Alan Bischoff, Andrew Nicol, Marcos Rossetti, Ben Kennedy

Buried volcanoes occur in great numbers within sedimentary basins globally. Knowledge of ancient buried volcanic systems has improved significantly over the past two decades. The in-depth understanding of these buried systems was mainly possible due to increasing availability of high-quality seismic reflection and sub-surface borehole data. This paper examines a cluster of Miocene volcanoes now buried by ca 1000 m of sedimentary strata in the Canterbury Basin, New Zealand. These volcanoes were imaged by 2D seismic lines and perforated by the Resolution-1 borehole. We refer to this group of volcanoes and related intrusive bodies as the Maahunui Volcanic Field (MVF). Here, we present detailed petrographic and seismic reflection interpretation of some representative volcanoes of the MVF, and of the strata that enclose them, to constrain the environments in which intrusions and eruptions occurred. Intrusive rocks penetrated by the Resolution-1 comprise a monzogabbro body with a saucer-shape geometry emplaced in organic-rich sedimentary layers. The monzogabbro contains miarolitic cavities and ophitic textures which, together with decompaction of its overburdened sedimentary strata, suggest an emplacement depth around 950 m below the paleo-seafloor. Seismic lines show an array of faults at the tips of the saucer-shaped monzogabbro. These faults are connected with the root of some volcanoes, and may have formed feeder systems for eruptions and hydrothermal fluids onto the Miocene paleo-seafloor. Volcaniclastic rocks comprise abundant glassy shards, relics of bubble walls, spheroidal fragments enveloped in a palagonite film, broken phenocrysts, and lithics. These volcaniclastic rocks are interbedded with lower bathyal siltstones, indicating that eruptions near the location of the Resolution-1 occurred in a deep-submarine environment (1000-1500 m). Integration of petrographic, geochemical and seismic reflection interpretations suggest that the volcaniclastic rocks have a genetic relationship with the saucer-shaped monzogabbro, which may have served as a shallow stationary magma chamber for some volcanoes in the MVF. The available data indicate processes of intense material fragmentation and particle dispersion, consistent with phreatomagmatic eruptions, although globally this eruptive style is rarely interpreted to occur at water depths around 1000 m. The emplacement of intrusions into organic-rich sedimentary rocks could incorporate thermogenic gases into the magmatic system, providing supplementary driving forces to form large deep-water pyroclastic eruptions.