中上扬子地区晚奥陶世－早志留世岩相古地理演化与黑色页岩的关系

晚奥陶世-早志留世时的中上扬子地区,是一个周边被前陆隆起围限的浅海陆棚,沉积环境受武陵-雪峰及黔中前陆隆起带的控制。前隆间的隆后盆地为滞留、还原环境的海域,为黑色页岩沉积提供了空间。上奥陶统凯迪阶五峰组黑色页岩厚度薄而稳定,含笔石生物层和丰富的放射虫,沉积环境为浅海深水盆地;下志留统鲁丹阶龙马溪组下部的黑色页岩为五峰组的继承性沉积,其上部及埃隆阶的龙马溪组为向上变浅的沉积序列,由潮下带潮汐沙坝-潮坪环境黑色含粉砂质页岩、灰色粉砂岩薄互层所构成。沉积序列的变化和时空演化受边缘古隆起的制约。晚奥陶世前陆隆起范围小(平缓),至早志留世鲁丹期,前陆隆起不断扩大并导致盆内构造分隔,发育了大致与前隆平行的坳陷,造成龙马溪组黑色页岩厚度变化大;至埃隆期晚期,海平面相对下降,水体变浅。除个别地区外,中上扬子地区基本结束了黑色页岩沉积。

Late Ordovician-Early Silurian sedimentary facies and palaeogeographic evolution and its bearings on the black shales in the Middle-Upper Yangtze area

During the Late Ordovician-Early Silurian, the Middle-Upper Yangtze area appeared as a shallow shelf surrounded by foreland uplifts, where the sedimentary environment was controlled by the Wuling-Xuefeng and central Guizhou foreland upwarped zones. The stagnant and reducing conditions in the inter-foreland back-uplift basins once provided vast accommodation spaces for the deposition of the black shales. The Katian black shales of the Late Ordovician in the Wufeng Formation are thin in thickness, and contain graptolite biostromes and abundant Radiolarias, suggesting a shallow-marine deep-water basin environment. The Rhuddanian black shales as the black graptolite shales in the lower part of the Lower Silurian Longmaxi Formation are considered as the inherited deposits of the Wufeng Formation. The overlying Aeronian Longmaxi Formation displays the shallowing-upward depositional sequences composed of the thin interbeds of black silty shales and grey siltstones, and records the transition from subtidal to intertidal and finally to tidal-flat environments. The space-time evolution of the depositional sequences was constrained by the surrounding palaeouplifts. The space-time range of the foreland uplifts was relatively small and gentle during the Late Ordovician. Till the Rhuddanian (Early Silurian), the steady expansion of the foreland uplifts led to the structural separation, development of the depressions nearly parallel to the foreland uplifts, and considerable variations in the thickness of the black shales in the Longmaxi Formation. Finally during the late Aeronian, the relative falling of sea levels and shallowing of sea water marked the termination of the deposition of the black shales in the Middle-Upper Yangtze area.