扬子克拉通北缘中元古界神农架群台子组沉积特征

Sedimentary characteristics of the Mesoproterozoic Taizi Formation of Shennongjia Group in northern margin of Yangtze Craton

有关扬子克拉通北缘神农架地区中元古界神农架群台子组沉积环境的认识一直存有分歧,文中基于野外和室内的综合研究讨论了台子组的沉积环境。台子组底部为蛋青色砂砾岩,下部为灰色、肉粉色细粉晶白云岩,中部为灰绿色、浅灰色石英中—细砂岩、粉砂岩与深灰色粉砂岩、粉砂质泥岩和泥质粉砂岩组合,上部则是粉砂岩—泥岩韵律。同时,台子组石英砂岩中以发育块状层理和平行层理为主,粉砂岩和泥岩则发育丰富的水平层理和透镜状层理。砂岩粒度概率累计曲线显示斜率较大,跳跃组分为主,悬浮组分含量低,无滚动组分,其中跳跃组分又以二段或多段式为特征。上述沉积指相标志和粒度分析均表明,台子组形成于滨岸与潮坪沉积环境,同时纵向沉积序列上,碎屑岩沉积物粒度表现为粗—细—粗—细演化特征,沉积相由下部到上部呈现滨岸相—碳酸盐岩缓坡台地相—潟湖相—潮坪相—潟湖相的演替规律,总体上构成由退积—进积—退积—进积的沉积序列。

Sedimentary environments of the Mesoproterozoic Taizi Formation of Shennongjia Group in Shengnongjia area,northern margin of Yangtze Craton are all along in debate.In this study,we further constrain the sedimentary environments of the Taizi Formation based on the comprehensive field and laboratory experiments.The lower part of the Taizi Formation consists of gray green silty-fine dolostone.The middle part of the Taizi Formation is composed of gray green,grayish quartz medium-fine sandstones,siltstones and dark grey siltstones,silty mudstones,muddy siltstones rhythms,while its upper part comprises siltstone-mudstone rhythm.Massive beddings and parallel beddings occur in the quartz sandstone,while siltstones and mudstones contain the horizontal and lenticular beddings.The cumulative probability curve of sandstones shows a steep slope,which are made up mainly of jumping components with two or more groups and little suspended components without rolling components.The sedimentary structures and grain size characteristics suggest that the Taizi Formation was formed in the coastal and tidal flat depositional environments.Meanwhile,the vertical sedimentary sequences are represented by the grain size evolution of coarse-fine-coarse-fine,which is combined with that the sedimentary facies from the bottom to the top show littoral facies-carbonate ramp platform facies-lagoon facies-tidal flat lagoon facies succession,constitute a complete transgressive-regressive sedimentary sequence with patterns of progradation-retrogradation-progradation.