拉萨地区林周盆地设兴组上部(古近系)年代研究的新进展及意义

拉萨地区林周盆地典中那玛剖面设兴组上部新发现的孢粉组合以落叶、阔叶植物为主体,主要为桦科的Alnipollenites,Betulaepollenites,Carpiniptes,山毛榉科的Quercoidites,胡桃科的Juglanspollenites,榆科的Ulmipollenites,椴科的Tiliapollenites等,孢粉化石多为古近纪常见分子,未发现白垩纪的特征分子。古近纪早期大量出现的三孔沟,网面三孔沟等花粉少量出现;孢粉组合更接近于古近纪中晚期的面貌,其时代可能属于晚始新世。因此,设兴组上部的年代可能是始新世晚期。由此推论,设兴组和林子宗群之间的角度不整合不能代表白垩纪古近纪之间的构造运动,而是代表始新世晚期后的构造运动。同时,林周盆地可能沉积有古近纪的河湖相地层。

ADVANCE OF GEOCHRONOLOGICAL DATING OF SHEXING FORMATION IN THE LHUNZHUB BASIN, LHASA AREA AND ITS SIGNIFICANCE

New materials of sporopollen from the upper part of Shexing Formation of the Dzhong-Nama section in the Lhunzhub Basin of Lhasa block are reported in this paper. Characteristics of the sporopollen assemblage are:Most of the found sporopollen are common elements of Paleogene, so the related strata are impossible to be Cretaceous sediments; The assumed deciduous and latifoliate vegetation including Alnipollenites, Betulaepollenites, Carpiniptes, Quercoidites, Juglanspollenites, Ulmipollenites, Tiliaepollenites et al., are dominant in this assemblage. It indicates that the climate was not droughty when they deposited; Triporopollenites and Tricolporopllenites which are dominant in Early Paleogene, occurred in little amount in the present assemblage. The sporopollen assemblage is closer to those of the middle and late Paleogene, and its age may be Late Eocene. The Shexing Formation contains not only the upper part of the Upper Cretaceous, but also part of Paleogene. The latest age of the Shexing Formation may be Late Eocene. Based on the aboved conclusion, the unconformity between the Shexing Formation and Linzizong Group may represent the post-Eocene tectonic movement. The unconformity does not represent tectonic movement at the Cretaceous- Paleogene boundary. At the same time, Paleogene fluvial-lacustrine strata might have been deposited in the Lhunzhub Basin.