Biostratigraphic Implications of the Calcareous Nannofossils from the Spiti Formation at Langza, Spiti Valley

Calcareous nannofossil biostratigraphy has been attempted for the first time on black shale of the Lower Member of the Spiti Formation exposed in Langza area, Spiti Valley, Himachal Pradesh. Ammonites and dinoflagellate cysts biostratigraphy usually assign the age of the Spiti Formation as Oxfordian to Tithonian. The present study provides an older, Callovian age (part of NJ12 Nannofossil Zone) to the black shale (‘Belemnites gerardi beds’) belonging to the belemnites-rich Lower Member of Spiti Formation based on biostratigraphic ranges of Ansulasphaera helvetica, Lotharingius hauffii, Watznaueria contracta and Watznaueria ovata. Reworking of nannofossil taxa has also been reported by the sporadic occurrences of Carinolithus magharensis (LAD in Bathonian) in a few samples. The rare occurrence of Faviconus multicolumnatus (FAD in Oxfordian) in the uppermost fossiliferous sample indicates the leaking of this taxa from the younger horizons.