Reevaluating the Guam Welded Tuff, Southern Goheung Peninsula, Korea: Evidence for Pyroclastic Origin, Rheomorphic Deformation, and Complex Ignimbrite Facies

Reevaluating the Guam Welded Tuff, Southern Goheung Peninsula, Korea: Evidence for Pyroclastic Origin, Rheomorphic Deformation, and Complex Ignimbrite Facies | 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 Reevaluating the Guam Welded Tuff, Southern Goheung Peninsula, Korea: Evidence for Pyroclastic Origin, Rheomorphic Deformation, and Complex Ignimbrite Facies Uk Hwan Byun, Sang Koo Hwang This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6226127/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract The Guam Welded Tuff in the southern Goheung Peninsula, Korea, exhibits a striking lava-like appearance, historically causing misinterpretation as an effusive rhyolitic lava flow. However, detailed field investigations, microstructural observations, and volcanic facies analyses indicate that this deposit originated from a pyroclastic density current generated by explosive silicic volcanism, subsequently modified by rapid welding, intense rheomorphic deformation, and crystallization. The planar basal contact with underlying units such as the Dohwa Andesite or Hado Formation and the absence of basal vitrophyre strongly argue against a lava flow origin. Extensive and uniform welding, accompanied by a continuous parallel fabric, supports rapid compaction and high-temperature adhesion of pyroclastic material. Microscopically, the presence of strongly elongated fiamme, distinct parataxitic textures, and low-angle stretching lineations confirms significant ductile deformation under hot conditions. Additionally, systematic vertical variations in volcanic facies—from the basal lithic-rich layers to the upper vitric zones—reflect progressive changes in depositional mechanisms, welding intensity, and cooling history. Regional geological context, including the Yujusan caldera setting and a SHRIMP U-Pb age of 83.2 Ma, further supports an explosive eruptive origin. These comprehensive observations necessitate reinterpretation of the Guam Welded Tuff as a high-temperature ignimbrite, whose lava-like textures result from complex post-depositional processes rather than effusive emplacement. Pyroclastic flow Welded tuff Rheomorphic deformation Volcanic facies Explosive silicic volcanism Guam Welded Tuff Goheung Peninsula geology Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 1. INTRODUCTION Silicic volcanic deposits are predominantly preserved as pyroclastic materials produced by explosive eruptions. In many cases, rapid welding and subsequent ductile deformation modify the original depositional textures, creating a lava-like appearance that can lead to misinterpretations of these deposits as effusive lava flows (Henry and Wolff 1992 ; Manley 1995 ). Numerous field and petrographic studies have demonstrated that intense welding and rheomorphic deformation can overprint primary pyroclastic textures, complicating the interpretation of the original eruptive processes (Andrews and Branney 2011 ; Branney and Kokelaar 1992 ; Sumner and Branney, 2002 ; Bullock et al. 2018 ). The complex interplay between welding, cooling, and deformation in rheomorphic ignimbrites has been highlighted in similar deposits (Sumner and Branney, 2002 ). Recent research has further refined our understanding of these phenomena; for example, Ellis et al. ( 2015 ) and Scarani et al. ( 2023 ) have provided new insights into the cooling and deformation histories of high-temperature ignimbrites. The Guam Welded Tuff on the Goheung Peninsula, Korea, exemplifies such deposits. Despite its pyroclastic origin, it exhibits a striking lava-like appearance, as evidenced by its well-defined vertical stratigraphy, which ranges from a basal lithic-rich zone to progressively more vitric upper zones. Similar facies variations and welding characteristics have been documented in other high-temperature pyroclastic density current (PDC) deposits, such as those in the Snake River Plain (Bachmann et al. 2000 ; Branney et al. 2008 ). Additionally, studies of rhyolite lava flows and large-volume ash-flow tuffs have shown that features such as welding intensity and ductile deformation are critical indicators of rapid deposition and cooling (Bonnichsen and Kauffman 1987 ; Ekren et al. 1984 ). Recent geochronologic and stratigraphic investigations in Korea (Hwang et al. 2022 , 2024 ; Kim et al. 2015 ) further support the interpretation of the Guam Welded Tuff as a high-temperature PDC deposit. This study integrates field observations and petrographic analyses to document the internal stratigraphy and deformation characteristics of the deposit, thereby enhancing our understanding of deposition, welding, and cooling processes in silicic explosive volcanism. 2. GEOLOGICAL BACKGROUND AND REGIONAL CONTEXT The Goheung Peninsula comprises a diverse assemblage of sedimentary, volcanic, and plutonic rocks that formed during the Late Cretaceous. The sedimentary rocks, predominantly of Early Cretaceous age, are mainly exposed in the northern part of the peninsula (Fig. 1 ; Chae et al. 2019 ; Kim et al. 2015 ; Park et al. 2021 ). In contrast, the southern portion contains spatially and compositionally zoned volcanic rocks, classified into three distinct volcanic subgroups based on major element chemistry: the lower felsic (predominantly rhyolitic) Goheung Subgroup, the middle andesitic Palyoung Subgroup, and the upper felsic Jijukdo Subgroup. These volcanic units are intruded by plutonic rocks (Hwang et al. 2022 ). The Goheung Subgroup, which represents the lower felsic volcanic sequence, includes the Goheung Tuff and the Wondae Rhyolite (Fig. 1 ; Kim et al. 2015 ; Hwang et al. 2022 ). The Palyoung Subgroup, composed of intermediate volcanic rocks, is subdivided into several units: the Udou Formation, Guryongsan Andesite, Palyoung Tuff, Podu Andesite, Balpo Tuff, and Dohwa Andesite (Hwang et al. 2022 ; Kim et al. 2015 ). The Jijukdo Subgroup, which represents the upper felsic volcanic sequence, comprises the Hado Formation, Guam Welded Tuff, Sadongri Formation, and Byeolhaksan Rhyolite (Figs. 1 and 2 ; Hwang et al. 2022 ). Notably, the Guam Welded Tuff, which exhibits a pyroclastic flow texture closely resembling a lava flow (Fig. 3 ), has been dated using the SHRIMP U-Pb zircon method to 83.2 ± 1.1 Ma (Hwang et al. 2024 ). While this unit was previously interpreted as a porphyritic rhyolite, quartz-feldspathic volcanic rock, or trachyte (Yamanari, 1924 ), recent studies have classified it as a welded ignimbrite due to the presence of phenocryst-rich matrices, abundant fiamme, and occasional lithic fragments (Hwang et al. 2022 ). The Byeolhaksan Rhyolite predominantly exhibits mm- to cm-scale banded structures and occasionally contains spherulites and lithophysae. The SHRIMP U-Pb zircon dating of this unit yields an age range of 81.9–80.3 Ma (Fig. 2 ; Kim et al. 2015 ; Hwang et al. 2024 ). The plutonic rocks in the region mainly consist of fine-grained diorite, which occurs as stocks and dikes. SHRIMP U-Pb zircon dating indicates that these intrusive rocks have an emplacement age of 74.8 Ma (Hwang et al. 2024 ). Table 1 Summary of facies in Guam Welded Tuff Facies Description Key Characteristics Approx. Thickness Interpretation Basal lithic-rich zone High abundance of matrix-supported lithic fragments (1–20 cm) embedded in a tuffaceous matrix; gradational decrease in clast size upward; presence of fine glass shards in the matrix; weak welding Lithic fragments (1–20 cm in diameter); subangular to rounded morphology; lateral continuity; good sorting; matrix composed of shards, crystal fragments, lapilli Basal layer; undefined thickness Represents an early, high-energy depositional phase characterized by a high input of entrained debris (Henry & Wolff, 1992 ; Manley, 1995 ). Basal eutaxitic zone Thin interval immediately above the lithic-rich zone; nearly complete welding (moderate welding intensity); homogeneous, well-cemented fabric; dominated by reworked tuff fragments with minimal lithic content (< 10 vol%); flattened shards; continuous microstructure Dominance of reworked tuff fragments; very low lithic content (< 10 vol%) Approximately 1 m thick Rapid welding; rapid consolidation of hot pyroclastic material ( Manley, 1996 ; Branney and Kokelaar, 1992 ) Lower vitric zone Continuous parallel fabric defined by strongly stretched and elongated fiammes (strong welding intensity); preservation of volcanic glass shards; uniform fiamme alignment; fiammes with high aspect ratios (> 40:1) Uniform parallel arrangement; high aspect ratio (> 40:1); fiammes wrapping around small lithic clasts; fiber-like aligned porosity Approximately 5 m thick Indicates deposition under high-temperature conditions with significant ductile deformation ( Bull & McPhie, 2007 ; Branney & Kokelaar, 2002 ). Middle lithoidal zone Light-gray to light-brown appearance; subtle parallel banding; intermingled lithic fragments and fiamme remnants (modrate-strong welding intensity); incipient subvertical columnar joints (30–60 cm); early crystalline development; signs of devitrification; mineral variability Subtle parallel banding; incipient subvertical columnar joints (30–60 cm); felsophyre-like texture. Up to 200 m in some exposures. Suggests a transitional phase marked by increasing cooling and crystallization; variation in mineral crystallinity and the development of subtle flow banding possibly influenced by residual volatile effects during deposition, indicative of transitional cooling (Sumner and Branney, 2002 ; Streck and Grunder, 1995 ; Seaman, Darby Dyar, & Marinkovic, 2009). Upper vitric zone Blue-gray to gray, well-welded fabric; stretched and uniformly arranged fiammes (moderate welding intensity); localized autoclastic brecciation; minor thermal contraction features (e.g., small-scale columnar joints) Uniform parallel fabric; localized autoclastic brecciation; minor brittle deformation features (e.g., small-scale columnar joints). Approximately 5 m thick. Represents the final stage of deposition, characterized by rapid cooling and minor brittle deformation ( Manley, 1995 ; Bonnichsen & Kauffman, 1987 ). 