十勝平野西・南部の火山灰層序

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In the Tokachi Plain, the alluvial fans are widely developed, and now dissected by some rivers into many terraces. These surfaces are covered with tephras named as Tokachi Loam (Tephra) by authors. The authors have tried to determine the exact age of aggradation, for the final purpose of establishing the relationships between the development of fans in the Tokachi Plain and the glaciation in the Hidaka Range. For this study, it was most important to clear the tephra stratigraphy at first. The main results of field and laboratory works are as follows:1) The Tokachi Loam is divided into two members of the Lower and the Upper Loam which are unconformably separated.2) The Lower Loam Member (about 3m in thickness) consists of highly weathered reddish brown ash, which is rather hard and contains no conspicuous pumice and scoriae.3) The Upper Loam Member (about 7m in thickness) is relatively loose, and consists of fine to medium-grained brown ash, pumiceous or scoriaceous ash, pumices and scoriae. Below the pumice horizon of well known Spfa, the authors recognized 9 pumice and scoriae beds which are useful for the correlation of the geomorphic surfaces. These are Rakko Pumice I (RP I), Rakko Pumice II (RP II), Tokachi Black Scoriae (TBS), Rakko Pumice III (RP III), Rakko Pumice IV (RP IV), Hiroo Pumice I (HP I), Hiroo Pumice II (HP II), Hiroo Pumice III (HP III), Hiroo Pumice IV (HP IV) in descending order.4) Judging from their distribution, the source vent of TBS may be in the northern region, while other 8 pumice beds are originated from the western volcanoes. RPI contains much amount of hornblende and other 8 beds consist mainly of ortho- and clino-pyroxene and magnetite.5) The Lower Loam Member is seen only on the surfaces higher than the Kamisarabetsu II, and looks more reddish on the surfaces higher than the Kamisarabetsu I.6) Some pumice beds of the Upper Loam Member are intercalated in the fan or fill gravels of the Kamisatsunai I surfaces. Using these pumice beds as Key, the relative age of fan formation or valley filling is determined.7) The tephra stratigraphy proposed by Tokachi Research Group (1972 a) is rather confusing with regard to the pumice beds of op-1, op-2, and op-3, so that it is necessary to revise. As RP IV is intercalated into the Biraotori Formation, the latter is correlated to the middle part of the fan gravels of the Kamisatsunai I surfaces.
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十勝平野西・南部の火山灰層序

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