Eruptive and depositional processes of a low-aspect-ratio ignimbrite (the Southern Kusandong Tuff, South Korea) inferred from magnetic susceptibility variability

https://doi.org/10.1016/j.jvolgeores.2021.107374Get rights and content

Highlights

  • Magnetic susceptibility helps to characterize the massive parts of an ignimbrite.

  • There exist magnetic susceptibility modifications by post-depositional alterations

  • The massive parts were made by accumulation of multiple cooling units.

  • Our results support generation of sustained lateral blasts by shallow-level explosion.

Abstract

Low-aspect-ratio ignimbrites, produced by the most lethal volcanic eruptions, are thin but laterally extensive and can serve as good stratigraphic markers in many sedimentary basins. They are commonly structureless, and require diverse analytical tools to reveal the related geological processes. This study aims to understand better the eruptive and depositional processes of the Cretaceous Kusandong Tuff, which is a several meter-thick and laterally extensive silicic ignimbrite in Korea, through rock-magnetic analyses of magnetic susceptibility (k) and isothermal remanent magnetization (IRM) together with optical and electron microscopic observations. Massive parts of the tuff show systematic vertical variation in k within each locality, and the vertical variation patterns vary from locality to locality, being classified broadly into three types: a single cycle of upward-decreasing k (type A), repetition of two or more upward-decreasing k (type B), and consistently low k (type C). A linear relationship between k and IRM indicates predominant contribution of ferri- and antiferro-magnetic minerals to k in the majority of the tuff. Integrated results of microscope images, temperature variations of k, and IRM unmixing analyses indicate that the major magnetic minerals are coarse magnetite (with occasionally partial maghemitization) of both primary and secondary origins, very fine secondary magnetite, and secondary hematite and goethite. Quantitative comparison of the unmixed magnetic mineral components reveals that those vertical k variations are related to some combination of magnetite destruction and subsequent precipitation of antiferromagnetic minerals by post-depositional alterations at different stages (types A, B, and C) and magnetite supply by deposition of additional cooling unit (type B). Despite the adverse post-depositional modifications of k, this suggests that the massive part of this tuff is a composite unit formed by accumulation of multiple cooling units from multiple pyroclastic current pulses probably associated with fluctuations in eruption rate at one or multiple source vent(s). Comparison by a k-based proxy between localities implies independence of primary magnetic mineral content on the distance from vent(s), potentially implying convection-dominated behavior in the pyroclastic current dynamics. This possible flow dynamics is inferred to be generated by sustained laterally-directed blasts from shallow-level explosion and could, in turn, lead to long-distance travel. This study exemplifies how the methodological approach using magnetic susceptibility can contribute to characterize the nature of low-aspect-ratio ignimbrite, and will help make more use of the Kusandong Tuff as a key regional stratigraphic marker as well.

Introduction

Ignimbrites are welded or unwelded deposits of pyroclastic density currents generated by explosive volcanic eruptions. They consist of pumice, crystals, and rock fragments in a glass-shard matrix and can extend over 100 km from their sources (Branney and Kokelaar, 2002; Sulpizio et al., 2014). Due to their unique lithology and extensive areal distribution, ignimbrites often serve as key markers that can aid in correlating basinfill strata over wide areas and establishing the chronostratigraphic framework of the basinfill as well as in understanding the nature of the eruptions and delineating the related volcanic hazards (de Silva and Francis, 1989; Fackler-Adams et al., 1997; Piper et al., 2002; Sohn et al., 2005; Lebti et al., 2006; Jeong et al., 2008; Sohn et al., 2009; Ort et al., 2013; Alva-Valdivia et al., 2019). Detailed characterization of the spatio-temporal lithofacies changes of an ignimbrite is necessary in order to draw meaningful stratigraphical and volcanological information. However, ignimbrites are mostly structureless and ancient ones can be chemically altered and lithified, thereby necessitating a number of analytical tools to be applied for an investigation of ignimbrites.

