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

Abstract

Carbon and oxygen isotope values (δ¹³C and δ¹⁸O) of carbonate cement from the Siwalik sandstones were measured in the Surai Khola section (age: 13 Ma to 1 Ma) of western Nepal. The δ¹⁸O values of the cement show three evolutionary phases. From 12 Ma to ~6 Ma, the average δ¹⁸O (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 δ¹⁸O values show a sharp increase with less scatter, with maximum δ¹⁸O value of ~ - 7‰. The average δ¹⁸O value for this period is - 10.7 ± 1.6‰ (n = 25). From 4 to 2 Ma, δ¹⁸O values remain fairly uniform with an average value of - 8.8 ± 1.2‰ (n = 17). The increase in δ¹⁸O values of carbonate cement in the sandstone occurs concurrently with that of pedogenic carbonate measured earlier by Quade et al. (1995), indicating a major role of meteoric water in controlling the δ¹⁸O value of diagenetic carbonate cement. The δ¹⁸O 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 δ¹³C (VPDB) value of calcite cement does not show any definite trend. The δ¹³C values during 12 to 7 Ma range from - 3.3‰ to - 9.9 ‰ with an average of - 7.1 ± 1.5‰ (n = 91). Higher δ¹³C 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 C4 plants, which have higher ¹³C / ¹²C, compared to C₃ plants. The large spread in δ¹³C values of the cement probably indicates production of CO₂ 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.