Identification and Characterization of a Buried Volcanic Field Using Seismic Reflection and Borehole Data

Abstract

5 alan.bischoff@canterbury.ac.nz *corresponding author\ 6 7This is a non-peer reviewed preprint submitted to EarthArXiv. Abstract 10Buried volcanoes occur in great numbers within sedimentary basins globally. 11Knowledge of ancient buried volcanic systems has improved significantly over the past 12 two decades. The in-depth understanding of these buried systems was mainly possible 13 due to increasing availability of high-quality seismic reflection and sub-surface 14 borehole data. This paper examines a cluster of Miocene volcanoes now buried by ca 15 1000 m of sedimentary strata in the Canterbury Basin, New Zealand. These volcanoes 16 were imaged by 2D seismic lines and perforated by the Resolution-1 borehole. We refer 17 to this group of volcanoes and related intrusive bodies as the Maahunui Volcanic Field 18 (MVF). Here, we present detailed petrographic and seismic reflection interpretation of 19 some representative volcanoes of the MVF, and of the strata that enclose them, to 20 constrain the environments in which intrusions and eruptions occurred. Intrusive rocks 21 penetrated by the Resolution-1 comprise a monzogabbro body with a saucer-shape 22 geometry emplaced in organic-rich sedimentary layers. The monzogabbro contains 23 miarolitic cavities and ophitic textures which, together with decompaction of its 24 overburdened sedimentary strata, suggest an emplacement depth around 950 m below 25 the paleo-seafloor. Seismic lines show an array of faults at the tips of the saucer-shaped 26 monzogabbro. These faults are connected with the root of some volcanoes, and may 27 have formed feeder systems for eruptions and hydrothermal fluids onto the Miocene 28 paleo-seafloor. Volcaniclastic rocks comprise abundant glassy shards, relics of bubble 29 walls, spheroidal fragments enveloped in a palagonite film, broken phenocrysts, and 30 lithics. These volcaniclastic rocks are interbedded with lower bathyal siltstones, 31 indicating that eruptions near the location of the Resolution-1 occurred in a deep-32 submarine environment (1000-1500 m). Integration of petrographic, geochemical and 33 seismic reflection interpretations suggest that the volcaniclastic rocks have a genetic 34 relationship with the saucer-shaped monzogabbro, which may have served as a shallow 35 stationary magma chamber for some volcanoes in the MVF. The available data indicate 36 processes of intense material fragmentation and particle dispersion, consistent with 37 phreatomagmatic eruptions, although globally this eruptive style is rarely interpreted to 38 occur at water depths around 1000 m. The emplacement of intrusions into organic-rich 39 sedimentary rocks could incorporate thermogenic gases into the magmatic system, 40 providing supplementary driving forces to form large deep-water pyroclastic eruptions. 41 42

Paleogeographic and Morphologic Reconstroction of a Buried Monogenetic Volcanic Field

“…Rock-types and interpretations of cuttings and borehole cores from the Resolution-1 172 well are presented in Bischoff et al (2019a). 173…”

“…The magmatic products of MVF melts 154 are primarily basaltic-alkaline in composition. Deep-water volcaniclastic rocks of the 155 MVF correlate both in age and in volcanic lithologies with the Wairiri Volcaniclastite, 156 which was erupted in shallow-water and outcrops near Coalgate (Milne 1975;Carlson 157 et al 1980;Bischoff et al 2019a; Figure 1). 158…”

“…stage (11.5 to 11 Ma) 304After volcanism ceased around 11.5 Ma(Bischoff et al 2019a), the MVF was 305 progressively buried by an increase in sediment influx from the NW, which is 306 interpreted to be derived from the early uplift events that built the NewZealand 307 Southern Alps(Field et al, 1989). Chronostratigraphic and isochron maps show that the 308 presence of the extinct submarine volcanic edifices had a local influence on the 309 distribution of sediments in the area(Figure 6 and 7b).…”