FAULTS AND JOINTS IN THE VICINITY OF LAKE SAGAMI, KANAGAWA PREFECTURE

Abstract

The geologic structure in the vicinity of Lake Sagami is most complicated in the Katsuragawa Valley. For example, a sandwich structure is recognized in the south of the lake (Fig. 1.). The main subjects of this article are sa-mmerized as follows (1) The fault between the Kobotoke and the Misaka group is a high angle thrust fault. The fault planes are observable in the localities 1-3, north of 5, 6 and 7. On the other hand, the ATisaka group itself has thrus-ted over the conglomerate of Katsuragawca formation from the north (loc. 8-10). therefore, the imbricate thrust-faulting in the area took place after the deposition of the Katsuragawa formation (Miocene). (2) There develop numerous joints, which are classified into two types according to the joint patterns (Fig. 5), in the conglomerate bed of the Katsuragawa and the Misaka member. “A” type joint is locally deve-loped in the upper bed of the Katsuragaw-a formation. “C” type joint is, however, most prominent and most widely developed in the conglomerate bed both of the Katsuragawa and the Misaka group, and always cut through pebbles and matrix alike. It has a smooth plane surface, generally, no evi-dence of granulation or slickensiding, and it occurs in pallel to each other and in crowds in the adjacent zone to the thrust fault. Judging from these facts, the type joint may be a tension joint or a fracture which occu-rred parallel to the direction of compression and the age of jointing is nearly the same as that of thrusting. (3) There. are some relation between geologic. structure and landforins (Fig.3, Fig. 6). Especially, the micro-landforra in the south of the lake, such as ravines and spurs has almost the same trend as that of the joint (Fig. 6). These facts possibly show that the tension joints have acted to promote weathering, and mass-wasting along them, which are now going on as seen in the loc. 5, 6 in Fig. 6.