Speight. — Geological Excursion to Lake Tekapo. 37 



Akt. IV.— Notes on a Geological Excursion to Lake Tekapo. 



By R. Speight, M.A., M.Sc. F.G.S., F.N.Z.Inst., Curator of the Canter- 

 bury Museum. 



[Read before the Philosophical Institute of Canterbury, 7th July, 1920 ; received by Editor, 

 31st December, 1920 ; issued separately, 27th June, 1921.] 



During the Easter recess of the present year the author paid a visit of 

 several days' duration to the country lying to the east and north of Lake 

 Tekapo, in the Mackenzie country, the visit being primarily to determine 

 the stratigraphical relations of the coal reported to occur in Coal River, 

 and its bearing on the origin of the Mackenzie intermontane basin. The 

 question of the origin of this basin, the greatest in the alpine region of 

 Canterbury, was discussed to some extent by Kitson and Thiele (1910, p. 431), 

 when these authors concluded that it was of structural origin, a conclusion 

 largely based on the observations of McKay on the Tertiary sedimentaries 

 which occur near Lake Ohau and in the lower part of the area. This 

 lower part, however, they do not appear to have visited ; while the 

 structural origin of the upper part in the vicinity of Lakes Pukaki and 

 T>'kapo, which they did examine, was stated as a probability, without 

 giving distinct evidence. Largely influenced by the great weight of Captain 

 Hutton's opinion, they concluded that the tectonic movements which 

 initiated its formation dated from pre-Cainozoic times ; that a depression 

 of the land took place in mid-Cainozoic times, and that the sea then invaded 

 the valleys and deposited marine sediments ; that the area was raised at 

 the close of the Cainozoic era with some slight deformation, and that the 

 resulting surface was modified by glacier erosion and deposition. This 

 is a brief summary of the position as far as the origin of the basin is 

 concerned. 



Since their paper appeared there has been a general swing of mature 

 geological opinion in the direction of the hypothesis that the chief structural 

 movements in the alpine region of the South Island took place in late 

 Jurassic or early Cretaceous times, when the Alps were raised as a folded 

 mountain-chain and during a subsequent period of stillstand of the land a 

 peneplain was formed as the result of prolonged subaerial erosion ; that on 

 lowering this surface below sea-level a mors or less continuous veneer of 

 Tertiary marine sedimentaries was laid down on it ; and that at the close of 

 the Tertiary era an epeirogenic movement ensued, with attendant faulting, 

 warping, and, in some cases, of folding of the beds, which resulted in the 

 formation of an elevated tract known as the Southern Alps. Included in 

 this are several remarkable intermontane basins, of which the Mackenzie 

 country is one. The second hypothesis is the one favoured by the author, 

 and the visit to the district under consideration was made in order to 

 ascertain if the facts furnished by it fitted in with this hypothesis. 



