盆山转换与沉积地质记录——以楚雄前陆盆地分析为例

Basin-range transition and depositional records:an example from the Chuxiong foreland basin in Yunnan

关键词:  盆山转换  复理石与磨拉石  楚雄盆地

Key words:  basin-range transition  flysch and molasse  Chuxiong Basin

摘要:

楚雄盆地位于扬子陆块的西南边缘,为一中生代周缘型前陆盆地。根据沉积相特征、层序地层结构和古地理演化的详细研究,结合古哀牢山造山带的构造演化,笔者认为楚雄盆地经历了从古生代被动大陆边缘沉积到中生代前陆盆地沉积的演化。前陆盆地演化的阶段性明显:晚三叠世卡尼期(云南驿组沉积期)和诺利早、中期(罗家大山组沉积期)为前陆复理石沉积;诺利晚期(花果山组沉积期)—古新世(赵家店组沉积期)为前陆磨拉石沉积。磨拉石沉积可分为海相含煤磨拉石和陆相红色磨拉石两种类型。其中陆相磨拉石沉积时间跨度长,分布面积广,沉积厚度大,沉积演化可细分为盆地成形、强烈沉降、回返充填和萎缩消亡四个阶段。随着逆冲造山楔的不断向上生长和向克拉通方向加载,楚雄前陆盆地经历了一个早期向上突然加深、变细和晚期向上变浅、变粗的沉积充填过程;盆地由早期复理石沉积演变为晚期磨拉石沉积;盆地基底形态由早期的窄而深演化为晚期的宽而浅;分布于造山楔前缘的盆地沉降与沉积中心也不断地向北东克拉通方向迁移。古流向、岩石学和岩石地球化学数据都显示楚雄前陆盆地沉积物的主要物源区为古哀牢山造山带,其次为东部隆起带,因此,盆地沉积物的供给具有明显的双物源特征。

Abstract:

The Chuxiong Basin located in central Yunnan on the southwestern margin of the Yangtze landmass is a Mesozoic peripheral foreland basin,which is bounded on the southwest by the Honghe fault,on the west by the Chenghai fault and on the east by the Puduhe fault.Tectonically,three units maybe distinguished for the basin.They are:(1)western thrusting zone;(2)central depression zone,and (3)eastern uplifted zone.In terms of sedimentary facies,sequence stratigraphy and palaeogeographic evolution in combination with tectonic evolution of the Ailaoshan orogenic zone,the basin shows a depositional evolution from the Palaeozoic passive continental margin to the Mesozoic foreland basin.Several phases of sediments have been identified in the basin.For example,during the Carnian (Late Triassic) and the early and middle Norian (Late Triassic),the foreland flysch sediments were accentuated.while during the late Norian (Late Triassic) to the Paleocene,the foreland molasse sediments were highlighted.The molasse sediments consist of two categories:marine and continental.The latter are characterized by long time duration,widespread occurrence and greater thickness.On the whole,the basin may have experienced four stages of evolution:basin formation,rapid subsidence, inversion and filling,and final termination.The upward growth and cratonward overburden of the over-thrusted orogenic wedges led to the fomiation of sudden deepening- and fining-upward sequences during the early stage and shallowing- and coarsening-upward sequences in the basin during the late stage.In other words,there are gradations from the early flysch sediments to the late molasse sediments,and from deep and narrow basal relief during the early stage to wide and shallow basal relief during the late stage.The centres of subsidence and deposition of the basin once in front of the thrusting orogenic wedges gradually migrated northeastwards (cratonwards).All the data on palaeocurrents,petrography, lithogeochemistry indicate multiple sources of the detritus in the basin,i.e.the detritus were derived dominantly from the ancient Ailaoshan orogenic zone,and subordinately from the eastern uplifted zone.