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What lithic clasts and lithic-rich facies can tell us about diatreme processes: An example at Round Butte, Hopi Buttes volcanic field, Navajo Nation, Arizona.

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Latutrie, Benjamin et Ross, Pierre-Simon ORCID: https://orcid.org/0000-0002-5302-698X (2021). What lithic clasts and lithic-rich facies can tell us about diatreme processes: An example at Round Butte, Hopi Buttes volcanic field, Navajo Nation, Arizona. Journal of Volcanology and Geothermal Research , vol. 411 . p. 107150. DOI: 10.1016/j.jvolgeores.2020.107150.

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Résumé

Round Butte (Hopi Buttes volcanic field, Arizona) exposes a diatreme 170–190 m across, 190 m below the pre-eruptive surface. The central part of the massif is 130–150 m in diameter, displaying 20–30 m-high subvertical cliffs. The well-known layer-cake stratigraphy of the sedimentary rocks of the Colorado Plateau permits identification of the largest lithic fragments preserved in the Round Butte diatreme. We define three main groups of pyroclastic facies: undisturbed beds, disturbed beds and non-bedded rocks. Two other minor facies groups were mapped: megablocks (blocks over 2 m in long axis), and small-volume debris avalanche deposits. Pyroclastic megablocks are finer grained and richer in lithic clasts than most diatreme rocks surrounding them. These pyroclastic megablocks are interpreted as subsided portions of the maar ejecta ring. Sedimentary megablocks originate either from above, or from the same level, relative to their current location, i.e. no megablock has a net upward displacement. Small-volume debris avalanche deposits are poorly sorted deposits resulting from gravitational destabilization of the surrounding country rocks into the syn-eruptive crater. Small-volume debris avalanches and individual megablock collapse are the main ways in which the crater grew in size laterally during the eruption.

We combine the componentry of the disturbed bedded pyroclastic facies, the non-bedded pyroclastic facies and the pyroclastic megablocks with a series of conceptual models for country rock fragmentation. This exercise further allows us to estimate diatreme wall slopes of 70° below the Bidahochi Formation to approximately the depth of the root zone around 440 m below the pre-eruptive surface. Lithic fragments at the current level of exposure come from elevations up to 190 m above (i.e., up to the pre-eruptive surface) and up to 250 m below (i.e., down to the root zone) their current locations. Pyroclastic units displaying the richest content of lithic clasts with a deep origin are typically the non-bedded facies interpreted to have formed from debris jets during the eruption.

Type de document: Article
Mots-clés libres: maar-diatreme; lithic clasts; debris jets; debris avalanches; megablocks
Centre: Centre Eau Terre Environnement
Date de dépôt: 10 juin 2021 15:17
Dernière modification: 14 févr. 2022 16:25
URI: https://espace.inrs.ca/id/eprint/11754

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