鄂西—渝东地区早寒武世克拉通内裂陷演化及对古地理格局的控制

Evolution of Early Cambrian Intracraton Rift and Its Influence on the Paleogeographical Pattern,Western Hubei-Eastern Chongqing

克拉通内裂陷对四川盆地及周缘海相碳酸盐岩油气勘探有利区带具有重要的控制作用,大量露头证据揭示鄂西—渝东地区早寒武世克拉通内裂陷的存在。研究充分利用野外露头与钻井资料,结合相关构造背景与地球动力学成因,在前人研究基础上明确提出早寒武世鄂西渝东克拉通内裂陷发育的综合识别标志,并刻画了裂陷的沉积充填演化过程以及对古地理格局的控制。结果表明:1)裂陷呈南宽北窄、东缓西陡的不对称箕状形态;2)受裂陷西缘同沉积断裂控制,裂陷内地层沉积厚度与深—浅水沉积、斜坡相沉积均具明显分异性,分异程度与断裂活动性呈正相关关系;3)在早寒武世裂陷以补偿沉积充填为主,其内部由初期陆棚沉积向末期碳酸盐岩台地过渡,根据充填特性可将裂陷演化划分为裂陷(水井沱组沉积期)、坳陷(沧浪铺组沉积期)、填平补齐(石龙洞组沉积期)3个阶段;4)裂陷的“开启—闭合”是“罗迪尼亚大陆裂解—冈瓦纳大陆聚合”在中、上扬子之间的响应,该构造背景下的古地貌分异、同沉积断裂活动及海平面变化共同控制其古地理格局。鄂西—渝东地区早寒武世克拉通内裂陷的提出不仅为四川盆地克拉通内裂陷作用构造—沉积格局提供了新的认识,也对中上扬子地区油气勘探具有重要的指导意义。

Intracraton rifts have had an important influence on favorable areas for marine carbonate oil and gas exploration in the Sichuan Basin and surrounding areas.A large amount of outcrop evidence reveals the existence of Early Cambrian intracraton rifts in western Hubei-eastern Chongqing.In this study,full use was made of field outcrop and available drilling data,combined with related its structural background and geodynamic origin.Based on previous studies,the study describes the identification marks of cratonic rifts in the western Hubei-eastern Chongqing area during the Early Cambrian.The evolution process of the sedimentary filling of the rift and its influence on the paleogeographic pattern was as follows.(1)The rift was wide in the south and narrow in the north,and gently sloping in the east and steep in the west.(2)It exhibits obvious differentiation which is positively correlated with fault activity.(3)The rift was mainly filled with compensation sediments that transitioned from the initial shelf sedimentation to finally a carbonate platform.The evolution of the rift occurred in three stages:rift(Shuijingtuo Formation deposition period),depression(Canglangpu Formation deposition period),and filling(Shilongdong Formation deposition period).(4)The rift opened and closed in response to the fragmentation/aggregation activity of the supercontinent between the Middle and Upper Yangtze.The structural background of paleogeographic differentiation,syn-sedimentary fault activity and sea level change determined its geographic pattern.This proposed Early Cambrian intracraton rift in the western Hubei-eastern Chongqing area provides a new understanding of the tectonic/sedimentary pattern of the internal craton rift in the Sichuan Basin and is also an important guide for oil and gas exploration in the Middle-Upper Yangtze area.