3. METHODS Field investigations were conducted at key outcrop locations along the southern Goheung Peninsula to document stratigraphic relationships, structural features, and the spatial distribution of volcanic units within the Guam Welded Tuff. Detailed field observations—including systematic photography and sketch mapping—were employed to record vertical and lateral variations within the deposit. Representative samples from each facies were collected for petrographic analysis. Thin sections were prepared using standard procedures and examined under both plane-polarized light (PPL) and cross-polarized light (XPL) to document key textural features, such as welding intensity, fiamme morphology, and evidence of ductile deformation (Henry and Wolff 1992 ; Quane and Russell 2005 ). The integration of detailed field mapping and petrographic observations provided a comprehensive understanding of the depositional architecture of the Guam Welded Tuff. 4. RESULTS 4.1 Hado Formation The Hado Formation is locally exposed below the Guheong Consolidated Tuff (Figs. 1 and 2 ). At the southern coastal landing of Hado Village, it reaches a thickness of approximately 20 m, increasing southward and thinning toward the north. In this area, the lower portion of the formation consists primarily of tuffaceous mudstone, while the upper portion comprises interbedded tuffaceous sandstone, mudstone, and shale (Hwang et al. 2022 ; Fig. 4 ). On the northern coast of Hado Village, the formation is ~ 7 m thick and gradually pinches out. Here, the lower part consists predominantly of tuffaceous sandstone, while the upper part contains tuffaceous conglomerate. Additionally, a ~ 1.5 m thick lens of conglomerate is present immediately below the Guheong Consolidated Tuff. The stratification is laterally continuous, with well-sorted clastic material. 4.2. Guam Welded Tuff and Its Volcanic Facies Field observations indicate that the Guam Welded Tuff is a silicic, layered deposit with a SiO₂ content of 71–75 wt.%, cropping out along a north–south-trending belt within the Yujusan caldera region (Hwang et al. 2022 ; Hwang et al. 2024 ). In some areas, the tuff is conformable with the overlying Hado Formation, whereas in other locations, it rests unconformably on underlying andesitic units (Fig. 1 ). The deposit reaches a maximum thickness of up to 220 m in the southern part of the volcanic province but thins to only a few tens of meters toward the north. It forms a sinuous ridge approximately 7.5 km long, characterized by steep, near-perpendicular slopes. At coastal exposures, such as near Jijukdo Island, well-developed vertical cooling joints are observed, with individual columns measuring 0.5–1.2 m in width. The internal stratigraphy of the deposit is expressed as a vertically zoned sequence of volcanic facies (Fig. 3 ), transitioning from the Basal lithic-rich zone (Fig. 5 ), through the Basal eutaxitic zone (Fig. 5 ), through the Basal eutaxitic zone (Fig. 5 ) and Lower vitric zone (Fig. 6 ), to the Middle lithoidal zone (Fig. 7 ) and Upper vitric zone (Fig. 8 )—as summarized in Table 1 . These facies exhibit systematic variations in lithic content, welding intensity, and fabric development, reflecting changes in depositional conditions during pyroclastic flow emplacement. Thin-section (optical) analysis provides additional insights into the deposit's microstructural characteristics. Examination under plane- and cross-polarized light reveals the absence of a distinct quenched vitrophyre at the base, with the lower portion dominated by matrix-supported lithic fragments. Abundant volcanic glass shards and well-developed fiammes—flattened, elongated zones with high aspect ratios—are preserved in a continuous parallel fabric, indicating ductile deformation under high-temperature conditions. Microstructural analysis further reveals localized deformation features, including: Slight rotation of phenocrysts Incipient microcracking Early-stage devitrification Autoclastic brecciation and minor thermal contraction in the upper portions Similar welding texture alterations due to devitrification of pumice-rich materials have been documented by Gifkins et al. ( 2005 ) and Ellis et al. ( 2015 ), supporting the interpretation of the observed microstructural changes. These thin-section observations corroborate the field-scale facies analysis, providing a comprehensive record of welding, deformation, and cooling processes during and after deposition. 5. Discussion This study has revealed that the silicic unit in the southern Goheung Peninsula exhibits a well-defined internal architecture, prompting the critical question: Does this deposit represent an effusive silicic lava flow or a pyroclastic ignimbrite formed by an explosive eruption? Discriminating between these origins is challenging, as parallel lamination, stretched fiammes, and ductile deformation fabrics can form in both rheomorphic pyroclastic deposits and silicic lava flows (Henry and Wolff 1992 ; Manley 1995 , 1996 ). The following discussion synthesizes field observations, petrographic data, and quantitative fabric analyses to support a pyroclastic origin, later modified by intense rheomorphic deformation. 5.1 Eruptive Conditions and Deposition The deposit’s widespread lateral continuity, thickness variations (up to 220 m in the south), and steep ridges are indicative of rapid deposition from a high-density pyroclastic flow (Henry and Wolff 1992 ; Manley 1995 ). The basal lithic-rich zone, containing abundant lithic fragments, likely formed during the initial phase of PDC emplacement, incorporating entrained debris (Hwang et al. 2022 ). Above this, the basal eutaxitic zone, characterized by near-complete welding and a homogeneous fabric, suggests rapid compaction of hot pyroclastic material (Branney and Kokelaar 1992 ; Manley 1996 ). These observations align with experimental and field studies of high-temperature ignimbrites, where rapid deposition and welding are defining characteristics. 5.2 Vertical Facies Variation and Cooling History The internal stratigraphy—comprising basal lithic-rich, basal eutaxitic, lower vitric, middle lithoidal, and upper vitric zones—records a systematic evolution in depositional conditions and cooling history. The lower vitric zone, with a continuous parallel fabric and strongly stretched fiammes (aspect ratios > 40:1), suggests that the deposit remained at high residual temperatures, allowing extensive ductile deformation (Branney and Kokelaar 2002 ; Bull and McPhie 2007 ). The middle lithoidal zone, exhibiting light-gray to light-brown tones, subtle banding, and incipient subvertical columnar jointing, represents a transition phase where the deposit began to cool and crystallize (Streck and Grunder 1995 ; Sumner and Branney 2002 ). The upper vitric zone, with a well-welded blue-gray to gray fabric and localized brittle features (autoclastic brecciation, small-scale columnar joints), reflects the final stage of cooling and a shift toward brittle deformation (Sumner and Branney, 2002 ). These vertical facies transitions mirror cooling histories documented in other high-temperature PDC deposits (Andrews et al. 2008 ; Branney et al. 2008 ). 5.3 Rheomorphic Deformation and Flow Dynamics Field and thin-section analyses reveal strongly stretched fiammes, low-angle stretching lineations, and nearly horizontal foliation, all indicative of significant ductile deformation during emplacement (Branney and Kokelaar 1992 ; Wolff and Wright 1981 ; Branney et al., 2004 ). These features suggest that the pyroclastic density current retained sufficient heat, allowing for internal strain accumulation and ductile flow. Similar effects of heterogeneity in magma water concentration on the development of flow banding and spherulites have been documented in rhyolitic lavas (Seaman et al. 2009 ). Comparative studies (e.g., Andrews and Branney 2011 ; Knott et al. 2016a ) confirm that rheomorphic deformation is typical of ignimbrites emplaced under rapid cooling conditions with minimal atmospheric entrainment 5.4 Synthesis and Broader Implications The facies architecture of the Guam Welded Tuff—from the lithic-rich basal unit to the well-welded upper vitric unit—records an evolving depositional environment influenced by rapid deposition, high-temperature welding, and post-depositional cooling and crystallization. This depositional model aligns with the 'lava-like' appearance of ignimbrites from other volcanic regions (Manley 1995 ; Andrews et al. 2008 ; Branney et al. 2008 ; Ellis et al. 2013 ), such as the Snake River Plain (Manley 1995 ; Andrews et al. 2008 ; Branney et al. 2008 ), similar high-grade ignimbrites (Branney et al. 2004 ; Knott et al. 2016b ; Bullock et al. 2018 ; Scarani et al. 2023 ). Additionally, heterogeneities in magma water concentration have been shown to influence the development of flow banding and spherulites in rhyolitic lavas (Seaman et al. 2009 ), which may contribute to the 'lava-like' appearance of these deposits. Moreover, the transition from ductile to brittle deformation, as evidenced by localized brittle structures in the upper vitric zone, provides key constraints on eruption dynamics and post-depositional thermal evolution. These findings not only refine our understanding of silicic explosive volcanism on the Goheung Peninsula but also offer broader insights into the interpretation of similar high-grade ignimbrite deposits worldwide. 