Paleosols in the Cretaceous Goshoura and Mifune groups, SW Japan and their paleoclimate implications

Cretaceous sedimentary basins in Kyushu Island, Japan, are developed as half-grabens, which accommodated very thick clastic wedges of non-marine to shallow-marine facies. The Albian to Cenomanian Goshoura Group in western Kyushu (ca. 1000 m thick) consists of interbedded channel sandstone, conglomerate, and mudstone. The Cenomanian to Turonian Mifune Group in central Kyushu (ca. 2300 m thick) comprises red and drab sandstone, and intercalated siliceous ash-fall tuff. Different types of paleosols occur in the Goshoura and Mifune groups. Paleosols in the Goshoura Group contain diagnostic vertic features such as subangular blocky peds, irregularly directed slickensides, and downward tapering fissures. Few obvious calcic horizons are developed except for the calcretes of rhizogenic origin. Paleosols in the Upper Formation of the Mifune Group are composed of cyclic acidic tuff–vertic paleosol intervals with abundant pedogenic calcretes. Paleosols of the Goshoura Group belong to the order of vertic Inceptisols, and those of the Mifune Group can be classified as calcic Vertisols. In spite of diagenetic alteration of the paleosols, paleo-vertic soils of the Goshoura and Mifune groups show the exceptional preservation of a number of pedological features. Among various soil-forming factors, diversity in each paleosol is mainly attributed to differences in paleoclimatic conditions during the Middle Cretaceous. Our results show that the paleoclimatic conditions in the subtropical–warm-temperate region had changed from subhumid to semi-arid with time.