Characteristics of, Topography, Geology and Alteration in Slide Areas

Abstract

Characteristics of twenty-nine ancient slides which were surveyed in the eastern region of the Ikutahara district in northeastern Hokkaido, Japan are as follows;The scale of the slides is about 120-1, 170 m wide and 190-1, 280 m long. Most of the slides can be classified as “Concave Hill Type”, “Slides Occurring in Hydrothermal Alteration Zones” and “Detritus and Clayey Soil Slides” based on their geomorphological features, bedrock geology and displaced materials, respectively.It is believed that the “Slides Occurring in Hydrothermal Alteration Zones” situated on volcaniclastic rocks of the Upper Miocene Ikutahara Formation, moved due to a stage I vein-related interstratified sericite/smectite zone and a stage II vein-related argillized kaolin mineral zone in hydrothermal alteration haloes and a homoclinal structure, while those of the Upper Miocene Yahagi Formation moved due to a stage I regional smectite zone. The slides located on mudstones of the Ikutahara Formation are believed to have occurred due to complex folding structures and distribution of the mudstones which make up the interstratified sericite/smectite zone. “Slides Occurring in Hydrothermal Alteration Zones” are closely related in space to interstratified sericite/smectite, smectite, and argillized kaolin mineral zones.“Slides Occurring in Crushed Zones” situated on rocks of the Upper Cretaceous -Paleocene Yubetsu Group, are thought to have occurred due to a fault, a homoclinal structure, and distribution of shales.“Granitic Rocks -Type Slides” probably occurred due to weathering along fault and joint planes in the hydrothermally altered or the least altered nevaditic biotite rhyolite intrusives.