索瓦组上段是羌塘盆地演化晚期的一套潮坪—潟湖相沉积,分布于北羌塘盆地中部.通过对索瓦组上段区域地层对比,发现索瓦组上段在盆地东部、西部和南部的岩性变化较大,海相油页岩的出露地仅分布于北羌塘坳陷南部胜利河—长蛇山—托纳木一线的局限海湾—潟湖的边缘带,其形成明显受到了古地理条件的控制.根据索瓦组上段以及其下伏地层产出的生物化石组合,并结合油页岩的Re-Os同位素年龄,认为索瓦组上段的时代可能为早白垩世.对该组产出的孢粉化石组合和硅化木分析表明,索瓦组上段在沉积早期和晚期的古气候条件发生了明显的变化,早期为温暖和湿润气候,而晚期为半干旱—干旱气候,这种垂向上的古气候变化特征不仅仅局限于胜利河地区,可能在整个北羌塘盆地都普遍存在.索瓦组上段发育的油页岩及其上部膏岩沉积组合的发现,对于评价羌塘盆地演化晚期的石油地质条件具有重要的意义.%The upper part of the Suowa Formation in the central district of the North Qiangtang depression is the sediment during the late-stage evolution of the Qiangtang basin, which deposits in tidal flat-lagoonal environment. By strata correlation of the upper part of the Suowa Formation, it is found that its lithology is different in the eastern, western and southern areas, and the places where the oil shale exposed only aligned around the epi-continental restricted gulf-lagoon areas (e. G. Shengli River area, Changshe Mountain area and Tuolamu area) in the southern margins of the North Qiangtang basin. The deposition of the oil shale is obviously controlled by differential palaeotopography. Based on the fossil assemblages in the upper part of the Suowa Formation and its underlying strata, and combined with the Re-Os dating of the Shengli River oil shale, the geological age of the upper part of the Suowa Formation is suggested to be Early Cretaceous. The Cretaceous spores and pollen assemblages and silicified wood reveal that the climate during the early period of the upper part of the Suowa Formation was warm and humid, while the climate during the later period was hot and arid. Furthermore, the vertical changed climate may be not restricted to the Shengli River region, but the whole North Qiangtang basin. The discovery of the oil shale-gypsum sedimentary associations is of important implications for our knowledge of the hydrocarbon accumulation and preservation of the Qiangtang basin during the late stage evolution.
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