Foraminiferal stratigraphy and paleoenvironments of Late Jurassic to Early Cretaceous deposits in Thakkhola, Nepal

This is the first detailed account of Late Jurassic, deeper marine, southern hemisphere, agglutinated benthic faunas, and their close taxonomic affinity to coeval assemblages from northern high latitudes. Over 1100m of dark shale, siltstone and sandstone in Thakkhola (Nepal) are dated by means of ammonites, dinoflagellate cysts and foraminifera as Oxfordian through latest Albian. The succession was deposited along the northern Gondwana margin, bordering Tethys, while Thakkhola lay at mid-latitudes (30-41°S). A highly diversified agglutinated faunal record in the Oxfordian lower Nupra Formation, with 44 taxa, is segmented (in ascending order) into the Eomarssonella paraconica assemblage, Verneuilinoides graciosus assemblage and Spiroplectammina suprajurassica assemblage. The assemblages are of restricted deeper shelf nature, in agreement with geochemical, sedimentary and ammonite-based depositional interpretation. The Tithonian upper Nupra Formation is attributed to a prodelta depositional setting, and contains a low-diversity, mainly agglutinated assemblage named after Trochammina annae. The Early Cretaceous Tangbe Formation shows a change to coarser terrigenous clastics deposited under shallow shelf conditions characterized by the Trochammina aff. schaimica assemblage of agglutinated taxa with extremely low diversity. The Late Jurassic assemblages of Nepal have a majority of taxa in common with those of Western Siberia, Svalbard and the Canadian Arctic Archipelago, which suggests that these "boreal faunas" contain many cosmopolitan taxa, suitable for global stratigraphic correlations.