鄂尔多斯盆地东北缘神木地区浅湖三角洲沉积作用及煤聚集

鄂尔多斯盆地东北缘延安组含有大量的优质煤,属稳定大地构造条件下的浅湖三角州沉积物。三角洲体系有以下单元组成:(1)前三角洲;(2)三角洲前缘(水下分流河道相和河口坝相);(3)三角洲平原(分流河道相,天然堤、决口扇、沼泽和具决口三角洲的分流间湾);(4)废弃三角洲平原.通过详细的露头观察和岩心研究及沉积岩的地质填图工作,浅湖三角洲特征可归纳如下:(1)推进速度非常快、舌形体长而窄,间湾在其间发育,决口事件频繁发生;(2)前三角洲非常薄(0.2～0.5m),三角洲剖面几乎绝大部分由三角洲前缘和三角洲平原构成;(3)河口坝相由砂岩和泥岩互层组成;(4)水下分流河道砂岩厚度在0.5～3.5m范围内,被河口坝沉积物围绕;(5)在废弃三角洲平原中,由于极少沉积物带入沼泽,因此沉积了厚的低灰煤层。

SEDIMENTATION OF SHALLOW LACUSTRINE DELTAS AND COAL ACCUMULATION OF JURASSIC YAN'''' AN FORMATION,NORTHEASTERN ORDOS BASIN, CHINA

The shallow lacustrine delta in stable tectonic setting suggests to be the origin of Yan'an formation(J1-2)with extensive and good quality coal beds at northern Shenmu area in the northeastern part of Ordos basin.Delta system includes following components: 1 ) prodelta; 2 ) delta front (subaqueous channel and mouth bar fades ); 3 ) delta plain ( distributary channel, levee, crevasse splay, swamp and interdistributary bay with crevasse deltas) and 4 ) abandoned delta plain. Based on detailed outcrop and core study and sedimentologic mapping,the characteristics of shallow lacustrine delta are summerized as follows.1 ) The progradation is very rapid. The lobes are long and narrow and the interdistributary bay is developed between them.Crevasse events are frequent.2 ) The delta front and delta plain deposits consist of the main parts of the whole delta sequence.3 ) Mouth bar facies consists of the alternation of sandstones and mudstones.4 ) Subaqueous channel sandstones with the thickness of 0.5-3.5m are surrounded by the mouth bar deposits.5 ) The abandoned delta plain deposits are characterized by a thick and low-ash coal bed which was accumulated in a large area of swamp, into which very few sediments were carried by current flow.