北海道南西部，濁川火山におけるカルデラ形成期～後カルデラ期の噴出物層序及び噴火推移

抄録

Nigorikawa volcano in southwestern Hokkaido, Japan, has a small caldera approximately 2 km in diameter. We carried out geologic, petrologic, and paleomagnetic studies of pyroclastic deposits on Nigorikawa volcano to reveal its eruption sequence and geologic history with high resolution. Nigorikawa pyroclastic deposits are divided into two eruption stages (1 and 2) separated by a paleosol layer ¹⁴C-dated at 12,901-12,750 calendar years. This suggests a ca. 1000 year hiatus between the Syn-caldera-forming eruption (Stage 1) and post-caldera activity (Stage 2). Stage 1 is composed of 7 units (Ng-1～Ng-7, in ascending stratigraphic order), while Stage 2 (Ng-8) is represented by a small (＞0.01 km³) pyroclastic flow deposit. The minimum total volume of the whole units (Ng-1 - Ng-8) is estimated to be 8.2 km³. Ng-1 consists of a lower (Ng-1 a) 0.11 km³ ash unit and an upper (Ng-1b) 0.53 km³ ash and pumiceous fall deposit, respectively. Ng-2 is a small (＜0.02 km³) pyroclastic flow deposit narrowly distributed in the northeast of the volcano. Ng-3 (0.02 km³) and Ng-5 (0.07 km³) are respectively sub-Plinian to Plinian pumice falls, that sandwich the Ng-4 (0.01 km³) intra-Plinian flow deposit. Ng-6 is a climactic ignimbrite (7.35 km³) further divided into Ng-6 a (lower) and -6b (upper) units based on the existence of a lithic concentration zone (LCZ) between the 2 units. In addition, co-ignimbrite ash (Ng-6c) widely covers the eastern distal area of the volcano. Ng-7 is a lithic-dominated pyroclastic surge deposit (0.07 km³) characterized by cross/parallel laminations. Ng-1 and Ng-7 commonly contain silty ash and blocky glass shards with moss-like morphology suggesting that they were formed by magma-water interaction (phreatomagmatic eruption) that occurred at the initiation and termination of Stage 1. Ng-8 (Stage 2) is a newly discovered eruption unit. Paleomagnetic features demonstrate that Ng-8 is a lateral flow deposit that was emplaced at a high (350-400 °C) temperature. Stage 2 can be stratigraphically and chronologically correlated with the post-caldera activity of the volcano that generated (1) tephra in the caldera-lake (lacustrine) deposits and (2) intrusive rocks (lava and dykes) through the caldera-fill deposits, both of which are described in borehole samples by previous work. Amphibole andesite lava fragments showing oxidization coating and having slightly different chemistry than co-existing juvenile pumice are generally included in all Nigorikawa pyroclastic deposits. We speculate that the older edifice of the andesitic lava dome (1.7 km³) existed before the Nigorikawa caldera formed.