Deposition from pyroclastic surges partially blocked by a topographic obstacle: an example from the Ilchulbong tuff cone, Jeju Island, Korea

초록

Ilchulbong (Jeju Island, Korea) is a UNESCO World Heritage site that has outstanding coastal exposures of a tuff cone formed by a phreatomagmatic eruption of basaltic magma in a shallow sea. Wet fallout and subsequent resedimentation processes were predominant means of emplacement of the tuff cone. The marginal deposits of the tuff cone show, however, peculiar deposit features which resulted from the interaction of wet pyroclastic surges and a topographic obstacle, a 30 m-high dissected scoria cone. These features include steeply dipping, inversely graded strata, a breccia bed composed of thin-laminated fine tuff clasts, and isolated tuff clasts within some lapilli tuff beds. These features indicate that the depositional system (underflow or bedload layer) and the transport system (overlying suspension current) of the pyroclastic surges underwent different responses upon colliding with the topographic obstacle. The coarse-grained materials in the depositional system were mostly blocked by the obstacle, producing a scree-like wedge of deposits containing inversely graded grain-flow strata. In contrast, the transport system was not significantly affected by the obstacle, maintaining its momentum and original flow direction after passing over the dissected scoria cone. Some of the fine-grained load in the transport system was, however, plastered onto the cliff wall. The plastered tuffs were later detached and fell from the wall, resulting in an intraformational breccia bed and exotic tuff clasts within some lapilli tuff beds. The studied site, located within a scenic World Natural Heritage site, can be utilized as an important geosite for the education of the processes which occur during the collision of a pyroclastic density current with a topographic obstacle and the different fates of the depositional and transport systems of a pyroclastic density current.

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