Abstract
This article deals with engineering properties and diagenesis process of the Ryukyu Limestone belonging to the Pleistocene Ryukyu Group widely exposed on south-east part of Miyakojima of the Ryukyu Islands.
Karst topography found within Miyakojima includes limestone wall and mound. Limestone wall is the row of ridges along normal faults.
On the basis of fossils contained and matrix of the rocks, the Ryukyu limestone is classified principally into four sedimentary facies: coral detrital limestone which contains corals and detritus, coral algal-mat limestone which contains autochthonous corals and algal-mat, algal ball limestone containing algal ball or rhodolites, detrital limestone which consists of detritus of calcareous organic remains.
Compressive strength of the Ryukyu limestone shows a wide scatter from 1 to 400 kgf/cm2. To know regional engineering properties of the Ryukyu limstone, we measured rebound value of limestone on outcrops in Miyakojima by a Schmid-Rock-Hammer and petropysical properties such as density, specific gravity, porosity and compressive strength.
The results are summarized as follows:
1) On the basis of frequency of rebound values, three patterns are recognized: Coral algal-mat limestone and algal ball limestone showe hihger rebound value. Coral detrital limestone shows lower rebound value. Detrital limestone shows a wide range rebound values.
2) The higher rebound value zones correspond to the karst topography of limestone wall and distribution zones composed of coral algal-mat limestone and algal ball limestone. Whereas lower rebound value zones correspond to coral detrital limestone.
3) Rebound values have relation with porosities taking into account of the cavities.
These results led us to the following hypothesis: Diagenesis of the carbonate sediments is influenced by initial porosity of each sedimentary facies, cementation and solution caused by infiltration of meteoric water. Cementation would have occured in a early stage of diagenesis process, and brought on the decrease of primary porosity and the increase of strength. Formation of secondary porosity by solution would cause the some decrease of strength.
