Upper Jurassic to Lower Cretaceous benthic foraminifera from South Tibet (Tethyan Himalayas): systematic, biostratigraphic, and palaeoecologic implications

The present study represents the first attempt to described foraminiferal fauna of the Gucuo Locality and is among the first work to illustrate the Upper Jurassic to Lower Cretaceous benthic foraminifers of South Tibet. The material consists in a combination of unpublished data on the Gucuo Section and published data on the Gyangze; Weimei, and Bandingsi sections.

A total of 239 specimens have been currently recorded in the Upper Jurassic to Lower Cretaceous Menkadun and Gucuo formations of the Gucuo locality. To facilitate further researches on foraminifers from the Tethyan Himalayas, a total of 59 species from 38 genera have currently been repertory and described. Among the recorded specimens, a few species are significant biostratigraphic markers used to identify and constrain the age of four foraminiferal assemblages. The Textularia aff. haeusleri assemblage contains a few numbers of agglutinated species very similar to the one described in the Oxfordian to Kimmeridgian Nupra Formation of the Thakkola Region. The Tithonian Trochammina quinqueloba assemblage is a widespread biostratigraphic horizon recorded in both southern and northern parts of South Tibet, as well as in the Indian and the Proto-Atlantic oceans. The Berriasian to Valanginian Trochammina abrupta assemblage contains typical Early Cretaceous foraminifers. It confirms the Jurassic/Cretaceous boundary should be delineated at the base of the shale Unit of the Gucuo Formation, although it was previously attributed to the base of the volcaniclastic sandstone units using ammonite data. This latest being currently associated to the Valanginian to Hauterivian Ammobaculites crepinae–Textularia bettenstaedti foraminiferal assemblage.

A palaeoecological reconstruction along a southwest to northeast transect of South Tibet, Himalayas, is proposed to understand the composition and dynamic of foraminiferal assemblages along with palaeoenvironmental changes. The morphogroup approach has been selected as the best methods to illustrate both sea-level and oxic changes. The northern tectonic zone of South Tibet is associated with the abundance of deep-water agglutinated foraminifers, which suggests a deep environment beyond the shelf break. The southern tectonic zone of South Tibet is rich in typical species of shallow environment and was located on the neritic shelf. The morphogroup analysis reveals localized, periodic disaerobic conditions on the sea-floor. The Jurassic to Cretaceous Boundary interval was associated with a drop of the sea-level, whereas the rise in diversity and abundance of marginal-sea taxa during the Valanginian to Hauterivian times underline a higher sea-level and a pulse in the development of epeiric seas in the southern part of South Tibet.