Art. XVI.—On the Age of the Alpine Chain of Western Otago.
[Read before the Otago Institute. 9th October, 1917; received by Editors, 22nd December, 1917; issued separately, 24th May, 1918.]
The alpine chain of Western Otago consists of folded altered rocks of older Palaeozoic age. Deeply involved in the eastern folds of this chain there occurs a remarkable wedge of Camozoic marine strata that can be traced as a narrow band from Bob's Cove, on the north shore of the middle arm of Lake Wakatipu, across the Richardson Mountains to the sources of the Shotover River, a distance of over twenty-five miles. The trend of this band is north-north-east, and its limits in that direction have not yet been defined. As exposed in the deep gorges with which the mountains are scored, the visible involvement exceeds 4,500 ft. At its southern end the thickness of the infolded beds is about 80 ft., and in the Shotover Mountains 12 ft.
At Bob's Cove, where these beds cover an area about half a square mile in extent, the succession is: Breccia-conglomerate (bottom); sandy clay; limestone; sandstone, in places pebbly.
Fossil mollusca are fairly abundant, but usually badly preserved. The few forms collected by me during my survey* of the Queenstown district in 1908–9 indicated an Oamaruian (Miocene) age, but the absence of certain molluscs that are held to be characteristic of that period left the matter of their age in some doubt; and in view of the profound involvement of these beds and the bearing this involvement has on the date of the tectonic movement that culminated in the building of the alpine chain I revisited Bob's Cove last January, and on that occasion collected from the sandstone lying below the limestone good examples of the following:—
Pecten huttoni Park.
Cucullaea alta Sowerby.
Limopsis zitteli Iher.
Cardium huttoni Iher.
Venericardia purpurata (Desh.).
Ostrea wüllerstorfi Zittel.
Polinices ovatus (Hutton).
Ancilla hebera Hutton.
Dentalium mantelli Zittel.
Of these, Pecten huttoni, Cucullaea alta, Limopsis zitteli, Cardium huttoni, Ostrea wüllerstorfi, and Dentalium mantelli are, so far as at present known, confined to the Oamaruian, and their presence may be regarded as satisfactory evidence that the Bob's Cove beds belong to the higher portion of that system, and the mountain-building movement which led to the deep involvement† of these beds took place in post-Miocene times, probably in the early Pliocene.
[Footnote] * James Park, The Geology of the Queenstown Subdivision, Bull. No. 7 (n.s.), N.Z. Geol. Surv., p. 66, 1909.
[Footnote] † Loc. cit., pp. 60–66.