Alan Bischoff, Andrew Nicol, Andrea Barrier, Hanfei Wang

Technological advances of our modern society motivated an unprecedented necessity to find natural resources in the subsurface of our planet. The search for these valuable resources has revealed an unexpected number of ancient volcanoes buried and preserved within sedimentary basins globally. Continuous improvements in remote sensing techniques such as seismic reflection can provide a valid opportunity to observe these extinct buried volcanic systems. In this paper, we present the Maahunui Volcanic Field (MVF), a cluster of 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 was imaged by high-quality 2D seismic lines and representative igneous rocks were recovered by the exploration borehole Resolution-1. Here, we present the reconstructed regional paleogeography in which eruptions and shallow (<2 km) intrusions occurred, as well as the original morphology of the volcanoes found in the MVF. Volcanism in the MVF occurred over an area of ca 1,520 km2, comprising at least 31 crater-and cone-type volcanoes. Eruptions in the MVF typically produced small-volume volcanoes (< 1 km3), controlled by a plumbing system that fed magma to disperse eruptive centres, a characteristic of monogenetic volcanic fields. The MVF plumbing system emplaced a number of shallow intrusive bodies up to 2.5 km3 in volume, typically within the Cretaceous-Paleocene sedimentary strata. In many cases, these intrusions have served as a shallow stationary magma chamber that possibly fed eruptions onto the paleo-middle Miocene sea-floor. Eruptions were entirely submarine (500 to 1500 m), producing deep-water morphologies equivalent of maar-diatreme and tuff cones. The morphology of the volcanoes is interpreted to be primarily 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 has changed the original volcanic morphology, 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 paleo-sea surface, forming an archipelago of nine small extinct volcanic islands. This study demonstrates that despite a number of perceived limitations, the geological history of ancient volcanoes now buried and preserved in sedimentary basins can be reconstructed by detailed seismic stratigraphic mapping and analysis of borehole data.