A new geological map and illustrated text covering 864 square kilometres of East Otago has been published by GNS Science.
GNS Science says the area encompassing Strath-Taieri Plain, the Rock and Pillar Range, and Middlemarch township was chosen for a new 1:50,000-scale geological map series as it combines great geological interest and resource potential and is next to the world-class Macraes gold mine.
Dunedin-based earth scientists Adam Martin, Simon Cox and Belinda Smith Lyttle combined satellite data with ground-based fieldwork to make the map, which provides new insights into the geological history of East Otago.
Geologists are increasingly using airborne geophysical data because it can quickly provide high-precision information over large areas. To acquire equivalent amounts of data from land-based techniques would take many weeks, say the authors.
Available from GNS Science, the map is the first in a series that will combine remote sensing techniques and fieldwork in parts of Otago, as well as Westland and Northland. By examining the geophysical signals of the rocks in the area Dr Martin and colleagues were able to map and model rock types over distances of more than 30 kilometres.
“This is remarkable because prior to this study the furthest we could trace any individual rock unit with any confidence was only a few metres,” says Martin.
The ability to trace rock types over significant distances has allowed researchers to interpret the history of how the rocks ended up where they have, and this in turn is essential in understanding the occurrence of precious metals such as gold and tungsten in Otago.
Historic gold occurrences in the map area include Barewood and Nenthorn, and the Macraes gold mine, which is only a few kilometres north of the mapped area.
The map also has the potential to help model groundwater aquifers and is important for defining quarrying areas for aggregate and diatomite. This latter mineral is the fossilised remains of tiny marine organisms called diatoms. It has many uses in industry and construction.
“In examining the new data a lot of other interesting things have become apparent,” says Martin.
This includes a new sub-division of the volcanic rocks in the area depending on whether they were lava flows or explosive eruptions, and more detailed mapping of ancient landslides. The volcanic rocks relate to episodes of volcanism about 20 million years ago.
Martin says this new work will be important for anyone who is interested in groundwater, finding new mineral or resource deposits, understanding natural hazards, or those with a natural curiosity for the geological world around them.
The map is available as a CD from GNS Science, and can be bought for $35 plus $5 post and packaging.