Diagenesis of dioctahedral and trioctahedral smectites from alternating beds in Miocene to Pleistocene rocks of the Niigata basin, Japan

Clay mineral diagenesis in the Niigata basin is documented by mineralogical and chemical analysis of clay minerals from cuttings from the Shinkumoide SK-1D (SSK-1D) well which is characterized by alternating beds containing d ioctahedral and trioctahedral smectite minerals with increasing depth. Dioc tahedral smectite shows a progressive increase in illite interstratificatio n with increasing depth. The transition of dioctahedral smectite to interst ratified illite-smectite (I-S) is supported chemically by an increase in K and Al and a decrease in Si with increasing depth. In contrast, trioctahedr al smectite (saponite) reacts to form a 1:1 interstratified chlorite-smecti te (C-S) with increasing burial depth and temperature. Considering the geol ogy and the occurrence of smectite, the SSK-1D smectites probably altered d iagenetically from two different parent materials: dioctahedral smectite is derived from elastic sediments and transforms to interstratified illite-sm ectite, whereas trioctahedral smectite is derived from andesitic pyroclasti c rocks and transforms to interstratified chlorite-smectite. The C-S occurs at the same depth of similar to 3200 m as the conversion of randomly interstratified (R = 0) I-S to (R = 1) I-S. Furthermore, the depth is compatible with a T-max temperature of 430-435 degreesC, which indicate s the starting temperature for oil generation from organic matter. The temp erature of the conversion of (R = 0) I-S to (R = 1) I-S and the start of co rrensite formation is estimated at 110-120 degreesC based on the time-tempe rature model suggested by others. The clay-mineral diagenesis in the SSK-1D further suggests that I-S and C-S can act as geothermometers in elastic an d pyroclastic sediments provided that the effect of time is considered.