Little has been known about the time-dependent rheological properties of clastic rocks. especially those on the long-term deformation or flow, the knowledge of which is keenly needed for interpretation and discussion of formative mechanisms of Some geological structures in nature. Among the properties. creep feature is of prime importance for mechanical analysis of deformed structures. because creep is considered to be an essential process of deformation at an extraordinary low strain rate just like a tectonic deformation. As is well known, the secondary creep represents a flow of constant strain rate. Accordingly. even as for rocks, there is a possibility of obtaining the equivalent viscosity from the creep test. or rather. the test is a unique method to be able to measure the equivalent viscosities of rocks. A creep test on shale under high confining pressure by Griggs (1939) was a very rare case on clastic rocks, but its result was not understood as stated by Griggs himself. In order to discuss the folding mechanism of sedimentary strata in younger geologic age, the creep test of semi-indurated clastic rocks under confining pressure is required by all means.
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In this paper, the writer intends to describe the technique and to report the results of the creep test of soft mudstone from the Niigata region along the Sea of Japan. and to discuss the growth process of fold on the basis of these results.
The tri-axial compression apparatus used for test is the one manufactured by Shimazu Seisaku-sho in Japan. Confining pressure of 3000 kg/cm2 is obtainable at maximum. and in addition to this. axial load of 50 tons is available at maximum. Strain rate is changeable in the range of lxlO−n/sec. n=3. 4. ···.7. Essential part of the apparatus is schematically illustrated in Fig. 1.
Test piece is trimmed in a cylindrical shape with height of 39.0 mm and diameter of 19.5 mm. Pallalelism of top and bottom surfaces is within accuracy of 1/500–1/250. Rock mater1a1s. from which the test pieces are made. are taken from the mudstone as dominant facies of the Nishiyama Formation in the Niigata region. The Formation has been considered to be early Pliocene in age. but its chronology is now debated wi.th another opinion that its upper part is the Pleistocene. Anyway. the Formation is undoubtedly included in a geologic system forming the folded structures with the general trend of NNE-SSW direction in the oi1 field of the Niigata region. The mudstone from the Formation is grey in colour, and the mean values of bulk densities in an air-dried state and in a water-saturated state are 1.88 and 2.08, respectively.
Testing conditions were arranged similar to those at the folding of the system including the Nishiyama Formation namely, confining pressure is 300 kg/cm2 and temperature is 20°C. In order to keep the axial stress constant, increase of the cross-sectional area of the test piece should be continuously estimated according to the progress of creeping.