Flute-like marks at the base of Plinian pumice-fall deposits at Ohachidaira caldera, Hokkaido, Japan

Flute-like marks at the base of Plinian pumice-fall deposits at Ohachidaira caldera, Hokkaido, Japan | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Flute-like marks at the base of Plinian pumice-fall deposits at Ohachidaira caldera, Hokkaido, Japan Yuki Yasuda, Fukashi Maeno This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4379634/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 15 Feb, 2025 Read the published version in Bulletin of Volcanology → Version 1 posted 5 You are reading this latest preprint version Abstract We describe flute-like marks at the boundary between the sub-Plinian and overlying Plinian units of the 34 ka eruption at the Ohachidaira caldera and discuss their formation mechanisms. These scours are observed at a proximal section ~ 1.8 km from the caldera center (~ 0.9 km from the rim) and show asymmetric cross-sectional shapes that closely resemble flute marks typically found in subaqueous sedimentary rocks; they cut into underlying strata of the immediately preceding sub-Plinian fallout deposits and are filled with pumice lapilli of the Plinian phase. Another characteristic erosional feature is shear deformation of the top ash layer of the sub-Plinian deposits; this ash layer is locally bent, folded, split, or fragmented. These erosional features appear to be coeval with the basal part of the Plinian unit. We interpret these features, based upon their field characteristics, to have been formed by dilute turbulent pyroclastic currents that occurred at the beginning of the Plinian phase. Observations allow the estimation of timescales for formation of the flute-like marks to be at ~ 0.5–5 min, which is rarely obtained from flute marks in sedimentary rocks. The only deposit left by the erosive currents is a thin fine ash lens that occurs at the bases of the flute-like marks, and no pyroclastic current deposits or erosional features were observed at the base of the Plinian sequence beyond 1 km from the caldera rim, suggesting short runout distances of the dilute pyroclastic currents; such small currents might have resulted from a local collapse along the margin of the Plinian column. Flute-like mark Shear deformation structure Erosion feature Proximal Plinian fall deposit dilute turbulent pyroclastic current Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Introduction Flute marks are erosive features typically observed at the bases of marine sandstones and can be used as an indicator of paleocurrent directions (e.g., Collinson and Mountney 2019 ; Peakall et al. 2020 ; Baas et al. 2021 ); they have an asymmetric concave-up shape with the deepest point located near their upstream end, and from the deepest point features flare away and decrease in depth toward the downstream end (Allen 1982 ). Such features are formed by differential erosion by turbulent eddies of an overlying turbidity current, in which the rates of erosion in growing flute marks are larger than those for the surrounding beds due to flow separation (Allen 1968 , 1969 ; Collinson and Mountney 2019 ). Flute marks could also be formed during aeolian sediment transport (Lancaster 1984 ; Sebe et al. 2015 ), but such systems are not rapidly covered by subsequent flows resulting in the scarcity of these marks in the aeolian settings. Although pyroclastic currents could potentially produce flute marks on the substrate as other types of sole marks (e.g., groove and impact marks at the base of the pyroclastic current deposits, Tenerife; Pittari and Cas 2004 ) or other erosional structures (e.g., U-shaped channels in the base surge deposits at Koko Crater, Hawaii; Fisher 1977 ), flute marks have rarely been reported in the volcanological literature. Recognition of flute marks in pyroclastic sequences would provide information on flow directions and on the flow dynamics of pyroclastic currents that produced the marks. Cole et al. ( 2002 ) is the only known example that recognized flute marks beneath the block-and-ash flow deposits at Soufrière Hills Volcano, Montserrat, but their geometries and features were not documented. This paper describes scours that resemble flute marks (here referred to as flute-like marks) and related erosional structures at the base of proximal Plinian fall deposits from the Ohachidaira caldera (Fig. 1 ). Detailed observations of such features provide evidence that they were formed during the passage of dilute turbulent pyroclastic currents at the beginning of the Plinian eruption. Grain size terminology in this paper follows that of White and Houghton ( 2006 ). Plinian phase of the Sounkyo eruption The pumice-fall deposits, in which flute-like marks are observed, are products of the initial fall phase of the 34-ka Sounkyo eruption that subsequently produced ~ 6.5 km 3 of ignimbrite, resulting in the formation of an ~ 2-km-diameter caldera (Fig. 1 ; Yasuda and Suzuki-Kamata 2018 ) at the center of the Taisetsu volcano cluster. The fall deposits are dispersed to the east, have an estimated bulk volume of ~ 1 km 3 and maximum column height of 25 km, and are subdivided into two units (Fig. 2 ; SK-A1 and -A2 in Yasuda and Suzuki-Kamata 2018 ). The lower unit (A1) is well stratified, thin (up to 7 m thick ~ 200 m away from the rim and ~ 50 cm thick at ~ 11 km downwind), and consists of alternation of fine to coarse ash beds (a few cm thick) and pumice and scoria lapilli beds that are centimeters to decimeters thick; these are interpreted to record low-intensity, discrete, short-lived eruption columns, or unsteady, oscillating columns. The upper unit (A2) is less stratified, much thicker (> 60 m thick at the caldera rim and ~ 2 m thick at ~ 11 km downwind), and consists mainly of pumice lapilli and blocks and minor scoria clasts. This subunit is interpreted to have been emplaced during the formation of higher-intensity, sustained eruption columns. The contrasting features indicate that the eruption transitioned from an unstable sub-Plinian phase to a stable Plinian phase. Flute-like marks at the base of the Plinian fall deposits Flute-like marks are found in only a section 1840 m east of the caldera center (940 m from the rim; red dot marked F in Fig. 1 ) at the boundary between the sub-Plinian and Plinian units (Fig. 3 ); it is one of nine sections (ranging 1.1–11.7 km from the caldera center) where the base of the overlying Plinian fall deposits is exposed. Bedding orientations and elevations of the deposits indicate that the fall units here were emplaced on a relatively flat basin east of the steep outer slope of the caldera. Exceptionally thick (> 60–30 m) accumulation of the proximal facies of the Plinian unit, east of the caldera, forms an east-elongate hill (Fig. 1 ); the south side of the hill is largely covered by the proximal portion of a later lava flow that extends to the northeast, while the north side is extensively gullied such that the base of the Plinian sequence is well exposed in a tributary of Hokkaisawa Creek. Here there are at least seven isolated flute-like marks along a narrow east-northeast trending, ~ 10-m-wide exposure (Fig. 3 ), all of which show cross-sectional shapes that curve downward; they cut into underlying layers of the sub-Plinian phase and are filled with a thin ash lens, in turn overlain by pumice lapilli from the later Plinian phase (Figs. 3 – 5 ). The upstream slopes of the marks are steeper than the downstream slopes with the lowest point near their upstream ends (mostly at 7–18% backward positions of the full length from their upstream ends, but one at ~ 35%); such an asymmetry implies the paleocurrent direction to the east-northeast (i.e., from the caldera). Some of the marks have near vertical or overhanging headwalls at their upstream ends (e.g., Fig. 4 ), typical of flutes in bedrock channels (Hancock et al. 1998 ). The flute-like marks have lengths of 20–130 cm and are 3–20 cm deep, roughly an order of magnitude larger than most flute marks observed in sedimentary rocks; sedimentary flutes are typically from several centimeters to 50 cm in length (rarely > 15 cm) and their depths are rarely > 2 cm (Allen 1971 ; Pett and Walker 1971 ). The widths of the flute-like marks cannot be measured because the exposure provides only a longitudinal profile of the structures. The ratios of length to depth for the flute-like marks are 5.0–7.5, values similar to flutes associated with turbidites (Baas et al. 2021 ). All the marks include a thin lens