New age constraints for the upper zongshan Formation: a morphometric study of the late cretaceous orbitoidal foraminifera in Gamba area of southern Xizang (Tibet)

This study refines the biostratigraphic framework of the upper Zongshan Formation in southern Xizang (Tibet) by conducting a meticulous morphometric analysis of larger benthic foraminifera (LBF) from the Gamba area. Focusing on the genera Orbitoides and Omphalocyclus, we employed a comprehensive morphometric approach to delineate the stratigraphic transition within the formation. A total of 42 samples were collected across pivotal lithological intervals, leading to the identification of species groups that provide precise age constraints for the upper Zongshan Formation. Specifically, the O. tissoti–O. media group and Om. anatoliensis within Limestone II to Calcareous Marl II support a late Campanian age, while the presence of the O. megaloformis–O. gruenbachensis group and potential Om. cideensis in Limestone III suggest a younger age bracket extending into the early Maastrichtian. Our findings not only clarify the stratigraphic position of the upper Zongshan Formation but also contribute significantly to the ongoing investigations of final evolutionary stages of the eastern Neotethys.