160 T ransacUons. 



Art. XYI. — On the Age of the Alpine Chain of Western Otago. 



By Professor James Park, F.G.S. 



[Bead before the Otago Institute. 9th October, 1917 ; received bij Editors, 22nd December, 



1917 ; issued separately, 24th May, 191S.] 



Plate VIII. 



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 Cainozoic 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 move- 

 ment 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 alt a Sowerby. 



Limopsis zitteli Iher. 



Cardium huttoni Iher. 



Venericardia purpurata (Desh.). 



Ostrea wullerstorfi, Zittel. 



Polinices ovatus (Hutton). 



Ancilla hebera Hutton. 



Dentaliimi mantelli Zittel. 



Of these, Pecten huttoni, Cucullaea alta, Limopsis zitteli, Cardium huttoni, 

 Ostrea wullerstorfi, and Dentalium mantelli are, so far as at present known, 

 confined to the Oamaruian, and their presence may be regarded as satis- 

 factory 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. 



* James Park, The Geology of the Queenstown Subdivision, Bull. No. 7 («•*.), 

 N.Z. Geol. Surv.. p. 66, 1909. 

 f hoc. e*7., pp. 60-66. 