of light-yellow massive ash at their bases that is absent elsewhere (“flute-related ash lens” in Figs. 4 , 5 ); it only occurs in the flute-like depressions. Exception is a limited area where it occurs on the uneroded surface of the sub-Plinian deposits near but outside the margin of a flute-like mark. Ash lenses are typically < 1 cm thick and show no systematic variation in thickness between the marks of different sizes. Some ash lenses are nearly constant in thickness along their length (Fig. 5 ), while others thicken near the deepest point of the depressions; for example, at a flute-like mark labeled 5 in Fig. 3 , the ash lens is ~ 0.5 cm thick along most of the downstream side of the mark, but thickens toward the upstream side and reaches 2.5 cm thick (Fig. 4 ). These ash lenses are composed mainly of very fine ash (1/16–1/8 mm in size; White and Houghton 2006 ) and minor coarse ash and fine lapilli and lack internal stratification. The lower contact is sharp and smooth, whereas the upper contact is irregular and uneven, caused by deposition of pumice lapilli on the tops of just-deposited ash lenses (Fig. 6 ). This contrasts with ash-fall layers observed in the sub-Plinian deposits below, where both the upper and lower contacts are irregular as a result of settlement of ash on bumpy top surfaces of lapilli beds and coverage by subsequent pumice fall lapilli. These features are consistent with the flute-related ash lenses having been deposited from the base of erosive currents, rather than by fallout, that in some places progressively incised the underlying deposits and formed smooth erosive surfaces. The fact that the ash lenses occur exclusively at the bases of flute-like marks, along with the lack of systematic variation in thickness between individual lenses, suggests that the very fine ash rapidly accumulated and were deposited at the base of the deepening scours. The preferential thickening of the ash lens near the deepest portions of some flute-like marks may indicate that ash particles accumulated preferentially in the headwalls of these scours where overriding currents likely expanded and separated, and at least in part, recirculated. Such recirculation may have facilitated decoupling and deposition of suspended sediment of the currents (very fine ash in this case; Hancock et al. 1998 ). Within a few to 10 cm of the bases of flute-like marks, the depositional structures of the underlying strata are typically disturbed (“disturbed zone” in Figs. 4 , 5 ). It is possible that these disturbed zones are included as part of erosional structures and the base of the zones could be interpreted as a basal erosion surface; however, we favor an interpretation that the base of the erosional structure (where overriding currents directly erode) is delimited by the lower contact of the ash lenses, and dynamic pressure of the eroding currents was transmitted to, and eventually deformed, the strata near the eroding surfaces. This interpretation is consistent with several observations: (1) materials from the ash lens or from overlying pumice lapilli appear to be absent within the disturbed zones; these zones include lenticular remnants of individual layers and irregular pods or lenses of mixture of multiple layers of distinct grain sizes, all of which appear to be material of the sub-Plinian unit. (2) The outer margins of the disturbed zones are irregular in shape, and in some cases form a wedge that extends up to ~ 15 cm beyond the upstream edge of flute-like marks. (3) Original bedding structures are mostly destroyed but are locally recognizable due to incomplete mixing. Such deformation and rearrangement of layers would have been facilitated when deposits were fresh and retained sufficient interstitial air; thus, the time gap between emplacement of the topmost parts of the sub-Plinian deposits and subsequent erosion events is short (hours to days). Pumice lapilli in the lowest part of the Plinian unit are commonly inversely graded, recording an increasing intensity of the eruption; a few centimeters of medium-coarse ash underlies the reverse-graded fallout lapilli at the base of the Plinian unit in medial to distal localities (“basal ash horizon” in Fig. 2 ) but appears to be absent in the most proximal sections (within 2 km of the caldera center; including the section described here). Here the top of the sub-Plinian deposits appear to be overlain by 10 cm of fine lapilli that grade upward into medium lapilli (left column in Fig. 2 ); however, the lowermost fine lapilli are missing in many flute-like marks and, instead, medium lapilli directly overlie the flute-related ash lens (Fig. 4 ). In places the topmost ash layer (~ 1 cm thick; labeled a in Figs. 2 , 4 , 5 , 7 ) of the sub-Plinian deposits shows shear deformation structures such as stretching, tearing, and ripping (Fig. 7 ). This ash layer is light purple in color, is composed of fine to coarse ash, and contains extremely thin (a millimeter or less) lenses of well-sorted medium ash; these features make it easy to distinguish from the flute-related ash lens that is finer grained, massive, and light yellow in color (Fig. 4 ). Where the layer is split into two or more pieces (Fig. 7 a–d), the interstitial spaces are filled with ash and lapilli coming from the underlying sub-Plinian unit, which are mixed with pumice lapilli coming from the overlying Plinian unit (materials of the two different origins in and near the mixed areas could be distinguished by grain size; Fig. 7 b), indicating that the layer was deformed at the same time as the Plinian pumice lapilli were being emplaced. Some pieces of the detached ash layer were transported some distance from, and resedimented above, their original positions and appear to be set within the overlying Plinian unit (Fig. 7 c). In some cases, the layer is fragmented into rigid pieces (Fig. 7 a–c), while in other cases it is stretched and plastically deformed (Fig. 7 d–f). These structures are consistent with the ash layer having been partly cohesive but partly still soft during deformation, and hence the deformation occurred soon after (within days to weeks) the sub-Plinian phase ceased. Most of the shear deformation structures occur independently of the observed flute-like marks, but one is adjacent to the downstream end of a flute-like mark (Fig. 7 d). Similar shear deformation is only found in a location 1120 m northeast of the caldera center (220 m from the rim; dot marked S in Fig. 1 ), where, although the lower contact of the Plinian deposits is exposed over several meters, flute-like marks are missing. These shear deformation features may represent only the lateral margins of flute structures, or alternatively they may record early stages of formation of flute-like marks. Discussion Despite the lack of planform data, the morphological similarities in cross section suggest that the flute-like marks would likely have been formed in a similar manner to flutes in sedimentary rocks. We infer that the marks record primary volcanic processes rather than water erosion. There is no evidence of surface water runoff prior to the Plinian phase in the section detailed here and other sections (e.g., rills or gullies on the sub-Plinian sequence). Moreover, because the underlying strata of the sub-Plinian sequence are nearly horizontal in attitude or dip shallowly (~ 4°) toward the west-northwest (toward the caldera), it seems unlikely that they are “flutes” formed by surface runoff during a hiatus between the sub-Plinian and Plinian phases. Also excluded is a simultaneous surface water flushing caused by the eruption, as there are no signs of the incorporation of water into the Plinian mixtures (e.g., fines-rich tuffs and accretionary lapilli or ash pellets; Wilson 2001 ; Carey et al. 2010 ). We suggest that the flute-like marks and other deformation structures at the base of the Plinian deposits resulted from the passage of dilute pyroclastic currents. The currents left nothing but the thin ash lenses in the flute-like marks, indicating that they were primarily erosional rather than depositional. It is likely that these currents were dominated by fine-grained ash with minor coarser material; otherwise, poorly sorted lapilli tuff would have been left behind by the currents. The location of individual flute marks is controlled by prior surface irregularities of the substrate, or the patterns of turbulent eddies of the parent currents (Allen 1971 ). The flute-like marks described here might not have been related