西藏羌塘托纳木上侏罗统索瓦组地层学特征

Stratigraphic Features of Upper Jurassic Suowa Formation in Tuonamu Region, Tibet

羌塘盆地地处藏北高原,其南北界分别为班公错-怒江缝合带与拉竹龙-金沙江缝合带,盆地面积约180000km2。进一步可分为羌北坳陷、中央隆起带和羌南坳陷3个二级构造单元。双湖托纳木地区地处羌北坳陷与中央隆起带毗连地带,索瓦组分布于研究区北部大部分地区,根据地层层序及岩性组合特征,同时结合岩石、古生物、沉积旋回、接触关系等对地层进行了较系统的划分,将索瓦组划分为4个岩性段;索瓦组一段到四段自下而上反映出一个完整的海侵—海退序列,分别为碳酸盐台地—混积缓坡—混积碳酸盐台地—混积外滨浅水陆棚沉积环境;索瓦组三段礁灰岩和油页岩的发现对古环境的恢复以及油气的生储盖研究都具有重要意义;根据区内索瓦组的古生物地层分布和组合特征,建立了珊瑚组合带和双壳类组合带,确定索瓦组为上侏罗统,大致与Oxfordian-Kimmeridgian相当。

The Qiangtang Basin in an area of 180 000 km2 is situated in the Northern Tibet Plateau. Its northern and southern boundaries are the Lazhulong-Jinshajiang suture zone and Bangongcuo-Nujiang suture zone, respectively. The basin can be divided into the northern Qiangtang depression,the central uplift zone and the southern Qiangtang depression. The Tuonamu region where the upper Jurassic Suowa Formation is extensively developed is in between the central uplift zone and northern Qiangtang depression. This study proposes a new division of four lithological members for the Suowa Formation based on its stratigraphic sequence and assemblage. The member 1 to member 4 of the Suowa are carbonate platform, mixing accumulated gentle slope, mixing accumulated carbonate platform and mixing accumulated shallow water out site shift environments, respectively and reflect an complete transgressive to regressive sequences, The finding of biogenic reef and oil shale from the member 3 of the Suowa Formation is significant for reconstructing ancient environmental and petroleum geology studies; Also, the study establishes coral combination and bivalve combination according to the distribution and integrated characteristics of paleobiotologic strata, which means that the Suowa Formation is of upper Jurassic, to be exact, the Oxfordian to Kimmeridgian.