Abstract
We reassessed the lithostratigraphy and spatial distribution of the Miocene Narisawa Formation and its associated extrusive and intrusive igneous rocks in the Sakazukiyama–Atagoyama area along the middle reaches of the Mamigasakigawa River, Yamagata City, northeast Japan. The Narisawa Formation consists mainly of gray to pale-gray pumiceous lapilli tuff that locally exhibits hyaloclastite facies. Field mapping, petrographic observations, and whole-rock X-ray fluorescence (XRF) analyses allowed us to redefine three volcanic rocks and several intrusive rock units in the Narisawa Formation: Sakazukiyama Lava, Atagoyama Lava, Atagoyama East Lava, and a suite of late-stage dark-gray dikes. The Atagoyama Lava occurs as an isolated rhyolitic lava dome, whereas the other two lava units form isolated rhyolitic domes (lava domes or cryptodomes) that occur within the outcrop area of the Narisawa Formation. The dark-gray dikes are andesitic to dacitic in composition and intrude both the basal pyroclastic facies of the Miocene Narisawa Formation and the base of the Atagoyama Lava, indicating the youngest magmatic activity in the study area. The presence of hyaloclastite facies in the pyroclastic rocks indicates that volcanic activity occurred in a submarine and sea-floor setting, and a subaqueous environment persisted from deposition of the Narisawa Formation through to emplacement of the coherent volcanic bodies. The volcanic sequence began with accumulation of the basal pyroclastic facies, followed by formation of the Sakazukiyama Lava, Atagoyama Lava, and Atagoyama East Lava, and then intrusion of the dark-gray dikes. K-Ar ages for all four of the coherent volcanic rocks cluster at ca. 12 Ma, suggesting they are nearly coeval with respect to the errors on the K-Ar dating (±0.3 Myr). These results provide a refined chronology and depositional model for the Sakazukiyama–Atagoyama area and advance our understanding of the Miocene subaqueous (submarine) volcanism in northern Japan.
