Art. XX.—On the Age of the Waikouaiti Sandstone, Otago, New Zealand.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 50, 1918, Page 196

Art. XX.—On the Age of the Waikouaiti Sandstone, Otago, New Zealand. By J. Allan Thomson, M.A., D.Sc., F.G.S., Director of the Dominion Museum, Wellington, New Zealand. [Received by Editors, 31st December, 1917; issued separately, 10th June, 1918.] Although contradictory opinions were held by Hutton and Haast on the one hand, and Cox, McKay, and Hector on the other, as to the relative age and relationships of the Notocene rocks of the north and south sides of the Shag River, all these geologists were in agreement in correlating the Waikouaiti sandstone on the one hand with the Caversham sandstone, and on the other with the Ototara limestone. Both these correlations were accepted also by Park (1910); but it is necessary to remember that at that time he placed the Ototara stone as the uppermost member of the Oamaruian. Marshall, in 1906, did not attempt a more detailed correlation than that the Caversham sandstone belonged to the Oamaru system, but in 1916 he referred to the foraminiferal limestone at Sandymount, which he had previously correlated with the Caversham sandstone, as a representative of the younger limestone of New Zealand—i.e., Ototaran. Thus practically all geologists who have written on the subject have agreed that the Caversham sandstone and Waikouaiti sandstone are the same horizon and are Ototaran. The rightness or wrongness of this conclusion has more than a merely local interest, for on it hang two other questions of a more general nature. First, the age of the Dunedin volcanic series can only be limited as regarding its commencement by reference to the Caversham sandstone—until a detailed palaeobotanical investigation of the intervening Fraser's Gully plant-beds is available. Secondly, the Miocene age of the Oamaruian is based very largely upon Chapman's conclusions regarding the Foraminifera collected by Park from the clays underlying the Waikouaiti sandstone, and if the latter is Ototaran the clays are lower Ototaran, or more probably Waiarekan, and the Middle, or more probably Lower Oamaruian, is Miocene. Opinions to the contrary, however, have recently been independently expressed by Marshall and myself (1917). Discussing the Hampden beds, I stated that “the percentage of Recent species in the Waiarekan is not inconsistent with an older age than Miocene for this stage,” while Marshall concluded that “these Onekakara [i.e., Hampden] beds seem to be more rightly classed with the Eocene than with any other European system.”

Obviously, then, if Chapman's correlation of the clays of Waikouaiti with the Miocene is admitted, either (a) the lower Ototaran or Waiarekan is Miocene, and the opinions stated by Marshall and myself err in ascribing too great an age to the Waiarekan, or (b) the Waikouaiti sandstone is not Ototaran. During a visit to Waikouaiti at Easter, 1917, in company with Professors J. Park and W. N. Benson, of Otago University, I collected from the Waikouaiti sandstone at the North Head a number of brachiopods, of which seventeen were referable to Pachymagas abnormis Thomson, while the remaining three belonged to a small species of Pachymagas with mesothyrid foramen, probably nearly related to the more orbicular forms of P. parki which occur in the Hutchinsonian of All Day Bay. Pachymagas abnormis was one of the species which I adduced in 1917 as evidence of the Upper Oamaruian age of the beds in the Takaka Valley, and subsequent discoveries have not invalidated its usefulness in this respect. I have since collected it in the uppermost bed of the Mount Brown limestone at the foot of the dip slope of the cuesta opposite Weka Pass — i.e., at a slightly higher horizon than the holotype, but still probably Hutchinsonian — and a single specimen in the Hutchinsonian greensands of All Day Bay. It occurs abundantly in the Awamoan mudstones of All Day Bay, and thus ranges in the Oamaru district from Hutchinsonian to Awamoan, but has not been found in the Ototaran. Now, a larger number of species are known from the Ototaran of the Oamaru district than from any other stage in any locality in New Zealand, so we are quite justified on the present evidence in considering P. abnormis a purely Upper Oamaruian species. Mr. S. S. Buckman, of Thame, England, has suggested in correspondence that Pachymagas abnormis should not be referred to Pachymagas, but should be made the type of a new genus on account of its beak characters, and if this course is followed it would be possible to differentiate a number of species within the somewhat variable series I have referred to P. abnormis. The specimens from the Awamoan mudstones of All Day Bay and from the Waikouaiti sandstone would, however, still have to be retained in the same species. The conclusion to be drawn from the presence of this brachiopod, then, is that the Waikouaiti sandstone is not Ototaran (i.e., Middle Oamaruian), but Upper Oamaruian, and it may well be Awamoan, and the underlying clays Hutchinsonian. In this connection an examination of the brachiopods from the sandstone at Seacliff, and from the Caversham sandstone and the greensands underlying the latter rock at the back of Flagstaff, would be of considerable interest, and I should be glad to receive specimens from these localities. List OF Papers Cited. Marshall,P., 1906. The Geology of Dunedin (New Zealand), Quart: Journ. Geol. Soe., vol. 62, pp. 381–424(ref. to pp. 389–90). Marshall,P., 1916. The Younger Limestones of New Zealand, Trans. N.Z. Inst., vol 48, pp. 87–99 (ref. to p. 93). Marshall,P., 1917. Fossils and Age of the Hampden (Onekakara) Beds, Trans. N.Z. Inst., vol. 49, pp. 463–66 (ref. to p. 465). Pabk, J., 1904. On the Geology of North Head, Waikouaiti, and its Relation to the Geological History of New Zealand, Trans. N.Z. Inst., vol. 36, pp. 418–30. Park, J., 1910, The Geology of New Zealand, Christchurch (ref. to p. 139). Thomson, J. A., 1917. Diastrophic and other Considerations in Classification and Correlation, and the Existence of Minor Diastrophic Districts in the Notocene, Trans. N.Z. Inst., vol. 49, pp. 397–413 (ref to pp. 409–10).