to prior surface irregularities because the surface of the underlying substrate appears to have been flat and smooth (Fig. 3 ). Localized stronger eddies may have disrupted the top ash layer of the underlying strata (such as those seen in Fig. 7 ), which then served as irregularities necessary for flute formation. The timing of growth of each flute-like mark likely varied depending on the appearance of such irregularities, with larger marks having started to grow earlier but smaller ones having initiated from later irregularities. The large flute-like marks (> 60 cm long and > 10 cm deep; flute-like marks labeled 2 and 4–7 in Fig. 3 ) observed at the outcrop would likely have begun to form early and grown over relatively long periods of time, while smaller marks (20 cm long and 3–4 cm deep; flute-like marks labeled 1 and 3 in Fig. 3 ) may record shorter periods of erosion. Alternatively, the small ones could represent crosscut near the lateral outer margins of larger flute structures. Within the growing marks, the flow separated and partly recirculated, enhancing deposition of suspended fine ash at the bases of the marks. The surrounding beds near the margins of the growing marks deformed and partly mixed with each other, due to dynamic pressure of the overriding currents and/or shear along the freshly exposed soft interior of the eroded beds. The absence of lowermost fine lapilli in some flute-like marks may be related to timing of dilute pyroclastic currents. These currents arrived this location during the beginning of the Plinian phase such that initial pumice lapilli would settle and mix with the moving currents, without producing a basal unit. Alternatively, the fine lapilli were deposited but subsequently eroded where flute-like marks formed. The presence of Plinian material beneath the ripped-up pieces of the top ash layer of the sub-Plinian unit (Fig. 7 b) indicates that the erosive currents did not precede the Plinian phase, but were, at least in part, coeval with initial Plinian fall deposits. Assuming that pumice lapilli accumulated at a rate of 1–10 m/h (typical of historically observed proximal Plinian products; Hildreth and Drake 1992 ; Sable et al. 2006 ; Carey et al. 2007 ), the currents would have occurred over a period of ~ 0.5–5 min at the beginning of the Plinian phase as the basal 10 cm of the Plinian deposits were being emplaced but eroded where flute-like marks developed. As the eruption waxed, pumice lapilli accumulated at progressively higher rates, which prevented the marks from continuing to grow and resulted in burial of these marks by pumice-fall deposits. Sections with erosion features (two localities: one is the red dot marked F and another is the dot marked S in Fig. 1 ) are constrained within ~ 1–2 km of the caldera center (< 1 km from the rim), and there is no evidence in more distal sections for erosion or deposition by these currents, suggesting that they could have traveled short distances. These most proximal outcrops do not have a basal ash horizon of the Plinian unit that commonly occurs at sections > 2.5 km of the caldera (Fig. 2 ); it would seem likely that the basal ash was emplaced in these proximal areas but was almost completely eroded by the subsequent turbulent currents, or falling ash was entrained into the moving currents. Alternatively, this absence may simply indicate that the ash did not fall in the immediate vicinity of the vent. These currents spread northeastward to eastward, but further interpretation of its directional distribution is hindered by limited outcrops. Erosive features similar to those described here seem uncommon at the base of proximal Plinian fall deposits elsewhere (e.g., the 1912 Novarupta deposits; Fierstein et al. 1997 ; Houghton et al. 2004 ). Formation and recognition of such features in this Sounkyo case may have been favored by the following conditions: (1) the underlying substrate was sufficiently soft to be eroded or deformed; and (2) the thinly stratified fine-grained substrate visually enhances the features. Dilute pyroclastic currents occurred during an initial waxing stage of the Sounkyo Plinian phase; it is uncertain whether such currents continued to occur during later stages of the eruption when relatively massive pumice lapilli and blocks were being deposited, a condition unfavorable for the formation and recognition of any erosional features. These currents were probably associated with the Plinian vent rather than a separate vent; the paleocurrent direction inferred from the morphology of the flute-like marks is consistent with these flows derived from the caldera, and it would seem fortuitous for a new vent to open at the almost same time as the Plinian activity started. Blasts formed by collapse of a dome in a Plinian vent (e.g., 2014 eruption of Kelud volcano, Indonesia; Maeno et al. 2019 ) or vent-opening lateral blasts (e.g., 1912 eruption of Novarupta in Alaska, USA; Hildreth and Fierstein 2012 ) might erode the substrate, but such currents would have emplaced a lava-lapilli-bearing unit beneath or interbedded with the basal parts of Plinian deposits; no such deposits are observed at sections where the base of the Plinian unit is exposed, and lithic clasts are rare in its basal part. Alternatively, and perhaps more likely, dilute pyroclastic currents may have resulted from a local collapse along the margin of the Plinian column. This type of collapse of the eruption column may occur when the column conditions are transitional between buoyant plume and column collapse, and likely produces small pyroclastic currents while the bulk of the column remains stable and buoyant (Koyaguchi et al. 2018 ). This may be consistent with the limited lateral extent of outcrops (in the most proximal locations, < 1–2 km from the vent) that show evidence of erosion at the base of the Plinian deposits. Conclusions The proximal section at the Ohachidaira caldera exhibits exceptional erosive structures at the contact between underlying sub-Plinian and overlying Plinian deposits, including asymmetric scours that closely resemble, in cross-sectional view, flute marks in sedimentary rocks. Such flute-like marks have not previously been described in Plinian fall deposits or are rarely reported in the volcanological literature. While flute marks are common in aqueous sedimentary rocks and thus are formed during water erosion, field evidence indicates that the flute-like marks and other deformation features were formed by erosion due to the passage of dilute, turbulent pyroclastic currents fed from the Plinian eruption column. The contact relationship of these erosion features with overlying pumice lapilli indicates that erosive events and initial pumice fallout deposits are largely coeval; such an interpretation allows an estimation of the time required for the flute-like marks to form, which would be difficult to estimate for flute marks in subaqueous environments. The presence of erosion features exclusively within the most proximal sections and the lack of evidence of the passage of pyroclastic currents at the base of the Plinian unit in more distal areas suggest short runout distances of the erosive pyroclastic currents. Such small dilute pyroclastic currents may be consistent with a local collapse of the margin of the Plinian column, as seen in numerical simulations for a transitional regime between stable column and total collapse (Koyaguchi et al. 2018 ). Declarations Funding This work was supported by Fukada Grant-in-Aid from the Fukada Geological Institute and JSPS KAKENHI Grant Number 21K14004 to YY. Conflicts of interest/Competing interests The authors declare no competing interests. Acknowledgments We acknowledge Yoshiro Ishihara for sharing his knowledge of flute marks. Greg Valentine and anonymous reviewers are thanked for thoughtful comments. References Allen JRL (1968) Flute marks and flute separation. 