Chemical diagenesis of Siwalik sandstone: Isotopic and mineralogical proxies from Surai Khola section, Nepal

Abstract

Carbon and oxygen isotope values ( δ 13C and δ 18O) of carbonate cement from the Siwalik sandstones were measured in the Surai Khola section (age: 13 Ma to 1 Ma) of western Nepal. The δ 18O values of the cement show three evolutionary phases. From 12 Ma to ∼6 Ma, the average δ 18O (VPDB) values are − 13.6 ± 1.9‰ ( n = 114) with a large spread from − 10‰ to − 18‰. This large spread probably indicates dissolution and re-precipitation of carbonate at various stages during burial. From 6 to 4 Ma, the δ 18O values show a sharp increase with less scatter, with maximum δ 18O value of ∼ − 7‰. The average δ 18O value for this period is − 10.7 ± 1.6‰ ( n = 25). From 4 to 2 Ma, δ 18O values remain fairly uniform with an average value of − 8.8 ± 1.2‰ ( n = 17). The increase in δ 18O values of carbonate cement in the sandstone occurs concurrently with that of pedogenic carbonate measured earlier by Quade et al. (1995) [Quade, J., Cater, M.L.J., Ojha, P.T., Adam, J., Harrison, M.T., 1995. Late Miocene environmental change in Nepal and the northern Indian subcontinent: stable isotopic evidence from paleosols: GSA Bulletin. 107 (12), 1381-1397.], indicating a major role of meteoric water in controlling the δ 18O value of diagenetic carbonate cement. The δ 18O value of carbonate cement of sandstones, however, is lower than that of pedogenic carbonate in the 6 to 4 Ma time range due to precipitation of sandstone cement at a temperature higher than that of pedogenic carbonate. The δ 13C (VPDB) value of calcite cement does not show any definite trend. The δ 13C values during 12 to 7 Ma range from − 3.3‰ to − 9.9 ‰ with an average of − 7.1 ± 1.5‰ ( n = 91). Higher δ 13C values are more common in samples younger than 7 Ma; they vary from − 2.8‰ to − 9.2‰ with an average of − 5.7 ± 1.5‰ ( n = 65) during 7 to 2 Ma. The increase in the post-7 Ma period is attributable to appearance of C 4 plants, which have higher 13C / 12C, compared to C 3 plants. The large spread in δ 13C values of the cement probably indicates production of CO 2 at various stages of diagenesis of the organic matter at different depths. Mineralogical composition also shows that diagenesis increases with depth. Clay minerals (< 2μ) separated from sandstone include smectite, illite, chlorite and kaolinite. Relative increase in the abundance of illite and decrease in the abundance of smectite indicate illitization of smectite with increase in burial depth. Presence of corroded features in K-feldspar suggests that dissolution of this mineral supplied potassium for illite formation.