5.5 Timing of Deformation in the Guam Welded Tuff A critical consideration in interpreting the emplacement processes of the Guam Welded Tuff is the timing relationship between rheomorphic deformation and deposition. Recent studies (e.g., Scarani et al., 2023 ) indicate that shear deformation can initiate within lower portions of ignimbrites even as pyroclastic density currents (PDCs) are still actively depositing material above. This challenges the conventional view that rheomorphic deformation occurs exclusively after deposition, instead supporting a model where deformation begins syn-depositionally due to sustained loading and shear stress from the overriding hot current. In the case of the Guam Welded Tuff, distinct elongation and uniform parallel alignment of fiamme observed within the basal eutaxitic and lower vitric zones strongly suggest that rheomorphic deformation commenced immediately upon initial deposition while upper zones remained actively depositional (Fig. 9 ). This interpretation is consistent with observations of other high-temperature ignimbrites, which retain sufficient heat and exhibit low enough viscosities to accommodate such early deformation. However, post-depositional deformation also played a significant role in modifying the ignimbrite. As the deposit cooled, rheomorphic deformation gave way to progressive devitrification, contraction, and brittle fracturing. The transition from the lower vitric zone to the middle lithoidal zone marks the onset of cooling-induced changes, where devitrification led to the gradual loss of original glassy textures and the formation of a more lithified matrix. In the upper vitric zone, rapid cooling near the surface further enhanced autoclastic brecciation and columnar jointing, distinguishing it from the more ductile zones below. Thus, the Guam Welded Tuff preserves a continuum of deformation styles, from syn-depositional rheomorphic flow to post-depositional brittle fracturing. Recognizing the interplay between syn-depositional and post-depositional deformation provides a refined framework for distinguishing ignimbrite emplacement dynamics beyond traditional interpretations that emphasize only post-depositional welding and cooling (Fig. 9 ). 6. Conclusions This study demonstrates that the Guam Welded Tuff of the Goheung Peninsula preserves a complex depositional history, characterized by a well-defined vertical stratigraphy and distinctive volcanic facies. Integrated field observations and petrographic analyses reveal systematic vertical transitions, including: A basal lithic-rich zone, characterized by graded bedding and entrained debris indicative of sustained pyroclastic density currents (PDCs). A basal eutaxitic zone, representing the onset of significant welding and initial rheomorphic deformation under sustained heat and loading conditions. A lower vitric zone, exhibiting strongly elongated fiamme and uniform parallel alignment, suggesting maximum welding intensity and high-temperature ductile deformation. A middle lithoidal zone, marked by progressive devitrification, subtle parallel banding, incipient columnar jointing, and gradual transition toward brittle deformation. An upper vitric zone, reflecting rapid cooling near the surface, evident from significant brittle deformation features including prominent columnar joints and autoclastic brecciation. Critically, this study distinguishes the ignimbrite's lava-like appearance from actual effusive rhyolitic lava flows. Although features such as parallel lamination, stretched fiamme, and ductile fabrics can occur in both ignimbrites and silicic lava flows, the absence of a basal vitrophyre, presence of distinct fiamme textures, and clearly defined vertical facies transitions firmly support a pyroclastic rather than effusive origin. Recognition of syn-depositional deformation, as evidenced by elongated fiamme and eutaxitic fabrics in the lower zones, enhances understanding of the complex interactions between welding intensity, deformation timing, and cooling processes. This refined interpretation resolves longstanding ambiguities concerning the Guam Welded Tuff’s emplacement mechanisms and provides critical insights into eruption dynamics, cooling histories, and deformation processes within silicic explosive volcanic settings. Furthermore, these findings contribute broadly to improving the interpretation of similar high-grade rheomorphic ignimbrites globally, offering valuable implications for volcanic hazard assessment and paleoenvironmental reconstructions. Declarations Competing Interests The authors declare that they have no competing interests. Funding declaration This research was supported by the Korean Ministry of Trade, Industry and Energy and the Korea Institute of Energy Technology Evaluation and Planning (RS-2024-00426295). Additional support was provided by the Basic Research Project (GP2020-003) of the Korea Institute of Geoscience and Mineral Resources (KIGAM), funded by the Ministry of Science, ICT and Future Planning of the Republic of Korea. Acknowledgements This study was sponsored by the Korean Ministry of Trade, Industry and Energy and the Korea Institute of Energy Technology Evaluation and Planning (RS-2024-00426295). This study was also supported by the Basic Research Project (GP2020-003) of the Korea Institute of Geoscience and Mineral Resources (KIGAM), funded by the Ministry of Science, ICT and Future Planning of the Republic of Korea. Data availability statement The data presented in this study are available on request from the corresponding author and the GIS data for the geological map are available at https://doi.org/10.22747/data.20241129.5419 under a CC BY-NC 4.0 license." References Andrews GDM, Branney MJ (2011) Emplacement and rheomorphic deformation of a large, lava-like rhyolitic ignimbrite: Grey’s Landing, southern Idaho, Geolological Society of America bulletin. Geol Soc Am Bull 123:725–743. https://doi.org/10.1130/B30167.1 Andrews GDM, Branney MJ, Bonnichsen B, McCurry M (2008) Rhyolitic ignimbrites in the Rogerson Graben, southern Snake River Plain volcanic province: volcanic stratigraphy, eruption history and basin evolution. Bull Volcanol 70:269–291. https://doi.org/10.1007/s00445-007-0139-0 Bachmann O, Dungan MA, Lipman PW (2000) Voluminous lava-like precursor to a major ash-flow tuff: low-column pyroclastic eruption of the Pagosa Peak dacite, San Juan volcanic field, Colorado. 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J Volcanol Geotherm Res 10:13–34. https://doi.org/10.1016/0377-0273(81)90052-4 Yamanari F (1924) Geological map of Korea (1:50,000), Series 1, Institute of Geological Survey, the government-general of Korea Cite Share Download PDF Status: Posted Version 1 posted 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. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. 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-6226127","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":435851346,"identity":"2c0cb3a4-36b3-4bef-aecc-2de16dc77f80","order_by":0,"name":"Uk Hwan Byun","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAuElEQVRIiWNgGAWjYBACAwYexocNBjDuAeK0MBuSrIVNsgHOJUaLuUTuscoZBbXyug28Dx8wnLlHWIvljLy0mxsMjhtuO8BubMBwo5gIh93IMbv5wOAY47YDbGwSDB8SiNNSCNRiD9TC/oNoLYwbDGoSQbYwMNwgQotlzxtjyRkGB5K3HWZjlkg4Q4QWc/Ycw489f+pstx1vY/zw4RgRWqDgMAMDM5AiXgMDQx0JakfBKBgFo2DEAQCi+Tx78ggFJAAAAABJRU5ErkJggg==","orcid":"https://orcid.org/0009-0003-4128-4046","institution":"Korea Institute of Geoscience and Mineral Resources","correspondingAuthor":true,"prefix":"","firstName":"Uk","middleName":"Hwan","lastName":"Byun","suffix":""},{"id":435851347,"identity":"2cc29412-dda1-465a-885a-fd5726eb4217","order_by":1,"name":"Sang Koo Hwang","email":"","orcid":"","institution":"Andong National University","correspondingAuthor":false,"prefix":"","firstName":"Sang","middleName":"Koo","lastName":"Hwang","suffix":""}],"badges":[],"createdAt":"2025-03-14 12:08:44","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6226127/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6226127/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":80919425,"identity":"892d7b20-a9a3-4b6b-b8cb-6a69df617c94","added_by":"auto","created_at":"2025-04-18 19:39:58","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":6503860,"visible":true,"origin":"","legend":"\u003cp\u003eGeological map of the Yujusan caldera region (modified from Hwang et al., 2022), showing major lithostratigraphic units and key localities. Lower panels display representative cross-sections illustrating volcanic and sedimentary sequences, while the inset map (lower right) situates the study area within Korea.