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J Volcanol Geotherm Res 357:287–305. https://doi.org/10.1016/j.jvolgeores.2018.04.017 Cite Share Download PDF Status: Published Journal Publication published 15 Feb, 2025 Read the published version in Bulletin of Volcanology → Version 1 posted Editorial decision: Moderate revision (possibly re-reviewed) 17 Sep, 2024 Reviewers agreed at journal 17 Jun, 2024 Reviewers invited by journal 17 Jun, 2024 Editor invited by journal 26 May, 2024 First submitted to journal 06 May, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4379634","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":315529844,"identity":"9eb7729e-acee-48d7-912e-32dc4211262f","order_by":0,"name":"Yuki Yasuda","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABCElEQVRIiWNgGAWjYNACNgY5BgbGBjBbAirGTEiLMVSLAfFaEiFWIGnBCQxuJD/+zFNml94vfbiBmYfhj5zkjOQDDD9qGNjNcWpJM5PmOZecO7MvEaTFwFhaIi2BsecYA7NlAy4tCWbMvG3MuRvOMDYw8/4zSJwnkWPAwNvAwGxwAJeW9M+fedvq0+1BWoC2gLUw/sWrJcdAmrftcIIBD1TLbKAWZny2SJ55UyY559xxwxlAWw7OYTA2lux5lnBY5pgETr/wHU/f/OFNWbU8fw/7wwdvGOTkJI4nH3z4psYmGVeIKVxIQHAgLhFIADEkkg1waJHvx3AxP0TEDpeWUTAKRsEoGHEAAOMlU0M3xfQtAAAAAElFTkSuQmCC","orcid":"https://orcid.org/0000-0001-9633-7377","institution":"The University of Tokyo: Tokyo Daigaku","correspondingAuthor":true,"prefix":"","firstName":"Yuki","middleName":"","lastName":"Yasuda","suffix":""},{"id":315529845,"identity":"e069a01c-7c18-4288-94d7-87cac52d1607","order_by":1,"name":"Fukashi Maeno","email":"","orcid":"","institution":"The University of Tokyo: Tokyo Daigaku","correspondingAuthor":false,"prefix":"","firstName":"Fukashi","middleName":"","lastName":"Maeno","suffix":""}],"badges":[],"createdAt":"2024-05-07 02:21:16","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4379634/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4379634/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s00445-025-01805-4","type":"published","date":"2025-02-15T15:58:03+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":60018780,"identity":"827ff7a3-a8ab-4e98-9a42-e27b71d04fdb","added_by":"auto","created_at":"2024-07-10 15:16:54","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":598551,"visible":true,"origin":"","legend":"\u003cp\u003eShaded relief map around the Ohachidaira caldera, showing location of proximal outcrops (dots) that expose the base of the Plinian deposits, and isopachs (dashed lines; from Yasuda and Suzuki-Kamata 2018) for the total (Plinian and sub-Plinian) fall deposits. Red dot marked F shows a section where flute-like marks are observed. Location marked S is where shear deformation structure is observed, but without flute-like marks; location marked C shows a section for the center column in Fig. 2. Shaded relief from https://www.gsi.go.jp/bousaichiri/hillshademap.html. Inset shows location of the study area (yellow star) in central Hokkaido, Japan\u003c/p\u003e","description":"","filename":"Fig1.png","url":"https://assets-eu.researchsquare.com/files/rs-4379634/v1/5dae69bcabbaeb02a4833373.png"},{"id":60018778,"identity":"08556139-4d5f-4f2b-a553-64561a31c41d","added_by":"auto","created_at":"2024-07-10 15:16:54","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":106456,"visible":true,"origin":"","legend":"\u003cp\u003eStratigraphic columns for representative sites of the fall deposits (left, 43.6831°N, 142.9072°E; center, 43.6676°N, 142.9143°E; right, 43.6982°N, 143.0169°E), highlighting only an ~60 cm interval near the boundary between the sub-Plinian and Plinian units (connecting line between the columns shows the unit boundary). The tops and bases of the fall deposits are not shown. Distance from caldera center is shown on each column. Left column represents the section (red dot in Fig. 1) detailed in the text; the column does not include any erosion features, but instead shows original textures of beds where preserved. Labels for some layers at the side of the left column as in Fig 4. Center column represents the section lower right in Fig. 1; the section for right column is located east of Fig. 1. Irrespective of distance from vent, the lowest part of the Plinian unit is reversely graded\u003c/p\u003e","description":"","filename":"Fig2.png","url":"https://assets-eu.researchsquare.com/files/rs-4379634/v1/5d4c5e053cf6ef783132593e.png"},{"id":60018779,"identity":"d2422f0f-9db8-4d96-bc2d-f230ba6aa1bd","added_by":"auto","created_at":"2024-07-10 15:16:54","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1702802,"visible":true,"origin":"","legend":"\u003cp\u003eThe boundary (white lines) between the sub-Plinian and Plinian deposits in a section ~1.8 km east of caldera center (caldera is to the right), showing seven flute-like marks (dashed lines; labeled 1–7) and a sag structure. The plane of the section is oriented ~N75°E, parallel to the radial direction from caldera center. Hammer is 33 cm long\u003c/p\u003e","description":"","filename":"Fig3.png","url":"https://assets-eu.researchsquare.com/files/rs-4379634/v1/e69c04c9093f364d108ae9ea.png"},{"id":60018781,"identity":"5697b352-8aef-4b97-bd17-9af70f887a1c","added_by":"auto","created_at":"2024-07-10 15:16:54","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":2091575,"visible":true,"origin":"","legend":"\u003cp\u003ePhotograph (\u003cstrong\u003ea\u003c/strong\u003e) and sketch (\u003cstrong\u003eb\u003c/strong\u003e) of a flute-like mark labeled 5 in Fig. 3. Dashed line in \u003cstrong\u003ea\u003c/strong\u003e denotes the flute structure; it is 75 cm long and 10 cm deep, incising into underlying layers of fine to medium ash and fine lapilli from the sub-Plinian phase, with some layers labeled and also colored in \u003cstrong\u003eb\u003c/strong\u003e for clarity. At the base of the mark is a flute-related ash lens (here up to 2.5 cm thick; colored yellow in \u003cstrong\u003eb\u003c/strong\u003e), which is different in color, grain size, and internal structure from the uppermost ash layer (labeled a) of the sub-Plinian deposits that is eroded out in the mark. Overlying the ash lens is fallout lapilli from the Plinian phase. Near the margin of the flute structure, the underlying layers are locally disrupted and mixed with each other (disturbed zones). The beige fine ash layer (labeled b) is missing near the deepest point of the mark, but a lenticular remnant of the layer is preserved in the disturbed zone\u003c/p\u003e","description":"","filename":"Fig4.png","url":"https://assets-eu.researchsquare.com/files/rs-4379634/v1/400db0c66912d35f53a99cf9.png"},{"id":60018783,"identity":"0aa7f2bc-da1f-4ccb-a4fa-586d22ba2062","added_by":"auto","created_at":"2024-07-10 15:16:54","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":2086241,"visible":true,"origin":"","legend":"\u003cp\u003ePhotograph (\u003cstrong\u003ea\u003c/strong\u003e) and sketch (\u003cstrong\u003eb\u003c/strong\u003e) of a flute-like mark labeled 6 in Fig. 3. Dashed line and labels as in Fig. 4. This flute-like mark, 130 cm long and 20 cm deep, is the largest observed in this outcrop. An ash lens (colored yellow in \u003cstrong\u003eb\u003c/strong\u003e) at the base of the mark is \u0026lt;1.5 cm thick and shows little lateral variation in thickness. The flute-like mark cuts into underlying strata; this contrasts with the impact sag immediately to the left of the mark, below which the strata are completely preserved. Hammer is 33 cm long. Darker zone left of the hammer handle is a scoria-block-rich pod surrounded by pumice lapilli fallout deposit (not detailed in this paper)\u003c/p\u003e","description":"","filename":"Fig5.png","url":"https://assets-eu.researchsquare.com/files/rs-4379634/v1/146ed3ea727a1efe0031fd61.png"},{"id":60019427,"identity":"68a015ae-328f-4d52-9067-e9d51b12a622","added_by":"auto","created_at":"2024-07-10 15:24:54","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":754388,"visible":true,"origin":"","legend":"\u003cp\u003eClose-up of the flute-related ash lens in Fig. 4. The light-yellow ash lens is massive and composed dominantly of very fine ash (1/16–1/8 mm in size). While the lower contact of the ash lens is relatively smooth, the upper contact is irregular due to deposition of pumice lapilli of the Plinian unit\u003c/p\u003e","description":"","filename":"Fig6.png","url":"https://assets-eu.researchsquare.com/files/rs-4379634/v1/e141384c4ce3b15109b4f8f4.png"},{"id":60018784,"identity":"7a392433-4021-4a4a-b81a-6a781f0ebf04","added_by":"auto","created_at":"2024-07-10 15:16:55","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":5597944,"visible":true,"origin":"","legend":"\u003cp\u003eExamples of shear deformation of the top ash layer (dashed; labeled a, as in Fig. 4) of the sub-Plinian deposits in the same section as Fig. 3. The scale in \u003cstrong\u003ed\u003c/strong\u003e–\u003cstrong\u003ef\u003c/strong\u003eis in centimeters. The top ash layer is ~1 cm thick, but its thickness appears to be ~2–3 cm in \u003cstrong\u003ec\u003c/strong\u003e,\u003cstrong\u003e e\u003c/strong\u003e, and \u003cstrong\u003ef\u003c/strong\u003edue to oblique angles of the outcrop faces to the bedding plane. White arrow in upper right of each panel indicates current direction (inferred to come from the caldera). \u003cstrong\u003ea\u003c/strong\u003e The ash layer was fragmented and ripped up, with some pieces resedimented above its original level (see also Fig. 7c). Rectangle indicates location of Fig. 7b. \u003cstrong\u003eb\u003c/strong\u003e Detail of uplifted and fragmented nature of the ash layer. The space between pieces of the layer is filled with material both from overlying Plinian and underlying sub-Plinian units (transparent white arrows), but the exact contact between these two components is obscured by mixing. \u003cstrong\u003ec\u003c/strong\u003e The top ash layer (labeled a) was highly fragmented, while a beige fine ash layer below (labeled b, as in Fig. 4) wasfolded and split into two. The ~15-cm-long slab of the detached ash layer “a” in upper center was uplifted, beneath which lapilli from overlying Plinian unit occur. Hammer is 33 cm long. \u003cstrong\u003ed\u003c/strong\u003eRip-up structure with the tip of the layer on the left side bent upward. This structure is located at the downstream end of a flute-like mark labeled 7 in Fig. 3. \u003cstrong\u003ee\u003c/strong\u003e The layer was separated into two with its right side folded up. \u003cstrong\u003ef\u003c/strong\u003e The layer was nearly split into two but remained attached to each other\u003c/p\u003e","description":"","filename":"Fig7.png","url":"https://assets-eu.researchsquare.com/files/rs-4379634/v1/9b14e9d83fa5588458a07e1a.png"},{"id":76487831,"identity":"d98e9a26-0bec-4ff5-a17e-a4423b5ca566","added_by":"auto","created_at":"2025-02-17 16:12:43","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":13400508,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4379634/v1/ede4fcd8-2e8f-47bc-baa8-57acfd64f65a.pdf"}],"financialInterests":"","formattedTitle":"Flute-like marks at the base of Plinian pumice-fall deposits at Ohachidaira caldera, Hokkaido, Japan","fulltext":[{"header":"Introduction","content":"\u003cp\u003eFlute marks are erosive features typically observed at the bases of marine sandstones and can be used as an indicator of paleocurrent directions (e.g., Collinson and Mountney \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Peakall et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Baas et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2021\u003c/span\u003e); they have an asymmetric concave-up shape with the deepest point located near their upstream end, and from the deepest point features flare away and decrease in depth toward the downstream end (Allen \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e1982\u003c/span\u003e). Such features are formed by differential erosion by turbulent eddies of an overlying turbidity current, in which the rates of erosion in growing flute marks are larger than those for the surrounding beds due to flow separation (Allen \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1968\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e1969\u003c/span\u003e; Collinson and Mountney \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Flute marks could also be formed during aeolian sediment transport (Lancaster \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e1984\u003c/span\u003e; Sebe et al. \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2015\u003c/span\u003e), but such systems are not rapidly covered by subsequent flows resulting in the scarcity of these marks in the aeolian settings.\u003c/p\u003e \u003cp\u003eAlthough pyroclastic currents could potentially produce flute marks on the substrate as other types of sole marks (e.g., groove and impact marks at the base of the pyroclastic current deposits, Tenerife; Pittari and Cas \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2004\u003c/span\u003e) or other erosional structures (e.g., U-shaped channels in the base surge deposits at Koko Crater, Hawaii; Fisher \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e1977\u003c/span\u003e), flute marks have rarely been reported in the volcanological literature. Recognition of flute marks in pyroclastic sequences would provide information on flow directions and on the flow dynamics of pyroclastic currents that produced the marks. Cole et al. (\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2002\u003c/span\u003e) is the only known example that recognized flute marks beneath the block-and-ash flow deposits at Soufri\u0026egrave;re Hills Volcano, Montserrat, but their geometries and features were not documented. This paper describes scours that resemble flute marks (here referred to as flute-like marks) and related erosional structures at the base of proximal Plinian fall deposits from the Ohachidaira caldera (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Detailed observations of such features provide evidence that they were formed during the passage of dilute turbulent pyroclastic currents at the beginning of the Plinian eruption. Grain size terminology in this paper follows that of White and Houghton (\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2006\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Plinian phase of the Sounkyo eruption","content":"\u003cp\u003eThe pumice-fall deposits, in which flute-like marks are observed, are products of the initial fall phase of the 34-ka Sounkyo eruption that subsequently produced ~ 6.5 km\u003csup\u003e3\u003c/sup\u003e of ignimbrite, resulting in the formation of an ~ 2-km-diameter caldera (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e; Yasuda and Suzuki-Kamata \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2018\u003c/span\u003e) at the center of the Taisetsu volcano cluster. The fall deposits are dispersed to the east, have an estimated bulk volume of ~ 1 km\u003csup\u003e3\u003c/sup\u003e and maximum column height of 25 km, and are subdivided into two units (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e; SK-A1 and -A2 in Yasuda and Suzuki-Kamata \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). The lower unit (A1) is well stratified, thin (up to 7 m thick ~ 200 m away from the rim and ~ 50 cm thick at ~ 11 km downwind), and consists of alternation of fine to coarse ash beds (a few cm thick) and pumice and scoria lapilli beds that are centimeters to decimeters thick; these are interpreted to record low-intensity, discrete, short-lived eruption columns, or unsteady, oscillating columns. The upper unit (A2) is less stratified, much thicker (\u0026gt; 60 m thick at the caldera rim and ~ 2 m thick at ~ 11 km downwind), and consists mainly of pumice lapilli and blocks and minor scoria clasts. This subunit is interpreted to have been emplaced during the formation of higher-intensity, sustained eruption columns. The contrasting features indicate that the eruption transitioned from an unstable sub-Plinian phase to a stable Plinian phase.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"Flute-like marks at the base of the Plinian fall deposits","content":"\u003cp\u003eFlute-like marks are found in only a section 1840 m east of the caldera center (940 m from the rim; red dot marked F in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) at the boundary between the sub-Plinian and Plinian units (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e); it is one of nine sections (ranging 1.1–11.7 km from the caldera center) where the base of the overlying Plinian fall deposits is exposed. Bedding orientations and elevations of the deposits indicate that the fall units here were emplaced on a relatively flat basin east of the steep outer slope of the caldera. Exceptionally thick (\u0026gt; 60–30 m) accumulation of the proximal facies of the Plinian unit, east of the caldera, forms an east-elongate hill (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e); the south side of the hill is largely covered by the proximal portion of a later lava flow that extends to the northeast, while the north side is extensively gullied such that the base of the Plinian sequence is well exposed in a tributary of Hokkaisawa Creek. Here there are at least seven isolated flute-like marks along a narrow east-northeast trending, ~ 10-m-wide exposure (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), all of which show cross-sectional shapes that curve downward; they cut into underlying layers of the sub-Plinian phase and are filled with a thin ash lens, in turn overlain by pumice lapilli from the later Plinian phase (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e–\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). The