中扬子地区早寒武世构造—沉积样式与古地理格局

Tectono-depositional patterns and palaeogeography in the Middle Yangtze River region during the Early Cambrian

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 transition is a key interval in Earth＇s history,as it records the advent,extinction and accelerated diversification of metazoans （＇Cambrian explosion＇）and coevolution of tectonic re-organization of global plates, a prolonged global oceanic anoxia and swift shifts in ocean geochemistry. To understand better the fundamental environmental changes on Earth＇s surface system, detailed analyses on depositional facies and sequences of the Lower Cambrian（Stages 1 and 2）were carried out across the western Hubei and northwestern Hunan in the Middle Yangtze River region, South China, and 16 types of lithofacies in three main lithological categories of carbonate, chert and fine-grained clastic mudrock were identified, which can be grouped into four depositional sequences （ S1 - S4） in terms of tempo-spatial facies variations. Just at the onset of Cambrian,the carbonate platform in northwestern Hunan was subject to subaerial exposure, its southern margin, however, underwent rapid subsidence, forming the perched terrace; the northern flank also suffered large-scale subsidence, plunging down to the basin in the northwestern Hunan（Yangjiaping） ,where the basement relief begun to rise once again to the north and evolved into carbonate platform in Three Gorges area in western Hubei. The vast lateral topographic differences shaped by the two uplift-depressions point toblock tilting in the context of extensional tectonism,which could have episodically occurred and thereby controlled the deposition, facies patterns and distributions in the Early Cambrian as recorded by the depositional sequences. A depositional sequence thus was likely formed in responses to the tectonic episode from enhanced to waned extensional tectonism. At the onset of extensional episode, due to strong basement block tilting, the carbonate platform （ i. e. in western Hunan） underwent relative uplift and subaerial exposure, in contrast,the platform margin on the south was subject to sudden downthrowing （subsidence） , forming the perched terrace. The deep-seated hydrothermal fluids could have been channeled and migrated upward along the fault/fracture conduits onto the seafloor on the perched terrace,forming shallow marine silicarich hydrothermal vent fields （ silica chimneys） , so that a special chert association （ i. e. mounded, vein and/or funnel-shaped brecciated chert bodies） was precipitated, constituting the basal part of sequences. Farther basinward southeast, due to waned hydrothermal activity and enhanced seawater influences, bedded cherts commonly predominated. On the other side, from northwestern Hunan to Three Gorges area,western Hubei,bedded cherts commonly intercalated with mudstones, more or less, comprise the basal portions of sequences,which tended to be less voluminous,and finally replaced by mudstones （i. e. at Jiuqunao）toward the platform. In the late stages of tectonic episodes, due to waning of extensional activity, the hydrothermallyoriginated silica influx decreased significantly, so that the carbonate factories could have been re-established and restored carbonate production except in the basinal setting. In such a case, the upper parts of sequences commonly comprise, more or less, carbonates. After the fourth hydrothermal episode, hydrothermal （or igneous）activities waned significantly beneath the basement,resulting in basement cooling and flexural subsidence, thereby leading to large-scale sea-level rise（transgression） and drowning of carbonate platforms, thereby depositing extensive black mudstones/shales. Afterwards,the Middle Yangtze River region evolved into a new stage of tectono-sedimentary evolution.