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Geology of the Richmond Range

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    Johnston, M.R. 1994 Geology of the Richmond Range. Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences geological map 12 32 p. + 1 map

    Abstract: Geological map O28AC Richmond Range extends from the ranges of east Nelson, through the head of the Pelorus valley, and across the Richmond Range to the Wairau valley. The sheet area consists of a variety of rocks of Permian to Mesozoic age which are placed in the following terranes, from northwest to southeast: Dun Mountain-Maitai, Caples, Rakaia, and Pahau. The Dun Mountain-Maitai terrane contains the Dun Mountain Ophiolite Belt which consists of the Dun Mountain Ultramafics Group, the mafic Lee River Group, and the unconformably overlying sedimentary Maitai Group. The Caples terrane contains only one group - the sedimentary Pelorus Group, whereas the Rakaia and Pahau terranes (to the northwest and southeast of the Alpine Fault respectively) contain the sedimentary Torlesse Supergroup. Within the mapped area only the basal part of the Maitai Group, comprising Wooded Peak Limestone and Tramway Sandstone (prehnite-pumpellyite, locally lawsonite-albite-chlorite facies) outcrops. Much of the ophiolite belt is faulted out, particularly the Lee River Group. Separating the Dun Mountain-Maitai terrane from the Caples tcrrane is the up-to-3km-wide serpentinitic Patuki Melange. The similar but less well developed Croisilles Melange outcrops within the Caples terrane close to the Patuki Melange. The Pelorus Group is divided into the Ward (grey and green sandstone and siltstone), Wether (green and red sandstone and siltstone), and Star (green sandstone) formations, but in the east much of it is undifferentiated. In the southeast the Pelorus Group rocks, which are dominantly non-quartzose sandstone and siltstone, become increasingly affected by moderate pressure-low temperature metamorphism. This is initially shown by transposition of bedding, then the development of penetrative foliation, and ultimately segregation laminae. These grey and green schistose rocks (up to greenschist facies) of the Caples terrane are in contact with the Rakaia terrane along a moderately gently northwest-dipping zone, up to several hundred metres thick, containing lenses of greenschist and talcose schist. The Rakaia terrane, which lies tectonically beneath the contact, consists of segregated grey schists derived from the quartzofeldspathic Torlesse Supergroup. The schistose rocks of both terranes constitute the Marlborough Schist Zone which is, on the basis of progressive increase in metamorphism towards the southeast, subdivided into textural zones (t.z. I-IV). Within the schist zone, foliation defines the synmetamorphic Goulter Synform, a northeast-trending regional structure whose axis lies within the Caples terrane. The Alpine Fault, a major near-vertical late Cenozoic eastnortheast-striking active dextral transcurrent structure, now separates the Rakaia terrane from the Pahau terrane which comprises weakly metamorphosed (zeolite facies) quartzofeldspatbic sandstone and mudstone. In the northwest of the mapped area is the eastnortheast-trending Queen Charlotte Fault Zone of late Cenozoic age. Another major fault is the Northbank Fault Zone which extends northeast from the Alpine Fault before trending eastnortbeast and progressively faulting out the Caples-Torlesse contact. Aggradation gravels (Manuka, Harry, Tophouse, Roundell, and Speargrass formations) flank the Wairau valley and, except for the lower Quatemary Hillersden Gravel, are of middle to late Quaternary age. The aggradation gravels, along with floodplain gravels filling the valley floor, provide the only arable land in an otherwise mountainous area. Slope instability is common in the mountains, particularly in the schistose rocks and the melanges. One large landslide has dammed the Goulter River, a tributary of the Wairau River, to form Lake Chalice. Minor deposits of copper and chromite occur in the Dun Mountain Ultramafics, and quartz reefs, mostly trending northnorthwest, in the schist zone have yielded gold and scheelite. (auth)

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