upstream slopes of the marks are steeper than the downstream slopes with the lowest point near their upstream ends (mostly at 7–18% backward positions of the full length from their upstream ends, but one at ~ 35%); such an asymmetry implies the paleocurrent direction to the east-northeast (i.e., from the caldera). Some of the marks have near vertical or overhanging headwalls at their upstream ends (e.g., Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e), typical of flutes in bedrock channels (Hancock et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e1998\u003c/span\u003e). The flute-like marks have lengths of 20–130 cm and are 3–20 cm deep, roughly an order of magnitude larger than most flute marks observed in sedimentary rocks; sedimentary flutes are typically from several centimeters to 50 cm in length (rarely \u0026gt; 15 cm) and their depths are rarely \u0026gt; 2 cm (Allen \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e1971\u003c/span\u003e; Pett and Walker \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e1971\u003c/span\u003e). The widths of the flute-like marks cannot be measured because the exposure provides only a longitudinal profile of the structures. The ratios of length to depth for the flute-like marks are 5.0–7.5, values similar to flutes associated with turbidites (Baas et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2021\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e \u003c/p\u003e\u003cp\u003e \u003c/p\u003e\u003cp\u003e \u003c/p\u003e\u003cp\u003eAll the marks include a thin lens of light-yellow massive ash at their bases that is absent elsewhere (“flute-related ash lens” in Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e); it only occurs in the flute-like depressions. Exception is a limited area where it occurs on the uneroded surface of the sub-Plinian deposits near but outside the margin of a flute-like mark. Ash lenses are typically \u0026lt; 1 cm thick and show no systematic variation in thickness between the marks of different sizes. Some ash lenses are nearly constant in thickness along their length (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e), while others thicken near the deepest point of the depressions; for example, at a flute-like mark labeled 5 in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, the ash lens is ~ 0.5 cm thick along most of the downstream side of the mark, but thickens toward the upstream side and reaches 2.5 cm thick (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). These ash lenses are composed mainly of very fine ash (1/16–1/8 mm in size; White and Houghton \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2006\u003c/span\u003e) and minor coarse ash and fine lapilli and lack internal stratification. The lower contact is sharp and smooth, whereas the upper contact is irregular and uneven, caused by deposition of pumice lapilli on the tops of just-deposited ash lenses (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). This contrasts with ash-fall layers observed in the sub-Plinian deposits below, where both the upper and lower contacts are irregular as a result of settlement of ash on bumpy top surfaces of lapilli beds and coverage by subsequent pumice fall lapilli. These features are consistent with the flute-related ash lenses having been deposited from the base of erosive currents, rather than by fallout, that in some places progressively incised the underlying deposits and formed smooth erosive surfaces. The fact that the ash lenses occur exclusively at the bases of flute-like marks, along with the lack of systematic variation in thickness between individual lenses, suggests that the very fine ash rapidly accumulated and were deposited at the base of the deepening scours. The preferential thickening of the ash lens near the deepest portions of some flute-like marks may indicate that ash particles accumulated preferentially in the headwalls of these scours where overriding currents likely expanded and separated, and at least in part, recirculated. Such recirculation may have facilitated decoupling and deposition of suspended sediment of the currents (very fine ash in this case; Hancock et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e1998\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e \u003c/p\u003e\u003cp\u003eWithin a few to 10 cm of the bases of flute-like marks, the depositional structures of the underlying strata are typically disturbed (“disturbed zone” in Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). It is possible that these disturbed zones are included as part of erosional structures and the base of the zones could be interpreted as a basal erosion surface; however, we favor an interpretation that the base of the erosional structure (where overriding currents directly erode) is delimited by the lower contact of the ash lenses, and dynamic pressure of the eroding currents was transmitted to, and eventually deformed, the strata near the eroding surfaces. This interpretation is consistent with several observations: (1) materials from the ash lens or from overlying pumice lapilli appear to be absent within the disturbed zones; these zones include lenticular remnants of individual layers and irregular pods or lenses of mixture of multiple layers of distinct grain sizes, all of which appear to be material of the sub-Plinian unit. (2) The outer margins of the disturbed zones are irregular in shape, and in some cases form a wedge that extends up to ~ 15 cm beyond the upstream edge of flute-like marks. (3) Original bedding structures are mostly destroyed but are locally recognizable due to incomplete mixing. Such deformation and rearrangement of layers would have been facilitated when deposits were fresh and retained sufficient interstitial air; thus, the time gap between emplacement of the topmost parts of the sub-Plinian deposits and subsequent erosion events is short (hours to days).\u003c/p\u003e\u003cp\u003ePumice lapilli in the lowest part of the Plinian unit are commonly inversely graded, recording an increasing intensity of the eruption; a few centimeters of medium-coarse ash underlies the reverse-graded fallout lapilli at the base of the Plinian unit in medial to distal localities (“basal ash horizon” in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) but appears to be absent in the most proximal sections (within 2 km of the caldera center; including the section described here). Here the top of the sub-Plinian deposits appear to be overlain by 10 cm of fine lapilli that grade upward into medium lapilli (left column in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e); however, the lowermost fine lapilli are missing in many flute-like marks and, instead, medium lapilli directly overlie the flute-related ash lens (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eIn places the topmost ash layer (~ 1 cm thick; labeled a in Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e) of the sub-Plinian deposits shows shear deformation structures such as stretching, tearing, and ripping (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e). This ash layer is light purple in color, is composed of fine to coarse ash, and contains extremely thin (a millimeter or less) lenses of well-sorted medium ash; these features make it easy to distinguish from the flute-related ash lens that is finer grained, massive, and light yellow in color (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). Where the layer is split into two or more pieces (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea–d), the interstitial spaces are filled with ash and lapilli coming from the underlying sub-Plinian unit, which are mixed with pumice lapilli coming from the overlying Plinian unit (materials of the two different origins in and near the mixed areas could be distinguished by grain size; Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eb), indicating that the layer was deformed at the same time as the Plinian pumice lapilli were being emplaced. Some pieces of the detached ash layer were transported some distance from, and resedimented above, their original positions and appear to be set within the overlying Plinian unit (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ec). In some cases, the layer is fragmented into rigid pieces (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea–c), while in other cases it is stretched and plastically deformed (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ed–f). These structures are consistent with the ash layer having been partly cohesive but partly still soft during deformation, and hence the deformation occurred soon after (within days to weeks) the sub-Plinian phase ceased. Most of the shear deformation structures occur independently of the observed flute-like marks, but one is adjacent to the downstream end of a flute-like mark (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ed). Similar shear deformation is only found in a location 1120 m northeast of the caldera center (220 m from the rim; dot marked S in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), where, although the lower contact of the Plinian deposits is exposed over several meters, flute-like marks are missing. These shear deformation features may represent only the lateral margins of flute structures, or alternatively they may record early stages of formation of flute-like marks.