Paleogeography and volcanic morphology reconstruction of a buried monogenetic volcanic field (part 2)

Over the last two decades, a growing number of studies have identified the presence of ancient volcanic systems buried and preserved within sedimentary basins. Continuous improvements in remote-sensing techniques such as seismic reflection offer an opportunity to analyze how these extinct buried volcanoes have formed and evolved. In this paper, we reveal the Maahunui Volcanic Field (MVF), a cluster of middle Miocene volcanoes and shallow intrusions currently buried by more than 1000 m of sedimentary strata of the Canterbury Basin, New Zealand. This fossil volcanic field is imaged by high-quality 2D seismic lines and has representative igneous rocks penetrated by the Resolution-1 borehole. These data have been used to reconstruct the original morphology of the volcanoes and the regional paleogeography in which eruptions and shallow (< 2 km depth) intrusions occurred. Volcanism in the MVF covered an area of ca 1520 km(2), comprising at least 31 cone- and crater-type volcanoes. Eruptions typically produced small-volume volcanoes (< 1 km(3)) controlled by a plumbing system that fed magma to disperse eruptive centers, a characteristic of monogenetic volcanic fields. The MVF plumbing system emplaced a number of shallow intrusive bodies up to 2.5 km(3) in volume, typically within the Cretaceous-Paleocene sedimentary strata of the Canterbury Basin. These intrusions may have served as shallow stationary magma chambers that fed eruptions onto the middle Miocene seafloor. The resulting eruptions were entirely submarine (500 to 1500 m), producing deep-water morphologies equivalent to maar-diatreme and tuff cones. The primary morphology of the volcanoes is interpreted to be mainly controlled by high-energy pyroclastic eruptions, in which coeval thermogenic gases and CO2 incorporated in the magmatic system could have had an important role in the fragmentation and dispersion of erupted material. In addition, post-eruptive degradation changed the morphology of some edifices in the MVF, which was controlled by the height of the edifices and by their location in relation to a major base-level fall. By the late Miocene, high volcanic edifices (> 200 m) located in a neritic setting were possibly emergent at the paleosea surface, forming an archipelago of nine small extinct volcanic islands. This study demonstrates the value of detailed seismic reflection mapping and borehole analysis for improving understanding of the processes that formed, transformed, and preserved ancient volcanoes now buried in sedimentary basins.