Magnetic susceptibility (k), which is the degree of magnetization of a material in response to an applied magnetic field strength, is one of the major properties of minerals and has been used to resolve a wide range of geologic issues in relation to orogeny, magma emplacement and genesis, origin of ore deposits, metamorphism and sedimentary processes (e.g., Cho and Kwon, 1994; Ellwood et al., 2001; Saint-Blanquat et al., 2001; Geoffroy et al., 2002; Lanci et al., 2002; Mattei et al., 2002). Analytical methods using k may also be useful for studying ignimbrites. Indeed, bulk k and its temperature variation, and anisotropy of k (AMS) have been proven useful in identifying (ferri- and antiferro-)magnetic mineral types in ignimbrite, detecting ignimbrite’s post-depositional alteration which commonly leads to destruction and transformation/formation of magnetic minerals, constraining flow direction and location of source vent of the ignimbrite-forming pyroclastic density current, and correlating separated ignimbrite exposures (Rosenbaum and Spengler, 1986; Palmer et al., 1996; Cagnoli and Tarling, 1997; Le Pennec et al., 1998; Zanella et al., 1999; Piper et al., 2002; Ort et al., 2003; Lebti et al., 2006; Paquereau-Lebti et al., 2008; Ort et al., 2013; Alva-Valdivia et al., 2017).

The Kusandong tuff (KT) is a crystal-rich and pumice-free ignimbrite, about 1-5 m thick and distributed over 200 km in the Gyeongsang Basin (Jeong et al., 2005; Sohn et al., 2005; Jeong, 2006; Sohn et al., 2009), which is the largest Cretaceous sedimentary basin in South Korea (Chough and Sohn, 2010). Because of its unique lithology and lateral continuity, the tuff has been used as a key bed in the Gyeongsang Basin for basinwide stratigraphic correlation and reconstruction of the Cretaceous tectonic and sedimentary evolution in the Korea Peninsula (Chang, 1975; Chang et al., 2003; Jeon and Sohn, 2003; Chough and Sohn, 2010). The KT was once presumed to be a single tuff bed. However, the KT was revealed to have different lithofacies characteristics between the northern and southern parts, and has, therefore, been divided into the Northern Kusandong Tuff (NKT; not shown in Fig. 1a) and the Southern Kusandong Tuff (SKT; shown in Fig. 1a) by Jeon and Sohn (2003). The NKT is richer in crystals and consists of a massive division (MD) and a stratified upper division (SUD), whereas the SKT is poorer in crystals and consists of a basal layered division (BLD) and an overlying massive division (MD) (Jeong et al., 2005; Sohn et al., 2005, Sohn et al., 2009). The BLD and MD of the SKT are interpreted to be deposits of a pyroclastic surge (dilute pyroclastic density current) immediately followed by a pyroclastic flow (relatively dense pyroclastic density current; Sohn et al., 2005, Sohn et al., 2009).

Sohn et al., 2005, Sohn et al., 2009 inferred that the source vent of the SKT was located several tens of kilometers to the east of the present exposures based on both grain fabric and AMS data, and that the present exposures represent a transverse section of the SKT (Fig. 1a). The absence of pumice and dense juvenile/cognate clasts and the extremely low content of accidental components derived from the pre-Cretaceous basement rocks in the SKT were interpreted to be due to rapid ascent of silicic magma and its shallow-level fragmentation, followed by immediate generation of pyroclastic currents (Sohn et al., 2009) by immediate collapse of a low eruption column (e.g., Hildreth and Mahood, 1986) or directly by explosive expansion (e.g., Legros and Kelfoun, 2000).

After the pioneering work of Sohn et al. (2009), the MD of this ignimbrite has been barely investigated, especially for its eruptive and depositional processes. It is essential to confirm the previous interpretation via additional independent methodological approach where possible, and to further improve our knowledge on ignimbrite emplacement processes. This study presents vertical and lateral variations of bulk magnetic susceptibility (k) and magnetic minerals (contributing to k) from MD exposures of the SKT at 12 localities, to clarify the eruption and deposition processes of the MD in more detail.

