It has been inferred that Lake Tazawa (Tazawa-Ko or Tazawako in Japanese) is a caldera lake, but the products of the caldera-forming eruption have yet to be confirmed. Here, we report the discovery of the rhyolitic pyroclastic density current deposits (PDCs) that erupted from Tazawa-Ko between 2 and 1.8 Ma and shall propose to call this confirmed caldera the Tazawako caldera after the name of Tazawako Town (Tazawako-Machi), where Lake Tazawa (Tazawa-Ko) is located. The lower PDCs are wavy to parallel-stratified deposits that are dominated by polyhedral and platy glass shards, suggesting that the caldera-forming eruption began with phreatomagmatic explosions. The upper PDCs are poorly sorted, largely non-stratified deposits that are closely associated with the debris avalanche deposits around Tazawa-Ko, and are welded where they occur in a valley 30 km to the north of Tazawa-Ko, thereby representing the climactic eruption. These caldera-forming eruption products are limited in volume even at proximal locations, presumably due to a long-term erosion. However, the total eruption volume is estimated to be ~1011 m3 with the ~8-km-diameter of the source caldera. This volume is supported by correlative fallout ash observed up to 800 km from Tazawa-Ko. Two lava domes erupted on the caldera floor, and two lava flows were effused on the somma between 1.8 and 1.6 Ma, following caldera formation. While pre-caldera volcanism remains largely unknown, a small volume of andesite lava flow was extruded from a site on the southeastern caldera rim. Finally, we shall propose to call this confirmed caldera the Tazawako caldera after the name of Tazawako Town (Tazawako-Machi), where Lake Tazawa (Tazawa-Ko) is located.

We report new uranium-lead (U-Pb) zircon ages for strata in the Sanjakugoshaku Formation in the lower part of the Eocene Nogata Group of the Chikuho Coalfield, Fukuoka, northern Kyushu, western Japan, to provide a better geochronological control for the group than a previously reported fission-track date (44.2±3.4 Ma) from the Uwaishi Formation in the upper part of the Nogata Group. The U-Pb zircon dates were obtained via laser ablation-inductively coupled plasma-mass spectrometry (LA-ICPMS) for four different tuff and tuffaceous layers in the Sanjakugoshaku Formation, yielding ages of 46.18±0.59, 44.21±0.49, 41.53±0.28, and 40.34±0.20 Ma, in stratigraphic order. These new dates indicate that deposition of the Sanjakugoshaku Formation lasted for more than 5 Myr (including hiatuses), with this period corresponding to most of the chronological interval of the Okinoshiman molluscan Stage (middle middle Eocene to late Eocene?) in Kyushu. Furthermore, the new dates suggest that the basal part of the Sanjakugoshaku Formation and underlying Oyake Formation (the lowest unit of the Nogata Group) are chronologically correlated with the older Takashiman molluscan Stage (early Eocene to early middle Eocene). Therefore, the tectonic processes responsible for the formation of the sedimentary basin of the Chiku-ho Coalfield appear to have started at latest by the early middle Eocene.

We measured the uranium-lead (U-Pb) and fission-track (FT) ages of single zircon grains from three samples (TKB3, TKB6, and TKB7) of tephras that are intercalated with Neogene to Quaternary deposits in the Horonobe area, Hokkaido, Japan, to understand their eruption ages. The weighted mean U-Pb ages and pooled FT ages of the youngest zircon grains are 1.43±0.18 Ma (2σ) and 1.14±0.40 Ma (1σ) for TKB3, and 7.55±0.14 Ma (2σ) and 6.62±0.43 Ma (1σ) for TKB6, respectively. Their U-Pb and FT ages overlap within their respective 2σ uncertainties, which suggests that their ages correspond to the eruption ages. Specifically, the TKB6 tephra age is consistent with the diatom fossil ages from the overlying and underlying silt deposits. The FT ages for most of the zircons in TKB7 are reset to their U-Pb ages or younger. The pooled FT age for all of the analyzed zircons (1.52±0.19 Ma; 1σ) and weighted mean U-Pb age for zircon grains within the youngest age range (1.34±0.16 Ma; 2σ) are coincident within their respective 2σ uncertainties. Therefore, their ages could also show the eruption age. TKB3 and TKB7 are from the Quaternary Sarabetsu Formation, and their U-Pb and FT ages correspond to the stratigraphy. Although TKB6 lies at the boundary between the Yuchi and Koetoi formations, its U-Pb and FT ages are significantly older than the reported depositional age of the Yuchi Formation. The Neogene to Quaternary depositional age of this horizon tends to be older from west to east in the Horonobe area. The older age of TKB6, which is located in the easternmost part of the study area, is consistent with this stratigraphic trend.

Here, we report zircon uranium-lead (U-Pb) dating by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) to better constrain the eruption ages of seven widespread Quaternary tephras distributed across Niigata Prefecture, Japan. Two samples (HK10, Og) are from well-documented tephras in the literature, and the other five samples (Odaira, Yokokura, Kimigaeri, Oguriyama, Shimizu Pass) are from local districts in Niigata Prefecture. We applied three different U-Pb age calculations which yielded ages of 0.4-1.7 Ma for the samples. The U-Pb ages for the >0.5 Ma tephras are similar among the three methods, whereas a small discrepancy exists for the <0.5 Ma tephras. Our U-Pb ages are compared with previous age estimates in the literature, and five tephras (HK10, Og, Yokokura, Kimigaeri, Oguriyama) yield consistent ages between the present and previous studies. The ages of the other two tephras (Odaira, Shimizu Pass) differ from previous estimates, whose reasons are also mentioned.