Basin analysis of the Lower Cretaceous Toyonishi and Kanmon Groups, Southwest Japan

YOSHIDOMI Ken-ichi

Journal of science of the Hiroshima University. Series C, Earth and planetary sciences

Neritic to fluvial sediments of 6,000 m thick are distributed in the Jurassic to Lower Cretaceous Kanmon Basin in the westernmost Honshu (Yamaguchi Prefacture), Inner Zone of the Southwest Japan. Sedimentological and geological investigations clarify the geometry, and reconstruct the tectono-sedimentary development process of the basin. The results may provide clues to elucidate the late Mesozoic tectonics in the East Asian continental margin.
The Jurassic to Lower Cretaceous sediments in the Kanmon Basin consist of the following three groups in ascending order; the Lower to Middle Jurassic Toyora Group comprising mainly of neritic shale, the uppermost Jurassic to lowest Cretaceous Toyonishi Group composed of the paralic, plant bearing lower and brackish molluscs bearing upper formations, and the middle to upper Lower Cretaceous Kanmon Group consisting of the lower clastic and the upper volcano-clastic subgroups.
Permian accretionary complexes and Upper Triassic shelf sediments are distributed on the southeast of the thick sediments of the Kanmon Basin, bounded by the NE-SW trending Nagato Tectonic Zone with serpentine melange. Provenance analyses of the Jurassic to Lower Cretaceous sediments indicate that the Permian accretionary complexes and the Upper Triassic shelf sediments supplied sandy and gravely clasts into the sediment basin during the Jurassic to Early Cretaceous time for ca. 100 m.y. Northwestward decrease in sediment thickness apart from the tectonic zone indicates that the geometry of the basin is a tilting basin (or half-graben) dipping southeastward.
Results of sedimantary facies analysis suggest that the differential movements between the basin and the provenance were caused by the intermittent growth dip-slip movement of the Nagato Tectonic Zone during the Jurassic to the Early Cretaceous. Volcanism of intermediate composition initiated and intensified in the late Early Cretaceous. Successive events around the Early/Late Cretaceous, i.e., acceleration of the differential movement along the Nagato Tectonic Zone, intensified volcanism, and plutonic emplacement, may represent a significant tectono-magmatic phase of the Yanshanian Movement in East Asia.
The Late Jurassic to Early Cretaceous Kyongsang Basin in the southeastern Korean Peninsular is a southeastward tilted basin and has a sedimentary history similar to that of the Kanmon Basin. These two large basins appear to occupy the northeastern end of the extensive area crowded by tilted basins in the East China Sea. These tilted basins dip commonly southeastward and are bounded by deep antithetic faults on their southeastern margins. The coming issue to be clarified is the dynamics generating such systematic arrangement of the late Mesozoic tilted basins gregarious in the East Asian continental margin.

eng

Hiroshima University

[ISSN] 0075-4374

[NCID] AA11095875