\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eDespite the lack of planform data, the morphological similarities in cross section suggest that the flute-like marks would likely have been formed in a similar manner to flutes in sedimentary rocks. We infer that the marks record primary volcanic processes rather than water erosion. There is no evidence of surface water runoff prior to the Plinian phase in the section detailed here and other sections (e.g., rills or gullies on the sub-Plinian sequence). Moreover, because the underlying strata of the sub-Plinian sequence are nearly horizontal in attitude or dip shallowly (~\u0026thinsp;4\u0026deg;) toward the west-northwest (toward the caldera), it seems unlikely that they are \u0026ldquo;flutes\u0026rdquo; formed by surface runoff during a hiatus between the sub-Plinian and Plinian phases. Also excluded is a simultaneous surface water flushing caused by the eruption, as there are no signs of the incorporation of water into the Plinian mixtures (e.g., fines-rich tuffs and accretionary lapilli or ash pellets; Wilson \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Carey et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2010\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eWe suggest that the flute-like marks and other deformation structures at the base of the Plinian deposits resulted from the passage of dilute pyroclastic currents. The currents left nothing but the thin ash lenses in the flute-like marks, indicating that they were primarily erosional rather than depositional. It is likely that these currents were dominated by fine-grained ash with minor coarser material; otherwise, poorly sorted lapilli tuff would have been left behind by the currents. The location of individual flute marks is controlled by prior surface irregularities of the substrate, or the patterns of turbulent eddies of the parent currents (Allen \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e1971\u003c/span\u003e). The flute-like marks described here might not have been related to prior surface irregularities because the surface of the underlying substrate appears to have been flat and smooth (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Localized stronger eddies may have disrupted the top ash layer of the underlying strata (such as those seen in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e), which then served as irregularities necessary for flute formation. The timing of growth of each flute-like mark likely varied depending on the appearance of such irregularities, with larger marks having started to grow earlier but smaller ones having initiated from later irregularities. The large flute-like marks (\u0026gt;\u0026thinsp;60 cm long and \u0026gt;\u0026thinsp;10 cm deep; flute-like marks labeled 2 and 4\u0026ndash;7 in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e) observed at the outcrop would likely have begun to form early and grown over relatively long periods of time, while smaller marks (20 cm long and 3\u0026ndash;4 cm deep; flute-like marks labeled 1 and 3 in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e) may record shorter periods of erosion. Alternatively, the small ones could represent crosscut near the lateral outer margins of larger flute structures. Within the growing marks, the flow separated and partly recirculated, enhancing deposition of suspended fine ash at the bases of the marks. The surrounding beds near the margins of the growing marks deformed and partly mixed with each other, due to dynamic pressure of the overriding currents and/or shear along the freshly exposed soft interior of the eroded beds. The absence of lowermost fine lapilli in some flute-like marks may be related to timing of dilute pyroclastic currents. These currents arrived this location during the beginning of the Plinian phase such that initial pumice lapilli would settle and mix with the moving currents, without producing a basal unit. Alternatively, the fine lapilli were deposited but subsequently eroded where flute-like marks formed. The presence of Plinian material beneath the ripped-up pieces of the top ash layer of the sub-Plinian unit (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eb) indicates that the erosive currents did not precede the Plinian phase, but were, at least in part, coeval with initial Plinian fall deposits. Assuming that pumice lapilli accumulated at a rate of 1\u0026ndash;10 m/h (typical of historically observed proximal Plinian products; Hildreth and Drake \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e1992\u003c/span\u003e; Sable et al. \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Carey et al. \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2007\u003c/span\u003e), the currents would have occurred over a period of ~\u0026thinsp;0.5\u0026ndash;5 min at the beginning of the Plinian phase as the basal 10 cm of the Plinian deposits were being emplaced but eroded where flute-like marks developed. As the eruption waxed, pumice lapilli accumulated at progressively higher rates, which prevented the marks from continuing to grow and resulted in burial of these marks by pumice-fall deposits. Sections with erosion features (two localities: one is the red dot marked F and another is the dot marked S in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) are constrained within ~\u0026thinsp;1\u0026ndash;2 km of the caldera center (\u0026lt;\u0026thinsp;1 km from the rim), and there is no evidence in more distal sections for erosion or deposition by these currents, suggesting that they could have traveled short distances. These most proximal outcrops do not have a basal ash horizon of the Plinian unit that commonly occurs at sections\u0026thinsp;\u0026gt;\u0026thinsp;2.5 km of the caldera (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e); it would seem likely that the basal ash was emplaced in these proximal areas but was almost completely eroded by the subsequent turbulent currents, or falling ash was entrained into the moving currents. Alternatively, this absence may simply indicate that the ash did not fall in the immediate vicinity of the vent. These currents spread northeastward to eastward, but further interpretation of its directional distribution is hindered by limited outcrops.\u003c/p\u003e \u003cp\u003eErosive features similar to those described here seem uncommon at the base of proximal Plinian fall deposits elsewhere (e.g., the 1912 Novarupta deposits; Fierstein et al. \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e1997\u003c/span\u003e; Houghton et al. \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2004\u003c/span\u003e). Formation and recognition of such features in this Sounkyo case may have been favored by the following conditions: (1) the underlying substrate was sufficiently soft to be eroded or deformed; and (2) the thinly stratified fine-grained substrate visually enhances the features. Dilute pyroclastic currents occurred during an initial waxing stage of the Sounkyo Plinian phase; it is uncertain whether such currents continued to occur during later stages of the eruption when relatively massive pumice lapilli and blocks were being deposited, a condition unfavorable for the formation and recognition of any erosional features. These currents were probably associated with the Plinian vent rather than a separate vent; the paleocurrent direction inferred from the morphology of the flute-like marks is consistent with these flows derived from the caldera, and it would seem fortuitous for a new vent to open at the almost same time as the Plinian activity started. Blasts