In search of the origin of cement in siliciclastic sandstones: An isotopic approach

Abstract

In siliciclastic sediments only a minor amount of cement is internally derived. This implies that an external source has to be sought in sandstone diagenesis. Controversy exists about the external source of cement and the nature of the aqueous solution, viz. meteoric, hydrothermal and marine, and/or dewatering of shale. Isotope geochemistry may prove useful in solving this controversy. The oxygen isotopic compositions of neoformed clay minerals are related to the isotopic composition of the coexisting aqueous solution. Aqueous solutions of diverse origin such as meteoric, hydrothermal and marine, and dewatering of shale, in general, bear characteristic isotopic signatures. Thus, the δ 18O-values of authigenic clay minerals may be used to identify the nature of the original aqueous solution, provided the temperature of the environment is known and the extent and the nature of isotopic exchange between the minerals and fluids following crystallization can be evaluated. Oxygen isotopic compositions of early authigenic clay minerals in Permo-Triassic Gondwana sandstones of Peninsular India have been used to infer the cement source. Sandstones of Sakmarian age have an average δ 8O-value of +5.0%o, whereas a relatively high average δ 8O-value of +13.2%o is observed in sandstones of Rhaetic age. δ 18O-values of sandstones ranging in age between Sakmarian and Rhaetic have values between +5.0 and +13.2% 0. This gradual increase of δ 180-values with decreasing age shows a strong correlation with the changing latitudinal location of the sample site from a position around 60°S during Sakmarian time to a position around 38°S during Rhaetic time. The changing pattern of oxygen isotopic composition of suthigenic clay has been interpreted as a result of corresponding change in isotopic composition of meteoric water due to the northerly drift of the Indian plate during Gondwana sedimentation. This suggests that the aqueous solution, involved in the early diagenesis of Gondwana sandstones, is of meteoric origin.