四川南江地区上震旦统灯影组混积层系特征及成因

陆源碎屑和碳酸盐混积层系是一种沉积机理特殊而又有重要意义的沉积现象。四川南江新立村下震旦统灯影组剖面碎屑岩-碳酸盐岩混积层系发育良好。在该剖面上共识别出4类混积层系岩石组合类型:细砂岩-泥晶白云岩组合(Ⅰ)、粉砂岩-泥晶白云岩组合(Ⅱ)、粉细砂岩-含粉砂白云岩组合(Ⅲ)、含砾砂岩-白云岩组合(Ⅳ)。通过岩石类型、岩石组合特征的分析,显示出灯影组二段上部-三段由下向上发育混积中潮坪、潮下浅滩、潮道、低潮坪4种沉积亚相,而混积层系主要产出于混积中潮坪和潮道亚相。微量元素指示研究地层整体形成于氧化环境,但发生混积层位的形成环境相对缺氧。根据当时海平面变化和大地构造演化,认为灯影组陆源碎屑和碳酸盐混积层系的出现与桐湾运动Ⅰ幕之后海平面下降导致的陆源碎屑输入增强有关。

Characteristics and genesis of the mixed siliciclastic-carbonate successions in the Upper Sinian Dengying Formation in the Nanjiang region, Sichuan

The mixed siliciclastic-carbonate successions represent a special type of depositional mechanisms and significant deposition. They are well developed in the Upper Sinian Dengying Formation in the Xinli section, Nanjiang region, Sichuan. Four lithologic associations have been distinguished, including (1) fine-grained sandstone-dolomicrite association; (2) siltstone-dolomicrite association; (3) silty sandstone-silty dolostone association, and (4) gravelly sandstone-dolostone association. Four sedimentary subfacies types are also recognized on the basis of the rock types and associations, including the mesotidal flat, subtidal shoal, tidal channel and lower tidal flat subfacies. The mixed siliciclastic-carbonate successions occur primarily in the mesotidal flat and tidal channel subfacies. The trace element data show that the studied strata are generally formed in the oxidation environments. However the mixed siliciclastic-carbonate successions are formed in the anoxic environments. In the light of the sea-level changes and tectonic evolution, the development of the mixed siliciclastic-carbonate successions in the Dengying Formation in the Xinli section, Nanjiang region, Sichuan may be companied by the siliciclastic input caused by the negative movements following the phase I of the Tongwan Movement.