\u003c/p\u003e","description":"","filename":"Figure1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6226127/v1/fc7a794bcf3e233ab60f0400.jpg"},{"id":80919111,"identity":"bd027f4a-2882-4a89-87dd-61d3f8b08b47","added_by":"auto","created_at":"2025-04-18 19:31:58","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1536785,"visible":true,"origin":"","legend":"\u003cp\u003ePanoramic southward view from Yujusan’s summit, with approximate geologic boundaries overlaid. The landscape is dominated by Kgwt (Guam Welded Tuff), with minor exposures of Kbr (Byeolhaksan Rhyolite) and Kda (Dohwa Andesite), identified through field observations and regional mapping. See Figure 2 for further details.\u003c/p\u003e","description":"","filename":"Figure2Panoramicview.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6226127/v1/c132af24c0246c6715e858d0.jpg"},{"id":80919114,"identity":"eb959ba0-b490-435c-ac5f-a78fcd5d0b37","added_by":"auto","created_at":"2025-04-18 19:31:58","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":577302,"visible":true,"origin":"","legend":"\u003cp\u003eStratigraphic column of the Guam Welded Tuff in the southern Goheung Peninsula, overlying the Hado Formation (tuffaceous sandstone, mudstone, and shale). The deposit is subdivided into the Basal lithic-rich zone, Basal eutaxitic zone, Lower vitric zone, Middle lithoidal zone, and Upper vitric zone, with a local breccia lens in certain areas. Each zone exhibits distinct variations in lithic content, welding intensity, glass content, and rheomorphic fabrics, indicative of rapid deposition from a high-temperature pyroclastic flow, followed by intense welding and ductile deformation. Detailed descriptions and interpretations are in Table 1.\u003c/p\u003e","description":"","filename":"Figure3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6226127/v1/d70363e058e4b8ad0952c2ca.jpg"},{"id":80919117,"identity":"e8667842-042e-41ad-9bfe-ef31a99a1d65","added_by":"auto","created_at":"2025-04-18 19:31:58","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1972065,"visible":true,"origin":"","legend":"\u003cp\u003eField photographs of key outcrops in the Hado. (A) Alternating tuffaceous sandstone, mudstone, and shale in the upper Hado Formation, exposed along the quay at Hado Village. (B) Polymict tuffaceous conglomerate with clast- to matrix-supported textures, containing subrounded to subangular andesite clasts of various sizes, found between the Hado Formation and the Guam Welded Tuff. (C) Prominent columnar joints in the Guam Welded Tuff, exposed along the southern coastal cliffs of Jijukdo Island.\u003c/p\u003e","description":"","filename":"Figure4FeaturesofmajoroutcropsinHadoFormation.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6226127/v1/637b8e3910d73c7a809aade8.jpg"},{"id":80919116,"identity":"b1377d5b-157a-40a8-a9da-a13d513bd8b3","added_by":"auto","created_at":"2025-04-18 19:31:58","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":2605658,"visible":true,"origin":"","legend":"\u003cp\u003eKey features of the Basal lithic-rich zone and Basal eutaxitic zone. (A) Normal grading of lithic fragments in the Basal lithic-rich zone, northern coast of Hado Village. (B) Photomicrograph (plane-polarized light) of the Basal lithic-rich zone, southwestern Wondodong Village, showing poorly packed vitric shards, some with “tuning-fork” morphologies (black arrows), and subrounded volcanic lithic clasts (yellow arrows). (C) Eutaxitic fabric in the Basal eutaxitic zone, northern coast of Hado Village, characterized by well-aligned, flattened fiammes, indicative of welding. (D) Photomicrograph (plane-polarized light) of the Basal eutaxitic zone, northern coast of Hado Village, showing flattened shards with minimal additional deformation. These images illustrate subtle deformation variations within the eutaxitic fabric, resulting from welding of hot pyroclastic material.\u003c/p\u003e","description":"","filename":"Figure5basallithicrichzoneandbasaleutaxiticzone.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6226127/v1/bff3590859a11db82310ccf7.jpg"},{"id":80919424,"identity":"5ac54e8a-f11f-4daa-a9f1-d048108dd4a9","added_by":"auto","created_at":"2025-04-18 19:39:58","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":1314933,"visible":true,"origin":"","legend":"\u003cp\u003eFeatures of the Lower vitric zone, southern Sudeoksan. (A) Parataxitic fabric defined by dark brownish-gray, highly stretched fiammes wrapping around rotated lithic clasts. The yellow arrow highlights an altered volcanic lithic fragment, likely modified by devitrification or hydrothermal processes. (B) Photomicrograph (plane-polarized light) showing a parataxitic fabric with strongly stretched shards. The yellow arrow again marks an altered volcanic lithic fragment displaying similar signs of alteration.\u003c/p\u003e","description":"","filename":"Figure6lowervitriczone.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6226127/v1/a49d66aa76c39952534214d3.jpg"},{"id":80919112,"identity":"c80741c8-ee7f-4523-8b0d-68f5c45ac514","added_by":"auto","created_at":"2025-04-18 19:31:58","extension":"jpg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":3754842,"visible":true,"origin":"","legend":"\u003cp\u003eRepresentative features of the Middle lithoidal zone. (A) Roadside outcrop in Dango-ri, resembling a felsophyre but exhibiting an indistinct parataxitic fabric defined by extremely stretched fiammes. (B) Outcrop on the western coast of Gain-dong, resembling flow-banded rhyolite, yet revealing an indistinct parataxitic fabric where stretched vesicles mimic flow banding. (C) Outcrop at the northern forest roadside of Hadong Village, resembling a felsophyre but displaying a parataxitic fabric with stretched fiammes surrounding a rotated lithic clast. (D) Subvertical columnar joints on the western coast of Gain-dong. (E) Photomicrograph (plane-polarized light) from Hadong Hill, showing a relict parataxitic fabric formed by devitrification, with microcrystalline fiammes and cryptocrystalline shards pseudomorphed by microcrystalline aggregates (yellow arrow). (F) Photomicrograph (plane-polarized light) from Guam Reservoir, displaying a relict parataxitic fabric with localized deflection around rotated phenocrysts due to devitrification of cryptocrystalline shards. The black arrow marks cryptocrystalline shards within former fiammes, while the yellow arrow highlights rotated phenocrysts.\u003c/p\u003e","description":"","filename":"Figure7themiddlelithoidalzone.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6226127/v1/d0dad9f8f34a643b84cee6c0.jpg"},{"id":80919120,"identity":"e8d5852b-7a66-436a-a33b-c0705149321b","added_by":"auto","created_at":"2025-04-18 19:31:58","extension":"jpg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":2847500,"visible":true,"origin":"","legend":"\u003cp\u003eFeatures of the Upper vitric zone. (A) Parataxitic fabric with highly stretched fiammes and wispy terminations, exposed at the northern forest roadside of Hadong Village. (B) Photomicrograph (plane-polarized light) from the northern forest roadside near Hadong Hill, showing a parataxitic fabric of stretched shards and devitrified fiammes (pseudomorphed by cryptocrystalline aggregates), along with vitroclastic textures where cuspate shards remain uncompressed around broken and rotated phenocrysts. (C) Photomicrograph (plane-polarized light) from the southern forest roadside of Sangdong Village, illustrating an eutaxitic fabric formed by flattened cuspate shards and fiammes with wispy or ragged terminations. (D) Autoclastic breccia composed of highly angular vitrophyre clasts of various sizes, exposed at the southern forest roadside of Sangdong Village.\u003c/p\u003e","description":"","filename":"Figure8uppervitriczone.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6226127/v1/be40e2c345903efdf8720ab4.jpg"},{"id":80919427,"identity":"f934a758-0188-4fa7-9e34-fe876b5b1150","added_by":"auto","created_at":"2025-04-18 19:39:58","extension":"jpg","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":1441246,"visible":true,"origin":"","legend":"\u003cp\u003eSimplified conceptual model depicting syn-depositional and post-depositional deformation stages within the Guam Welded Tuff. Syn-depositional zones form progressively during active deposition from pyroclastic density currents (PDCs), reflecting initial depositional conditions and early-stage welding processes. Post-depositional zones represent further modification due to continued compaction, cooling, crystallization, and increasing deformation intensity, highlighting the transition from ductile rheomorphic deformation to brittle deformation and fracturing toward the top of the sequence.\u003c/p\u003e","description":"","filename":"Figure9model.