5:15 PM - 6:45 PM
[SCG54-P02] Lithofacies and rock-magnetic properties of borehole cores drilled on the submarine caldera wall of the Kikai Caldera
Keywords:Kikai Caldera, Deep-sea Boring Machine System (BMS) , pillow lava, flow direction, AMS
The whole picture of geological setting of the Kikai Caldera in southern Kyushu remains poorly understood because almost entire caldera is submerged except for Satsuma-Iojima and Take-shima, which are located on the caldera rim. Therefore, several research cruises for seafloor survey have been conducted in elucidating detailed eruption history, such as eruptive sequence of past large-scale eruptions. In the research cruise KM22-01, borehole drilling was conducted using the Deep-sea Boring Machine System (BMS) equipped on research vessel "KAIMEI" of JAMSTEC to clarify the history of volcanic activity in Kikai Caldera (Tanaka et al., 2023). In this presentation, we report on lithofacies and rock-magnetic properties of 25-m borehole cores recovered from top of the caldera wall (180 m below sea level) northeast side of the caldera.
The lithofacies of borehole cores recovered from 0 to 6 m below sea floor (mbsf) consist mainly of sediment with pumice up to several centimeters in grain size, fine-grained volcanic glass shards and minerals, shell fragments and foraminifera. The cores below about 6 mbsf are composed of dense volcanic rocks. The cores from 6 to 10 mbsf consist of hyaloclastite characterized by fractured volcanic rock blocks embedded in fine-grained clastic material. The cores below about 10 mbsf consist of pillow lavas which are characterized by a structure of overlapping rounded clasts (lobes) surrounded by glassy rims. The lobes of pillow lavas tend to be larger at deeper at depth, with lobes about 50 cm thick at around 21 mbsf. These dense rocks contain about 20 vol.% plagioclase with maximum size of 6 mm, and the amounts of phenocryst of these rocks do not change with depth.
To estimate the flow direction of the dense volcanic rocks as emplacement progressed, we collected samples from several lobes of pillow lavas and measured the anisotropy of magnetic susceptibility (AMS), a rock-magnetic technique which contributes to fabric analysis. Because the cores were not azimuthally oriented, we arbitrarily defined azimuth of the characteristic remanent magnetization (ChRM) for each sample as having an azimuth of 0° (i.e., the north). At the results, the declination of the maximum axis of AMS (Kmax), which suggests the direction of flow, were scattered in lobes shallower than about 15 mbsf, whereas lobes deeper than about 15 mbsf were generally ESE-WNW alignment. The trend in the degree of anisotropy (P’) changed around 15 mbsf and deeper depth samples trend to have larger values of P’.
The drilling site is located on southeastern slope of a topographic high on caldera rim under the sea. The volcanic rocks sampled in this study were determined from their lithofacies to be part of lava flow erupted onto seafloor. Based on rock-magnetic properties, furthermore, the flow direction of lava is estimated to be ESE-WNW alignment, which is consistent with the slope direction of drilling site. These results suggest that this topographic high may be the source of the eruption. As for the rock-magnetic properties, a relationship between declination of Kmax and the P’ was observed, indicating more fluid results for deeper units. At least, AMS measurements provide useful information as a means of elucidating eruptive phenomena. Although the volcanic rocks indicate andesite composition (Hanyu et al., 2023), the lithofacies shows the characteristics of pillow lava, suggesting that the magma that erupted may have been of low viscosity (high temperature).
This study was conducted as part of a joint research project by the Nuclear Regulation Authority, Ibaraki University, and JAMSTEC.
References
Hanyu et al. (2023) VSJ 2023 Fall Meeting, B1-16.
Tanaka et al. (2023) Ocean and Earth Symposium 2022, 6-1.
The lithofacies of borehole cores recovered from 0 to 6 m below sea floor (mbsf) consist mainly of sediment with pumice up to several centimeters in grain size, fine-grained volcanic glass shards and minerals, shell fragments and foraminifera. The cores below about 6 mbsf are composed of dense volcanic rocks. The cores from 6 to 10 mbsf consist of hyaloclastite characterized by fractured volcanic rock blocks embedded in fine-grained clastic material. The cores below about 10 mbsf consist of pillow lavas which are characterized by a structure of overlapping rounded clasts (lobes) surrounded by glassy rims. The lobes of pillow lavas tend to be larger at deeper at depth, with lobes about 50 cm thick at around 21 mbsf. These dense rocks contain about 20 vol.% plagioclase with maximum size of 6 mm, and the amounts of phenocryst of these rocks do not change with depth.
To estimate the flow direction of the dense volcanic rocks as emplacement progressed, we collected samples from several lobes of pillow lavas and measured the anisotropy of magnetic susceptibility (AMS), a rock-magnetic technique which contributes to fabric analysis. Because the cores were not azimuthally oriented, we arbitrarily defined azimuth of the characteristic remanent magnetization (ChRM) for each sample as having an azimuth of 0° (i.e., the north). At the results, the declination of the maximum axis of AMS (Kmax), which suggests the direction of flow, were scattered in lobes shallower than about 15 mbsf, whereas lobes deeper than about 15 mbsf were generally ESE-WNW alignment. The trend in the degree of anisotropy (P’) changed around 15 mbsf and deeper depth samples trend to have larger values of P’.
The drilling site is located on southeastern slope of a topographic high on caldera rim under the sea. The volcanic rocks sampled in this study were determined from their lithofacies to be part of lava flow erupted onto seafloor. Based on rock-magnetic properties, furthermore, the flow direction of lava is estimated to be ESE-WNW alignment, which is consistent with the slope direction of drilling site. These results suggest that this topographic high may be the source of the eruption. As for the rock-magnetic properties, a relationship between declination of Kmax and the P’ was observed, indicating more fluid results for deeper units. At least, AMS measurements provide useful information as a means of elucidating eruptive phenomena. Although the volcanic rocks indicate andesite composition (Hanyu et al., 2023), the lithofacies shows the characteristics of pillow lava, suggesting that the magma that erupted may have been of low viscosity (high temperature).
This study was conducted as part of a joint research project by the Nuclear Regulation Authority, Ibaraki University, and JAMSTEC.
References
Hanyu et al. (2023) VSJ 2023 Fall Meeting, B1-16.
Tanaka et al. (2023) Ocean and Earth Symposium 2022, 6-1.