Nelson comprises rocks more varied in type than in any other New Zealand land district: in age they represent every geological period since the Cambrian, with the possible exceptions of the Silurian and the Carboniferous. Nelson's rocks include a great diversity of minerals of scientific and possibly, of some economic importance. In Nelson, as in Marlborough, the active Alpine Fault and its probable north-eastern continuation, the Whangamoa Fault (seediagram 8) separates two areas of dissimilar geology. To the east of the fault, for example in the Spenser Mountains, St. Arnaud Range, and Richmond and Bryant Ranges, Upper Paleozoic and Lower Mesozic greywackes and argillites are the predominant rocks exposed. The main part of Nelson lying north-west of the fault is made up chiefly of Paleozoic granites, sediments, and metamorphic rocks, partly obscured by patches of Cretaceous and Tertiary covering strata.

Most of Nelson is mountainous. The largest area of plains and low hill country is the Moutere Depression, which extends from Tasman Bay to the Alpine Fault. It is a major structural depression occupied in its upper levels by thick accumulations of deeply weathered and dissected gravels. The Moutere Depression separates two regions of contrasting geology–East Nelson, which is made up largely of north-east-trending belts of Upper Paleozoic sedimentary, volcanic, and ultramafic rocks; and West Nelson, which comprises mainly granites and Lower Paleozoic sedimentary and metamorphic rocks of exceedingly complex structure. These two regions are shown in section C of diagram 6.

West Nelson

West Nelson contains the largest granite masses in New Zealand. They are disposed in three more or less continuous belts trending approximately north-south. An eastern belt lies immediately to the west of the Moutere Depression and can be traced for 70 miles from Separation Point almost to Murchison. Typically this granite is massive and light coloured, and sand derived from it forms beautiful beaches at Kaiteriteri, Totaranui, and other bays in Tasman Bay. At Canaan and other places a variety of minerals of scientific interest have been formed by contact of this granite with metamorphic rocks; they include a small wollastonite body at present being worked. A central granitic belt, the largest in New Zealand, extends from the northern extremity of Karamea Bight for 120 miles through the Victoria Range to the Ahaura River in Westland. This granite, like the western granite belt, comprises types ranging from pink alkali-granites with large orthoclase crystals to light-coloured, biotite calc-alkali granites. The western belt is exposed discontinuously from the Mokihinui River to the Buller River and from here continuously south through much of the Paparoa Range. These granites are more gneissic than the belts further east. Mention has been made of the Lower Paleozoic basic and ultramafic intrusive rocks of West Nelson. One belt of these, containing useful deposits of serpentine, talc-magnesite rock, and a little asbestos, forms a complex between the lower Cobb and Takaka Valleys. A folded sill or laccolith of gabbro, norite, and amphibolite is exposed in Rameka Creek on the Pikikiruna Range, and has formed skarns by contact with adjacent marble masses.

In south-west Nelson, possibly Precambrian greywackes and argillites of the Waiuta Group form the southern part of the Paparoa Range, and a second belt of them lies east of the Grey-Inangahua Valley, continuing northward to beyond the Mokihinui River. At Reefton and Waiuta these rocks contain gold-bearing quartz veins.

In north-west Nelson Lower Paleozoic sedimentary and metamorphic rocks form a broad wedge, narrowing to the south, between the eastern and central granite belts. The structure is complex: overfolds, thrusts, and nappes have been interpreted there. Broadly, the oldest rocks – the Cambian Haupiri Group – form a central belt that takes in much of the Haupiri, Anatoki, Douglas, Snowden, and Lockett Ranges and continues, displaced by faults, through the Cobb Reservoir area to the Wangapeka River. The Cambrian belt is flanked to the west by Ordovician sedimentary rocks, and to the east by Ordovician, Devonian, and possibly Silurian rocks. Section H shows some of the Cambrian and Ordovician sediments folded and faulted and intruded by granites. Trilobites of Middle Cambrian age, the oldest New Zealand fossils, are known from limestone lenses in the Cobb Valley.

The bulk of the covering strata of Nelson is Tertiary in age, but there are small areas of covering rocks that are older. At the Buller Gorge, Big River, Fox River mouth, and Punakaiki are patches of coarse breccia (the Hawks Crag Breccia) and of nonmarine sandstones and shales, that are probably Jurassic or Cretaceous in age. The special interest of these rocks is that the Hawks Crag Breccia is the host rock of New Zealand's only known uranium deposits. At the base of Farewell Spit an expanse of Cretaceous non-marine conglomerates, sandstones, and mudstones has yielded coal from mines at Puponga and Mangarakau. The Tertiary limestones at Golden Bay and Cape Foulwind are used for cement.

The Tertiary rocks of Nelson are the remnants of a once continuous cover. On a peneplain cut in later Cretaceous and early Eocene times small areas of coal measures accumulated in south-west Nelson in the Eocene; these have been mined at Ngakawau, Stockton, Millerton, Denniston, and other mining areas of the Buller coalfield. The largest areas of marine Tertiary rocks in Nelson are the Murchison Basin, the Karamea Syncline, and the Grey-Inangahua Depression (shown in section I). In northern Nelson most of the Tertiary rocks have been removed except for a few strips preserved in tectonic depressions, such as the Aorere Valley and the Takaka Valley.

East Nelson

In East Nelson Triassic and Upper Paleozoic sedimentary and marine volcanic rocks of the New Zealand Geosyncline form north-east-trending belts that extend from D'Urville Island to the Wairau Fault. Associated with them is a belt of basic and ultramafic intrusive rocks – the so-called Nelson “mineral belt”. Youngest of these basement rocks are fossiliferous Triassic sandstones and conglomerates, which are preserved by faulting as a narrow strip adjoining the northern part of the Moutere Depression; they are well exposed in the Wairoa Gorge. A major fault separates the Triassic rocks from a belt of Permian strata (Maitai Series) which form a synclinal fold to the east. The Maitai Series include a number of formations, the most distinctive of which is a finely banded red and green sediment. The Maitai rocks rest in some areas on “mineral belt rocks”, in others on marine volcanic and sedimentary rocks of the Te Anau Group. The “mineral belt” consists largely of serpentine masses, but includes two large masses of dunite, an almost pure olivine rock. Dun Mountain is the massif that gives this rock its name; it also forms the Red Hills to the south. Chromium and copper ores were mined last century from many small mines at Dun Mountain and other areas in the “mineral belt”, but with little commercial success. South, east, and north of Nelson city is a belt of Paleozoic volcanic rocks (the Brook Street Volcanics); west of these, between McKays Bluff and Pepin Island, is a strip of syenites and similar intrusive rocks that has yielded the hard boulders that form the Nelson Boulder Bank.

The Nelson Upper Paleozoic and Triassic sedimentary and volcanic rocks together form the Nelson Syncline, a regional feature complicated by faulting which can be matched with similar rocks in the Southland Syncline. It has been suggested that both synclines were formerly continuous and have been displaced horizontally by the Alpine Fault. This implies a horizontal movement of 300 miles, perhaps since late Jurassic and early Cretaceous times. Further support for this theory stems from the fact that the plutonic and metamorphic rocks of west Nelson can be matched with those of Fiordland and the Marlborough schists (flanking the Nelson Paleozoic strata to the east) and with the Otago schists.

A small area of Tertiary sedimentary rocks, including thin coal measures, lies south of Nelson city. The most prominent exposure of them is in the waterfront cliffs of the Port Hills.

The known active faults in Nelson district are shown in diagram 8. Earthquakes associated with movement on the White Creek Fault in 1929 produced much damage in the Murchison area and spectacular landslides occurred along much of the West Nelson coastline.