Paleogene carbonate microfacies and sandstone provenance (Gamba area, South Tibet): Stratigraphic response to initial India-Asia continental collision

The Paleogene marine strata in the Gamba area of south Tibet comprise carbonates of the Zongpu Formation and siliciclastic rocks of the Enba and Zhaguo Formations, documenting the final stages of marine deposition in the Tethyan Himalaya. The ~350-m-thick Zongpu Formation was dated as late Danian to Ypresian based on larger benthic foraminifers. Thirteen distinct microfacies identify three sedimentary environments. Mudstone, wackestone with Udoteacean algae, bioclastic-peloidal packstone, packstone with Rotaliids and green algae, floatstone with Alveolina and Orbitolites were deposited in restricted lagoonal environments. Bioclastic packstone and grainstone with Rotaliids were deposited in high-energy shoal environments. Floatstones with Nummulitids or Alveolinids were deposited in shallow open-marine environments. The Zongpu Formation was accumulated on a carbonate ramp. It documents two deepening-upward sequences separated by an unconformity corresponding to the Palaeocene/Eocene boundary and marked by a conglomerate with limestone clasts. The overlying Enba Formation comprises greenish grey calcareous shales intercalated with litho-quartzose sandstones in the upper part and capped by subaerial litho-quartzose red beds of the Zhaguo Formation. Petrographic analysis, detrital zircon geochronology and Hf isotopic data indicate that detritus in the Enba and Zhaguo Formations, deposited on the Indian passive margin, was derived from the Asian active margin in the north. These clastic units were thus deposited after the onset of the India-Asia continental collision in the early Himalayan foreland basin. Major lithological and paleoenvironmental changes occurred at three stratigraphic levels: the Jidula/Zongpu boundary (~62. Ma), the Paleocene/Eocene boundary (~56. Ma) and the Zongpu/Enba boundary (~51. Ma). Our provenance study confirms that the India-Asia collision was already under way during the deposition of the Enba Member (i.e., by ~51. Ma) and, along with facies analysis and general palaeogeographic considerations, indicates that Neo-Tethys was still wide open during the Early-Middle Paleocene. It is thus argued, consistently with previous studies, that the Paleocene/Eocene disconformity documented in the Gamba area as in the northwestern Tethyan Himalaya is likely to record flexural uplift consequent to initial underthrusting of the Indian continental margin beneath Asia at, or just a little earlier than, 56. Ma.