# Waiareka-Deborah volcanic field

The Waiareka-Deborah volcanic field is a group of sub-alkaline basalt to basaltic andesite composition volcanics, most of which erupted about 34 million years ago. There is a range of determined ages by various methods and sites although most have very similar timings. At Bridge Point one deposit has an age of 39.5 ± 1.8 and another 34.3 ± 0.5 million years ago. They are found near Oamaru, South Island New Zealand, and are small Surtseyan volcanoes that erupted originally on a submerged continental shelf.
The former term, the Waiareka-Deborah volcanic group should not be used as any alkali basalt volcanoes in this group and all of those in the former Waiareka volcanic field are now assigned to the Dunedin volcanic group and its monogenetic volcanic field.

## Geography

They extend on the present Otago coast south from just north of Oamaru, but well south of the Waitaki River, to the Moeraki Peninsula. The field has an area on land of about 890 square kilometres (340 sq mi). There are probably Southern Pacific Ocean subsurface components from sonar studies but these have not yet been characterised by core sampling. Accordingly, a further area of up to 3,500 square kilometres (1,400 sq mi) could yet be assigned to these volcanics. The furtherest inland deposit is at Basalt Hill just beyond the Maerewhenua River. There are Dunedin volcanic group eruptives between this and the coast and indeed most of the field is coastal.

## Geology

Deposits without an age may need reclassification due to complexity. The presence of six overlapping Surtseyan volcanoes at one site, Cape Wanbrow, is an example of this complexity. At least two examples of more recent alkaline Dunedin volcanic group eruptives through Waiareka-Deborah volcanic field crystalline rock have been characterised to date.
Waiareka-Deborah volcanic field :
| Feature | Age | Geology/Comments |
| Basalt Hill | - | sub-alkaline basalt to basaltic andesite crystalline rock on underlying Cenozoic non-volcanic sediments. |
| Tokarahi Sill | - | sub-alkaline basalt to basaltic andesite crystalline rock on underlying Cenozoic non-volcanic sediments. Doleritic sills in limestone. |
| Boatmans Harbour | 34.2 ± 0.4 Ma, 34.3 ± 0.9 Ma | sub-alkaline basalt to basaltic andesite volcaniclastic rocks on underlying Cenozoic non-volcanic sediments, basaltic pillows in the Ototara Limestone matrix with pillow rinds of sideromelane enclosing labradorite and olivine, with clinopyroxene present in the pillow interiors. |
| Cape Wanbrow | 34.2, 36, 38 Ma | Six volcanoes have been defined that erupted over a period of more than 3 million years with overlapping eruptives. Layered sub-alkaline basalt to basaltic andesite volcaniclastic projection into sea. One alkaline basanitic ash horizon. Pillows have interstitial bryozoan limestone. |
| Enfield | - | sub-alkaline basalt to basaltic andesite crystalline rock on underlying Cenozoic non-volcanic sediments. |
| Round Hill | 33.6 ± 1.8 Ma | sub-alkaline basalt to basaltic andesite crystalline rock surrounded by volcaniclastic rocks on underlying Cenozoic non-volcanic sediments. |
| Awamoa Creek | - | sub-alkaline basalt to basaltic andesite volcaniclastic rocks on underlying Cenozoic non-volcanic sediments. |
| Alma | - | sub-alkaline basalt to basaltic andesite. Alma Group is another name for these volcanics in the literature. |
| Clarks Mill Sill | - | sub-alkaline basalt to basaltic andesite volcaniclastic rocks on underlying Cenozoic non-volcanic sediments. Doleritic sills in limestone. |
| Trig S, Maheno | 34.0 ± 0.6 Ma | sub-alkaline basalt to basaltic andesite. |
| Kakanui | 34.1 ± 0.1 Ma, 33.7 ± 0.3 Ma | alkaline melanephelinite and basanitic clasts crystalline rock on underlying Cenozoic non-volcanic sediments, sideromelane pyroclasts. |
| Aorere Point, Bridge Point | 39.5 ± 1.8 Ma, 34.3 ± 0.5 Ma | sub-alkaline basalt to basaltic andesite, crystalline rock on underlying Cenozoic non-volcanic sediments. |
| Mount Charles Sill | - | sub-alkaline basalt to basaltic andesite, volcaniclastic rocks on underlying Cenozoic non-volcanic sediments. Olivine dolerite at base overlaid with quartz dolerite. Doleritic sills in mudstone. |
| Lookout Bluff | 8 Ma | This is from composition characterisation a smaller Dunedin volcanic group later eruption in the middle of an earlier volcaniclastic rock on underlying Cenozoic non-volcanic sediments. |
| Moeraki Sill | - | sub-alkaline basalt to basaltic andesite, crystalline rocks on underlying Cenozoic non-volcanic sediments. Doleritic sills in siltstone and mudstone. A Porcellanite deposit was quarried by the Māori but note that some deposits may be dykes from Dunedin volcanic group. |
| Tawhiroko Sill | - | sub-alkaline basalt to basaltic andesite, volcaniclastic rocks on underlying Cenozoic non-volcanic sediments. Olivine dolerite at the base and a pegmatitic quartz dolerite core. |

## History

The first geological description of volcanics associated with the group was made in 1850 by Dr. Gidon Algernon Mantell in the Quarterly Journal of the Geological Society. This is acknowledged in the first comprehensive geology review of Oamaru district that uses extensively the term "Deborah limestone" with respect to sedimentary strata in relationship to the volcanics.