四川盆地北部二叠系栖霞组陆源碎屑岩的发现及其指示意义

Discovery of terrigenous siliciclastic rocks and its indicative significance of Permian Qixia Formation from Northern Sichuan Basin

在四川盆地北部旺苍双汇剖面二叠系栖霞组碳酸盐地层中发现了陆源碎屑岩沉积,其区内对二叠纪瓜德鲁普世早期古地理演化及二叠系油气勘探具有重要的指示意义。笔者等通过沉积学和锆石U-Pb年代学的方法,对碎屑岩的沉积环境和物源条件进行了分析。栖霞组陆源碎屑岩的沉积特征表明其形成于滨岸沉积环境,指示区内经历过短暂的海平面下降。岩石学及锆石形态特征表明其具有较高的成分成熟度和结构成熟度,碎屑锆石UPb测年结果显示年龄主要集中在2600~2400 Ma、1800~1200 Ma、1000~700 Ma和500~400 Ma 4个峰值区间,反映物源成分较为复杂,指示碎屑颗粒经历了较长距离搬运或再旋回。通过对比锆石年龄频谱发现,栖霞组碎屑岩与扬子地块志留系—二叠系碎屑岩及前寒武系结晶基底均具有相似的锆石年龄分布特征。结合区域构造演化,认为扬子地块前寒武系结晶基底是碎屑锆石的初始来源,上扬子北缘志留系—石炭系可能为栖霞组碎屑岩的形成提供了直接的物源供给。栖霞组碎屑岩的出现指示研究区在瓜德鲁普世早期由于海平面的下降,局部水下隆起短暂暴露形成了海岛或古陆并遭受剥蚀,为碎屑岩形成提供物源。基于栖霞组碎屑岩所指示的沉积环境和物源条件,恢复了研究区瓜德鲁普世早期的古地理格局。栖霞组陆源碎屑岩的发现对揭示四川盆地北部瓜德鲁普世早期地貌分异及古地理格局转换提供了新的证据,为区内下一步油气勘探提供重要指示。

Objectives:Terrigenous siliciclastic deposits have been found in the carbonate strata of the Permian Qixia Formation in the Shuanghui section,Wangcang area,northern Sichuan Basin,which has important indicative significance for the Permian Early Guadalupian paleogeographic evolution and oil and gas exploration of the northern Sichuan Basin.Methods:By means of sedimentology and zircon U-Pb chronology,the sedimentary environment and provenance conditions of siliciclastic rocks have been analyzed.Results:The sedimentary characteristics of the terrigenous siliciclastic rocks in the Qixia Formation indicate that they were formed in a coastal sedimentary environment,indicating that the area has experienced a short period of sea level decline.The petrological and morphological characteristics of zircons indicate that they have high compositional maturity and structure maturity.U-Pb dating results of detrital zircons show that their ages are mainly concentrated in the four peak ranges of 2600~2400 Ma,1800~1200 Ma,1000~700 Ma and 500~400 Ma,which reflects the complex provenance composition,and indicates that the clastic particles have undergone long distance transport or recycling.By comparing the zircon age spectrum,it is found that the zircon age distribution characteristics of the Qixia Formation siliciclastic rocks is similar to the Silurian—Permian siliciclastic rocks and Precambrian basement rock of the Yangtze block.Combined with regional tectonic evolution,it is believed that the Pre-Cambrian crystalline basement in the Yangtze Block is the initial source of detrital zircons,and the Silurian—Carboniferous System in the northern margin of Yangtze Block may provide direct provenance supply for the formation of the Qixia detrital rocks.The emergence of siliciclastic rocks in the Qixia Formation indicates that in the Early Guadalupian,due to the drop of sea level,local underwater uplift was briefly exposed to form an island or old land and suffered denudation,which provided the source for the formation of siliciclastic rocks.Based on the sedimentary environment and provenance conditions indicated by the siliciclastic rocks of the Qixia Formation,the paleogeographic pattern of the study area in the Early Guadalupian is restored.Conclusions:The discovery of terrigenous siliciclastic rocks of the Qixia Formation provides new evidence for revealing the Early Guadalupian geomorphic differentiation and paleogeographic pattern transformation in northern Sichuan Basin,and provides an important indication for the next oil and gas exploration in the area.