Clay Science 6, 149-157 (1985)

CLAY MINERALOGY OF SOME CENOZOIC SEDIMENTS AND SEDIMENTARY ROCKS IN KAKEGAWA-OMAEZAKI REGION, SHIZUOKA PREFECTURE, JAPAN

A. N. LIYANAGE Institute of Geosciences, Faculty of Science, Shizuoka University, Shizuoka 422, Japan

(Received May 23, 1984)

ABSTRACT

About 40 samples of argillaceous sediments and sedimentary rocks collected from some formations of Pleistocene to Oligocene age in the Kakegawa-Omaezaki region, Shizuoka Prefecture, were mineralogically examined. On the basis of X-ray diffractograms obtained from the < 2ƒÊm clay fraction of the samples, major clay minerals were identified and their relative abundances were estimated. Smectite is the most dominant clay mineral in some formations, while chlorite in others. Illite occurs in significant amounts in every sample and kaolinite is always less than 10% or absent. Irregularly mixed-layered illite/smectite and chlorite/vermiculite phases were observed only in highly consolidated mudstone. The low abundance of kaolinite and high abundance of smectite and chlorite are in harmony with the fact that these sediments are marine. Estimated clay mineral compositions show no systematic variation from Pleistocene Furuya Formation to Middle Miocene Saigo Group. However, there is a gradual variation between abundance of mixed-layer phases and that of discrete smectite in the sedimentary rocks older than Late Middle Miocene; mixed-layer phases increase from Kurami Group to Oligocene Setogawa Group with the corresponding de-

crease in discrete smectite. This variation can be ascribed to the burial diagenesis.

INTRODUCTION

The investigated area, Kakegawa-Omaezaki region in the Shizuoka Prefecture, is situated in the Pacific Ocean side of the southwest Japan. It is a sedimentar terrain with rugged topography and comprises a thick sequence of sediments and sedimentary rocks ranging in age from Eocene to Recent. In the northern and northwestern parts of the area Quaternary sediments conformably cover the Neogene rocks, and they become thin towards south unconformably overlying the older sediments (Tsuchi, 1961). In this study, surface or near-surface samples were collected from argillaceous sediments and sedimentary rocks in this area (locations are shown in Fig. 1) and those samples were analysed in order to identify and to estimate the relative abundances of major clay minerals in them. Attempts had also been made to estimate the origin and the post-depositional changes of the formations from which the samples were taken.

*Present address: Department of Earth Sciences , Nagoya Univ., Nagoya 464, Japan. 150 A. N. Liyanage

FIG. 1. Simplified geological map of the Kakegawa-Omaezaki area showing the locations from which samples were collected (after Tsuchi, 1977 and Geological Survey of Japan, 1961). 1-Late Pleistocene gravel. 2- Furuya Formation. 3—Soga Group. 4-Kakegawa Group. 5-Sagara Group. 6-Saigo Group. 7-Kurami Group. 8-Oigawa Group. 9-Seto- gawa Group. 10-Mikura Group.

METHODS

Analyses were performed by separating the < 2ƒÊm clay fraction of each sample by allowing larger particles to settle out of a suspension and then siphoning out apart of the suspension. The consolidated samples were pulverized into fine powders before preparing suspensions. Then the siphoned-out fractions were centrifuged and the Clay Mineralogy of Cenozoic Sediments 151

resulted clay portions (< 2ƒÊm fractions) were used to prepare oriented aggregates. A part of the separated clay was dried at the room temperature and ground into fine powders. Oriented specimens for X-ray diffractometry were prepared by dropping a few drops of a clay suspension on a glass slide. On the other hand, randomly oriented specimens were prepared by mounting fine powder on an aluminium holder. In order to make Mg- and K-saturated specimens, samples were washed 4 times with 1N MgC12 or KC1 solutions, respectively; and then washed once with distilled water. Prior to Mg- and K-saturations two samples were treated with 1N sodium citrate solution by the method proposed by Tamura (1958) for removing interlayer contaminant. Initially natural, Mg-saturated and K-saturated samples were X-rayed. Then natural and K-saturated samples were treated with ethylene glycol by spraying the liquid, and Mg-saturated samples were treated with glycerol by exposing to the vapour for 4 hours at 110•Ž and X-rayed again (Nagasawa et al., 1981). To avoid the overlapping of chlorite (002) peak with kaolinite (001) peak, each sample was boiled in 6N HC1 for 30 minutes (Brown and Brindley, 1980), washed with distilled water and prepared an oriented glass slide. Transmission electron micrography of some selected samples was carried out. To confirm the absence of kaolinite (revealed by the X-ray patterns) some samples were examined through an infrared spectrophotometer by the KBr disc method (Oinuma and Kodama, 1967). With the aid of the 17A, 14A, 9A and 7A basal reflections of the Mg-saturated and HCl treated samples, as stated by Oinuma (1968), quantitative estimations of the relative abundances of major clay minerals of the samples were carried out.

Width of the 10A peak at its half height in the X-ray pattern of natural specimen was measured and these values were considered as the degree of crystallinity of illite

(Segonzac, 1970). The method proposed by Togashi (1979) was used to determine the expandability of illite: natural and ethylene glycol treated slides were X-rayed and the differences in d-spacings of 10A peaks (Ad) were measured.

