A dataset for distribution and characteristics of Holocene pyroclastic fall deposits along the Pacific coasts in western Hokkaido, Japan

The tephra layers known with eruption ages play an important role in an investigation of tsunami history and archaeology in addition to volcanic history in Hokkaido, Japan. We investigated the event and tephra layers of the Late Holocene in the Pacific coast of western Hokkaido, where the stratigraphy of the Late Holocene has not been clarified. Surveys in coastal peatlands, mostly undisturbed deposits, have allowed for the discovery of thin tephra layers. The newly discovered tephra layers at the unexplored site were used to describe facies, observation under a polarization microscope, refractive index measurement of volcanic glasses, and chemical analysis, and correlated with the reported widespread tephras. We conducted wide-area field surveys and succeeded in revealing a wider distribution of tephra layers than previously known. The distribution of volcanic ash in the coastal area will contribute to the investigations of future volcanic and coastal hazards.


Specifications
Stratigraphy; Tephrochronology Specific subject area Volcanology; Geochemistry  Type of data  Tables and figures  How data were acquired Geological fieldwork (sampling procedure) JEOL JXA 8900R (Electron Probe Micro Analyzer: EPMA). JEOL JSM-T330A, Link ISIS300 (Energy Dispersive X-ray Spectrometer: EDS) RIMS 20 0 0 (Refractive Index Measuring System) Data format Raw Parameters for data collection The refractive index of volcanic glass was measured by dehydrating the samples so that the variation due to hydration was reduced. The chemical analysis of volcanic glass by EPMA and EDS was carried out by adjusting the current value and beam diameter to prevent the ionization of light elements. Description of data collection Layer thickness and stratigraphy were described from sampling core and coastal outcrops. The core samples were collected at several nearby sites and checked for variability. Data source location Samples were analyzed at the University of Tokyo, Kashiwa, Japan and Hokkaido University of Education, Sapporo, Japan. Sampling locations are listed in Fig. 1 and Tables 2. Data accessibility All the data sets are available with this article.

Value of the Data
• Tephra distribution data are commonly used as chronological markers in Hokkaido.
• These data can contribute to reconstruct the magnitude and intensity of past explosive eruption in Hokkaido, Japan, and to model possible future eruptive scenarios for hazard assessment. • The data can be used to constrain better the chronology of past coastal hazards (i.e., tsunami, storm) to assist archeological investigations for temporal evolution.

Data Description
Fig . 1 shows the study area and selected stratigraphic columns. Each volcanic ash layer was comprehensively correlated by comparing the layer facies (stratigraphic sequence, grain size, and coloration), mineral compositions, refractive index, and chemical composition of volcanic glass with those reported in Hokkaido [1][2][3] . The tephras widely distributed along the Pacific coast of western Hokkaido (Hidaka, Iburi, Uchiura bay, and Kameda Penisula) correspond to Komagatake c2 tephra (Ko-c2: AD1694), Tarumae b tephra (Ta-b: AD1667), Usu b tephra (Us-b: AD1663), Komagatake d tephra (Ko-d: AD1640), Baegdusan Tomakomai tephra (B-Tm: AD946), and Tarumae c2 tephra (Ta-c2: approximately BC400). The Us-b tephra is subdivided into units. The distribution is different between the Plinian eruption deposits (unit B) and the phreatomagmatic deposits (units A, C, E, F, and G: [ 1 , 4 ]). Fig. 2 shows photographs of representative cores and outcrops in each region. Table 1 shows the chemical composition of the confirmed tephra, and Fig. 3 shows the percentage of constituent minerals and the histogram of the refractive index. The scatter plots of K 2 O and TiO 2 , which are useful for the identification of volcanic ash [ 1-3 , 5 ], are presented as one of the premises on which we correlated unknown tephras of known age.
We describe the correlation of each tephra for each region where the combination of tephra layers is similar. In Kameda Peninsula and western Uchiura bay region, volcanic ash layers of Komagatake and B-Tm were identified. B-Tm was easily determined from the unique chemical composition (high potassium) and fine and good sorting grain at all sites. Komagatake tephras (Ko-d and Ko-c2) at Sites 2 and 3, which were difficult to identify from the stratigraphy and mineral compositions, were identified by the scattered plots of SiO 2 , K 2 O, and CaO [8] . From western Iburi to eastern Uchiura Bay, we identified B-Tm, Us-b: fine-grained phreatomagmatic   units, Ko-c2 as coarse-grain and mafic minerals are widely observed. Ko-d tephra is occasionally found in patches below Us-b. Tokui [2] and Nakamura et al. [9] described in detail the stratigraphy of the eastern part of Iburi to the northern part of Hidaka; thus, we do not report here the details of the stratigraphy of this area. In the southern part of Hidaka, we identified Ta-c2, B-Tm, Us-b, Ta-b, and Ko-c2. Ta-c2 was found below B-Tm as dark orange and fine-grain volcanic ash. Peat layers rarely separate the three tephras deposited in the 17th century are rarely separated because of gaps of only a few years to a few decades. However, they can be distinguished based on the combination since unit B of Us-b is mainly pumice and Ta-b is fine-grain in contrast. The identification supported by the K 2 O-TiO 2 diagrams is plotted in different areas due to the different source volcanoes of each tephra. Fig. 4 and Table 2 show the layer thickness distribution of tephras at each study site.

