Journal of science of the Hiroshima University. Series C, Earth and planetary sciences Volume 11 Issue 2
published_at 2003-08-25

Origin of Unconformities and Depositional Processes of the Pleistocene Carbonate Rocks in the Humid Subtropical Conditions, the Ryukyu Group on Southern Okinawa Island, Southern Japan

JIJU Katsutoshi
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Abstract
Unconformable boundaries of carbonate rocks were studied with the petrographic aspects in the Ryukyu Group mainly in southern Okinawa Island, southern Japan. There had been many studies about features of unconformable boundaries exposed in arid to sub-humid climate, because the active evaporation forms characteristic secondary calcite known as calcretes. However, exposure features under the humid conditions have been little studied like those found in the Ryukyu Group. In this study, the author reports some examples of the unconformities in the humid and subtropical Ryukyu Group, which is expected to have been affected by cyclic sea-level change and exposure events during the Pleistocene gracial-intergracial periods. Recognition of the unconformable horizons is important for understanding of the depositional processes and history. To clear the diagnostic features of unconformities, recent exposure surfaces were studied first. In inland places, unconsolidated reddish brown soil of several tens cm covers the rock surfaces. The bedrocks are often pedogenically brecciated. We can observe characteristic features of karstic topography in southern Okinawa Island, which are ramparts developed at fault scarps, and karstic planation surfaces developed at foot of the ramparts. These plane surfaces are probably responsible for the high permeability of the young limestone which has many initial and secondary voids. This feature leads to diffuse way of flowing groundwater, causing relatively uniform dissolution on the ground. The plane nature of the karstic surfaces is highly suggestive for the fact that unconformable boundaries in the Ryukyu Group could be flat. It is in contrast to the Palaeozoic limestones of the Motobu Peninsular, having low stratal permeability, which leads to represent complicated karstic surfaces like cockpit karst. On the contrary, in coastal areas, bedrocks lack the mantling of soil layers, and have jagged surfaces related to formation of phytokarst. In intertidal to subtidal zones, endolithic organisms like sipunculids actively bore into the bedrocks, changing it to be micritic, poorly sorted materials. In supratidal zone, plants can live utilising fractures and pores. The fractures are usually filled with coastal sand, which help the plants survive. The plants alter the sands and the bedrocks into brown pedogenetic materials, which are usually consolidated in coastal areas. Important features of the past exposure surface in the Pleistocene Ryukyu Group are, rhizolith, neptunian dykes, pedogenic breccias, and phytokarstic structures. Unconsolidated soils are also important, which often survived at only depressions and fissures. Cutting of the underlying limestone structres are also well observed just below the unconformities. The Ryukyu Group in the southern Okinawa Island can be divided into three, that are, the "Reddish Limestone" (equivalent to the "Chinen Sandstone"), the Naha Formation, and the Minatogawa Formation. It seems that different stratigraphic horizons have different features of the unconformities. It seems that preservation of the pedogenetic features is mainly related to the time-dependent factors in period of subaerial exposure. Sedimentation environments may also affect to the preservation of the diagnostic features like unconsolidated palaeosols. In this study, it is cleared that the Naha Formation can be divided into lower and upper parts in the western part of the southern Okinawa Island, where many quarries exist. A couple of sedimentary sequences are recognised in each of the lower and upper Naha Formation, but there is overall shallowing upward trend in each, which in many case ends by shallow types of coral limestones. The same trend was also reported from Motobu Peninsular, Ie and Toku-no-shima Islands. The distribution of coral limestone of the uppermost Naha Formation is limited just around Yoza to Maehira areas. However, the distribution of the centre of the coral limestone of the lower Naha Formation is at Kyan, though it has wide distribution compared to that of the upper Naha Formation. From this viewpoint, some crustal movement might have been related to form unconformities between the lower and upper Naha Formation.