Newly discovered Early to Middle Triassic Conodont faunas in the Shiquanhe area, western Lhasa Block, Xizang (Tibet) and their biostratigraphic and palaeobiogeographic implications

Abstract

Conodonts have taken on a significant role in elucidating the sedimentary setting as well as geological history of the Lhasa Block (LSB) of Xizang (Tibet). Conodont biostratigraphic research indicates that the LSB was in carbonate platform setting rather than an emergent land mass during Triassic. Lower Triassic sedimentary successions of the LSB are classified as limestone-dominated and dolomite-dominated types. A relatively complete Early to Late Triassic conodont succession has been established for the limestone-dominated sections. In contrast, only a rough Early Triassic Hadrodontina-Pachycladina fauna has been described from dolomite-dominated sections. Our investigations of the dolomite-dominated type succession of the Shiquanhe area of the western LSB document for the first time three Early Triassic and two Middle Triassic conodont faunas. They are, in ascending order, the Late Griesbachian to Early Smithian Hadrodontina (revised from previously described Hadrodontina-Pachycladina) fauna, the Smithian Pachycladina shiquania n. sp. fauna, the Spathian Triassospathodus homeri-Triassospathodus triangularis Assemblage Zone, the Early Ladinian Neogondolella transita-Neogondolella excelsa-Neogondolella cornuta fauna, and the Late Ladinian Budurovignathus cf. diebeli-Pseudofurnishius murcianus fauna. The recovered Early to Middle Triassic conodont faunas permit more robust redefinition of the dolomite-dominated Triassic strata. Correlations of the conodont succession of the Shiquanhe area with those in other areas confirm that ellisoniids-bearing conodont successions existed preferentially along dolomite-dominated strata of low paleo-latitude. They also confirm that the Shiquanhe area had a closer relationship with Europe than with the western USA in palaeobiogeographic distribution in yielding the Lower Triassic species Pachycladina fauna and Ladinian species Neogondolella excelsa and a more diverse Budurovignathus species. Moreover, well-preserved platform-like elements of a newly described species Pachycladina shiquania n. sp. provide clarification of the true P₁ and P₂ elements of the Pachycladina apparatus.