Waiareka-Deborah volcanic field
The Waiareka-Deborah volcanic field is a group of sub-alkaline basalt to basaltic andesite composition volcanics, that erupted 36.4 to 27.6 million years ago. They are found near Oamaru, South Island New Zealand,[1] and are small Surtseyan volcanoes that erupted originally on a submerged continental shelf.[2][3]
| Waiareka-Deborah volcanic field | |
|---|---|
| Stratigraphic range: | |
| Type | Igneous |
| Overlies | Rakaia Terrane |
| Area | 890 square kilometres (340 sq mi)[1] |
| Lithology | |
| Primary | Sub-alkaline basalt and basaltic andesite |
| Location | |
| Coordinates | 45.0°S 170.3°E |
| Region | Otago |
| Country | New Zealand |
| Type section | |
| Named for | Waiareka Valley, inland from Oamaru |
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.[4]
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. There are probably Southern Pacific Ocean subsurface components from sonar studies but no such have not yet been characterised by core sampling. Accordingly, a further area of up to 2,500 square kilometres (970 sq mi) could yet be assigned to these volcanics.[1] 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.[1]
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.[5] At least two examples of more recent alkaline Dunedin volcanic group eruptives through Waiareka-Deborah volcanic field crystalline rock have been characterised to date.[1]
| 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 | 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[5][6] | Six volcanoes have been defined that erupted over a period of more than 3 million years with overlapping eruptives.[5] 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 | alkaline melanephelinite and basanitic clasts crystalline rock on underlying Cenozoic non-volcanic sediments, sideromelane pyroclasts |
| Aorere aPoint | 39.5 ± 1.8 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[4] 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.[7] 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.[8]
References
- Scott, James M.; White, James D. L.; le Roux, Petrus J. (2020). "Intraplate volcanism on the Zealandia Eocene-Early Oligocene continental shelf: The Waiareka-Deborah Volcanic Field, North Otago". New Zealand Journal of Geology and Geophysics. 63 (4): 450–468. doi:10.1080/00288306.2020.1785896. S2CID 221380814.
- Simone Hicks, PhD proposal: Ecological and sedimentological evolution of the volcanically active Oligocene continental shelf, east Otago, New Zealand, Geology Department, University of Otago. Retrieved 19 April 2010.
- R. A. F. Cas; C. A. Landis; R. E. Fordyce (1989). "A monogenetic, Surtla-type, Surtseyan volcano from the Eocene-Oligocene Waiareka-Deborah volcanics, Otago, New Zealand: A model". Bulletin of Volcanology. 51 (4): 281–298. Bibcode:1989BVol...51..281C. doi:10.1007/BF01073517. S2CID 129657592.
- Scott, James M.; Pontesilli, Alessio; Brenna, Marco; White, James D. L.; Giacalone, Emanuele; Palin, J. Michael; le Roux, Petrus J. (2020). "The Dunedin Volcanic Group and a revised model for Zealandia's alkaline intraplate volcanism". New Zealand Journal of Geology and Geophysics. 63 (4): 510–529. doi:10.1080/00288306.2019.1707695. S2CID 212937447.
- Moorhouse, BL; White, JD; Scott, JM (2015). "Cape Wanbrow: A stack of Surtseyan-style volcanoes built over millions of years in the Waiareka–Deborah volcanic field, New Zealand". Journal of Volcanology and Geothermal Research. 298: 27–46. doi:10.1016/j.jvolgeores.2015.03.019.
- Moorhouse, BL; White, JD (2016). "Interpreting ambiguous bedforms to distinguish subaerial base surge from subaqueous density current deposits". The Depositional Record. 2 (2): 173–95. doi:10.1002/dep2.20.
- Gidon Algernon Mantell (1850). "Notice on the Remains of the Dinornis and other Birds, and of Fossils and Rock Specimens, recently collected by Mr Wanter Mantell in the Middle Island of New Zealand; with Additional Notes on the Northern Island". Quarterly Journal of the Geological Society. 6: 319–343.
- Park, James (1918). "The geology of the Oamaru district, North Otago (Eastern Otago division)". Bulletin of the Geological Survey Branch of the Mines Department. New Zealand Geological Survey Office (20): 4–5.