QM21 Dunedin

The Dunedin 1:250 000 geological map covers 24 000 sq. km. of southeast Otago and the adjacent offshore area. Geomorphology is dominated by the Otago peneplain, an exhumed Cretaceous to early Cenozoic erosion surface of complex origin. The peneplain is cut by the northeastward-trending Taieri-Tokomairiro depression which is bounded to the east by an upfaulted range of coastal hills. The Dunedin volcanic complex lies across the northeastern end of the depression.

The Clutha valley lies north of a series of strike ridges within the Southland Syncline, a major fold which trends inland from Nugget Point in the southwest. The continental shelf reaches depths of 150m at a distance of some 30km offshore. The continental slope drops to below 1100m and is deeply furrowed by canyon heads. Rocks of the Murihiku, Dun Mountain-Maitai, Caples and Torlesse terranes form basement beneath Cretaceous to Quaternary rocks. Murihiku Supergroup rocks of the Murihiku terrane (Triassic to Jurassic) form the northern limb of the Southland Syncline. The Dun Mountain-Maitai terrane (Permian to Early Triassic) consists of Maitai Group sediments tectonically mixed with Early Permian igneous rocks of the Dun Mountain Ophiolite Belt, and the distinctive Kaka Point Structural Belt. Northeast from the Livingstone Fault, sediments of the Permian to Triassic Caples Group are progressively metamorphosed from prehnite-pumpellyite facies to greenschist facies Haast Schist.

In the northeast the Torlesse Supergroup is also metamorphosed to laminated quartzofeldspathic schists of the biotite zone. Bands of metavolcanics and metachert occur in both Caples and Torlesse rocks. Haast Schist is structurally complex, and has recumbent folds defined by bedding in the Caples Group, and younger foliation in higher grade schists. All terrane boundaries are faulted, although the Caples-Torlesse boundary fault has been refolded and obliterated by metamorphism. Early Cretaceous terrane amalgamation was followed by widespread erosion to initiate the Otago peneplain. Local Late Cretaceous rifting was accompanied by deposition of the Matakea Group in fault-angle depressions. The peneplain and a local erosion surface cut on Matakea Group are overlain by Late Cretaceous to Eocene nonmarine sediments and shallow marine sediments. This sequence is truncated by the Late Oligocene Marshall Unconformity which is overlain by the Kekenodon Group. The overlying Otakou Group was deposited during regression and mild tectonism in the Early Miocene. Middle Miocene rocks of the Dunedin Volcanic Group form the Dunedin Volcano.

Several eruptive phases of trachytic to phonolitic lavas represent fractionation of basaltic magmas. Volcanic agglomerate and conglomerate are also present. Outlying flow and intrusive complexes occur on both sides of the Taieri-Tokomairiro depression. Late Miocene to Pliocene tectonism reactivated some Cretaceous faults and initiated others, and deformed the Otago peneplain and the overlying Cr etaceous to Cenozoic sequence. Open folds in this sequence are also of Miocene or younger age. Miocene to Pliocene gravel was deposited in paleovalleys on the peneplain at Lake Mahinerangi. Quaternary sediments now infill main valleys, and partly cover the continental shelf. A thin regolith of loess, colluvium, and weathered rock covers much of the area. Quaternary landslides are common.

Quaternary sea level fluctuations combined with movement on some faults, formed marine benches both onshore and on the continental shelf. The northeastward-trending Akatore, Titri, and Settlement faults were active in the Late Quaternary in response to NW-SE directed compression. Resources include lignite to sub-bituminous coal, quartz gravel and sand, limestone, phosphate, building stone, and aggregate. Metallic minerals include mercury, manganese, tungsten, gold, and antimony. Hydrocarbon potential offshore has been tested. Natural hazards include widespread slope instability in Cenozoic rocks, and small slides in surficial materials. Dip-slope failures and block falls occur on steep slopes in schists.

Seismotectonic hazards include possible ground shaking and liquefaction from distant earthquakes, and movements on local faults. Erosion, flooding tsunami, and subsidence over old mines constitute additional hazards.