Abstract:

The distribution of lithofacies and sedimentary facies within the third member of the Dengying Formation in the Sichuan Basin remains a topic of debate,hindering a comprehensive understanding of the tectonic framework of that period and limiting further exploration of oil and gas resources. To address this issue,this study conducts an in-depth analysis of multiple outcrop sections and drill cores from the third member of the Dengying Formation in the Sichuan Basin. Three primary depositional environments have been identified: deep-water shelf,shallow-water shelf,and coastal zones. The deep-water shelf environment is characterized by a tranquil setting,with shale and mudstone as the dominant lithologies. The shallow-water shelf,influenced by storm waves and backflow currents,is marked by the deposition of sandstone,siltstone,and mixed sedimentary rocks,which commonly exhibit storm-induced structures such as hummocky cross-bedding,graded bedding,and scour surfaces. The coastal zone,controlled by wave and tidal processes,with sandstone as the predominant lithology,is characterized by the well-developed cross-bedding. Furthermore,in the Xixiang-Nanzheng area,the fourth member of the Dengying Formation directly overlies the basement,indicating the presence of a palaeocontinent during the deposition of the third member. Based on the dominant facies identified from typical sections and using the single-factor method,a sedimentary facies distribution map for the third member of the Dengying Formation in the Sichuan Basin has been constructed. This map reveals a terrigenous siliciclastic-dominated epeiric marine environment controlled by the Hannan paleocontinent. As one moves away from the Hannan paleocontinent,the depositional environments transition sequentially from coastal to shallow-water shelf and then to deep-water shelf settings. Overall,the farther one is from the paleocontinent,the less clastic material is supplied,resulting in finer sediment grain size and thinner depositional thickness. This depositional pattern reflects the extensional tectonic background of the Sichuan Basin during the Dengying period. During this period,the South China Plate was drifting and did not receive terrestrial supplies from other plates. By the Early Cambrian,the northwest margin of the Sichuan Basin gradually approached the Gondwana continent,transitioning the tectonic background from extensional to compressional. This collisional orogeny influenced the depositional pattern of the Canglangpu Formation in the Sichuan Basin,resulting in an eastward-dipping geomorphology where clastic rocks change into carbonate rocks from west to east.

Key words: sedimentary facies, sedimentary pattern, tectonic background, Dengying Formation, Sichuan Basin

摘要：

四川盆地灯影组三段岩相及沉积相展布尚存争议,不利于深入理解该时期的构造格局及油气勘探工作的进一步开展。在此背景下,对四川盆地灯三段多个野外剖面和钻井进行详细研究,共识别出深水陆棚、浅水陆棚、滨岸3种沉积环境。其中,深水陆棚相为静水环境,主要发育泥页岩; 浅水陆棚相主要受风暴浪和回流作用影响,以砂岩、粉砂岩和混积岩为主,常见丘状交错层理、粒序层理及冲刷面等风暴成因构造; 滨岸相受控于波浪和潮汐作用,以砂岩为主,交错层理最为发育。此外,在西乡—南郑一带灯四段直接超覆于基底上,说明在灯三段沉积时期该区为古陆。依据典型剖面识别出的优势相,结合单因素法绘制出四川盆地灯三段沉积相展布图,重建了汉南古陆主控的碎屑岩陆表海沉积格局,具体表现为: 远离汉南古陆方向,自北向南依次发育滨岸、浅水陆棚和深水陆棚沉积,整体上离古陆越远,碎屑供给越少,沉积物粒度越细,沉积厚度越小。该沉积格局响应了晚震旦世灯影组沉积时期四川盆地的伸展构造背景,即该时期华南板块处于独立漂移状态,未接受其他板块的陆源供应。至早寒武世,四川盆地西北缘逐渐向冈瓦纳大陆靠近,伸展构造背景逐渐向挤压构造背景转换,碰撞造山致使沧浪铺组沉积期四川盆地形成了西高东低及西侧发育碎屑岩、向东过渡为碳酸盐岩的沉积格局。

关键词: 沉积相, 沉积格局, 构造背景, 灯影组, 四川盆地

CLC Number: 

• P531