Late Triassic‐Jurassic bivalves from volcanic sediments of the Lhasa block, Tibet

Abstract

Mesozoic volcanic sediments known as the Yeba Formation are widespread in the northeastern region of Lhasa. Recent fossil collections in the formation allow the establishment of a biostratigraphic framework for the volcanic sediments; bivalves are assigned to 21 genera and 22 species including a new genus and 5 new species: Lhasanella lhasaensis (gen. et sp. nov.), Trigonodus xiabolangensis (sp. nov.), Isocyprina lhasaensis (sp. nov.), Gervillaria tibetica (sp. nov.), and Jurassicorbula fuersichi (sp. nov.). Three benthic assemblages have been recognised: the Trigonodus‐Isocyprina assemblage (Rhaetian‐Hettangian), the Lhasanella‐Propeamussium assemblage (Toarcian‐Bajocian), and the Jurassicorbula‐Neomiodon assemblage (Middle Jurassic). The Yeba Formation ranges in age from latest Triassic to Middle Jurassic. Volcanism in the Lhasa block occurred frequently during this period, corresponding to the fragmentation episode in eastern Gondwanaland and to the northwards drift of the Lhasa block. A strong faunal affinity between the Lhasa block and southern Qinghai, Japan, and Vietnam suggests that the Lhasa block had been located within the northern marginal region of the East Tethys since Toarcian times. The existence of a deep ocean which might serve as a barrier to fauna dispersal between the Lhasa and Qingtang blocks remains questionable.