Takahisa Yoshimura

Smectite diagenesis in Neogene marine sandstone and mudstone of the Niigata basin, Japan

Illitization of smectite during progressive burial diagenesis occurs differently in sandstone and mudstone, which are interbedded in the sedimentary sequence of the Niigata basin. Reaction progress of illitization of smectite via mixed-layer illite-smectite (I-S) in the mudstone is more complete than in the sandstone. In sandstone, smectite converts to (Reichweit, R ≥ 3) I-S and illite via random (R = 0) I-S to ordered (R = 1) I-S, and authigenic chlorite and quartz form as products of the illitization of smectite. The original composition of detrital smectite and the occurrence of Na+, K+, Ca2+, Mg2+, and Fe2+ in pore fluids partly control both illitization of smectite and the resulting authigenic mineral products in the diagenetic process. In mudstone, detrital smectite is K- and Si-rich in composition, and the illitization of smectite indicates that the original composition is mostly inherited. Excess silica owing to illitization is released to produce authigenic quartz. In sandstone, smectite forms primarily by precipitation. The evolving compositions produced by early illitization form smectite, then random (R = 0) I-S, and then ordered (R = 1) I-S. These transitions are related to compositions of pore fluid. Changes in K/(K + Ca + Na) vs. K + Ca + Na imply that the increase of interlayer cations occurs by absorption and smectite transforms to random (R = 0) I-S, followed by the exchange of interlayer cations to ordered (R = 1) I-S with increasing diagenetic grade. Late illitization from (R = 1) I-S to R ≥ 3 suggests decomposition of smectite and early I-S with an increase in the number of illite layers. Dissolution experiments of host rocks with pure water and 0.01 M HCl solution reflect the differences in chemistry of the original pore fluid and authigenic carbonate in the process of diagenesis of clastic rocks. These results explain how chemical composition produces large variations in transformation temperature of smectite to illite in the diagenetic process.

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 dioctahedral and trioctahedral smectite minerals with increasing depth. Dioctahedral smectite shows a progressive increase in illite interstratification with increasing depth. The transition of dioctahedral smectite to interstratified illite-smectite (I-S) is supported chemically by an increase in K and Al and a decrease in Si with increasing depth. In contrast, trioctahedral smectite (saponite) reacts to form a 1:1 interstratified chlorite-smectite (C-S) with increasing burial depth and temperature. Considering the geology and the occurrence of smectite, the SSK-1D smectites probably altered diagenetically from two different parent materials: dioctahedral smectite is derived from clastic sediments and transforms to interstratified illite-smectite, whereas trioctahedral smectite is derived from andesitic pyroclastic rocks and transforms to interstratified chlorite-smectite.The C-S occurs at the same depth of ~3200 m as the conversion of randomly interstratified (R = 0) I-S to (R = 1) I-S. Furthermore, the depth is compatible with a Tmax temperature of 430-435°C, which indicates the starting temperature for oil generation from organic matter. The temperature of the conversion of (R = 0) I-S to (R = 1) I-S and the start of corrensite formation is estimated at 110-120°C based on the time-temperature 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 clastic and pyroclastic sediments provided that the effect of time is considered.

Transformation of montmorillonite into halloysite found from Hanetsu, Niigata Prefecture

At Nagamine-hara near Shibata hydrothermally altered pyroclastic rocks of the Miocene are overlain by terrace deposits composed of red soil and strongly weathered gravel. White clay deposits are found underneath the unconformity plane. They are weathering products of the altered pyroclastic rocks. Various stages of transition from unweathered pyroclastic rocks to white clays are observed at outcrops A and B (Fig. 2-a and b). X-ray diffraction data (Fig. 3) of unweathered rocks (specimen B-4) show that a clay mineral contained in them is montmorillonite, though it is “abnormal” type which gives double endothermic peaks in the range of 500-700°C (Fig. 9). Whereas white clays are composed of halloysite (E-3) or mixed-layer halloysitemontmorillonite (A-1). The products in the intermediate stages of weathering are mixed-layer halloysitemontmorillonite in whhch halloysite layer increases in process of weathering. At the intial stage of weathering clays are composed of heterogeneous phases (B-3). Chemical compositions by electron microscopic analysis (Table 1, Fig. 10, 11) are compatible with X-ray and thermal data. The observation by transmission electron microscope (Fig. 12) indicates that thinning of montmorillonite particles and curling of their edges occur inthe earlier stage of weathering and that formation of tube-like form and mosaicking such as cell division appear in the later one.