11:15 AM - 11:30 AM
[MIS13-09] Consolidation test for low-porosity siltstones and tectonic evolution of the Neogene Miyazaki forearc basin
Keywords:Forearc basin, Miyazaki Group, Consolidation test, Vitrinite reflectance, Calcareous nannofossil, Lithification
Consolidation tests give us clues to understand the tectonic evolution of forearc basins because the maximum effective stress that sediments have experienced can be directly calculated. They are applicable for both present/past-forearc basin sediments. However, most of the tests have been conducted for porous sediments (>~30% of porosity), and therefore validity of the tests for the less porous samples has not been verified. Here, we propose an application method of consolidation tests for low-porosity (< 20%) siltstones. Validity of the tests were also examined by comparing the paleo-geothermal gradient calculated from the consolidation test and layer thickness.
We applied this method to the siltstones of the Neogene Miyazaki forearc basin and discussed the formation history. The Miyazaki Group comprises forearc basin sediments in South Kyushu in Southwest Japan, is characterized by significant spatial variations in stiffness of sedimentary rock despite the minor differences in depositional ages. It is divided into three lithofacies (the Aoshima, Miyazaki, and Tsuma facies, from south to north).
To investigate the maximum burial depth (Dmax) of sedimentary rocks, consolidation tests were conducted. During a consolidation test under K0-conditions, elastic deformation firstly proceeds under the stress state that the samples have previously experienced. Secondly, the samples yield, and plastic deformation begins under the stress states that have not been previously experienced. Therefore, the maximum effective stress is calculated as the consolidation yield stress from the consolidation curve. However, consolidation yielding points of the low-porosity sediments are sometimes unclear because of the excess pore water pressure in the samples during the tests. To avoid it, we propose an unsaturated condition for the samples (50% of saturation) on which the tests are conducted at small strain rate (0.01%/min). Reproducibility of the consolidation tests for low-porosity siltstones was conformed, using the several samples from the same host rock.
Consolidation yield stress of the Aoshima Facies (38.2 MPa) was significantly larger than that of the Miyazaki and Tsuma facies (13.8-16.2 and 13.6-15.5 MPa, respectively). Dmax calculated from the consolidation yield stress indicated that the Aoshima Facies (3590 m) had been buried deeper than the Miyazaki and Tsuma facies (1390-1600 and 1370-1550 m, respectively). To validate the results of the consolidation tests, we compared the layer thickness and Dmax. For the comparison, we constrained the paleo-geothermal gradient using the paleo-temperatures from the vitrinite reflectance data and layer thickness data. The values of the Aoshima Facies were 17.3°C/km based on the layer thickness, and 17.8°C/km based on Dmax. The values of the Tsuma Facies were 26.7°C/km based on the layer thickness, and 28.8°C/km based on Dmax. The paleo-geothermal gradient based on Dmax is similar to that based on layer thickness. Therefore, Dmax are concordant with the stratigraphy and the consolidation test data for low-porosity siltstones (< 20%) are valid. The paleo-geothermal gradient of the Tsuma Facies was ~1.5 times steeper than that of the Aoshima Facies. It is because the thermal conductivity of siltstones of the upper Aoshima Facies (1.87 W/mK) was higher than that of the lower and upper Tsuma Facies (1.62 and 1.47 W/mK, respectively).
The spatial variation in consolidation reflects the sedimentary depositional environments and regional tectonics in the forearc region. The sediments of the Aoshima Facies were deposited in the deep-marine depocenter and deeply buried, whereas those of the Miyazaki and Tsuma facies were deposited in shallower marine facies and less buried. The large amount of uplift in the Aoshima Facies was due to counter-clockwise rotation of the South Kyushu microplates and subduction of the Kyushu–Palau Ridge beneath the forearc basin since 2 Ma.
We applied this method to the siltstones of the Neogene Miyazaki forearc basin and discussed the formation history. The Miyazaki Group comprises forearc basin sediments in South Kyushu in Southwest Japan, is characterized by significant spatial variations in stiffness of sedimentary rock despite the minor differences in depositional ages. It is divided into three lithofacies (the Aoshima, Miyazaki, and Tsuma facies, from south to north).
To investigate the maximum burial depth (Dmax) of sedimentary rocks, consolidation tests were conducted. During a consolidation test under K0-conditions, elastic deformation firstly proceeds under the stress state that the samples have previously experienced. Secondly, the samples yield, and plastic deformation begins under the stress states that have not been previously experienced. Therefore, the maximum effective stress is calculated as the consolidation yield stress from the consolidation curve. However, consolidation yielding points of the low-porosity sediments are sometimes unclear because of the excess pore water pressure in the samples during the tests. To avoid it, we propose an unsaturated condition for the samples (50% of saturation) on which the tests are conducted at small strain rate (0.01%/min). Reproducibility of the consolidation tests for low-porosity siltstones was conformed, using the several samples from the same host rock.
Consolidation yield stress of the Aoshima Facies (38.2 MPa) was significantly larger than that of the Miyazaki and Tsuma facies (13.8-16.2 and 13.6-15.5 MPa, respectively). Dmax calculated from the consolidation yield stress indicated that the Aoshima Facies (3590 m) had been buried deeper than the Miyazaki and Tsuma facies (1390-1600 and 1370-1550 m, respectively). To validate the results of the consolidation tests, we compared the layer thickness and Dmax. For the comparison, we constrained the paleo-geothermal gradient using the paleo-temperatures from the vitrinite reflectance data and layer thickness data. The values of the Aoshima Facies were 17.3°C/km based on the layer thickness, and 17.8°C/km based on Dmax. The values of the Tsuma Facies were 26.7°C/km based on the layer thickness, and 28.8°C/km based on Dmax. The paleo-geothermal gradient based on Dmax is similar to that based on layer thickness. Therefore, Dmax are concordant with the stratigraphy and the consolidation test data for low-porosity siltstones (< 20%) are valid. The paleo-geothermal gradient of the Tsuma Facies was ~1.5 times steeper than that of the Aoshima Facies. It is because the thermal conductivity of siltstones of the upper Aoshima Facies (1.87 W/mK) was higher than that of the lower and upper Tsuma Facies (1.62 and 1.47 W/mK, respectively).
The spatial variation in consolidation reflects the sedimentary depositional environments and regional tectonics in the forearc region. The sediments of the Aoshima Facies were deposited in the deep-marine depocenter and deeply buried, whereas those of the Miyazaki and Tsuma facies were deposited in shallower marine facies and less buried. The large amount of uplift in the Aoshima Facies was due to counter-clockwise rotation of the South Kyushu microplates and subduction of the Kyushu–Palau Ridge beneath the forearc basin since 2 Ma.