09:35 〜 09:50
[MGI28-03] Consolidation state of sediments in the Hellenic Arc Volcanic Field, Greece: Evidence for excess pore pressure caused by huge eruptions and mass wasting (IODP Expedition 398)
キーワード：過剰間隙水圧、IODP、IODP Expedition 398、サントリーニ
The consolidation state of sediments provides crucial information about the pore pressure in sediments, as well as the loading and unloading history of sedimentary basins. We performed consolidation tests on mudstones and calcareous oozes just below the thick volcaniclastics in the Hellenic Arc Volcanic Field, Greece. These sediments were sampled from the International Ocean Discovery Program (IODP) Expedition 398 in the Christiana, Santorini, and Kolumbo (CSK) volcanic field.
To understand the in-situ stress and pore pressure, we compared the preconsolidation stress (pc) from the consolidation test with the in-situ effective overburden stress (σ’v) calculated from the shipboard bulk density measurement of core samples. The overconsolidation ratio (OCR = pc/σ’v) is used to identify the state of underconsolidation (OCR<1) or overconsolidation (OCR>1) at each drill site.
In the IODP Sites U1589, U1590 and U1593 in the Anydros Basin, underconsolidation states were identified in the interval 200-600 m below sea floor (OCR = 0.59 to 0.85). A maximum of 40% of the effective in-situ overburden is supported by the excess pore pressure at 200 mbsf. These underconsolidated intervals are overlain by >200 m of volcaniclastics derived from the Santorini and the Kolumbo volcanoes. Therefore, the rapid sediment supply (0.8-1.0 m/ky) from the submarine volcanoes apparently leads to the excess pore pressure, which can make sedimentary basins unstable.
On the other hand, measurements from IODP Sites U1591 and U1598 in the Christiana Basin, and Sites U1592 and U1599 in the Anafi Basin showed normal consolidation (i.e., OCR = 1) and overconsolidation (OCR =1.27-2.52) states. Sediments which showed overconsolidation are mostly composed of dolomitic mudstones. The effect of cementation is identified from their consolidation curves, implying that the intergranular bonding contributes to the overconsolidation of sediments. In the presentation, the maximum amount of erosion is calculated to explain the overconsolidation states in the Cristiana and Anafi basins.
To understand the in-situ stress and pore pressure, we compared the preconsolidation stress (pc) from the consolidation test with the in-situ effective overburden stress (σ’v) calculated from the shipboard bulk density measurement of core samples. The overconsolidation ratio (OCR = pc/σ’v) is used to identify the state of underconsolidation (OCR<1) or overconsolidation (OCR>1) at each drill site.
In the IODP Sites U1589, U1590 and U1593 in the Anydros Basin, underconsolidation states were identified in the interval 200-600 m below sea floor (OCR = 0.59 to 0.85). A maximum of 40% of the effective in-situ overburden is supported by the excess pore pressure at 200 mbsf. These underconsolidated intervals are overlain by >200 m of volcaniclastics derived from the Santorini and the Kolumbo volcanoes. Therefore, the rapid sediment supply (0.8-1.0 m/ky) from the submarine volcanoes apparently leads to the excess pore pressure, which can make sedimentary basins unstable.
On the other hand, measurements from IODP Sites U1591 and U1598 in the Christiana Basin, and Sites U1592 and U1599 in the Anafi Basin showed normal consolidation (i.e., OCR = 1) and overconsolidation (OCR =1.27-2.52) states. Sediments which showed overconsolidation are mostly composed of dolomitic mudstones. The effect of cementation is identified from their consolidation curves, implying that the intergranular bonding contributes to the overconsolidation of sediments. In the presentation, the maximum amount of erosion is calculated to explain the overconsolidation states in the Cristiana and Anafi basins.