PASTAS Y PINTURAS Muros y tabiques

The Kellyville crater is much more magnetically complex than the Onewhero crater. This is clear from the percentage of the crater that is characterised by a positive anomaly (Figure 5.11). The measured magnetic anomalies are much greater than that of the Onewhero crater, and this can be attributed to the volcanic

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complexity of the Kellyville Volcanic Complex. It is likely that an even greater percentage of the crater would have a positive anomaly if it were not being masked by the magnetically negative diatomite deposits.

Figure 5.11: A map outlining the area in the Kellyville crater that is above what was calculated as the normal magnetic intensity for the Mercer region.

The magnetic anomaly map (Figure 5.9) reveals a mixture of low and high intensity magnetic features. Due to the erosion of much of the crater infill after the crater breach, some of the magnetic anomalies observed in the Kellyville Volcanic Complex can be confidently attributed to known geological features. These observations then make the interpretation of subsurface anomalies less prone to

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error. An example of this is Glass Hill cone. I am reasonably confident that the elongate, highly magnetic positive anomalies in the west of the crater (4.) are produced by deposits of the scoria cone nearby. I am less confident about the position of the vent in the Kellyville crater. The position of the circular magnetic anomaly (5.) in the middle of the crater and the long wavelength characteristics of this features make this interpretation the best fit for the data available.

The extent of high purity, or relative thickness, of diatomite in the Kellyville crater is also made apparent from the magnetic survey. Two regions with a negative magnetic anomaly less than -2000 nT were identified. These areas are likely to be deposits of diatomite that are thicker or of a higher purity than the surrounding post- eruption lake infill.

5.7

Chapter summary

Magnetic surveys of the Onewhero and Kellyville craters were undertaken in the 2014 – 2015 summer. 969 magnetic readings were recorded at Onewhero and 329 magnetic readings were recorded at Kellyville. The measurements were made with a G-856AX Memory-Mag Proton Precession Magnetometer. An effort was made to take readings away from metallic-based objects that could distort magnetic values. Aside from this, no further corrections were required to process the data.

An investigation of the Onewhero magnetic anomaly map yields six interesting features. The most intense positive anomaly belongs to a feature that is interpreted as an extrusive vent or scoria cone. Linked to this vent is a ridge of lower, but still positive, magnetic anomalies that stretch away across the crater to the east. This is interpreted as a lava flow extending out from the dyke-fed vent.

A separate region of high magnetic intensity occurs in the south of the crater and it is suggested that this belongs to the known lava flow that probably originates from the Klondyke cone. The lava flow does not crop out at the surface in the area covered by the magnetic survey, but it is likely that it dips below the younger post- eruption infill and into the magnetic survey area.

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The last major magnetic anomaly feature is a very low intensity region towards the north of the crater. The low magnetic anomaly in this area is interrupted by two locally high intensity magnetic readings. The feature as a whole is interpreted as dykes that erupted at the surface, creating a localised depression into which lake sediments were deposited that have a very low magnetic susceptibility.

An investigation of the Kellyville magnetic anomaly map yields five interesting features. Two negative magnetic anomalies occur near the middle of the crater. These are interpreted as being deposits of diatomite with a very low magnetic susceptibility. The evidence of one of the negative magnetic features can be confirmed due to a diatomite outcrop that is cut by Koheroa Road.

Valleys separate many of the low magnetic anomalies. These are interpreted as erosional valleys that formed when the crater was breached and allowed unconsolidated sediment with a negative magnetic susceptibility to be washed out into the river system.

A positive anomaly with a short wavelength feature occurs in the west of the crater. This feature can be identified from the surface; it is the scoria cone known as Glass Hill. Ridges that have a positive magnetic intensity radiate away from Glass Hill and these are interpreted as basalt flows.

The last magnetic feature is noticeably different to anything else observed in the crater. It is a circular region with a relatively long wavelength, this means it probably occurs deeper than any of the other high-intensity magnetic structures. It occurs in the centre of the Kellyville crater and could be the initial eruption vent or a basin of eroded iron-rich sediment.

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Chapter Six: Discussion

6.1

Introduction

Chapter 6 consists of five sections. The first section integrates the gravimetric and magnetic geophysical data for the Onewhero and Kellyville volcanic complexes. A final interpretation of the subsurface geology for these two craters will be provided based on the best fit available from the bore-hole data, the gravity data, and the magnetic data. I will also comment on how well the gravimetric and magnetic data supports or obscures each other. The second section will describe and discuss the syn- and post-eruption history for the Onewhero and Kellyville volcanic complexes. Comments will be made on whether these phreatomagmatic centres satisfy the geological conditions needed to classify them as tuff rings or maar volcanoes. The third section will discuss the similarities and differences between the Onewhero and Kellyville volcanic complexes and other phreatomagmatic localities in the nearby Auckland Volcanic Field. In the fourth section I will investigate and discuss similarities and differences between the Onewhero and Kellyville volcanic complexes and selected phreatomagmatic localities in a global context. In the fifth and final section of Chapter 6 I will propose a direction for further studies of the Onewhero and Kellyville craters as well as other volcanoes in the South Auckland Volcanic Field.