On the Upper Miocene Matsudayama Formation and the Kannawa Fault in the eastern part of the Ashigara Hill, central Japan

Abstract

The geostructural history of the Ashigara Hill has been interpreted in several ways. The sedimentary rocks of the Matsudayama Formation developed in the eastern part of this hill were once believed to be Lower Pleistocene in age. However, recent discovery by the writer of key fossils such as nannofossils, fossil pollens and larger foraminifera definitely places the age of these to Upper Miocene. These Upper Miocene rocks trend WNW and dip northward. They are bounded to the north by a fault bordering altered Middle Miocene volcanic rocks of the Tanzawa Group. The formation is overturned at the southern margin. The stratified block of small bodies of the Matsudayama Formation occur in fault contact with the above older volcanic rocks and with the younger sedimentary rocks of the Plio-Pleistocene Ashigara Group. To the south, the Upper Miocene units are bounded by a normal fault which dips steeply southward. This fault is newly defined as the Kannawa Fault. On the basis of the above, it is seen that this area was subjected to stress after tilting of the marine sediments of the Matsudayama Formation. This lead to the accumulation of the Ashigara Group in latest Pliocene. The folded layers of the Ashigara Group are cut by WNW normal fault (Kannawa Fault), young northeast-southwest wrench fault (Kosugezawa Fault), and still younger set of NNE-trending strike-slip faults (Shiozawa Fault System).