1995 Volume 41 Issue 41 Pages 65-83
The Upper Cretaceous Mifune Group, which is well exposed in the Axial Zone of Kyushu, is characterized by nonmarine, brackish water and shallow marine deposits. It overlies the Kiyama metamorphic rocks and serpentinite nonconformably at the northern part and unconformably the Upper Permian Mizukoshi Formation and disconformably the Higo metamorphic rocks at the southern part. The Mifune Group is divided into the Basal, Lower and Upper Formations in ascending order. The Basal Formation (20-50m thick) is composed of conglomerates with red-colored matrix. The Lower Formation (250-750m thick) is composed of sandstones and siltstones with plenty of brackish water and marine molluscan fossils. The Upper Formation (more than 1000m thick) is principally composed of red beds with sandstones and acidic tuff beds.
This paper describes sedimentological features of the Mifune Group, especially on the sedimentary facies and paleocurrent patterns. The results of the investigation are summarized as follows:
1. Seventeen distinct sedimentary facies are recognized, which are grouped into eight facies assemblages. The characteristics of these facies assembleges may suggest a variety of sedimentary environments, such as alluvial fan, lacustrine nearshore, lacustrine offshore, estuarine channel, sandy tidal flat, muddy tidal flat, shoreface and offshore.
2. Small-scale sedimentary structures are recognized in terrestrial red and fine-grained beds, such as graded-bedding, cross lamination, ripple mark and rhythmite-like parallel lamination.
3. Sedimentary facies and paleocurrents data suggest that depositional environments in the northern area were changed from alluvial fan through estuarine channnel and muddy tidal flat to lacustrine offshore in ascending, whereas in the southern area from distal alluvial fan through shallow-sea sandy tidal flat, offshore and shoreface to lacustrine nearshore and lacustrine offshore. During the marine depositional phase, the sea opened eastward and bay-like environments were developed on the west side of the basin.