Section snippets

Geological setting

In the Korean Peninsula, non-marine sedimentary basins were produced in the backarc region of the Japanese Arc by the subduction of the proto-Pacific plate (or the Izanagi plate) beneath the eastern margin of the Eurasian continent during the Cretaceous (Chough et al., 2000; Chough and Sohn, 2010; Fig. 1b). These basins experienced diverse volcanic activities resulting in a number of volcanic interbeds in the basinfill and cauldron structures in the basins (Fig. 1c; Cha and Yun, 1988). The

SKT

The SKT is distributed over a distance of 100 km (Fig. 1a). The sedimentary strata beneath the SKT are characterized by the alternation of purple and gray mudstones commonly containing mudcracks, carbonate nodules and thin sandstone interbeds, suggesting that the SKT-forming pyroclastic current(s) flowed on a wide, gentle-sloping alluvial plains and shallow lakes with little topographic relief (Sohn et al., 2005).

The glassy ash matrix of the SKT (see also Figs. 1d and e, which show field

Microscopic observations

Under optical microscope, selected MD samples from all localities contain about 0.1–0.7% of opaque minerals, showing various shapes within the tuff (Figs. 2a, b). Of which, the samples from locality 02-1 have the lowest concentrations of opaque minerals. The majority of the opaque minerals occur as solitary coarse (~0.1–0.3 mm) phenocrysts within the fine-grained volcanic ash matrix (Fig. 2a). Some of them are included in other phenocrysts (i.e., plagioclase, biotite), showing two different

Magnetic methods

Bulk magnetic susceptibility (k) for the studied samples was first measured at the outcrops using a portable SM-20 magnetic susceptibility meter (GF Instruments) in order to determine its variation on a vertical outcrop section. The SM-20 measurement was made three times at each point, and the mean values at individual points were used in the following considerations. To obtain more accurate magnetic susceptibility values and then cross-check the SM-20-derived values, cylindrical cores, 25 mm

k values

The ksm-20 measurements covered a wider range of vertical section within each locality, than those of the kminisep. Mean and standard deviations of these kminisep and ksm-20 values for individual localities are given in Table 1. We first address the kminisep values because of their higher precision. Individual kminisep values for specimens are in a wide range of 41–6,032 (3–480 ×4π) ×10−6 SI. For some stratigraphic levels within a locality, ksm-20 and kminisep values are generally in good

Major magnetic minerals contributing to k

Bulk magnetic susceptibility (k) of natural samples generally depends on the type, content, and grain size distribution of magnetic minerals. The major magnetic minerals that contributed to k could be determined by microscopic observations, thermomagnetic curves, and IRM progressive acquisitions (see Sections 4 and 6), which suggest that the analyzed samples mostly contain magnetite (with different types based on size and morphology and textural relationship with the surrounding minerals),

Conclusions

The massive division (MD) of the SKT, a Cretaceous low-aspect-ratio ignimbrite in Korea, exhibits three types of vertical variations in bulk magnetic susceptibility (k) at different localities: a single cycle of upward-decreasing k (type A), repetition by two or more upward-decreasing k (type B), and consistently low k (type C). We find that the majority of the MD contains coarse magnetite, occasionally with partial maghemitization, of both primary and secondary origins, much finer magnetite

Funding

This work was funded by the National Research Foundation of Korea (NRF-2017R1A2B2007773) and partly by the Korea Institute of Geoscience and Mineral Resources (GP2020-003).

Data availability

The data presented in this paper are available from JJ and HA on reasonable request.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We thank the editor Heidy M Mader for handling the reviews, and two anonymous reviewers for constructive comments which greatly improved the manuscript. We thank Yong-Hee Park of Kangwon National University for allowing us to use his paleomagnetic facilities for additional experiments of IRM progressive acquisition. HA is grateful to Yuhji Yamamoto, Myriam Kars, and research/technical staffs of Center for Advanced Marine Core Research, Kochi University, for their kind support. HA also thank

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    Present Affiliation: Geological Research Center, Geology Division, Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejeon 34132, Republic of Korea

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