摘要: 奥陶纪生物大辐射事件（Great Ordovician Biodiversification Event; GOBE）是海洋环境与生物相互作用的结果，通过对该时期碳酸盐台地沉积相和层序的研究可揭示其形成与演化过程及海平面变化历史，并为奥陶纪生物时空分布特征及演化规律的探讨提供沉积背景和等时地层格架。在野外露头剖面实测和显微镜下观察的基础上，识别了黔北地区瓢儿田剖面下奥陶统（桐梓组和红花园组）的岩相类型，进一步分析了沉积模式和高频米级沉积旋回及沉积层序，最后探讨了沉积演化过程中的控制因素。结果显示：（1）瓢儿田剖面下奥陶统发育10种岩相类型，主要形成于碳酸盐缓坡沉积体系，且桐梓组和红花园组沉积时期分别以非骨架碳酸盐颗粒和骨架碳酸盐颗粒为特征；（2）桐梓组和红花园组主要发育非对称性的开阔海沉积旋回；（3）识别出3个半三级层序（Sq1~Sq4），每个三级层序均为II型层序界面（即岩性转换面）所限。其中Sq1~Sq3为完整的三级层序，由TST和RST构成，但Sq4仅发育TST；（4）瓢儿田剖面下奥陶统的沉积演化和层序发育主要受不同级次的相对海平面变化和古地理格局的共同控制。其中不同级次相对海平面波动制约着沉积相的垂向演化，古地理格局则控制着沉积相带的空间分布。
Depositional Facies and Sequence Stratigraphy of The Lower Ordovician Successions in Northern Guizhou Province
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Abstract: The Great Ordovician Biodiversification Event (GOE) is the result of the interaction between marine environments and organisms. The analyses of sedimentary facies and depositional sequences of the Ordovician carbonate platform can reveal its development and evolution and the history of sea level fluctuations, which can provide a sedimentary background and isochronous stratigraphic framework for exploring the spatio-temporal distribution characteristics and the evolution of coeval organisms. Based on the measurement of the outcrop section and microscopic observation, the Lower Ordovician (Tongzi and Honghuayuan formations) successions of the Piaoertian section in northern Guizhou Province were investigated to recognize the lithofacies types, construct the depositional model, identify the sequence stratigraphy, and explore the controls of depositional evolution. Results are as follow: (1) Ten types of lithofacies were identified and further grouped into two depositional belts (or associations) in a carbonate ramp system. The depositional system was further divided into non-skeletal grain-dominated and skeletal grain-dominated end members for the Tongzi and Honghuayuan formations, respectively. (2) These lithofacies are vertically stacked into asymmetrical, meter-scale, shallowing-upward open-marine depositional cycles for both Tongzi and Honghuayuan formations. (3) In view of vertical facies and cycle stacking patterns, three and one half third-order depositional sequences (Sq1~Sq4) were distinguished, which are defined by II sequence boundaries (lithologic transition surfaces). Sq1~Sq3 are full sequences that consist of lower transgressive (TST) and upper regressive (RST) packages, but Sq4 only contains TST. (4) The depositional evolution and sequence development of the Lower Ordovician of the Piaoertian section were mainly controlled by relative sea-level oscillations of different orders and paleogeographic configuration. The relative sea-level oscillations of different orders dictated the vertical development of facies; the paleogeographic configuration primarily defined the spatial distribution of depositional belts (or associations).