羌塘盆地东部晚三叠世-侏罗纪构造-沉积演化

Tectonic-sedimentary evolution during Late Triassic-Jurassic period in the eastern part of the Qiangtang basin,Tibet

本文以野外实测剖面为基础,结合区域地质调查,通过地层学、岩石组合特征及沉积盆地分析,并综合区域构造演化背景及年代地层学等最新研究成果,对羌塘盆地东部上三叠统-侏罗系岩石组合、沉积类型和沉积相特征进行了对比研究,明确了羌塘盆地东部晚三叠世-侏罗纪的地层分布特征、沉积相类型、展布特征及构造-沉积演化过程。以区域性角度不整合为标志,将羌塘盆地东部上三叠统-侏罗系划分为2个构造层(卡尼阶-诺利阶构造层和瑞替阶-基默里阶构造层),并详细叙述了不同构造层内各地层的分布范围、厚度变化及岩性特征。研究区上三叠统-侏罗系以海陆过渡相和海相沉积为主,其中海陆过渡相包括三角洲相和潮坪-泻湖相,海相包括碳酸盐缓坡、蒸发台地、局限台地、滨海、浅海、半深海和深海相沉积。卡尼期主要发育滨海-浅海-半深海-深海相沉积(北羌塘坳陷)和滨浅海沉积(南羌塘坳陷)。诺利期早期以碳酸盐岩缓坡-混积陆棚沉积为主,诺利期晚期、赫塘期-巴柔期、卡洛期和基默里期主要发育潮坪-三角洲相沉积(北羌塘坳陷)和滨浅海沉积(南羌塘坳陷),巴通期和牛津期以蒸发台地-局限台地-混积陆棚沉积为主。沉积相东西向带状展布,东部低隆起是分割南北相带的重要构造单元,并在巴通期及牛津期为水下隆起,控制着羌塘东部局限台地、蒸发台地及台地边缘的分布。将区域构造背景与沉积记录相结合,总结出5个构造-沉积演化阶段:活动陆缘阶段(卡尼期,Ⅰ1)、前陆盆地演化阶段(诺利期,Ⅰ2)、裂陷-坳陷阶段(瑞替期-巴柔期,Ⅱ1)、被动陆缘-坳陷阶段(巴通期-卡洛期,Ⅱ2)和被动陆缘-坳陷阶段(牛津期-基默里期,Ⅱ3)。

Based on the field outcrops surveyed,combined with recent published the regional tectonic evolution and geochronology data,we analyzed the lithologies and rock associations of strata,identified the sedimentary facies types,and discussed the distribution sedimentary facies and the tectonic-sedimentary evolution in the eastern Qiangtang basin during the Late Triassic-Jurassic.Marked by regional unconformities,there are two tectono-stratigraphic units(from the Carnian to the Norian and from the Rhaetian to the Kimmeridgian,respectively)in the eastern part of Qiangtang basin.In this paper,we systematically described the distribution range,thickness variation and lithological characteristics of different formations in the tectono-stratigraphic units.The Late Triassic-Jurassic is dominated by marine facies and marine-continental transitional facies.The marine-continental transitional facies include deltaic and tidal-lagoon facies.Marine facies including gentle carbonate slope,evaporative platform,restricted platform,littoral,neritic,bathyal and abysmal facies.The Carnian Stage is dominated by littoral-neritic-bathyal-abysmal facies in the north Qiangtang depression otherwise the littoral-neritic facies in the south Qiangtang depression.The early Norian Stage is dominated by carbonate gentle slopemixed continental shelf facies.The late Norian,Bajocian,Callovian and Kimmeridgian stages are dominated by tidal flat-delta facies in the north Qiangtang depression and littoral-neritic facies in the south Qiangtang depression.The Bathonian and Oxfordian stages are dominated by evaporative platform-restricted platform-mixed continental shelf facies.The sedimentary facies formed zones from north to south and extended in an E-W direction.The Eastern Lower Uplift(ELU)played an important role in the division zones of sedimentary facies from north to south.During the Bathonian and Oxfordian,the ELU developed below the sea level and controlled the distribution of restricted platform,evaporative platform and platform margin.Combined regional tectonic evolution with sedimentary records,we proposed 5 tectonic-sedimentary evolution stages:Active continental margin stage(Carnian,Ⅰ1),foreland basin stage(Norian,Ⅰ2),rift-depression stage(Rhaetian-Bajocian,Ⅱ1),passive continental margin-depression stage(Bathonian-Callovian,Ⅱ2)and passive continental margin-depression stage(Oxfordian-Kimmeridgian,Ⅱ3).