The lower Paleogene shallow-water limestones in the Tethyan Himalaya of Tibet and their implications for larger foraminiferal evolution, India-Asia collision and PETM-CIE

Abstract

Fossiliferous limestones in shallow marine environments are important archives for the studies of paleontology, biostratigraphy, paleoenvironment and paleoclimatology as well as geodynamic evolution of their sedimentary basins. In the eastern Tethyan Himalaya, two areas, Tingri and Gamba, expose the best outcrops of the lower Paleogene larger foraminiferal limestones in Tibet. Based on studies of the limestones, ~70 species and ~20 genera of the Paleocene-Early Eocene larger foraminifera have been identified. Following the Oppel Zone s principle, ten Shallow Benthic Zones (SBZs) ranging from SBZ 1 to 10 have been divided at Tingri, and five SBZs comprising SBZ 2, 3, 4, 5, and 7 are recognized at Gamba. A high-resolution Carbon Isotope Excursion (CIE) curve and the SBZ from the same section at Tingri reveal that the Paleocene-Eocene (P-E) boundary in the shallow marine environment is located in the upper part of SBZ 5, where no evident biotic turnover of benthic foraminiferal communities happened. Notably, a transient but distinct Larger Foraminiferal Extinction and Origination (LFEO) event, marking the boundary between SBZ 5 and 6 in Tibet, occurred at the onset of the CIE recovery. Furthermore, based on studies of the larger foraminifera together with other published data in the Tethyan Himalaya, we propose the initial India-Asia continental collision in Tibet took place at the P-E boundary. Interestingly, the high-resolution CIE curve at Tingri shows similar patterns to the most representative one (from ODP 690) from the deep ocean, however, the magnitude is evidently larger in the shallow marine environment than in the deep sea. It reveals that the CIE in the entire ocean had followed certain regular steps to reach the lowest carbon isotope values during the PETM, and the magnitude of the negative CIE may increase from the deep seas to shallow marine environments. However, the mechanisms causing these phenomena are still enigmatic.