Geology of the Middle Cobb Area, N.W. Nelson.
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The Cambrian Haupiri. Group consists of a eugeosynclinal sequence, the younger part of which is characterised by contemporaneous facies changes. Interlayered volcanogenic sediments and spilitized basaltic andesite flows in the Devil River Volcanics are the oldest rocks, overlain by the Tasman Formation with one andesite flow (?) and fossiliferous limestone pods in both volcanogenic and non-volcanogenic sediments. Indicative of rapid uplift in the Haupiri Orogeny the Balloon Formation with extensive intraformational conglomerates, and the Lockett Conglomerate have a large proportion of acidic and intermediate igneous detritus along with basic, ultrabasic, metamorphic and sedimentary material.
This Cambrian sequence has been thrust along the Anatoki Thrust over the top of the Ordovician miogeosynclinal sequence, with the deep water Aorere and Golden Bay Groups younging east into the shallow water shelf facies that comprise the Mt. Arthur Group. The Cambrian rocks were first folded isoclinally about eastward plunging axes during the emplacement of nappes in the Tuhua Orogeny. At this time a slaty cleavage was formed in the underlying Ordovician rocks and they were deformed with northverging shear and drag folds.
Subsequent, largely post-Permian (Rangitata Orogeny(?)) compressional folding throughout the area formed north-south trending folds in both the Cambrian and Ordovician rocks, probably giving rise to a slaty. cleavage (axial planar) in the Haupiri Group and perhaps straining the Lockett Conglomerate. (A finite strain analysis of am~rientad speciman of Lockett Conglomerate is included herein).
It is possible the Cambrian sequence, now metamorphosed to prehnite-pumpellyite facies, lies in the axial region of a broad synclinorium. It is unlikely the Golden Bay Group· in the Cobb area is of higher metamorphic grade than the prehnite-pumpellyite facies, although no diagnostic Ca-Al silicates are present.
This Cambrian sequence has been thrust along the Anatoki Thrust over the top of the Ordovician miogeosynclinal sequence, with the deep water Aorere and Golden Bay Groups younging east into the shallow water shelf facies that comprise the Mt. Arthur Group. The Cambrian rocks were first folded isoclinally about eastward plunging axes during the emplacement of nappes in the Tuhua Orogeny. At this time a slaty cleavage was formed in the underlying Ordovician rocks and they were deformed with northverging shear and drag folds.
Subsequent, largely post-Permian (Rangitata Orogeny(?)) compressional folding throughout the area formed north-south trending folds in both the Cambrian and Ordovician rocks, probably giving rise to a slaty. cleavage (axial planar) in the Haupiri Group and perhaps straining the Lockett Conglomerate. (A finite strain analysis of am~rientad speciman of Lockett Conglomerate is included herein).
It is possible the Cambrian sequence, now metamorphosed to prehnite-pumpellyite facies, lies in the axial region of a broad synclinorium. It is unlikely the Golden Bay Group· in the Cobb area is of higher metamorphic grade than the prehnite-pumpellyite facies, although no diagnostic Ca-Al silicates are present.
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99 leaves. ill. photos. Map (folded in pocket). 27 cm.
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1971Coleman
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POLYGON ((172.664777296625402 -41.141775575474256,172.601972030380011 -41.142147699837722,172.508201643884036 -41.06704276257733,172.546798985325722 -41.058244074554551,172.63298816683735 -41.059742495830179,172.67108269080822 -41.076839251709465,172.674420120575519 -41.118558034711448,172.664777296625402 -41.141775575474256))
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Coleman, AC, “Geology of the Middle Cobb Area, N.W. Nelson.,” Otago Geology Theses, accessed May 15, 2025, https://theses.otagogeology.org.nz/items/show/50.