Larger Foraminiferal Biostratigraphy and High-Resolution Strontium Isotope Stratigraphy of the Paleocene Limestones in the Guru Area, South Xizang (tibet)

Larger benthic foraminifera (LBF) are among the most important microfossil groups for Paleogene biostratigraphy and flourished in shallow-marine environments of the Neo-Tethyan Ocean during the Paleocene–Eocene. Based on systematic and stratigraphic studies of LBF in the Neo-Tethyan realm (mainly the Mediterranean region) during this interval, the shallow benthic zonation (SBZ) was established in 1998. Although this scheme has been widely accepted, it is not impeccable. For example, the time intervals represented by the Paleocene SBZ are relatively long, and further subdivision is still possible. LBF-bearing limestones are well exposed in south Tibet, which was geographically located at the easternmost part of the Neo-Tethyan ocean. To further refine and supplement the Tethyan SBZ, we conducted detailed biostratigraphic and strontium isotopic studies on the Paleocene LBF-bearing limestones at Guru, south Tibet. About 69 Paleocene LBF species belonging to 30 genera were identified, of which 19 species are reported for the first time from south Tibet. Following the Oppel zone principle and adopting some key index fossils generally accepted in the Neo-Tethys, four SBZ ranging from SBZ 2 to SBZ 5 were recognised. According to the compositional changes of LBF within 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 Paleocene limestones. The high-resolution Sr isotope stratigraphy provides reliable age constraints for each SBZ. Durations 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. The newly established LBF biostratigraphy in the Guru area subdivides the Paleocene SBZ for the first time, presenting a division scheme for the Tethyan LBF biostratigraphy from Tibet.