Mayor Island, Bay of Plenty, New Zealand
Te Ananui Flat
| Te Ananui Flat is on the northern extremity of Mayor Island and it is a remakably flatish surface on the steep flanks of the cone. It has been proven to be a product of the last major phase of activity and caldera collapse on Mayor Island. This eruption dated by me at 6,340 yrs ago from a charcoal sample in Omapu Bay was so significant that up to 70 cm of ash (Tuhua Ash) was deposited on the mainland. However, here the flat is a valley-fill of a very thick and blocky ignimbrite. Here also the caldera wall on main cone (on the right) shows that the Te Ananui fan has flowed into a breach in the caldera. |
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View of the 6,340 year old ignimbrite with accompanying surge and Plinian fall deposits interbedded and on top. Houghton et al (1992) identified six phases in the 6,340 year eruption and this ignimbrite was deposited in the cataclysmic final phase of the eruption. It was an extremely powerful eruption depositing
nonwelded ignimbrite, wet and dry
surge deposits, and two coarse Plinian fall beds.The ignimbrite here ranges from very coarse blocky fine-poor lenses in the lower part of the deposit with fine -rich lenses above.
Welding of the ignimbrite is complex. This ignimbrite forms the coastal fans of Te Ananui and Te Kopua Flats. One fan in Taratimi Bay abuts a vertical sea
cliff on the east of the caldera and shows the
complex interrelationships between fines-rich
ignimbrite, lithic lag breccias, and fines-poor,
lithic-rich ignimbrite. In the distance is the main cone on which the ignimbrite abuts at Hurhurihang Bay. |
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While Te Ananui Flat is a "flat surface" in megascale. It is quite rolling and not an easy place to traverse, espeacially with the vegetation being very dense as well. However, when you are doing research the observation from my dinghy only went so far. This is the 6,340 year old ignimbrite in the central section of the Te Ananui fan. |
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| A good section through the 6,340 year old ignimbrite. The lower layers are fines-poor and in lenses that lap over each other. Flow direction would have been in the direction of the photographer. The upper layers are coarse-depleted and interbedded with surge, blocky lenses and Plinian fall deposits in the top-most layers. |
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| After a good hike we finally made the edge of Te Ananui Flat. A trek well worth the effort. Note the two persons on the edge of the flat on the left. Sitting on top of the 6,340 year old ignimbrite. Here it is the eastern margin of the flat and in the background is Opoupoto (Cathedral) Bay. |
A stack of the same sequence as on the cliff face shows that the 6,340 year old ignimbrite was more extensive than that as preserved. Erosion of the cliff face has been quite rapid considering 50 or so metres of the real estate has been removed in that short time. |
About as close as you can get to the outcrop of the 6,340 year old ignimbrite without abseiling. We did have to use a rope to get to this vantage point. The fallen block highlight the coarse blocky fine-poor deposits in the lower part of the flow. They also highlight the mode of cliff retreat. |
