11:15 〜 11:30
[SGL24-03] Variation of consolidation characteristics among 3 facies in Miyazaki forearc basin, SW Japan
To understand the main reasons that made the variation of consolidation characteristics in sedimentary basins, we investigated the maximum consolidation stress and paleo-temperature of sedimentary rocks in the Miyazaki Group: late Miocene-Pleistocene forearc basin in Kyushu, SW Japan.
The Miyazaki Group is divided into 3 facies: Tsuma, Miyazaki, and Aoshima facies (northern, central, southern parts of the body, respectively), based on lithology. In spite of the fact that there exists only minor difference of sedimentary age, the uniaxial strength of the sediments varied significantly, which implies indicatively the difference in the degree of consolidation. To validate the hypothesis, we conducted porosity/density analyses, consolidation tests, and maximum temperature estimation by means of vitrinite reflectance and ROCK-EVAL / Tmax measurements. Porosity of siltstone has large variation (21.1-45.6 %) in the Tsuma Facies, whereas small variation in the Miyazaki (13.9 -18.0 %) and the Aoshima facies (11.2-16.2 %). The consolidation tests were conducted using the consolidation apparatus (Nagoya Institute of Technology/JAMSTEC) and yield stress (pc) were calculated based on Casagrande method. The calculated pc are 4.1-7.8, 4.6-9.3, and 4.4-8.5 MPa in the Tsuma, Miyazaki, and Aoshima facies, respectively. The pc is getting to be smaller to the upper parts. Vitrinite reflectance (RO) was in range 0.27-0.42, 0.42-0.46, and 0.46-0.73 % in the Tsuma, Miyazaki, and Aoshima facies, respectively. ROCK-EVAL / Tmax also showed this trend of thermal maturity. Even though the calcareous Nannofossil assembly in the lower part of Aoshima and Miyazaki facies represent the same biostratigraphic zone (NN11; 5.5-8.5 Ma), RO has major variation (0.73 and 0.46 %, respectively). Assuming the constant geothermal gradient (25 °C/km) in the Miyazaki Group, 5700 and 4000 m maximum burial depth were expected in the lower part of Aoshima and Miyazaki facies. Based on above findings, we proposed the following interpretation on the geologic history of the Miyazaki Group. The westernmost of the Aoshima and Miyazaki facies are expected to be deposited in distal and proximal part of the forearc basin and they had major difference in burial depth. And the Tsuma Facies, the Northern part of the Miyazaki Group, started to be deposited after the lower part of Aoshima and Miyazaki facies was formed. The Aoshima and Miyazaki facies are divided by left lateral faults and the average attitude of the bedding planes are N5 °E 15 °E and N30 °E 10 °E, respectively. Therefore, a larger amount of eastward-tilting and uplifting of the Aoshima Facies occurred, which made approximately 1700 m difference in the maximum burial depth along the fault.
The Miyazaki Group is divided into 3 facies: Tsuma, Miyazaki, and Aoshima facies (northern, central, southern parts of the body, respectively), based on lithology. In spite of the fact that there exists only minor difference of sedimentary age, the uniaxial strength of the sediments varied significantly, which implies indicatively the difference in the degree of consolidation. To validate the hypothesis, we conducted porosity/density analyses, consolidation tests, and maximum temperature estimation by means of vitrinite reflectance and ROCK-EVAL / Tmax measurements. Porosity of siltstone has large variation (21.1-45.6 %) in the Tsuma Facies, whereas small variation in the Miyazaki (13.9 -18.0 %) and the Aoshima facies (11.2-16.2 %). The consolidation tests were conducted using the consolidation apparatus (Nagoya Institute of Technology/JAMSTEC) and yield stress (pc) were calculated based on Casagrande method. The calculated pc are 4.1-7.8, 4.6-9.3, and 4.4-8.5 MPa in the Tsuma, Miyazaki, and Aoshima facies, respectively. The pc is getting to be smaller to the upper parts. Vitrinite reflectance (RO) was in range 0.27-0.42, 0.42-0.46, and 0.46-0.73 % in the Tsuma, Miyazaki, and Aoshima facies, respectively. ROCK-EVAL / Tmax also showed this trend of thermal maturity. Even though the calcareous Nannofossil assembly in the lower part of Aoshima and Miyazaki facies represent the same biostratigraphic zone (NN11; 5.5-8.5 Ma), RO has major variation (0.73 and 0.46 %, respectively). Assuming the constant geothermal gradient (25 °C/km) in the Miyazaki Group, 5700 and 4000 m maximum burial depth were expected in the lower part of Aoshima and Miyazaki facies. Based on above findings, we proposed the following interpretation on the geologic history of the Miyazaki Group. The westernmost of the Aoshima and Miyazaki facies are expected to be deposited in distal and proximal part of the forearc basin and they had major difference in burial depth. And the Tsuma Facies, the Northern part of the Miyazaki Group, started to be deposited after the lower part of Aoshima and Miyazaki facies was formed. The Aoshima and Miyazaki facies are divided by left lateral faults and the average attitude of the bedding planes are N5 °E 15 °E and N30 °E 10 °E, respectively. Therefore, a larger amount of eastward-tilting and uplifting of the Aoshima Facies occurred, which made approximately 1700 m difference in the maximum burial depth along the fault.