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前寒武纪—寒武纪转折期是地球历史演化的重要阶段之一,不仅纪录了后生动物的产生、灭绝以及加速分异的过程,同时也伴随着全球构造重组、长期全球性海洋缺氧、海洋地球化学明显的变化等。为了更好地认识这一重要时期地球表层环境演变及动力机制,我们对鄂西—湘西北地区下寒武统地层(相当于第1、2阶)进行了详细的沉积相和层序分析,在3大岩相组合(碳酸盐岩、硅岩、黑色泥岩/页岩)中识别出了16种主要岩相类型,并根据岩相时空变化识别出4个沉积层序(S1～S4)。在寒武纪开始时,湘西碳酸盐台地发生暴露,而台缘南侧发生裂陷,台地北侧的湘西北地区(如石门杨家坪)也发生大幅沉陷,但往北至三峡地区变浅,过渡为碳酸盐台地,形成两隆两凹的古地貌格局,这种巨大侧向地形差异(起伏)指示伸展构造背景下的断块掀斜作用,这种构造作用在寒武纪早期呈幕式活动,从而控制了该时期的沉积作用、沉积样式及其空间分布,并以沉积层序体现,因此每一层序代表了一幕从构造(拉张)强化到衰减过程的沉积响应。在构造幕的开始阶段,由于强的拉张和断裂掀斜作用,湘西碳酸盐台地发生暴露或变浅,而在该台缘南侧发生断陷,形成断阶,深部富硅热液流体沿断裂向上运移到断阶海底形成浅海热液喷流系统(硅烟囱),所以此处层序底部往往由一套特殊的硅质沉积(如有多管—孔丘状硅岩、脉状硅岩或漏斗状/楔状硅岩体)组成。此带向东南盆地方向,由于热液活动衰减和海水影响的增强,主要发育层状硅岩。从湘西北至三峡地区,层序底部一般由薄层状硅岩(常含泥质)组成,且向台地方向量减少(但碳酸盐岩增多)。在构造幕晚期,由于构造活动衰减,热液硅通量大幅减少,除深水盆地外,碳酸盐工厂恢复生产,因此层序上部主要由碳酸盐岩组成(深水盆地除外)。从第四构造幕热液活动后,盆地基底热衰减迅速,深部硅输入通量大幅减少,并在扬子地块发生大范围均匀挠曲沉降和快速海平面上升,造成碳酸盐台地淹没,形成了牛蹄塘组(或同期)富有机质黑色岩系。从此后,中扬子地区进入了一个新的构造—沉积演化时期。
The Precambrian-Cambrian turning point is one of the important stages in the evolution of the Earth’s history. It not only records the generation and extinction of metazoans, but also speeds up the process of differentiation. At the same time, it is accompanied by global structural reorganization, long-term global oceanic anoxic , Obvious changes in marine geochemistry, etc. In order to better understand the evolution of the Earth’s surface environment and the dynamical mechanism during this important period, we have conducted detailed sedimentary facies and sequence analysis of the Lower Cambrian strata (equivalent to the first and second order) in the western Hubei and western Hunan regions, Sixteen major facies types have been identified in the three major facies assemblages (carbonate, silicalite, black mudstone / shale) and four sedimentary sequences (S1-S4) have been identified based on the temporal variation of lithofacies. At the beginning of the Cambrian, the carbonate platform in western Hunan was exposed and the south margin of the margin was rifted. The northwestern part of Hunan (such as Shimen Yangjiaping) on the north side of the platform also suffered a substantial subsidence, but the area north of the Three Gorges area became shallow. The transition to a carbonate platform, the formation of the two Long two concave ancient geomorphological pattern, this huge lateral topographic differences (ups and downs) indicate the extensional tectonic fault block tilt effect, this tectonic role in the early Cambrian curtain was curtain Which controls the sedimentation, depositional patterns and their spatial distribution during this period and is characterized by sedimentary sequence. Therefore, each sequence represents a depositional response from the tectonic (tension) strengthening to the decaying process. At the beginning of tectonic curtain, the carbonate platform in western Hunan is exposed or shallower due to strong extension and fault tilting, and faulted to the south of the platform margin to form fault-step, deep rich silicate hydrothermal fluid So that the bottom of the sequence here is often composed of a special set of siliceous sediments (such as multi-tube-hole-shaped silicalite, veins Silicon rock or funnel / wedge silicon rock body). The belt to the southeast basin direction, due to the decay of hydrothermal activity and sea water, the main development of layered silicalite. From the northwestern Hunan to the Three Gorges region, the bottom of the sequence is generally composed of thin-bedded silicic rocks (often containing muddy matter) and decreases toward the platform (with increased carbonate). In the late tectonic setting, due to the decay of tectonic activity, the flux of hydrothermal silicon decreased drastically. The carbonate plant was resumed production except for the deep-water basins. Therefore, the upper part of the sequence mainly consisted of carbonate rocks (excluding deep-water basins). After the tectono-hydrothermal activity from the fourth tectonic setting, the basal thermal decay rapidly and the input flux in the deep silicon decreased greatly. In the Yangtze block, a large range of uniform flexural subsidence and rapid sea level rise resulted in submergence of the carbonate platform The Niutitang Formation (or contemporaneous) organic-rich black rock series. Since then, the middle Yangtze region has entered a new structure - sedimentary evolution period.