古代阶梯状曲流砂坝河流沉积的识别

河流的沉积特征是多样的。实际上，多数河流沉积序列与经典曲流河沉积模式存在一定差别。在松辽盆地茂兴地区泉三段地层中阶梯状曲流砂坝河流较为发育，其特征为：发育２层曲流砂坝沉积，下曲流砂坝厚度较大，一般厚２－５ｍ，发育较完好的正韵律，与经典曲流砂坝沉积模式特征相似；上曲流砂坝亦为正韵律，但泥质含量偏高，层理发育较差。上、下曲流砂坝间多为突变接触，或被紫色或杂色泥质层隔开。

RECOGNITION OF ANCIENT STEPPED POINT-BAR SEDIMENTS

The classical model of fluvial facies is constructed based on the two followingassumptions: 1) the river course is dominantly controlled by the bankfull discharge,and 2) the helicoidal flow is fully developed. However, investigations of modernsedimentary environments show that the discharge of the meandering stream ischangeable and that the flow around meander bends is not completely helicoidal.So the sedimentary patterns of rivers are highly varied various. The ancient steppedpoint-bar sediments are well developed in the Quan-3 Member in the SongliaoBasin. They are characterized by the development of two sequences of point-bardeposits which are in abrupt contact and separated by the purple-red or variegatedmudistones. The lower point bar deposits, 2--5m thick, show a normal rhythmsimilar to the classical model; the upper point-bar deposits also show a normalrhythm but contain more clay and the bedding is less developed.