Geological and Palynological Studies on Quaternary Peat Deposits in Hirugano Tableland, Gifu Prefecture, Central Japan

Abstract

There are some small tablelands at 800-1, 000 meters above the present sea level of the surround ing areas of Mt. Hakusan, Central Japan. Hirugano tableland which is one of them occupies at about 1, 000 meters of the northwestern part of Gifu Prefecture, and at the tableland some small moores are distributed sporadically. The present writers investigated on the peat deposits of these moores from the viewpoints of palaeoclimate and the cause of formation of the tableland.
The conclusions of the study are summarized as follows:
(1) The stratigraphy of Hirugano tableland is shown as the following table.
Hirugano Member {Holocene Peat Deposits Middle-Late Pleistocene {Loam Layer Conglomerate Layer
unconformity
Early Pleistocene Dainichidake-Washigatake Andesite Group
unconformity
Pliocene Shirotori Lake Deposits
unconformity
Late Cretaceous Nôhi Rhyolite Group
unconformity
Late Jurassic Tetori Group
The lowermost part of the peat deposits was formed in about 7, 000 years before the present, 6, 680±130 years B. P. (GaK-3628), and the ratio of sedimentation of the peat deposits may be estimated as about 0.31mm/year.
(2) The cause of formation of Hirugano tableland is concluded as follows:
In the end of Early Pleistocene epoch the lava flows of andesite from Dainichidake and Washigatake damed up the south entrance of the basin which was formed by eroding the Late Jurassic Tetori Group, Late Cretaceous Nôhi Rhyolite Group and Pliocene Shirotori Lake Deposits. For the reason this lake named Lake Hirugano was formed here. The conglomerate and pyroclastics belonging to the Hirugano Member were deposited in the lake throughout the Middle and Late Pleistocene epoch. Many moores which are distributed sporadically in Hirugano tableland show the last stage of Lake Hirugano filled up from the viewpoint of the histroy of lake. That is, the moores which we can see in Hirugano tableland are a relic of Lake Hirugano into our day.
(3) The climatic change found in the peat deposits is summarized as follows.
a: Warm stage: This stage is between about 7, 000 years and about 4, 000 years ago. This stage corresponds to the Postglacial climatic optimum age and the Atlantic and early part of the Subboreal in the pollen chronology of Europe.
b: Cool stage: This stage is between about 4, 000 years and about 1300 years ago. An annual mean temperature of this stage is 0.5° to 1.0°C lower than that of the present time.
c: Present climatic stage: This stage is the time after about A. D. 700. The present annual mean temperature of Hirugano tableland is 7.6°C.