Transition zone between the upper diatreme and lower diatreme: origin and significance at Round Butte, Hopi Buttes volcanic field, Navajo Nation, Arizona

Round Butte is a small but complex Miocene diatreme that crops out 190m below the pre-eruptive surface, in the southeastern part of the Hopi Buttes volcanic field. Erosional remnants consist of a diatreme 170-190m in diameter, of which the central 130-150m is well-exposed in a massif featuring 20-30-m-high sub-vertical cliffs, and a 50-cm-thick basanite dike. Field mapping allowed us to define three main groups of pyroclastic rocks in the diatreme: undisturbed beds, disturbed beds, and non-bedded rocks. Pyroclastic rocks range in grain size from coarse tuff to tuff breccia and in componentry from juvenile-rich to lithic-rich, with a dominance of heterolithic lapilli tuffs. Rocks from the undisturbed bedded pyroclastic group are present above an unconformity found all around the massif, whereas the disturbed bedded and the non-bedded pyroclastic groups are always found below it. This unconformity was previously understood as the contact between the upper and the lower diatremes. The undisturbed beds above the unconformity indeed compose the upper diatreme, but the assemblage of non-bedded rocks (invasive columns) and disturbed beds (residual columns) below it is not typical of the lower diatreme. Instead, they represent a transition zone between the upper and lower diatremes. Such a transition zone also occurs in other diatremes, it is important genetically, and we propose to add it to the general model of maar-diatreme volcanoes.