Torlesse geology of Kakahu, South Canterbury, New Zealand.


Hitching, Keith David.


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Argillitic metasiltstone, metagreywacke, conglomerate, chert, metavolcanics and marble comprise the Torlesse rocks at Kakahu. These rocks are situated forty kilometres north west of Timaru, cover an outcrop area of twenty five square kilometres, are overlain unconformably by Tertiary sediments to the south and east, and are separated from Tertiary rocks to the north and west by the Rapuwai fault.
Metasiltstone and metagreywacke comprise approximately 60% and 30% of the Torlesse rocks of Kakahu respectively. These rocks belong to textural zone 2B, except for those on the far west of the field area, which are 2A. Pumpellyite is abundant in all metasiltstones and metagreywackes. Prehnite is common in the 2A rocks but occurs only spasmodically in those of textural grade 2B. Point counting indicates that the Kakahu metagreywackes are quartzofeldspathic and are broadly typical of Torlesse metagreywackes. The only sedimentary structure seen in these rocks is laminated, often graded, bedding.
Conglomerate is uncommon and was found at only three localities. Pebbles are predominantly well rounded intrabasinal Torlesse-type siltstones, but rare chert, granophyre, and rhyolite pebbles were also found.
Fourteen occurrences of chert are recorded, and these may be subdivided as follows:
i) Bedded, coloured cherts up to 160 m thick. These cherts are bedded on a scale of 2- 50 cm, and grading and alternations with thin laminae of chloritic slate occur at several localities. These features suggest that these cherts are probably siliceous turbidites. A relict radiolarian texture is observed in some thin sections.
ii) Non bedded, grey or yellow grey cherts from l m to approximately 20 m in thickness.
iii) Red and pink cherts intercalated with hematitic metavolcanics and white marble in the lower Kakahu River gorge.
Two lithologies of intercalated metavolcanics and marble crop out in the lower Kakahu River gorge. The westernmost lithology is a 140 m thick sequence of sixteen bedded units of tuffaceous greenschist, grey marble and metasiltstone. The second lithology which lies directly to the east of the former is 90.m thickness of intercalated poorly schistose hematitic metavolcanicsĀ·and pure white marble. In places the white marble is present as irregular lensoidal masses within the metavolcanics - these were probably originally masses of calcareous sediment which infiltrated into primary voids between submarine pillows.
The Kakahu Marble is a 30 m thick, pure grey marble which has yielded a fauna of Pennsylvanian conodonts. Conodonts of the same age have also been obtained from the grey marble which is interbedded with tuffaceous greenschist and metasiltstone in the lower Kakahu River gorge. No other identifiable fossils were found in the Torlesse rocks of Kakahu.
Three phases of folding are recognised. F1 folds are tight mesoscopic folds which are preserved only in the bedded cherts and interbedded metavolcanic and marble rocks. Macroscopic F2 folding was accompanied by the formation of the regional schistosity (S 2). S2 was macroscopically folded during F3 deformation.
The resistant marker lithologies (i.e. cherts, intercalated metavolcanics and marbles, the Kakahu Marble) are traceable along strike for up to 500 m; their boundaries appear to have been tectonically sheared off. Evidence of transposition and shearing is ubiquitous in the metasiltstones and metagreywackes, and the rocks comprise a tectonic melange in the sense of Cowan (1974). Some marker lithologies have boundaries with metasiltstone that are parallel to sand laminae observed in the metasiltstone. As the only two fossil localities at Kakahu, which are 1 kilometre apart, are Pennsylvanian in age, and as one of these fossil localities is interbedded with typical Kakahu metasiltstone, it is likely that all the Torlesse rocks of Kakahu are likewise of Pennsylvanian age.

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93 leaves : illus. (part. col.) map (in pocket) ; 30 cm.


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POLYGON ((171.044311836751405 -44.116279021172488,171.088587346195197 -44.117266834042972,171.103378618880583 -44.161343771946399,171.039864816155216 -44.160747704552463,171.02722374613748 -44.172678591188038,171.003738861770557 -44.172270471483905,171.005543466451428 -44.158009582896362,171.044311836751405 -44.116279021172488))

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Hitching, Keith David., “Torlesse geology of Kakahu, South Canterbury, New Zealand.,” Otago Geology Theses, accessed June 19, 2019,