关键词: 侏罗—白垩纪界线, 桑秀组, 西藏南部

Abstract:

The boundary between Jurassic and Cretaceous is the most problematical of all Phanerozoic system boundaries, has long been debated and not yet been resolved on a global scale. The marine boundary strata outcrop in southern Tibet, with controversy on the boundary horizon. Moreover, it is an untidy problem that influences the development of an accurate stratigraphic classification for the Mesozoic and, as a consequence, affects regional geological mapping and recovery of plate tectonics. This work studies three sections from Gyangze and Nagarze areas in Tibet.The Linxi Section of Nagarze that contains the uppermost Jurassic to Cretaceous strata is a new section recently measured by the authors. Two other sections of Kadong in Nagarze and Jiabula in Gyangze also provide complete stratigraphic sequences from Upper Jurassic to Cretaceous. At those two sections the uppermost Jurassic to Cretaceous transition beds were re-measured and re-studied as major part of this project.The strata in Gyangze are divided into the Weimei and Jiabula formations. In Nagarze, a large group of sedimentary-volcanic rocks occur between the Weimei and Jiabula formations, which were named as the Sangxiu Formation. The Weimei Formation is dominated by light grey coarse-fine grain quartz sandstone. The Sangxiu Formation is exposed as dark shale in the lower part and volcanic rocks in the upper.The Jiabula Formation consists of a group of black shales with intercalations of sandstone and sandy limestone. A few fossils as Haplophylloceras sp. and Himalayites sp. have been found from the Weimei Formation. Some ammonite species of Spiticeras, Berriasella and Haplophylloceras, and rich bivalve Inoceramus fauna occur in the lower Sangxiu Formation in Nagarze, while the upper volcanic rock was dated as 133 Ma by isotopic data. More ammonites have been found from the lower part of Jiabula Formation in Gyangze, which have been recognized as Spiticeras-Berriasella and Himalayaites-Haplophylloceras assemblages in ascending order. By the correlation with the biostratigraphy in Nyalam of the southernmost Tibet, the age of Weimei Formation is Tithonian of uppermost Jurassic; the Jiabula Formation in Gyangze belongs to Lower Cretaceous The ammonites from the lower shale and isotope age from its upper volcanic rock define the age of Sangxiu Formation from Berriasian to Hauterivian of Lower Cretaceous. The Jurassic/Cretaceous boundary, therefore, is situated between the Weimei and Sangxiu formations in Nagarze and between the Weimei and Jiabula formations in Gyangze. The boundary is marked by the disappearance of Virgatosphinctes, Aulocosphinctes and appearance of Spiticeras. During the Jurassic and Cretaceous boundary time, a large regression happened in the Tibet-Tethys Sea, which is represented by the widespread sandstone sedimentation of uppermost Jurassic.

Key words: Jurassic/Cretaceous boundary, Sangxiu Formation, southern Tibet