Himalayan Geology, Vol. 24(2), 2003, pp. 1-18, Printed in India
Sedimentology, Palaeobiology and Stable Isotope Chemostratigraphy of the Terminal Neoproterozoic Buxa Dolomite, Arunachal Pradesh, NE Lesser Himalaya
VINOD C. TEWARI Wadia Institute of Himalayan Geology, Dehra Dun - 248 001, India
The Terminal Neoproterozoic carbonates are represented as the Chillipam Dolomite, the Rupa Dolomite or the Dedza Dolomite in the Kameng valley and the Menga Limestone in the Subansiri valley of the Arunachal Lesser Himalaya. The Chillipam Dolomite (= Rupa Dolomite/Dedza Dolomite) of the Kameng valley and the Menga Limestone of the Subansiri valley in Arunachal Pradesh are eastward extension of the Buxa Dolomite of Bhutan. The term Buxa Dolomite has been used for these carbonates. A thick succession of pebbly quartzite – diamictite is exposed at the base of the Terminal Proterozoic Chillipam Dolomite. Eight major sedimentary facies variation have been recorded (1) Sandy tidal flat – beach complex (2) Pebbly Quartzite – Diamictite (glaciomarine) (3) Fenestral bird’s eye dolomite cherty ministromatolitic – oncolitic (4) Black shale (5) Columnar stratified stromatolitic (6) Brecciated cherty (7) Calcareous quartzite and (8) Domal stromatolitic. Primary sedimentary structures and stratified domal and columnar stromatolitic buildups suggest a shallow marine (tidal flat) depositional environment for the Buxa (Chillipam) Dolomite.
The discovery of Terminal Neoproterozoic ministromatolites, digitate and microstromatolites, cyanobacteria, micrometazoans (sponge spicule like structures) from the Chillipam Dolomite and vase shaped microfossils (Melanocyrillium or testate amoebae) is palaeobiologically significant. The occurrence of stromatolite assemblage Stratifera – Colleniella – Aldania – Collumnaefacta – Linella from the Chillipam Dolomite is also important since this assemblage is common in Terminal Proterozoic – Lower Cambrian sequences. The palaeobiological remains strongly suggest that the Precambrian – Cambrian transition lies in the Chillipam Dolomite/Menga Limestone of the Arunachal Lesser Himalaya. The carbon isotope chemostratigraphy of the Menga Limestone has revealed significantly positive and consistent isotopic ratios. The δ13C (carbonate carbon) values range from +3.7 to +5.45‰ (PDB). The oxygen isotopic data also shows remarkable consistency with the δ18O values fluctuating within a narrow range between -8.9 and -7.2‰ (PDB). The significantly positive C-isotopic results correspond to the Terminal Proterozoic C-isotopic evolution, substantiating the findings indicated by the palaeobiological remains. The biomolecules (amino acids) have been recorded from a sample of the Menga Limestone applying Laser Raman spectroscopic technique.