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6226127/v1/7abe3bcfb56bd98cee45bf40.jpg"},{"id":82976240,"identity":"e49d421c-0feb-4543-9024-4d58cee6622f","added_by":"auto","created_at":"2025-05-18 08:20:14","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":23204531,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6226127/v1/bfa0b228-adc3-45fb-9a46-6ffda07d9338.pdf"}],"financialInterests":"","formattedTitle":"Reevaluating the Guam Welded Tuff, Southern Goheung Peninsula, Korea: Evidence for Pyroclastic Origin, Rheomorphic Deformation, and Complex Ignimbrite Facies","fulltext":[{"header":"1. INTRODUCTION","content":"\u003cp\u003eSilicic volcanic deposits are predominantly preserved as pyroclastic materials produced by explosive eruptions. In many cases, rapid welding and subsequent ductile deformation modify the original depositional textures, creating a lava-like appearance that can lead to misinterpretations of these deposits as effusive lava flows (Henry and Wolff \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e1992\u003c/span\u003e; Manley \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e1995\u003c/span\u003e). Numerous field and petrographic studies have demonstrated that intense welding and rheomorphic deformation can overprint primary pyroclastic textures, complicating the interpretation of the original eruptive processes (Andrews and Branney \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Branney and Kokelaar \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e1992\u003c/span\u003e; Sumner and Branney, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Bullock et al. \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). The complex interplay between welding, cooling, and deformation in rheomorphic ignimbrites has been highlighted in similar deposits (Sumner and Branney, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2002\u003c/span\u003e). Recent research has further refined our understanding of these phenomena; for example, Ellis et al. (\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2015\u003c/span\u003e) and Scarani et al. (\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2023\u003c/span\u003e) have provided new insights into the cooling and deformation histories of high-temperature ignimbrites.\u003c/p\u003e \u003cp\u003eThe Guam Welded Tuff on the Goheung Peninsula, Korea, exemplifies such deposits. Despite its pyroclastic origin, it exhibits a striking lava-like appearance, as evidenced by its well-defined vertical stratigraphy, which ranges from a basal lithic-rich zone to progressively more vitric upper zones. Similar facies variations and welding characteristics have been documented in other high-temperature pyroclastic density current (PDC) deposits, such as those in the Snake River Plain (Bachmann et al. \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2000\u003c/span\u003e; Branney et al. \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). Additionally, studies of rhyolite lava flows and large-volume ash-flow tuffs have shown that features such as welding intensity and ductile deformation are critical indicators of rapid deposition and cooling (Bonnichsen and Kauffman \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e1987\u003c/span\u003e; Ekren et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e1984\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eRecent geochronologic and stratigraphic investigations in Korea (Hwang et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2022\u003c/span\u003e, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Kim et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2015\u003c/span\u003e) further support the interpretation of the Guam Welded Tuff as a high-temperature PDC deposit. This study integrates field observations and petrographic analyses to document the internal stratigraphy and deformation characteristics of the deposit, thereby enhancing our understanding of deposition, welding, and cooling processes in silicic explosive volcanism.\u003c/p\u003e"},{"header":"2. GEOLOGICAL BACKGROUND AND REGIONAL CONTEXT","content":"\u003cp\u003eThe Goheung Peninsula comprises a diverse assemblage of sedimentary, volcanic, and plutonic rocks that formed during the Late Cretaceous. The sedimentary rocks, predominantly of Early Cretaceous age, are mainly exposed in the northern part of the peninsula (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e; Chae et al. \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Kim et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Park et al. \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). In contrast, the southern portion contains spatially and compositionally zoned volcanic rocks, classified into three distinct volcanic subgroups based on major element chemistry: the lower felsic (predominantly rhyolitic) Goheung Subgroup, the middle andesitic Palyoung Subgroup, and the upper felsic Jijukdo Subgroup. These volcanic units are intruded by plutonic rocks (Hwang et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2022\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe Goheung Subgroup, which represents the lower felsic volcanic sequence, includes the Goheung Tuff and the Wondae Rhyolite (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e; Kim et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Hwang et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). The Palyoung Subgroup, composed of intermediate volcanic rocks, is subdivided into several units: the Udou Formation, Guryongsan Andesite, Palyoung Tuff, Podu Andesite, Balpo Tuff, and Dohwa Andesite (Hwang et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Kim et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). The Jijukdo Subgroup, which represents the upper felsic volcanic sequence, comprises the Hado Formation, Guam Welded Tuff, Sadongri Formation, and Byeolhaksan Rhyolite (Figs.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e; Hwang et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2022\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eNotably, the Guam Welded Tuff, which exhibits a pyroclastic flow texture closely resembling a lava flow (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), has been dated using the SHRIMP U-Pb zircon method to 83.2\u0026thinsp;\u0026plusmn;\u0026thinsp;1.1 Ma (Hwang et al. \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). While this unit was previously interpreted as a porphyritic rhyolite, quartz-feldspathic volcanic rock, or trachyte (Yamanari, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e1924\u003c/span\u003e), recent studies have classified it as a welded ignimbrite due to the presence of phenocryst-rich matrices, abundant fiamme, and occasional lithic fragments (Hwang et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). The Byeolhaksan Rhyolite predominantly exhibits mm- to cm-scale banded structures and occasionally contains spherulites and lithophysae. The SHRIMP U-Pb zircon dating of this unit yields an age range of 81.9\u0026ndash;80.3 Ma (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e; Kim et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Hwang et al. \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). The plutonic rocks in the region mainly consist of fine-grained diorite, which occurs as stocks and dikes. SHRIMP U-Pb zircon dating indicates that these intrusive rocks have an emplacement age of 74.8 Ma (Hwang et al. \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2024\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eSummary of facies in Guam Welded Tuff\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cspan type=\"SmallCaps\" class=\"SmallCaps\" name=\"Emphasis\"\u003eFacies\u003c/span\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cspan type=\"SmallCaps\" class=\"SmallCaps\" name=\"Emphasis\"\u003eDescription\u003c/span\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cspan type=\"SmallCaps\" class=\"SmallCaps\" name=\"Emphasis\"\u003eKey Characteristics\u003c/span\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cspan type=\"SmallCaps\" class=\"SmallCaps\" name=\"Emphasis\"\u003eApprox. Thickness\u003c/span\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cspan type=\"SmallCaps\" class=\"SmallCaps\" name=\"Emphasis\"\u003eInterpretation\u003c/span\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cspan type=\"SmallCaps\" class=\"SmallCaps\" name=\"Emphasis\"\u003eBasal lithic-rich zone\u003c/span\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eHigh abundance of matrix-supported lithic fragments (1\u0026ndash;20 cm) embedded in a tuffaceous matrix; gradational decrease in clast size upward; presence of fine glass shards in the matrix; weak welding\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eLithic fragments (1\u0026ndash;20 cm in diameter); subangular to rounded morphology; lateral continuity; good sorting; matrix composed of shards, crystal fragments, lapilli\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eBasal layer; undefined thickness\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eRepresents an early, high-energy depositional phase characterized by a high input of entrained debris (Henry \u0026amp; Wolff, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e1992\u003c/span\u003e; Manley, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e1995\u003c/span\u003e).