三角洲平原上网结河的发育与聚煤作用

Development of Anastomosed Fluvial Systems and Coal Accumulation on the Deltaic Plain

龙潭组上段（２４＃煤顶板到１２＃煤）是盘县地区含煤最好的层段，对该区西北部本段地层的研究表明其中发育有网结河体系。河道沉积由多层叠置的鞋带状砂体组成，湿地沉积主要由菱铁质泥质岩夹薄层板状细砂岩（决口扇沉积）和煤组成。河流的发育和聚煤作用都显示幕式性，可能主要受海平面变化的控制。网结状河道被认为是海侵期对低位下切河谷充填过程中经侧向冲裂水道并相互连通形成的；稳定、连续的厚煤层发育于河道大范围废弃后的海侵晚期，其次是高水位早期。

The upper member of Longtan Formation at northwestern Panxian, western Guizhou, consisting of about 11-18% coal seams and 82-89% clastic intervals, contains the most economic coal deposits in the Late Permian coal-bearing strata. Two episodes of channel-fill sedimentation are recognized in this coal-bearing unit, and they interrupt the extensive development of mires and the accumulation of peats. The widespread swamps, therefore, are developed after the regional channel abandonment. The channel-fill sandstones are 4-25m thick and 300-500m wide, have pronounced basal scours, internal erosional surfaces, sharp top contacts and abundant authigenic glauconites, and they are filled dominately by aggradational deposits, and show interconnecting shoestring-like patterns. An anastomosed channel complex is suggested by the coeval channels of different dimensions. The sheet (or tabular) sandstones, ranging from 0.2m to 3m in thickness, are interpreted as the crevasse-splay deposits. The laminated and/or massive mudrocks with abundant sideritic nodules or bands, constituting 60-80% of the deposits in sediment volumes, are assumed to the wetland deposits. Some mudrocks with marine body fossils are considered as the interdistributary bay or esturary deposits. It is suggested that the episodic development of fluvial systems and the accumulation of coals perhaps were mainly controlled by sea level fluctuations for lack of full evidences of the relative rapid subsidence near the Kangdian Uplift in the west of the study area. As the sea level rose in the early transgression, the sea water invaded and filled the incised valleys exposed at the sealevel lowstand. The water-logged incised valleys on the deltaic plain had to release the excessive water potentials by lateral avulsion. Interconnecting with the avulsion channels finally formed the anastomosed river system. The continued sea level rising led to the extensive channel abandonment and the mire development because the channel aggradation could not catch up the sea level regional channel desertion in the late transgression. However, the coal accumulation on mires developed during other stages of sea level variations became unstable, discontinuous, and are of minor economic importance because of the variable mire environments or the lowstand downward incision of channels.