The Block Stream Deposits Surrounding the Lake Ohta-ike, Mineyama Highland, Hyogo, Japan

Abstract

Block streams are distributed on the low-relief slopes near Lake Ota-ike, Mineyama highland (750 ～ 950m amsl), in the central part of Hyogo Prefecture, western Japan. Based on the aerial photo interpretation, the block streams were classified into the upper stream (Block stream I) and the lower stream (Block stream II). A 100 m long section of the block stream was observed by their morphological and sedimentological features. At the bottom of the geologic section, an older fan deposits composed of well-sorted and weathered sands and gravels were found. Deposits of block stream II about 2m thick covers the older fan deposits. This deposit is composed of non-sorted boulders (size: 60 ～ 80 cm in average, 160cm in maximum) with fine matrix and of patches of embedded AT tephra (26000 ～ 29000 y.B.P.). The surface of the block stream II is covered with Kuroboku soil whose origin is Ah tephra (6,300 y.B.P.) mainly. Clearly, as the tephra show, block stream II formed during the later substage of the Last Glacial Period. The formation of recent fans followed the dissection of the block stream II by channels. This landform development is suggests that predominant geomorphological processes change from periglacial to the fluvial ones in the area. The block stream I and older fans are considered to have been formed during or before the early part of the Last Glacial Period.