Late Jurassic–Early Cretaceous palynofloras in the Lhasa Block, central Xizang, China and their bearing on palaeoenvironments

Abstract

As a part of the Qinghai–Xizang Plateau, the Lhasa Block has experienced a very complicated geological history since it rifted from Gondwana. Little is known about how terrestrial ecosystems evolved on this block during this time. Our new materials collected from this block reveal for the first time the palaeovegetation and palaeoclimate during the Late Jurassic to Early Cretaceous. Three distinct palynological associations are recognized, including the Late Jurassic Cyathidites–Classopollis association, and the Early Cretaceous (Berriasian–Barremian) Dicheiropollis and Dicheiropollis-peak associations. An additional Classopollis–Cooksonites association recovered from Qubsang, which lacks Dicheiropollis, is coeval with the Dicheiropollis association. These associations are generally similar to those on the Qiangtang Block, southern Qinghai Province, which indicates that these blocks had merged before the Late Jurassic, facilitating establishment of a common palynoflora across the two blocks and adjacent regions. The palaeovegetation on the Lhasa Block in the Late Jurassic was characterized by abundant tree ferns, whereas during the earliest Cretaceous, cheirolepidiacean communities were more dominant; within these communities a new cheirolepidiacean plant that produced Dicheiropollis pollen emerged, implying a potential increase in temperature. The palaeoclimate was generally warm and semi-humid during the Late Jurassic and Early Cretaceous although fluctuations of temperature and humidity occurred. A reduction in precipitation from the Lhasa Block to the inland Qiangtang Block and Tarim Basin is reflected in the palynoflora. A brief history of the vegetation and climate of the Lhasa Block from Late Jurassic to Early Cretaceous is provided.