Abt. XXXIX.—The Geology of the Cape Runaway District.
[Read before the Otago Institute, 2nd December, 1913.]
The strip of country running due east and west from Cape Runaway to Matakaoa Point has not yet received much attention from the geological standpoint. Throughout this twenty miles of coast-line the country rises steeply, and soon reaches to an elevation of 1,300 ft. The country slopes down nearly as steeply on the southern side of the ridge into the valleys of the Wharekahika and Ngarue Streams, running east and west respectively. Thus this narrow bar of country is marked off from all that lying to the south, and constitutes a physiographical unit. In a north-north-east direction the ocean-floor dips down steeply, and within fifty miles there is a depth of 1,000 fathoms, and within two hundred miles the ocean is 4,380 fathoms deep. This profound abyssal depth marks the farthest known extension of that great earth-feature the Kermadec-Tonga trench; but, since soundings are relatively few in this part of the Pacific Ocean, it is quite possible that future work will show that this trench actually extends still farther to the south. This is the nearest known approach of abyssal depths to the coastline of New Zealand. The average slope throughout this distance of two hundred miles is about 137 ft. per mile—that is, 1 in 40.
Although, this small patch of country has never received much attention from geologists, it has generally been recognized that the rocks of which it is composed are of a different nature from those of the country that lies to the south. The rocks that are in contact with those which are now being considered are of the Oamaru system of marine sediments. Farther to the southward, if one may judge from the nature of the pebbles that are found in the gravels of all the tributary streams of the Wharekahika Stream, the Oamaru sediments are soon succeeded by the Maitai sediments of Trias-Jura age.
The following are the references that have been made to this country in previous geological work:—
1869: Hector's geological map of New Zealand represents them as formed of Cretaceo-Tertiary sediments.
1873: McKay, Geological Report, p. xvi.—The map indicates the occurrence of trachytic conglomerates throughout this area.
1873: Hector's geological map of New Zealand.—Coal-measures, Cretaceous to Miocene.
1877: Cox, Geological Report, p. 112.—Trachytes and trachytic agglomerates: “The trachytes in solid flows may be observed continuously until passing Matakaoa Point, Hicks Bay, where they pass by degrees into coarse angular conglomerates.” He thinks that the centre of eruption in all probability lay to the seaward of the present coast-line. “They appear to me to be of the same age as the trachytes of Whangarei Heads (andesites), which they resemble in a marked degree, and as they are overlaid by the oyster-bed, which is the lowest member of the Tawhiti series as here developed, they must be at least of Upper Eocene age.”
1883 and 1885: Hector's geological map of New Zealand.—Palaeozoic sediments.
1887: McKay, Geological Report, p. 184.—This geologist went by sea from Whangaparao to Awanui. He did not see these rocks, but on his map on page 192 he indicates them as acidic volcanic-rocks.
1907: Marshall, Trans. Aust. Assoc. Adv. Sci., p. 376, states that he has not examined any rocks from this district, but in the map indicates them as basalts.
1910: Park, “Geology of New Zealand.”—In the geological map this area is coloured as though formed of acidic volcanic rocks.
1912: Marshall, “Geology of New Zealand.”—In the geological map the area is tinted the same as those that are formed of basalts and andesites.
This brief résumé shows that the rocks have never been accurately examined. An opportunity of visiting the district occurred in February, 1912. Specimens were then collected from the south side of Hicks Bay and from the valley of the Wharekahika River, as well as the coast-line near Cape Runaway. Mr. E. de C. Clarke also kindly gave me specimens that he had collected from the coast-line at Potikirua and Arerotepa. The specimens from the valley of the Wharekahika River and Cape Runaway were not in situ, but all the others were. On the south side of Hicks Bay the volcanic rocks are interstratified with the limestone of Tertiary age, and they evidently represent the products of volcanic activity in relatively deep water, judging from the nature of the associated limestone and by the absence of oxidized material and of steam-pores in the fragmentary matter. Tuff and scoriaceous material is interstratified with the limestone.
All the specimens collected show a great similarity in composition and structure. There are two distinct types, holocrystalline and glassy. The glassy type is formed almost entirely of a dark-brown glass with incipient crystallization in black semi-opaque patches. There are no crystals or microlites present.
The holocrystalline rocks are somewhat diabasic in structure, often ophitic, with relatively little olivine, which is nearly completely changed into serpentine. The feldspar is mainly labradorite much decomposed. The augite is nearly colourless, and is always allotriomorphic. Iron-ore is plentiful, apparently entirely ilmenite. These two types closely resemble those that are found at Oamaru, where also they are interbedded with the limestone of Tertiary age. They have been described by Hutton.* Hutton† also describes several other glassy types of a similar nature. He afterwards states that the rocks occur interstratified with sedimentary rocks of Oamaru age at intervals from Look-out Point, near Hampden, to Castle Hill, a distance of 150 miles.‡ McKay and Sollas describe similar types from the Mason River, in South Nelson, where again they are interstratified with limestone rocks of Oamaru age.§ It thus appears that over a wide extent of the eastern coast-line of New Zealand submarine eruptions of a basic nature occurred at the time when the limestone of Oamaru age was under going deposition. It has been suggested in another paper that the lime stone of this series was deposited approximately at the same time throughout New Zealand, and that it was laid down when the country had reached its maximum of physical depression.∥ If this be the case, it is evident that this widespread series of submarine eruptions was practically simultaneous, and ushered in a period of general epeirogenic elevation in the New Zealand region.
[Footnote] * Trans. N.Z. Inst., vol. 19, 1886, p. 417.
[Footnote] † Jour. Boy. Soc. N.S.W., 1889, p. 152.
[Footnote] ‡ Trans. N.Z. Inst., vol. 32, 1899, p. 170.
[Footnote] § “Rocks of Cape Colville Peninsula,” vol. 2, p. 168.
[Footnote] ∥ Trans. N.Z. Inst., vol. 43, 1910, p. 404.