Sedimentological Study of the Ryukyu Group

The sedimentology of the Pleistocene Ryukyu Group in the Ryukyu Islands, southwestern Japan, was investigated. The deposition of the Ryukyu Group, which is characterized by limestone of organic origin, is an important event in the Late Cenozoic history of the Ryukyu Islands. The limestone of the Ryukyu Group is classified into a coral, algal mat, an algal ball and a detrital facies. Definition of each facies is based on both field and laboratory studies. A model of deposition for the limestone is presented and statistically examined applying the Markov chain analysis. The depositional sequence of the Ryukyu Group is strongly Markovian. Special attention is given to the explanation of the mechanism of cyclic sedimentation, which must have caused the growth of such a Markovian-type sequence. In the course of mathematical treatment of the problem, subtracting the independent trials probability matrix leaves a residual matrix of positive probabilities that define the fully developed cyclothem. The fully developed cyclothem of the Itokina Formation can be expressed as conglomeratic sandstone; (fine-grained) sandstone and mudstone; calcareous (fine-grained) sandstone and mudstone; limestone; calcareous (fine-grained) sandstone and mudstone; (fine-grained) sandstone and mudstone; and conglomeratic sandstone. Dynamical analysis of limestone deposition in this fully developed cyclothem was carried out. Considering the geologic history of the Ryukyu Islands, It is concluded from the analyses that sudden subsidence and isolation of the Islands provided a favorable environment for the development of the limestone under warm water conditions