2:00 PM - 2:15 PM
[SCG57-02] Pre-caldera-forming phases during the 7.3ka Kikai Caldera Eruption
Keywords:Kikai Caldera, plinian eruption , intraplinian pyroclastic flow, caldera-forming eruption
The 7.3 ka eruption (Akahoya Eruption) of the Kikai caldera off southern Kyushu is the largest in the Japanese Archipelago during the Holocene period. On this eruption, previous studies have shown the existence of plinian (Unit A) and intraplinian pyroclastic flow (Unit B) phases preceding the climax caldera-forming phase. However, detailed eruption sequences of these pre-caldera-forming phases have not been sufficiently studied. In this study, we attempted to more precisely understand the eruption deposits based on geological surveys and to compare the stratigraphy between the proximal areas (Satsuma Iwojima and Takeshima) and the distal areas (Osumi and Satsuma Peninsula, Tanegashima, and southernmost Miyazaki Prefecture).
Unit A in the proximal areas is basically divided into five subunits (Unit A0-4 from the lower part). Unit A0 is a well-sorted ash layer mainly composed of very fine pumice. Unit A1 is a single layer of white coarse pumice and shows typical facies of plinian deposits. Unit A2 is a poorly sorted peach-gray ash layer very rich in fine volcanic glass ash and is found only in Takeshima. Unit A3 is a well-sorted pumice layer very rich in altered lithics. Unit A4 is a coarse pumice layer similar to Unit A1 but is larger in both thickness and grain size. In some outcrops, pumice layers exceptionally large in both thickness and grain size compared to the basic stratigraphy described above are observed.
In the southern and southernmost Osumi Peninsula Unit A consists of multiple subunits, while it is a single layer in the other distal areas. Unit A in the southernmost Osumi Peninsula consists of at least seven pumice falls and intervening ash-rich layers. The lowest fine pumice fall is a single layer and is comparable to Unit A1 in the proximal areas by component analysis. Multiple layers overlying Unit A1 distal facies are stratigraphically comparable to Unit A3-4 in the proximal area. The lower part of the Unit A3-4 distal facies is composed of alternating beds of at least four lithic-rich fine pumice layers and intervening red-ash-rich layers. The upper part of Unit A3-4 distal facies is a coarse pumice layer that shows reversed grading. This subunit is notably larger than the other subunits both in thickness and grain size. Unit A distributed in distal areas other than the southern and southernmost Osumi Peninsula is traced to this uppermost subunit. The tephra volume of Units A1 and A3-4 is estimated to be ~0.4 km3 and ~10 km3, respectively.
Unit B is confirmed only in the proximal areas. In general, Unit B is a valley-filling welded tuff, and the lowermost part is intercalated by Unit A. In Takeshima, Unit B has poorly sorted, non- or weakly welded stratified facies. In Satsuma Iwojima, cross-stratified facies dominate in the lower part while stratified facies dominate in the upper part. They are generally densely welded, but sometimes the upper part horizontally transitions to poorly sorted non-welded facies.
Based on these results, we interpreted the deposits of pre-caldera-forming phases as below. Unit A0 is the product of a small event at the beginning of the 7.3 ka eruption. Unit A is divided into two plinian events (Units A1 and A3-4) by Unit A3, which is thought to indicate a vent-opening or rapid expansion of a conduit. Unit B is considered to be a dilute, high-temperature intraplinian pyroclastic flow deposit produced by the collapse of a plinian column that fed Unit A3-4. The interpretation by previous studies of the upper part of Unit B in Satsuma Iwojima as a fallout deposit should be reconsidered. Based on the local anomaly of Unit A in the proximal areas and discussion on the number of subunits of Unit A4 between proximal and distal areas, most of Unit A4 in the distal areas is considered to be coeval with Unit B. The coexistence of intraplinian pyroclastic flows and a plinian column due to partial column collapse may have started early in the Unit A3-4 event.
Unit A in the proximal areas is basically divided into five subunits (Unit A0-4 from the lower part). Unit A0 is a well-sorted ash layer mainly composed of very fine pumice. Unit A1 is a single layer of white coarse pumice and shows typical facies of plinian deposits. Unit A2 is a poorly sorted peach-gray ash layer very rich in fine volcanic glass ash and is found only in Takeshima. Unit A3 is a well-sorted pumice layer very rich in altered lithics. Unit A4 is a coarse pumice layer similar to Unit A1 but is larger in both thickness and grain size. In some outcrops, pumice layers exceptionally large in both thickness and grain size compared to the basic stratigraphy described above are observed.
In the southern and southernmost Osumi Peninsula Unit A consists of multiple subunits, while it is a single layer in the other distal areas. Unit A in the southernmost Osumi Peninsula consists of at least seven pumice falls and intervening ash-rich layers. The lowest fine pumice fall is a single layer and is comparable to Unit A1 in the proximal areas by component analysis. Multiple layers overlying Unit A1 distal facies are stratigraphically comparable to Unit A3-4 in the proximal area. The lower part of the Unit A3-4 distal facies is composed of alternating beds of at least four lithic-rich fine pumice layers and intervening red-ash-rich layers. The upper part of Unit A3-4 distal facies is a coarse pumice layer that shows reversed grading. This subunit is notably larger than the other subunits both in thickness and grain size. Unit A distributed in distal areas other than the southern and southernmost Osumi Peninsula is traced to this uppermost subunit. The tephra volume of Units A1 and A3-4 is estimated to be ~0.4 km3 and ~10 km3, respectively.
Unit B is confirmed only in the proximal areas. In general, Unit B is a valley-filling welded tuff, and the lowermost part is intercalated by Unit A. In Takeshima, Unit B has poorly sorted, non- or weakly welded stratified facies. In Satsuma Iwojima, cross-stratified facies dominate in the lower part while stratified facies dominate in the upper part. They are generally densely welded, but sometimes the upper part horizontally transitions to poorly sorted non-welded facies.
Based on these results, we interpreted the deposits of pre-caldera-forming phases as below. Unit A0 is the product of a small event at the beginning of the 7.3 ka eruption. Unit A is divided into two plinian events (Units A1 and A3-4) by Unit A3, which is thought to indicate a vent-opening or rapid expansion of a conduit. Unit B is considered to be a dilute, high-temperature intraplinian pyroclastic flow deposit produced by the collapse of a plinian column that fed Unit A3-4. The interpretation by previous studies of the upper part of Unit B in Satsuma Iwojima as a fallout deposit should be reconsidered. Based on the local anomaly of Unit A in the proximal areas and discussion on the number of subunits of Unit A4 between proximal and distal areas, most of Unit A4 in the distal areas is considered to be coeval with Unit B. The coexistence of intraplinian pyroclastic flows and a plinian column due to partial column collapse may have started early in the Unit A3-4 event.