RESULTS

Clay minerals such as smectite, chlorite, illite, kaolinite and irregularly mixed-layered illite/smectite and chlorite/vermiculite as well as quartz and feldspar in varying amounts were observed in the samples. Representative X-ray diagrams and electron micrographs are shown in Figs. 2 and 3, respectively. As shown in Fig. 4 the most dominant clay mineral of the Furuya Formation is smectite and the least abundant one is kaolinite. Specimens No. 6A and 7 contain expandable 2:1 clay mineral with hydroxy-aluminium interlayers. No. 6A shows a broad and diffuse peak between 15-17A in the diffractogram of the Mg-saturated and glycerol treated clay. When it was treated with sodium citrate the diffuse peaks were replaced by sharp ones at 17.5A on Mg-saturation followed by glycerol solvation. Hydroxy-aluminium interlayering in No. 7 was shown by the change of the broad and diffuse peaks between 12-14A and at 14A (chlorite) of the K-saturated clay to strong 152 A. N. Liyanage

FIG. 2. X-ray patterns for oriented aggregates of some natural specimens (Sample No. 33, 52, 3, 6A and 26). 12.6A and weak 14.2A by Na citrate treatment followed by K-saturation. The Soga and Kakegawa Groups, which underlie the Furuya Formation, contain larger amounts of illite and lesser smectite, chlorite and kaolinite (Fig. 4). The prominant clay mineral of the Sagara and Saigo Groups is smectite, and signifi- cant quantities of illite and chlorite with or without kaolinite also occur there (Fig. 4). As the age becomes older the relative abundance of smectite decreases and that of chlorite and illite increases in the Kurami, Oigawa and Setogawa Groups. There are only traces of irregularly mixed-layered illite/smectite in the Kurami Group, while it is the most dominant clay constituent in the Setogawa Group (Fig. 4). As shown in Fig. 5 the X-ray curves of the natural specimens with irregularly mixed-layered illite/ smectite have a broad but distinct peak around 11A (Weaver, 1956). This peak did not change by Mg-saturation, but it disappeared by Mg-saturation followed by glycerol solvation. After K-saturation the 11A peak disappeared and width and intensity of Clay Mineralogy of Cenozoic Sediments 153

FIG. 3. Electron micrographs of some specimens (Sample No. are shown in the upper right corner of each picture).

10A illite peak greatly increased (Nagasawa and Tsuzuki, 1974).

Some samples of the Oigawa Group have irregularly mixed-layered chlorite/vermicu- lite and Fig. 6 shows the X-ray patterns of such a sample. On these patterns, relatively strong 14A peak and weaker 2nd order 7A peak of the natural clay did not change by Mg-saturation or by glycerol solvation after Mg-saturation. But in the X-ray pattern of the K-saturated sample has a very weak 14A peak. Therefore the strong 14A peak of the natural and Mg-saturated samples could be due to chlorite/vermiculite and not to discrete chlorite. The expandability of illite is generally small but illite in the Furuya Formation has relatively large Ģd, about 0.25A. The crystallinity of illite is generally high in the older samples than that of the younger ones but no systematic variation is detected. 154 A. N. Liyanage

FIG, 4. Clay mineral percentage and lithology of the samples arranged accord- ing to the stratigraphic succession. *Highly consolidated

Contain •› small amounts of irregularly mixed-layered illite/smectite +Contain small amounts of irregularly mixed-layered chlorite/vermiculite Clay Mineralogy of Cenozoic Sediments 155

FIG. 5. X-ray patterns for oriented aggregates of natural , Mg-saturated, glycerol treated after Mg-saturation and K-saturated specimen of No . 61.

DISCUSSION

As illustrated in Fig. 4, the Furuya Formation and Sagara and Saigo Groups contain high percentage of smectite over the other clay minerals. Furthermore some samples in the Furuya Formation include small amounts of hydroxy-aluminium interlayered 2:1 clay minerals. The abundance of expandable layers in illite is generally high and the degree of crystallinity of illite is moderate in the Furuya Formation. The smectite, poorly crystallized illite and hydroxy-aluminium interlayered 2:1 clay minerals in the Furuya Formation may be the products of weathering of the older sedimentary rocks of the surrounding areas, and the short distance transportation and short time exposure to the sea water may facilitated little or no compositional changes during the deposi- tional process. The features such as clay mineral percentage (see Fig. 4) and crystallinity of illite of both Sagara and Saigo Groups are similar to those of the Furuya Formation. These similarities may be due to a common origin of the constituent clay minerals although the pyroclastic origin of a part of the smectite in Sagara and Saigo Groups cannot be 156 A. N. Liyanage

FIG. 6. X-ray patterns for oriented aggregates of natural , Mg-saturated, glycerol treated after Mg-saturation and K-saturated specimen of No . 74. ruled out. On the other hand, Soga and Kakegawa Groups have large amounts of illite and chlorite and small amounts of smectite and kaolinite (see Fig. 4). The process which led to the deposition of Soga and Kakegawa Groups might have been different from those of Sagara and Saigo Groups: probably the deposition of Soga and Kakegawa Groups had occurred under a calm environmental condition, as proposed by Tsuchi (1961), allowing detritus to remain in suspension for some time. The occurrence of mixed-layer phases as well as the corresponding decrease in discrete smectite in the well-consolidated mudstone of the Kurami, Oigawa and Seto- gawa Groups might be due to the burial diagenesis (Perry and Hower, 1970). The dominance of mixed-layer illite/smectite phases over the other clay minerals and the presence of well crystallized illite in Setogawa Group indicate an advanced stage of burial diagenesis.

ACKNOWLEDGMENTS

I offer my sincere gratitude to Professor Keinosuke Nagasawa, Faculty of Science , Shizuoka University for his invaluable help and constant guidance throughout the research. This research was made possible by a scholarship given to me by the Ministry of Education, Science and Culture of Japanese Government, for which I would like to record my thanks. Clay Mineralogy of Cenozoic Sediments 157

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