扬子西南缘滇中成冰系南沱组沉积特征及其上红层归属讨论^*

Sedimentary characteristics of the Cryogenian Nantuo Formation and stratigraphic attribution of red beds in central Yunnan, southwestern margin of Yangtze Craton

摘要

扬子克拉通滇中地区新元古界南沱组与陡山沱组之间过渡段“红层”不同学者认识尚有分歧。以扬子克拉通西南缘昆明晋宁鲁纳村剖面和飞大田剖面为主,对其成冰系南沱组开展了详细的地层学、沉积学研究。南沱组砾岩段及其上覆红层泥岩段具擦痕、坠石、韵律性纹层等典型冰川沉积特征,据此划分出4个沉积亚相,即冰下滞积亚相、冰缘河道亚相、冰前三角洲前缘亚相和滨岸亚相,反映了冰期演化的不同阶段。红层段顶部最年轻碎屑锆石年龄为636Ma,表明南沱组上段紫红色粉细砂岩顶部沉积时限可能晚于636Ma。区域地层对比发现,平面上,滇中地区南沱组砾岩段和泥岩段分布稳定; 垂向上,自下而上为冰川冰下堆积砾岩,向上过渡为冰缘河道砾岩,再向上为冰前三角洲前缘泥岩和滨岸细砂岩,表现为冰川逐渐融化并逐步海侵的规律。研究认为南沱组应当二分: 下段杂砾岩段,上段紫红色泥岩段,即南沱组—陡山沱组界限置于红层段顶部。论文深化了对扬子西南缘成冰纪沉积古地理的理解,为完善四川盆地周缘成冰纪—埃迪卡拉纪等时地层格架和四川盆地及周缘深层油气勘探提供了沉积学新资料。

Abstract

In the central Yunnan region of the Yangtze Craton,the “red beds” that develop in the transitional section between the Nantuo Formation and the Doushantuo Formation of the Neoproterozoic,are still debated among scholars. This study carried out detailed stratigraphic and sedimentological studies on the Nantuo Formation in the Lunacun and Feidatian sections in the central Yunnan region. The sections were located on the southwestern margin of the Yangtze craton. The conglomerate member and its overlying red mudstone member of the Nantuo Formation exhibit typical glacial sedimentary features such as striae,dropstone,and rhythmic laminae,base on which,four sedimentary subfacies can be divided,i.e.,subglacial lag,periglacial channel,pre-glacial delta front,and pre-glacial shoreline subfacies,reflecting different stages of glaciation. The youngest detrital zircon age at the top of the red mudstone is 636Ma,indicating that the deposition time of the top of the purple-red fine sandstone in the upper member of the Nantuo Formation may be later than 636Ma. The regional stratigraphic comparison reveals that the spatial distribution of the conglomerate and mudstone members of the Nantuo Formation in central Yunnan is stable;it represents accumulation of conglomerates under glacier ice,transitioning upwards to the conglomerates of periglacial river channels,and then upwards to the mudstones and fine sandstone of the pre-glacial delta front and the pre-glacial shoreline,exhibiting a pattern of gradual melting of glaciers and marine invasion. In summary,this study suggests that the Nantuo Formation should be divided into two parts,with the lower member consisting of conglomerate and the upper member consisting of purple-red mudstone. The boundary between the Nantuo Formation and the Doushantuo Formation should be placed at the top of the red layer. This study deepens the understanding of the Cryogenian palaeogeography,the southwestern margin of the Yangtze craton. It provides new sedimentological data for improving the isochronous stratigraphic framework from the Cryogenian to Ediacaran,and assists in deep oil and gas exploration in the Sichuan Basin and its surrounding areas.

关键词

扬子西南缘 / 南沱组 / 红层 / 冰川沉积相

Key words

southwestern margin of Yangtze Craton / Nantuo Formation / red beds / glacial sedimentary facies

基金资助

Co-funded by the National Natural Science Foundation of China(No.42072135),the China Geological Survey(No.DD20221649),and the Beijing Science and Technology Commission's International Innovation Resources Cooperation Project(No.Z201100008320007)