Larger benthic foraminifera and strontium isotope stratigraphy of the Palaeocene limestones in the Guru area, south Xizang (Tibet): an attempt to build a high-resolution biozonation for the eastern Neo-Tethys

Abstract

Larger benthic foraminifera (LBF) are one of the most important microfossil groups for Palaeogene biostratigraphy, particularly for shallow-marine carbonate deposits. During the Palaeocene, LBF thrived throughout the Neo-Tethyan Ocean, especially in the eastern Neo-Tethys. However, previous studies on Palaeocene LBF biostratigraphy in this area are relatively poor. This paper presents a detailed stratigraphic study of LBF and strontium isotope from the Palaeocene limestones at Guru, south Tibet. About 68 species of 29 genera were identified, among which 19 species are reported for the first time from south Tibet. Following an Oppel zone approach and adopting some key index fossils that are widely accepted in the Neo-Tethys, four Shallow Benthic Zones (SBZ) ranging from SBZ 2 to SBZ 5 were recognised. Based on detailed LBF changes in each SBZ, we tentatively divide SBZ 2 (uppermost Danian–upper Selandian), SBZ 3 (upper Selandian–middle Thanetian), and SBZ 5 (upper Thanetian) into 3, 3, and 2 sub-biozones, respectively. The ⁸⁷Sr/⁸⁶Sr ratios from 42 bulk carbonate samples were measured to construct an independent age framework for the Palaeocene limestones. Integration of LBF and ⁸⁷Sr/⁸⁶Sr data yielded a high-resolution stratigraphy and provided absolute age controls for each SBZ. Ages of SBZ 2, SBZ 3, SBZ 4, and SBZ 5 were estimated to be ∼62–60 Ma, ∼60–57.4 Ma, ∼57.4–57 Ma, and ∼57–56 Ma, respectively. Our newly built Palaeocene LBF biostratigraphy at Guru needs further testing from other areas in the Neo-Tethyan realm, and is open for refinement and correction in the future.