Volcanology of two maar-diatremes: Round Butte and Twin Peaks,Hopi Buttes volcanic field, Navajo Nation, Arizona.

Latutrie, Benjamin (2020). Volcanology of two maar-diatremes: Round Butte and Twin Peaks,Hopi Buttes volcanic field, Navajo Nation, Arizona. Thèse. Québec, Doctorat en sciences de la terre, Université du Québec, Institut national de la recherche scientifique, 231 p.

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Résumé

Maar-diatremes are small, monogenetic, short-lived volcanoes that are mainly phreatomagmatic in origin. They display a funnel morphology and are composed, from top to bottom, by an ejecta ring, a maar crater, a typically bedded upper diatreme, a typically non-bedded lower diatreme, a root zone and a feeder intrusion. The Miocene Hopi Buttes volcanic field in northern Arizona comprises many maar-diatreme volcanoes, and provides excellent exposures of diatremes. The well-known stratigraphy of the sedimentary country rocks in the area allows the depth of current diatreme remnants relative to the pre-eruptive surface to be estimated. From north to south (younger to older), the main sedimentary formations are the Bidahochi Formation, the Moenave Formation, the Chinle Formation (Owl Rock Member, Petrified Forest Member and Shinarump Member) and the Moenkopi Formation. Two months of field work were carried out on the Round Butte diatreme and on the Twin Peaks volcanic complex. Round Butte is a small diatreme exposed 190 m below the pre-eruptive surface and exposes the transition from upper diatreme to lower diatreme. Twin peaks is composed by two ‘plug’-dominated volcanic remnants reaching up to the pre-eruptive surface, and a small satellite diatreme. Detailed mapping, componentry measurements and sampling were realised at both sites. In the laboratory, geochemistry of coherent lavas, dikes and juvenile fragments was characterized. Thin sections of coherent and pyroclastic rocks were described and a subset of the pyroclastic thin sections was point counted. At Round Butte, three main groups of pyroclastic lithofacies were highlighted with undisturbed bedded rocks, disturbed bedded rocks and non-bedded rocks, as well as two minor groups comprising megablocks (blocks over 2 m in long axis) and debris avalanche deposits. Undisturbed bedded pyroclastic rocks were deposited on the floor of the syn-eruptive crater by fallback and pyroclastic surges. They are now part of the upper diatreme and sit on an unconformity. This unconformity formed during the eruption as a consequence of crater excavation, without requiring a significant pause in volcanic activity. Below this unconformity is the association of disturbed bedded and non-bedded pyroclastic rocks. The former are crater floor pyroclastic deposits in which the bedding has been progressively disturbed, whereas the latter form invasive columns created by passing debris jets. This association is interpreted as a transition zone toward the lower diatreme at depth. The transition zone has been integrated into the general model for maar-diatreme volcanoes, and was the missing link between upper diatreme rocks which are clearly phreatomagmatic at many sites, and the more enigmatic lower diatreme rocks. Both are shown to form at the same time, by the same suite of phreatomagmatic processes. Lithic fragment proportions measured with field componentry at Round Butte are compared with country rock fragmentation models based on the stratigraphy of surrounding country rocks and the size of the diatreme. The best models have a root zone within the Moenkopi Formation, up to 440 m below the pre-eruptive surface. Competent sedimentary formations/members (Moenave Formation, Owl Rock Member and Moenkopi Formation) are found in higher proportions in the field componentry than in the models because they are resistant to disaggregation. On the other hand, the less competent stratigraphic formations/members (Bidahochi Formation, Petrified Forest Member, Shinarump Member) show a depletion because they disaggregate easily; they are possibly included within the matrix of the pyroclastic rocks. In addition, Bidahochi clasts were preferentially expelled toward the ejecta ring during early maar formation. Sedimentary megablocks originate from the Bidahochi Formation, Moenave Formation and Owl Rock Member. These megablocks, and debris avalanche deposits, formed from slumps related to the destabilization of the crater walls. Not all maar-diatremes are purely phreatomagmatic. At Twin Peaks, this work describes ‘plug’- dominated volcanic remnants composed by two main peaks (North and South) and a satellite diatreme forming a SE-NW line. The two main peaks are maar-diatreme volcanoes because they display a round shape, gentle to steep contacts with the country rocks and contain some phreatomagmatic deposits. Detailed mapping highlights four stratigraphic units. Unit 1 is composed by grey to brown juvenile clasts with the highest proportion of lithic clasts of the massif; it is interpreted as phreatomagmatic. Unit 2 is less rich in lithic and brown juvenile clasts, but displays a higher proportion of moderately to highly vesiculated juvenile clasts; it is interpreted as phreato-strombolian. Unit 3a is characterized by non-welded spatter within a fine pyroclastic matrix and is interpreted as phreato-hawaiian. Unit 3b is composed by welded spatter to clastogenic lavas; the eruptive style was fire fountaining. Finally, Unit 4 is composed of black jointed coherent basanite and is interpreted as a lava lake. The lava-filled maar crater could have overflowed, forming lava flows, and this is an under-recognized hazard associated with maar-diatremes.

Type de document:	Thèse Thèse
Directeur de mémoire/thèse:	Ross, Pierre-Simon
Mots-clés libres:	transition zone; upper diatreme; lower diatreme; phreatomagmatism; magmatism; lithic clasts; lithic-rich lithologies; non-bedded rocks; disturbed beds; undisturbed beds
Centre:	Centre Eau Terre Environnement
Date de dépôt:	14 oct. 2020 19:31
Dernière modification:	29 janv. 2021 15:01
URI:	https://espace.inrs.ca/id/eprint/10410

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