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

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.