formed by collapse of a dome in a Plinian vent (e.g., 2014 eruption of Kelud volcano, Indonesia; Maeno et al. \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2019\u003c/span\u003e) or vent-opening lateral blasts (e.g., 1912 eruption of Novarupta in Alaska, USA; Hildreth and Fierstein \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2012\u003c/span\u003e) might erode the substrate, but such currents would have emplaced a lava-lapilli-bearing unit beneath or interbedded with the basal parts of Plinian deposits; no such deposits are observed at sections where the base of the Plinian unit is exposed, and lithic clasts are rare in its basal part. Alternatively, and perhaps more likely, dilute pyroclastic currents may have resulted from a local collapse along the margin of the Plinian column. This type of collapse of the eruption column may occur when the column conditions are transitional between buoyant plume and column collapse, and likely produces small pyroclastic currents while the bulk of the column remains stable and buoyant (Koyaguchi et al. \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). This may be consistent with the limited lateral extent of outcrops (in the most proximal locations, \u0026lt;\u0026thinsp;1\u0026ndash;2 km from the vent) that show evidence of erosion at the base of the Plinian deposits.\u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eThe proximal section at the Ohachidaira caldera exhibits exceptional erosive structures at the contact between underlying sub-Plinian and overlying Plinian deposits, including asymmetric scours that closely resemble, in cross-sectional view, flute marks in sedimentary rocks. Such flute-like marks have not previously been described in Plinian fall deposits or are rarely reported in the volcanological literature. While flute marks are common in aqueous sedimentary rocks and thus are formed during water erosion, field evidence indicates that the flute-like marks and other deformation features were formed by erosion due to the passage of dilute, turbulent pyroclastic currents fed from the Plinian eruption column. The contact relationship of these erosion features with overlying pumice lapilli indicates that erosive events and initial pumice fallout deposits are largely coeval; such an interpretation allows an estimation of the time required for the flute-like marks to form, which would be difficult to estimate for flute marks in subaqueous environments. The presence of erosion features exclusively within the most proximal sections and the lack of evidence of the passage of pyroclastic currents at the base of the Plinian unit in more distal areas suggest short runout distances of the erosive pyroclastic currents. Such small dilute pyroclastic currents may be consistent with a local collapse of the margin of the Plinian column, as seen in numerical simulations for a transitional regime between stable column and total collapse (Koyaguchi et al. \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2018\u003c/span\u003e).\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eFunding\u003c/h2\u003e \u003cp\u003eThis work was supported by Fukada Grant-in-Aid from the Fukada Geological Institute and JSPS KAKENHI Grant Number 21K14004 to YY.\u003c/p\u003e \u003cp\u003e \u003cstrong\u003eConflicts of interest/Competing interests\u003c/strong\u003e \u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eAcknowledgments\u003c/h2\u003e \u003cp\u003eWe acknowledge Yoshiro Ishihara for sharing his knowledge of flute marks. Greg Valentine and anonymous reviewers are thanked for thoughtful comments.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eAllen JRL (1968) Flute marks and flute separation. Nature 219:602\u0026ndash;604. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1038/219602a0\u003c/span\u003e\u003cspan address=\"10.1038/219602a0\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAllen JRL (1969) Erosional current marks of weakly cohesive mud beds. 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J Volcanol Geotherm Res 357:287\u0026ndash;305. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jvolgeores.2018.04.017\u003c/span\u003e\u003cspan address=\"10.1016/j.jvolgeores.2018.04.017\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"bulletin-of-volcanology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"buvo","sideBox":"Learn more about [Bulletin of Volcanology](http://link.springer.com/journal/445)","snPcode":"445","submissionUrl":"https://www.editorialmanager.com/buvo/default2.aspx","title":"Bulletin of Volcanology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Flute-like mark, Shear deformation structure, Erosion feature, Proximal Plinian fall deposit, dilute turbulent pyroclastic current","lastPublishedDoi":"10.21203/rs.3.rs-4379634/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4379634/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eWe describe flute-like marks at the boundary between the sub-Plinian and overlying Plinian units of the 34 ka eruption at the Ohachidaira caldera and discuss their formation mechanisms. These scours are observed at a proximal section\u0026thinsp;~\u0026thinsp;1.8 km from the caldera center (~\u0026thinsp;0.9 km from the rim) and show asymmetric cross-sectional shapes that closely resemble flute marks typically found in subaqueous sedimentary rocks; they cut into underlying strata of the immediately preceding sub-Plinian fallout deposits and are filled with pumice lapilli of the Plinian phase. Another characteristic erosional feature is shear deformation of the top ash layer of the sub-Plinian deposits; this ash layer is locally bent, folded, split, or fragmented. These erosional features appear to be coeval with the basal part of the Plinian unit. We interpret these features, based upon their field characteristics, to have been formed by dilute turbulent pyroclastic currents that occurred at the beginning of the Plinian phase. Observations allow the estimation of timescales for formation of the flute-like marks to be at ~\u0026thinsp;0.5\u0026ndash;5 min, which is rarely obtained from flute marks in sedimentary rocks. The only deposit left by the erosive currents is a thin fine ash lens that occurs at the bases of the flute-like marks, and no pyroclastic current deposits or erosional features were observed at the base of the Plinian sequence beyond 1 km from the caldera rim, suggesting short runout distances of the dilute pyroclastic currents; such small currents might have resulted from a local collapse along the margin of the Plinian column.\u003c/p\u003e","manuscriptTitle":"Flute-like marks at the base of Plinian pumice-fall deposits at Ohachidaira caldera, Hokkaido, Japan","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-07-10 15:16:49","doi":"10.21203/rs.3.rs-4379634/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Moderate revision (possibly re-reviewed)","date":"2024-09-17T16:16:13+00:00","index":"","fulltext":""},{"type":"reviewerAgreed","content":"","date":"2024-06-17T17:13:27+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-06-17T16:36:49+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"Bulletin of Volcanology","date":"2024-05-26T06:21:07+00:00","index":"","fulltext":""},{"type":"submitted","content":"Bulletin of Volcanology","date":"2024-05-06T22:20:23+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"bulletin-of-volcanology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"buvo","sideBox":"Learn more about [Bulletin of Volcanology](http://link.springer.com/journal/445)","snPcode":"445","submissionUrl":"https://www.editorialmanager.com/buvo/default2.aspx","title":"Bulletin of Volcanology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"f5ead7a4-6326-46bb-b721-53ddc3e7041a","owner":[],"postedDate":"July 10th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-02-17T16:07:36+00:00","versionOfRecord":{"articleIdentity":"rs-4379634","link":"https://doi.org/10.1007/s00445-025-01805-4","journal":{"identity":"bulletin-of-volcanology","isVorOnly":false,"title":"Bulletin of Volcanology"},"publishedOn":"2025-02-15 15:58:03","publishedOnDateReadable":"February 15th, 2025"},"versionCreatedAt":"2024-07-10 15:16:49","video":"","vorDoi":"10.1007/s00445-025-01805-4","vorDoiUrl":"https://doi.org/10.1007/s00445-025-01805-4","workflowStages":[]},"version":"v1","identity":"rs-4379634","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4379634","identity":"rs-4379634","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}