西组下段)开阔海域位于研究区南、中部,各期海相层由南、中部的灰岩向北变为泥灰岩、泥岩,在研究区北部最终尖灭。海水是从南东方向侵入的。各期海退碎屑沉积物由北部的河流沉积体系向南变为三角洲沉积体系,在南部为潮坪—潮道或沙坝体系,古海岸线逐渐南移。聚煤中心位于各期古岸线的附近或前方,并随时间的推移而向南移。

The Late Palaeozoic coal-bearing measures on the eastern margin of Ordos basin include Benxi fm, Jinshi fm, Taiyuan fm, and Shanxi fm, The coal-bearing measures are divided upward from bottom into ten cycles which laterally are persistent in the region according to the numbers of marine layers and the variation of sediment grains. All cycles are derived from the allocyclity,and mainly controlled by the regionally eustatic sea-level change. No. 1-3 cycles belong to Benxi fm and Jinshi fm. The open epicontinental sea covered the north-middle tracts of investigative area when No.l-3 cycles were deposited, and the transgression was invaded from east or northeast and palaeoslope trended northward at that time. The Zhongtiao palaeouplift which exsisted the southern part of the studying area caused this two formation thickness to thin and pinch out southward. The open epicontinental sea was located in the south-middle tract of the studied area when No.4-8 transgressive-regressive cycles (belonging to Taiyuan fm and the lower members of Shanxi fm) deposited. The transgreesion was invaded from southeast and northward, and palaeoslope trended southward. The lithological characters of marine beds of ever stage varied from south to north, from limestone in south-middle part, to marl limestone and mudstone in north area,and marine layers pinched out in the north part terminally.The regressive clastic rocks of each stage which are the thickest in the north and get thinner southward recorded progressive change in depositional system, from fluvial system in the north part to deltaic system in south-middle parts,to tidal flat-chan-nel or barried bar (islands) in the southmost. The sediments derived mainly from the northern Yinshan palaeouplift area dispersed southward into the open bay by a series of low-salinity southerly flowing river. Each vertically stackedfluvial sand body demostrated the existence of syndepositional tectonics during the No.4-8 cycle formation. Each coastline progressed southward as a series of shallow-water, arcuate of cuspate delta. Every stage coal-accumulating centre was located near or in the front of coastline,and progressed southward with the development of depositional system.