VENTANAS, PUERTAS DE ALUMINIO Y TABIQUERIA VIDRIADA

The reliability of a subsurface geophysical model can be increased by adding constraints. Many geophysical investigations combine magnetic and gravimetric data to interpret subsurface geology due to the speed and low cost of these surveys (Pirrung et al. 2003; Schulz et al., 2005; Cassidy et al., 2007; Loera et al., 2008; Mrlina et al., 2009; Blaikie et al., 2012; Jordan et al., 2013).

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The gravimetric and magnetic data obtained and interpreted in this investigation can be integrated to form best-fit geological models for the Onewhero and Kellyville craters.

At Onewhero the magnetic and gravimetric data supported each other to a mixed degree. Figure 6.1 is an overlay of the Bouguer anomaly with the magnetic values across the X – X’ transect in the Onewhero crater. Low density maar sediments are often composed of silica-rich diatomite or oil-shales that do not retain magnetisation (Pirrung et al., 2003). Therefore a two-dimensional profile comparing gravity and magnetic anomalies should show the same general trends. Figure 6.1 outlines this clearly: the low gravity anomaly in the northwest of the crater is mirrored by the low magnetic anomaly in the same area. This area of the crater is a depocentre for low density sediment, and due to the negative magnetic anomaly the sediments must have a low susceptibility to an external magnetic field. Due to the known presence of diatomaceous clay at the surface I can confidently interpret this negative magnetic and gravimetric region as a thick (modelled as 90 – 100 m thick) accumulation of low-density, diatom-rich sediment.

Figure 6.2 is a two-dimensional magnetic and gravimetric profile overlay for the Y – Y’ transect. The magnetic and gravimetric results here do not support each other. The gravity anomaly in this profile indicates the presence of low density sediments. The gravity minimum in this profile is -4.5 mGal, compared to almost -6 mGal for the X – X’ transect. The change in these minimum values comes from either a difference in the density of the underlying sediment in the area or a difference in the thickness of the sediment body. In Chapter 4, the post-eruption infill body is modelled as the same density throughout, and therefore it is assumed that the sediment is thinner in the Y – Y’ transect and this is the reason for the higher Bouguer anomaly. The magnetic profile for this region suggests that the sediment should be composed of material with a higher magnetic susceptibility. The post- eruption infill in the X – X’ transect produces a magnetic anomaly ranging from 0 to -1000 nT but in the Y – Y’ transect the values range from about 50 to -200 nT. This could be due to ferrimagnetic material below the post-eruption infill or alternatively the post-eruption infill in this region of the crater could contain a higher percentage of sediment derived from the ejecta ring or the nearby basalt.

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At Kellyville the magnetic and gravimetric data support each other to a mixed degree. Figure 6.3 is an overlay of the Bouguer anomaly with the magnetic values across the X – X’ transect that crosses the Kellyville crater in a west to east direction. In this profile the geophysical data is complementary: in general the highs and lows of the magnetic and Bouguer anomalies follow the same trends. The magnetic anomalies in the X – X’ transect appear to support the presence of a thick basalt body that is overlain by low density sediments that are diamagnetic. Between 400 m and 600 m, Glass Hill scoria cone is present as a peak in the maximum Bouguer and magnetic anomalies. Both of the anomalies decrease after leaving the slopes of Glass Hill as they cross over the known diatomite deposit that is dissected by Koheroa Road. Figure 6.4 is a Bouguer-magnetic anomaly overlay for the Y – Y’ transect that traverses the Kellyville crater in a north to south direction. The gravimetric and magnetic anomalies do not correlate as well as in Figure 6.3, although the general trends can still be made out. Between 300 m and 700 m, the Bouguer and magnetic anomalies both gradually increase. The increase in this region is interpreted in the two-dimensional gravity modelling as the start of subsurface basalt or basalt-derived sediment, probably originating from Glass Hill. The Bouguer-magnetic anomalies begin to decline again at 1200 m as the transect leaves the other side of Glass Hill and any of its volcanic products. The steep trench in the magnetic anomaly at 750 m is due to a deposit of negatively magnetised material that lies north-northeast of Glass Hill. This deposit is located in close proximity to the main diatomite outcrop and has a very similar negative magnetic anomaly. The two diamagnetic sediment bodies are probably very pure deposits of diatomite.

In this investigation, the creation and interpretation of a geological model for both the Onewhero and Kellyville volcanic complexes has been aided by integrating existing bore-hole data with gravimetric and magnetic surveys. Each layer of geological/geophysical data has strengthened the geological models for the two craters.

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