\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cspan type=\"SmallCaps\" class=\"SmallCaps\" name=\"Emphasis\"\u003eBasal eutaxitic zone\u003c/span\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eThin interval immediately above the lithic-rich zone; nearly complete welding (moderate welding intensity); homogeneous, well-cemented fabric; dominated by reworked tuff fragments with minimal lithic content (\u0026lt;\u0026thinsp;10 vol%); flattened shards; continuous microstructure\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eDominance of reworked tuff fragments; very low lithic content (\u0026lt;\u0026thinsp;10 vol%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eApproximately 1 m thick\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cspan type=\"SmallCaps\" class=\"SmallCaps\" name=\"Emphasis\"\u003eRapid welding; rapid consolidation of hot pyroclastic material (\u003c/span\u003eManley, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e1996\u003c/span\u003e; Branney and Kokelaar, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e1992\u003c/span\u003e\u003cspan type=\"SmallCaps\" class=\"SmallCaps\" name=\"Emphasis\"\u003e)\u003c/span\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLower vitric zone\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eContinuous parallel fabric defined by strongly stretched and elongated fiammes (strong welding intensity); preservation of volcanic glass shards; uniform fiamme alignment; fiammes with high aspect ratios (\u0026gt;\u0026thinsp;40:1)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eUniform parallel arrangement; high aspect ratio (\u0026gt;\u0026thinsp;40:1); fiammes wrapping around small lithic clasts; fiber-like aligned porosity\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eApproximately 5 m thick\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cspan type=\"SmallCaps\" class=\"SmallCaps\" name=\"Emphasis\"\u003eIndicates deposition under high-temperature conditions with significant ductile deformation (\u003c/span\u003eBull \u0026amp; McPhie, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Branney \u0026amp; Kokelaar, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2002\u003c/span\u003e\u003cspan type=\"SmallCaps\" class=\"SmallCaps\" name=\"Emphasis\"\u003e).\u003c/span\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cspan type=\"SmallCaps\" class=\"SmallCaps\" name=\"Emphasis\"\u003eMiddle lithoidal zone\u003c/span\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eLight-gray to light-brown appearance; subtle parallel banding; intermingled lithic fragments and fiamme remnants (modrate-strong welding intensity); incipient subvertical columnar joints (30\u0026ndash;60 cm); early crystalline development; signs of devitrification; mineral variability\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eSubtle parallel banding; incipient subvertical columnar joints (30\u0026ndash;60 cm); felsophyre-like texture.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eUp to 200 m in some exposures.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eSuggests a transitional phase marked by increasing cooling and crystallization; variation in mineral crystallinity and the development of subtle flow banding possibly influenced by residual volatile effects during deposition, indicative of transitional cooling (Sumner and Branney, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Streck and Grunder, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e1995\u003c/span\u003e; Seaman, Darby Dyar, \u0026amp; Marinkovic, 2009).\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eUpper vitric zone\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eBlue-gray to gray, well-welded fabric; stretched and uniformly arranged fiammes (moderate welding intensity); localized autoclastic brecciation; minor thermal contraction features (e.g., small-scale columnar joints)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eUniform parallel fabric; localized autoclastic brecciation; minor brittle deformation features (e.g., small-scale columnar joints).\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eApproximately 5 m thick.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cspan type=\"SmallCaps\" class=\"SmallCaps\" name=\"Emphasis\"\u003eRepresents the final stage of deposition, characterized by rapid cooling and minor brittle deformation (\u003c/span\u003eManley, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e1995\u003c/span\u003e; Bonnichsen \u0026amp; Kauffman, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e1987\u003c/span\u003e\u003cspan type=\"SmallCaps\" class=\"SmallCaps\" name=\"Emphasis\"\u003e).\u003c/span\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e"},{"header":"3. METHODS","content":"\u003cp\u003eField investigations were conducted at key outcrop locations along the southern Goheung Peninsula to document stratigraphic relationships, structural features, and the spatial distribution of volcanic units within the Guam Welded Tuff. Detailed field observations\u0026mdash;including systematic photography and sketch mapping\u0026mdash;were employed to record vertical and lateral variations within the deposit.\u003c/p\u003e \u003cp\u003eRepresentative samples from each facies were collected for petrographic analysis. Thin sections were prepared using standard procedures and examined under both plane-polarized light (PPL) and cross-polarized light (XPL) to document key textural features, such as welding intensity, fiamme morphology, and evidence of ductile deformation (Henry and Wolff \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e1992\u003c/span\u003e; Quane and Russell \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2005\u003c/span\u003e). The integration of detailed field mapping and petrographic observations provided a comprehensive understanding of the depositional architecture of the Guam Welded Tuff.\u003c/p\u003e"},{"header":"4. RESULTS","content":"\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e4.1 Hado Formation\u003c/h2\u003e \u003cp\u003eThe Hado Formation is locally exposed below the Guheong Consolidated Tuff (Figs.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). At the southern coastal landing of Hado Village, it reaches a thickness of approximately 20 m, increasing southward and thinning toward the north. In this area, the lower portion of the formation consists primarily of tuffaceous mudstone, while the upper portion comprises interbedded tuffaceous sandstone, mudstone, and shale (Hwang et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). On the northern coast of Hado Village, the formation is ~\u0026thinsp;7 m thick and gradually pinches out. Here, the lower part consists predominantly of tuffaceous sandstone, while the upper part contains tuffaceous conglomerate. Additionally, a\u0026thinsp;~\u0026thinsp;1.5 m thick lens of conglomerate is present immediately below the Guheong Consolidated Tuff. The stratification is laterally continuous, with well-sorted clastic material.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e4.2. Guam Welded Tuff and Its Volcanic Facies\u003c/h2\u003e \u003cp\u003eField observations indicate that the Guam Welded Tuff is a silicic, layered deposit with a SiO₂ content of 71\u0026ndash;75 wt.%, cropping out along a north\u0026ndash;south-trending belt within the Yujusan caldera region (Hwang et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Hwang et al. \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). In some areas, the tuff is conformable with the overlying Hado Formation, whereas in other locations, it rests unconformably on underlying andesitic units (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The deposit reaches a maximum thickness of up to 220 m in the southern part of the volcanic province but thins to only a few tens of meters toward the north. It forms a sinuous ridge approximately 7.5 km long, characterized by steep, near-perpendicular slopes. At coastal exposures, such as near Jijukdo Island, well-developed vertical cooling joints are observed, with individual columns measuring 0.5\u0026ndash;1.2 m in width. The internal stratigraphy of the deposit is expressed as a vertically zoned sequence of volcanic facies (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), transitioning from the Basal lithic-rich zone (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e), through the Basal eutaxitic zone (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e), through the Basal eutaxitic zone (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e) and Lower vitric zone (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e), to the Middle lithoidal zone (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e) and Upper vitric zone (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e)\u0026mdash;as summarized in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. These facies exhibit systematic variations in lithic content, welding intensity, and fabric development, reflecting changes in depositional conditions during pyroclastic flow emplacement.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThin-section (optical) analysis provides additional insights into the deposit's microstructural characteristics. Examination under plane- and cross-polarized light reveals the absence of a distinct quenched vitrophyre at the base, with the lower portion dominated by matrix-supported lithic fragments. Abundant volcanic glass shards and well-developed fiammes\u0026mdash;flattened, elongated zones with high aspect ratios\u0026mdash;are preserved in a continuous parallel fabric, indicating ductile deformation under high-temperature conditions.\u003c/p\u003e \u003cp\u003eMicrostructural analysis further reveals localized deformation features, including:\u003c/p\u003e \u003cp\u003e \u003cul\u003e \u003cli\u003e \u003cp\u003eSlight rotation of phenocrysts\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eIncipient microcracking\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eEarly-stage devitrification\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eAutoclastic brecciation and minor thermal contraction in the upper portions\u003c/p\u003e \u003c/li\u003e \u003c/ul\u003e \u003c/p\u003e \u003cp\u003eSimilar welding texture alterations due to devitrification of pumice-rich materials have been documented by Gifkins et al. (\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2005\u003c/span\u003e) and Ellis et al. (\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2015\u003c/span\u003e), supporting the interpretation of the observed microstructural changes. These thin-section observations corroborate the field-scale facies analysis, providing a comprehensive record of welding, deformation, and cooling processes during and after deposition.