Almost eroded but still there: Oldest diatreme volcanoes of pre-Quaternary age in the eastern Bohemian Massif, Czechia

Abstract

Young volcanic forms are often well pronounced in topography and preserve valuable geological record of paleoenvironmental evolution. However, in ancient volcanic fields (Ma old), some volcanic landforms may exist only as erosional remnants that are completely transformed by long-term denudation. The identification of such structures is challenging owing to their small size and subtle geomorphic signature. Earlier magnetic surveys from the 1960s identified a minor volcanic form of unclear origin at a site within the Bruntál Volcanic Field, a Late Cenozoic intraplate volcanic province in Central Europe (Czechia). Newly conducted high-resolution magnetic, gravity, and geoelectrical surveys of the site revealed remnants of two deeply eroded diatremes and their feeder dykes. Analysis of the collected samples and excavated material from shallow trenches supports the phreatomagmatic origin of these structures. The two analogous circular diatremes, separated by a distance of 750 m, exhibit pronounced magnetic and conductive responses. Their small diameters (<100 m), coupled with minor negative gravity anomalies and the presence of excavated diatreme breccias, suggest the exposure of a deep part of the diatremes, e.g. root-zones, at the current erosional surface. The material buried in the diatremes also allows for the deduction of the previous extent of concurrently eroded Neogene sediments in the area. Despite unsuccessful attempts at Ar–Ar dating, the Neogene age assumed based on the deep erosional level of the volcanic vents makes them the oldest known maar-diatreme volcanoes in the eastern Bohemian Massif. This study provides very rare information about explosive volcanism in the area of northeastern margin of the Bohemian Massif and is the first-ever study on regional pre-Quaternary phreatomagmatism in the Bruntál Volcanic Field.