Calcareous Nannofossils Revealing the Highest Marine Horizon of the Cenomanian Turonian Transition in Lhasa Terrane (xizang)

It has been disputed when seawater withdrew from the Lhasa Terrane or the Bangong-Nujiang suture zone, while Oceanic Anoxic Event 2 (OAE 2) has not been determined in the same terrane. This is because the Langshan Formation has been mainly assigned in the Aptian through the early Cenomanian by fossils of orbitoidal foraminiferas and rudist bivalves. Though marine mid-Cretaceous strata are widely distributed and well exposed in the Coqen Basin of the northern Lhasa Terrane, Xizang (Tibet), the top of the Langshan Formation has been covered by the Quaternary or nothing, leading to an uncertainty whether younger marine strata had deposited. To detect the youngest marine deposit, recent investigations in the Luoma Changbo (LMC) section of the Coqen Basin (west of Dong Co) were conducted and calcareous nannofossils were analyzed. Here we present new calcareous nannofossil biostratigraphic data from the top of the Langshan Formation. Results show that the nannofossil assemblages are relatively abundant and moderately preserved, allowing for a refined biostratigraphic subdivision. The first occurrence (FO) of the zonal marker species Quadrum intermedium and the last occurrence (LO) of Helenea chiastia, both defining subzone UC5c, provide a key constraint on the Cenomanian–Turonian transition in the studied section, implying the latest marine deposition could have not ceased until 93.9 Ma (the Cenomanian–Turonian boundary age). Our work reveals the youngest age of the top Langshan Formation and improves the correlation in the Neo-Tethys.