Shallow-water carbonate responses to the Paleocene–Eocene thermal maximum in the Tethyan Himalaya (southern Tibet): Tectonic and climatic implications

Abstract

This study presents a detailed stratigraphic record of the Paleocene–Eocene Thermal Maximum (PETM) in the Gamba area of the Tethyan Himalaya, a carbonate-platform succession originally deposited along the southern margin of the eastern Tethys Ocean. The Paleocene-Eocene boundary interval is marked by a negative carbon isotope excursion at the boundary between members 3 and 4 of the Zongpu Formation. The succession is erosionally truncated at this surface, which is overlain by an intraformational carbonate conglomerate, and only the upper part of the PETM interval is preserved. Foraminiferal assemblages of Shallow Benthic Zone 4 are present below the conglomerate bed, but are replaced by assemblages of Shallow Benthic Zone 6 above the conglomerate. Depositional facies also change across this surface; below the disconformity, floatstones and packstones containing nummulitid forams record progressive transgression in an open-marine environment, whereas restricted or lagoonal inner-ramp deposits containing Alveolina and Orbitolites are typical above the disconformity. The prominent negative excursion observed in the δ13C of whole-rock carbonate (−1.0‰ at Zongpu, −2.4‰ at Zengbudong) and organic matter (−24.7‰, at Zengbudong) is correlated to the characteristic PETM carbon isotope excursion. This major negative excursion in shallow-marine carbonates may have partly resulted from syndepositional alteration of organic matter. The erosional unconformity can be constrained to the lower PETM interval (between 56 and 55.5Ma), and is identifiable throughout the Tethyan Himalaya. This widespread disconformity is attributable to tectonic uplift associated with the southward migration of an orogenic wave, originated 3±1Ma earlier in the middle Paleocene at the first site of India-Asia continent-continent collision. A possible eustatic component of the pre-PETM sea-level fall, which resulted in the excavation of incised valleys filled during the subsequent sea-level rise when the conglomerate bed was deposited, remains to be assessed.