\u003c/p\u003e \u003c/div\u003e"},{"header":"5. Discussion","content":"\u003cp\u003eThis study has revealed that the silicic unit in the southern Goheung Peninsula exhibits a well-defined internal architecture, prompting the critical question: Does this deposit represent an effusive silicic lava flow or a pyroclastic ignimbrite formed by an explosive eruption? Discriminating between these origins is challenging, as parallel lamination, stretched fiammes, and ductile deformation fabrics can form in both rheomorphic pyroclastic deposits and silicic lava flows (Henry and Wolff \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e1992\u003c/span\u003e; Manley \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e1995\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e1996\u003c/span\u003e). The following discussion synthesizes field observations, petrographic data, and quantitative fabric analyses to support a pyroclastic origin, later modified by intense rheomorphic deformation.\u003c/p\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e5.1 Eruptive Conditions and Deposition\u003c/h2\u003e \u003cp\u003eThe deposit\u0026rsquo;s widespread lateral continuity, thickness variations (up to 220 m in the south), and steep ridges are indicative of rapid deposition from a high-density pyroclastic flow (Henry and Wolff \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e1992\u003c/span\u003e; Manley \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e1995\u003c/span\u003e). The basal lithic-rich zone, containing abundant lithic fragments, likely formed during the initial phase of PDC emplacement, incorporating entrained debris (Hwang et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Above this, the basal eutaxitic zone, characterized by near-complete welding and a homogeneous fabric, suggests rapid compaction of hot pyroclastic material (Branney and Kokelaar \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e1992\u003c/span\u003e; Manley \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e1996\u003c/span\u003e). These observations align with experimental and field studies of high-temperature ignimbrites, where rapid deposition and welding are defining characteristics.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e5.2 Vertical Facies Variation and Cooling History\u003c/h2\u003e \u003cp\u003eThe internal stratigraphy\u0026mdash;comprising basal lithic-rich, basal eutaxitic, lower vitric, middle lithoidal, and upper vitric zones\u0026mdash;records a systematic evolution in depositional conditions and cooling history.\u003c/p\u003e \u003cp\u003e \u003cul\u003e \u003cli\u003e \u003cp\u003eThe lower vitric zone, with a continuous parallel fabric and strongly stretched fiammes (aspect ratios\u0026thinsp;\u0026gt;\u0026thinsp;40:1), suggests that the deposit remained at high residual temperatures, allowing extensive ductile deformation (Branney and Kokelaar \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Bull and McPhie \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2007\u003c/span\u003e).\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eThe middle lithoidal zone, exhibiting light-gray to light-brown tones, subtle banding, and incipient subvertical columnar jointing, represents a transition phase where the deposit began to cool and crystallize (Streck and Grunder \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e1995\u003c/span\u003e; Sumner and Branney \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2002\u003c/span\u003e).\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eThe upper vitric zone, with a well-welded blue-gray to gray fabric and localized brittle features (autoclastic brecciation, small-scale columnar joints), reflects the final stage of cooling and a shift toward brittle deformation (Sumner and Branney, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2002\u003c/span\u003e).\u003c/p\u003e \u003c/li\u003e \u003c/ul\u003e \u003c/p\u003e \u003cp\u003eThese vertical facies transitions mirror cooling histories documented in other high-temperature PDC deposits (Andrews et al. \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Branney et al. \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2008\u003c/span\u003e).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e5.3 Rheomorphic Deformation and Flow Dynamics\u003c/h2\u003e \u003cp\u003eField and thin-section analyses reveal strongly stretched fiammes, low-angle stretching lineations, and nearly horizontal foliation, all indicative of significant ductile deformation during emplacement (Branney and Kokelaar \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e1992\u003c/span\u003e; Wolff and Wright \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e1981\u003c/span\u003e; Branney et al., \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2004\u003c/span\u003e). These features suggest that the pyroclastic density current retained sufficient heat, allowing for internal strain accumulation and ductile flow. Similar effects of heterogeneity in magma water concentration on the development of flow banding and spherulites have been documented in rhyolitic lavas (Seaman et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). Comparative studies (e.g., Andrews and Branney \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Knott et al. \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2016a\u003c/span\u003e) confirm that rheomorphic deformation is typical of ignimbrites emplaced under rapid cooling conditions with minimal atmospheric entrainment\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e5.4 Synthesis and Broader Implications\u003c/h2\u003e \u003cp\u003eThe facies architecture of the Guam Welded Tuff\u0026mdash;from the lithic-rich basal unit to the well-welded upper vitric unit\u0026mdash;records an evolving depositional environment influenced by rapid deposition, high-temperature welding, and post-depositional cooling and crystallization. This depositional model aligns with the 'lava-like' appearance of ignimbrites from other volcanic regions (Manley \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e1995\u003c/span\u003e; Andrews et al. \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Branney et al. \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Ellis et al. \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2013\u003c/span\u003e), such as the Snake River Plain (Manley \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e1995\u003c/span\u003e; Andrews et al. \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Branney et al. \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2008\u003c/span\u003e), similar high-grade ignimbrites (Branney et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Knott et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2016b\u003c/span\u003e; Bullock et al. \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Scarani et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Additionally, heterogeneities in magma water concentration have been shown to influence the development of flow banding and spherulites in rhyolitic lavas (Seaman et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2009\u003c/span\u003e), which may contribute to the 'lava-like' appearance of these deposits. Moreover, the transition from ductile to brittle deformation, as evidenced by localized brittle structures in the upper vitric zone, provides key constraints on eruption dynamics and post-depositional thermal evolution. These findings not only refine our understanding of silicic explosive volcanism on the Goheung Peninsula but also offer broader insights into the interpretation of similar high-grade ignimbrite deposits worldwide.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e5.5 \u003cb\u003eTiming of Deformation in the Guam Welded Tuff\u003c/b\u003e\u003c/h2\u003e \u003cp\u003eA critical consideration in interpreting the emplacement processes of the Guam Welded Tuff is the timing relationship between rheomorphic deformation and deposition. Recent studies (e.g., Scarani et al., \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2023\u003c/span\u003e) indicate that shear deformation can initiate within lower portions of ignimbrites even as pyroclastic density currents (PDCs) are still actively depositing material above. This challenges the conventional view that rheomorphic deformation occurs exclusively after deposition, instead supporting a model where deformation begins syn-depositionally due to sustained loading and shear stress from the overriding hot current.\u003c/p\u003e \u003cp\u003eIn the case of the Guam Welded Tuff, distinct elongation and uniform parallel alignment of fiamme observed within the basal eutaxitic and lower vitric zones strongly suggest that rheomorphic deformation commenced immediately upon initial deposition while upper zones remained actively depositional (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e). This interpretation is consistent with observations of other high-temperature ignimbrites, which retain sufficient heat and exhibit low enough viscosities to accommodate such early deformation.