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

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

Developmental history of a coral reef complex controlled by the sea-level change and terrigenous influx: the Pleistocene Ryukyu Group of Nakijin Village, Okinawa Prefecture.

The Pleistocene coral reef complex, the Ryukyu Group, is widely distributed in Nakijin Village, Okinawa Prefecture. The geology and stratigraphy of the group was investigated on the basis of sedimentological and topographic characteristics. Because the Quaternary carbonate was deposited under a strong control of the sea-level change, information on paleodepth was as much as extracted by use of the recent knowledge on the living subtropical reef biota. We subdivided the Ryukyu Group into three units. Unit 1 consists of alternating deposits of different facies in lower and thick coral limestone in upper. It is highly terrigenous in the northeastern Motobu Peninsula due to a strong influx of silisiclastics. The alternation seen in localities along the northern coast of the Motobu Peninsula indicates several cyclic change in paleodepth. Unit 2 covers a large area and exhibits a zoning in distribution of different facies parallel to the present altitude. From the coast to the high-lying area, the facies generally changes from rhodolith limestone, through clastic limestone, and to coral limestone. This distribution pattern is consistent with that of the present coral reef complex around the Ryukyu Islands. The unit 3 mainly consists of coral limestone and restricts its distribution in the lowermost terrace along the coast. Kouri Island shows much better development of the coral limestone than the northern Motobu Peninsula. Moat facies developed on the tops of units 1 and 2 in the island is obscure in the peninsula. This regional difference in reef development is associated with the difference in depositional settings; especially intensity of the terrigenous influx. The basement rocks are widely exposed in the middle-southern Motobu Peninsula, and the Ryukyu Group in the northern peninsula contains a large amount of silisiclastics which had probably been disturbing the growth of the reef-forming corals. In Kouri Island, where is separated from the peninsula by a relatively deep strait, the terrigenous influx did not sufficiently affect the carbonate deposition. The cyclic facies change found in unit 1 is hardly seen in units 2 and 3. The columnar sections of unit 1 indicate a short-term and small amplitude of sea-level change, which may correspond to the climatic characteristic of the Early Pleistocene (mostly before 0.7-0.5 Ma). From the Middle Pleistocene, both the interval and amplitude of sea-level increased. The amplitude was too large to record the cyclic facies change, and the younger units (units 2 and 3) were deposited mainly during sea-level highstands

Cement generations and diagenetic history of the upper Ordovician Cliefden Caves Limestone Group of New South Wales, Australia

Diagenetic features of the upper Ordovician (Caradocian) Cliefden Caves Limestone Group (N. S. W., Australia) were studied by the cathodoluminescence (CL) method, and their relation with depositional environments was discussed. The Cliefden Caves Limestone Group was deposited on a shallow marine carbonate platform developed on a volcanic island. The limestone succession reveals intertidal-subtidal lithofacies and consists of the Fossil Hill, Belubula, and Vandon Limestones, in ascending order. The Fossil Hill Limestone mostly consists of bedded limestone rich in brachiopods and sedentary organisms. The Belubula Limestone is a typical Middle-Upper Ordovician peritidal succession with syn-depositional dolomite and silicified fossil grains. The Vandon Limestone consists of fossiliferous stratified limestone occasionally containing red brown argillaceous limestone beds. The Cliefden Caves Limestone Group was subjected to various diagenetic processes. The investigation of CL is the best or only method to differentiate the diagenetic products. There are at least three cementation stages and a dissolution stage. The cements of the first stage are mainly dull fine-grained (10-30 µm) calcite crystals fringing inter- and intra-granular porosity, which typically indicate a marine phreatic environment. In some specimens of the Belubula Limestone, cement of the first generation exhibits meniscus fabrics suggesting precipitation in a marine vadose environment. The dissolution formed both molds of aragonitic skeletal grains and fabric-unrelated void spaces which can exceed several cm in diameter. Lack of the first generation cement within the dissolution voids indicates that the dissolution postdated the marine cementation. After the dissolution stage, the second cementation precipitated granular calcite crystals composed of non-luminescence, dull, and bright zones in the peripheral order. The relative thickness of non-luminescence and bright zones probably related with a redox condition during their diagenesis. The last generation of cements formed in a deep burial environment, is normally dull and filled almost all remained porosity. This diagenetic history fits to the change of depositional environment which may have been controlled by both of local and global environmental settings