Characterization and comparison of Cretaceous red beds in Kyushu and east Asia

世界各地的红层因其在野外的独特颜色而引起了研究人员的兴趣。然而，红层的成因是复杂的，也是持续研究的主题。从沉积学、矿物学和地球化学学的角度研究了九州中部白垩纪红层，包括小野川群、密丰群和戈肖诺拉群，并讨论了其成色假说。细粒赤铁矿分散在粘土基质中，是沉积物呈红色的原因，沉积物的来源是源岩中含铁矿物中的铁。供给陆相和咸水沉积盆地的铁物质在氧化条件下以铁的状态保存，并最终老化为着色的赤铁矿。输送到浅海海底的铁在溶液中被还原和去除，没有造成红色的影响。沉积盆地中铁的补给和有利于铁在铁状态下保存的氧化条件是导致岩层呈红色的重要因素。中国南部和中南半岛形成蒸发矿物的干热气候对九州中部红层的形成起着次要的作用。

Red beds at various localities of the world have attracted the interest of researchers owing to their distinctive color in the field. However, the genesis of red beds is complex and the subject of continuing research. Cretaceous red beds in middle Kyushu included in the Onogawa, Mifune and Goshonora Groups have been examined from sedimentological, mineralogical and geochemical viewpoints, and a hypothesis for their coloration is discussed.Field and laboratory studies on the red beds lead us to the following conclusions. Fine-grained hematite disseminated in clayy matrix is responsible for red coloration of sediments, which originated from iron in iron-bearing minerals in their souce rocks. Iron substances supplied to the terrestrial and brackish sedimentary basins were preserved in a ferric state under oxidizing conditions, and ultimately aged to the pigmenting hematite. Iron transported to the shallow sea bottom was reduced and removed in solution without contribution of red coloration. A supply of iron into a depositional basin, and an oxidizing condition favorable for preservation of iron in a ferric state are the important factors for red coloration of the beds. Hot and dry climates in which evaporitic minerals formed in southern China and Indochinaplayd a less important role in red beds formation in middle Kyushu.