Geophysical Characterisation of the Hindon Maar Complex

Bowie, Elliot

In 2007, airborne geophysical surveys identified four sub-circular features presenting high magnetic intensity near Hindon, 25 km NW of Dunedin, within the Miocene Waipiata Volcanic Field (WVF). Similar magnetic anomalies within the WVF are associated with maar-diatreme structures, e.g., Foulden Maar or Gladsmuir diatreme. Subsequent excavations produced evidence that at least two of the basins contain extremely fossiliferous laminated diatomite and/or carbonaceous mudstone, deposited in former maar-lakes. Surface exposure is very limited, which means that geophysical surveys (ground-based magnetic, microgravity and seismic) are essential for accurately representing the size, depth and sediment infill. The large magnetic susceptibility of the basaltic diatreme allows for ground-based magnetic surveys to accurately characterise the size and extent of all four structures. It was found that these structures extend over an area of 16 km2 and are individual anomalies probably related to the same magma source. The relative density of rock within the maar complex then allowed for microgravity surveys to be conducted. Schist typically has a density of 2.73, basalt 2.77, volcanic breccia 2.19 and diatomite 1.53 g/cm3. The relatively low density of diatomite in the subsurface produces a negative anomaly that has been characterised by a Worden Gravimeter survey. Two maars have yielded negative anomalies related to a thickness of diatomite in the subsurface. The final geophysical method was a seismic survey conducted in Maar One. The seismic plot has revealed the thickness and lithologies of the sediment infill within Maar One. This has clarified that at least one of these deposits is a suitable candidate for full coring and paleoenvironmental studies, as at Foulden Maar some 25 km to the northwest. An integrated geophysical interpretation of Maar One has indicated that 160 to 190 m of laminated sediment is present within Maar One. Seismic and microgravity surveys have allowed for selection of an appropriate location to recover this maximum thickness. These two methods provide a generally consistent interpretation of basin thickness (although the thickest locations lie 150 m apart in the two models. A site between these points is most suitable for a drill core to be located. The Foulden core yielded 127 m of diatomite, enabling the reconstruction of a 100,000- year-long terrestrial record of climate, fossil biodiversity and ecosystems. A potential 190 m core of diatomite at Hindon suggests this record could be extended and improved. New samples of volcanic rock have been selected for radiometric dating to confirm the early Miocene age established by preliminary palynological biostratigraphy.

xvi,149 pages A4

2015Bowie

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