豫西地区下三叠统刘家沟组砾岩成因

豫西地区刘家沟组中下部发育多层砾岩,其中扁平砾石与球形砾石共生,扁平砾多顺层排列,球形砾常具圈层结构。因该砾岩层发育普遍,特征明显,成为刘家沟组的区域划分对比标志之一。但迄今为止,其成因众说纷纭、未有定论。通过砾岩发育层段的野外观察,结合刘家沟组的沉积环境分析,认为宜阳地区刘家沟组砾岩层的砾石不是单纯的物理成因,其形成可能与微生物繁盛的席底底质有关。微生物席的存在使沉积物表面具有一定的粘性和韧性,在动荡水流的作用下,部分破坏作用形成的微生物席碎片卷曲、滚动,形成具有圈层结构的"砂球",在湖泊重力流作用下,扁平的微生物席碎片与"砂球"被搬运至较深水区域,形成特征明显的扁平砾石砾岩层。

The genesis of conglomerate in Lower Triassic Liujiagou Formation of western Henan Province

There are several layers of conglomerates in the Lower Triassic Liujiagou Formation in western He''nan Province.The con-glomerates consist of flat sand or mud gravels and round sand gravels, with some of the sand gravels having ring structures just like ooids in limestone.This obvious characteristic has become one of the important marker beds in dividing and correlating strata in the study area, though its cause remains a mystery and should be further ascertained.Based on the study of lithofacies of outcrops and lab experiments, the authors hold that the rings in gravels are microbially mediated structures, and the formation of conglomerates was probably associated with mats substrate after the Mass Extinction.Mat living caused the surface of sediments to have some viscosity and toughness, some destroying mat fragments caused curling, rolling and the development of"sand ball"with ring structure in vibrat-ing water, and those mat fragments and sand balls were transported into deeper water by lake gravity flow later, forming the obvious conglomerates in Liujiagou Formation of Lower Triassic in western He''nan Province.