In northwestern Hokkaido, where the Honshu and Kuril arcs intersect, an enormous pile of Neogene sediments (about 10km thick) is developed in close relation to the Cenozoic orogenesis. The Kotanbetsu Formation, representing the lower to middle Miocene sequence of these sediments, exceeds 3km. in thickness and is characterized by gravity-flow deposits. The strata are divisible into three major facies: 1) chaotic deposits or olistostrome facies, 2) graded-bed or turbidite facies, and 3) ripple-bed or contourite facies.
The chaotic deposit facies, rather restricted in distribution in the proximal parts of the basin, is further subdivided into three sedimentary types: pebbly mudstone, chaotic breccia, and matrix-deficient conglomerate. The graded-bed and the ripple-bed facies are predominant in the relatively distal parts of the basin. The chaotic deposits were generated by intense tectonic movements of the 'Mesozoic' basement rocks thrust up to towards the west. The graded beds were deposited from turbidity currents, although their current directions are not clearly defined, whilst the ripple beds were formed by southward-flowing contour currents.
These characteristic sediments are comparable in their tectonic position of an apparent back-arc belt to the Onerahi Chaos-Breccia of Northland, New Zealand, related to the Miocene to Pliocene Kaikoura Orogeny. The Kotanbetsu and its equivalents, however, may have been the deposits in basins produced by collision of two continental blocks, known generally as the Hidaka Orogeny, but not in the ancient back-arc belt.
Copyright© The Geodetic Society of Japan
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