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eHowever, post-depositional deformation also played a significant role in modifying the ignimbrite. As the deposit cooled, rheomorphic deformation gave way to progressive devitrification, contraction, and brittle fracturing. The transition from the lower vitric zone to the middle lithoidal zone marks the onset of cooling-induced changes, where devitrification led to the gradual loss of original glassy textures and the formation of a more lithified matrix. In the upper vitric zone, rapid cooling near the surface further enhanced autoclastic brecciation and columnar jointing, distinguishing it from the more ductile zones below.\u003c/p\u003e \u003cp\u003eThus, the Guam Welded Tuff preserves a continuum of deformation styles, from syn-depositional rheomorphic flow to post-depositional brittle fracturing. Recognizing the interplay between syn-depositional and post-depositional deformation provides a refined framework for distinguishing ignimbrite emplacement dynamics beyond traditional interpretations that emphasize only post-depositional welding and cooling (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e).\u003c/p\u003e \u003c/div\u003e"},{"header":"6. Conclusions","content":"\u003cp\u003eThis study demonstrates that the Guam Welded Tuff of the Goheung Peninsula preserves a complex depositional history, characterized by a well-defined vertical stratigraphy and distinctive volcanic facies. Integrated field observations and petrographic analyses reveal systematic vertical transitions, including:\u003c/p\u003e \u003cp\u003e \u003cul\u003e \u003cli\u003e \u003cp\u003eA basal lithic-rich zone, characterized by graded bedding and entrained debris indicative of sustained pyroclastic density currents (PDCs).\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eA basal eutaxitic zone, representing the onset of significant welding and initial rheomorphic deformation under sustained heat and loading conditions.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eA lower vitric zone, exhibiting strongly elongated fiamme and uniform parallel alignment, suggesting maximum welding intensity and high-temperature ductile deformation.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eA middle lithoidal zone, marked by progressive devitrification, subtle parallel banding, incipient columnar jointing, and gradual transition toward brittle deformation.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eAn upper vitric zone, reflecting rapid cooling near the surface, evident from significant brittle deformation features including prominent columnar joints and autoclastic brecciation.\u003c/p\u003e \u003c/li\u003e \u003c/ul\u003e \u003c/p\u003e \u003cp\u003eCritically, this study distinguishes the ignimbrite's lava-like appearance from actual effusive rhyolitic lava flows. Although features such as parallel lamination, stretched fiamme, and ductile fabrics can occur in both ignimbrites and silicic lava flows, the absence of a basal vitrophyre, presence of distinct fiamme textures, and clearly defined vertical facies transitions firmly support a pyroclastic rather than effusive origin.\u003c/p\u003e \u003cp\u003eRecognition of syn-depositional deformation, as evidenced by elongated fiamme and eutaxitic fabrics in the lower zones, enhances understanding of the complex interactions between welding intensity, deformation timing, and cooling processes. This refined interpretation resolves longstanding ambiguities concerning the Guam Welded Tuff\u0026rsquo;s emplacement mechanisms and provides critical insights into eruption dynamics, cooling histories, and deformation processes within silicic explosive volcanic settings. Furthermore, these findings contribute broadly to improving the interpretation of similar high-grade rheomorphic ignimbrites globally, offering valuable implications for volcanic hazard assessment and paleoenvironmental reconstructions.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eCompeting Interests\u003c/h2\u003e \u003cp\u003eThe authors declare that they have no competing interests.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eFunding\u003c/h2\u003e \u003cp\u003edeclaration\u003c/p\u003e \u003cp\u003eThis research was supported by the Korean Ministry of Trade, Industry and Energy and the Korea Institute of Energy Technology Evaluation and Planning (RS-2024-00426295). Additional support was provided by the Basic Research Project (GP2020-003) of the Korea Institute of Geoscience and Mineral Resources (KIGAM), funded by the Ministry of Science, ICT and Future Planning of the Republic of Korea.\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e \u003cp\u003eThis study was sponsored by the Korean Ministry of Trade, Industry and Energy and the Korea Institute of Energy Technology Evaluation and Planning (RS-2024-00426295). This study was also supported by the Basic Research Project (GP2020-003) of the Korea Institute of Geoscience and Mineral Resources (KIGAM), funded by the Ministry of Science, ICT and Future Planning of the Republic of Korea.\u003c/p\u003e\u003ch2\u003eData availability statement\u003c/h2\u003e \u003cp\u003eThe data presented in this study are available on request from the corresponding author and the GIS data for the geological map are available at \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.22747/data.20241129.5419\u003c/span\u003e\u003cspan address=\"10.22747/data.20241129.5419\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e under a CC BY-NC 4.0 license.\"\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eAndrews GDM, Branney MJ (2011) Emplacement and rheomorphic deformation of a large, lava-like rhyolitic ignimbrite: Grey\u0026rsquo;s Landing, southern Idaho, Geolological Society of America bulletin. 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Bull Volcanol 57:151\u0026ndash;169. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/BF00265035\u003c/span\u003e\u003cspan address=\"10.1007/BF00265035\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSumner JM, Branney MJ (2002) The emplacement history of a remarkable heterogeneous, chemically zoned, rheomorphic and locally lava-like ignimbrite: \u0026lsquo;TL\u0026rsquo; on Gran Canaria. J Volcanol Geotherm Res 115:109\u0026ndash;138. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/S0377-0273(01)00311-0\u003c/span\u003e\u003cspan address=\"10.1016/S0377-0273(01)00311-0\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWolff JA, Wright JV (1981) Rheomorphism of welded tuffs. J Volcanol Geotherm Res 10:13\u0026ndash;34. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/0377-0273(81)90052-4\u003c/span\u003e\u003cspan address=\"10.1016/0377-0273(81)90052-4\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eYamanari F (1924) Geological map of Korea (1:50,000), Series 1, Institute of Geological Survey, the government-general of Korea\u003c/span\u003e\u003c/li\u003e \u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Pyroclastic flow, Welded tuff, Rheomorphic deformation, Volcanic facies, Explosive silicic volcanism, Guam Welded Tuff, Goheung Peninsula geology","lastPublishedDoi":"10.21203/rs.3.rs-6226127/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6226127/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe Guam Welded Tuff in the southern Goheung Peninsula, Korea, exhibits a striking lava-like appearance, historically causing misinterpretation as an effusive rhyolitic lava flow. However, detailed field investigations, microstructural observations, and volcanic facies analyses indicate that this deposit originated from a pyroclastic density current generated by explosive silicic volcanism, subsequently modified by rapid welding, intense rheomorphic deformation, and crystallization. The planar basal contact with underlying units such as the Dohwa Andesite or Hado Formation and the absence of basal vitrophyre strongly argue against a lava flow origin. Extensive and uniform welding, accompanied by a continuous parallel fabric, supports rapid compaction and high-temperature adhesion of pyroclastic material. Microscopically, the presence of strongly elongated fiamme, distinct parataxitic textures, and low-angle stretching lineations confirms significant ductile deformation under hot conditions. Additionally, systematic vertical variations in volcanic facies—from the basal lithic-rich layers to the upper vitric zones—reflect progressive changes in depositional mechanisms, welding intensity, and cooling history. Regional geological context, including the Yujusan caldera setting and a SHRIMP U-Pb age of 83.2 Ma, further supports an explosive eruptive origin. These comprehensive observations necessitate reinterpretation of the Guam Welded Tuff as a high-temperature ignimbrite, whose lava-like textures result from complex post-depositional processes rather than effusive emplacement.\u003c/p\u003e","manuscriptTitle":"Reevaluating the Guam Welded Tuff, Southern Goheung Peninsula, Korea: Evidence for Pyroclastic Origin, Rheomorphic Deformation, and Complex Ignimbrite Facies","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-04-18 19:31:53","doi":"10.21203/rs.3.rs-6226127/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"7940c5b6-73d9-48fc-a9fb-c618ec77c030","owner":[],"postedDate":"April 18th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-05-18T08:12:00+00:00","versionOfRecord":[],"versionCreatedAt":"2025-04-18 19:31:53","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6226127","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6226127","identity":"rs-6226127","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}