Field survey
A total of 431 samples from cores and outcrops were obtained at the 42 sites along the Pacific coast by using a handy Geoslicer and a Peat Sampler (diameter of 7 cm) with lengths Table 1 Major element analyses of volcaniclastic glass. The result for each oxide is shown as the mean and deviation of normalized weight% and, N is a number of analyzed glass shards.     [2] , Aoki and Machida [5] , Furukawa and Nanayama [3] , and Nakamura [1] are color shaded. AT is a working standard sample analyzed in each analysis. Scatter plots for SiO 2 -K 2 O and SiO 2 -CaO contents of glass shards by EPMA to identify for historical Komagatake tephras. of either 0.6 m or 2.5 m. Photographs were taken of each core sample, and sedimentary facies (stratigraphic sequence, thickness, and presence of pumice) were described.

Sample preparation
To separate volcanic glass and rock-forming minerals from the clay, each sample was vibrated in an ultrasonic cleaning device and the clay components were removed with water elutriation. Samples were dried at 70 °C. The dried samples were divided into 0.063-0.125 phi and 0.125-0.25 phi using a sieve. Refractive index measurements and microscopic observations were conducted for samples of 0.063-0.125 phi and 0.125-0.25 phi, respectively. The samples measuring the glass refractive index were dehydrated by annealing at 400 °C for 12 h in an electric furnace [9] .  Fig. 4. The open circles and black or blue numbers show the location of survey sites and thickness (cm) distribution of each widespread tephra revealed by this study, respectively. Cross marks show patchy tephra layers or thickness less than 1 cm. For comparison, the isopachs of each tephra from Nakamura [1] are shown as gray solid and dashed lines. Gray numbers and cross marks indicate the tephra thickness distribution reported by Tokui (1993), Furukawa and Nanayama [3] , and Nakamura [1] . Measuring System (RIMS 20 0 0: Kyoto Fission Track Co., Ltd.). This method is possible to measure with an overall accuracy of ±2 × 10 −4 and a precision of ±1 × 10 −4 [10] .

Chemical analysis of volcanic glass
Chemical analysis of volcanic glass was performed using a JEOL JXA 8900R Electron Probe Micro Analyzer (EPMA) and a JEOL JSM-T330A (Link ISIS300) Energy Dispersive X-ray Spectrometer (EDS). EPMA operating conditions were 15 kV acceleration voltage, 7 nA beam current, beam scanned area of 10 μm, and counting time was 10-60 s at the peak position and 5-30 s at the background-position [11] . EDS operating conditions were 15 kV acceleration voltage, 1.1 nA specimen current, and the beam scanned an area of 3 μm. All analysis results were corrected using the oxide ZAF method. The AT tephra from Aira Caldera was used as an in-house standard to check any difference between the reference values [5] . The mean values of approximately 10 grain with a detection analysis values more than 90% by weight, and it was normalized to 100%.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships which have, or could be perceived to have, influenced the work reported in this article.