Art. XVII.—On the Occurrence of Striated Boulders in a Palaeozoic Breccia near Taieri Mouth, Otago, New Zealand.
[Read before the Otago Institute, 9th December, 1919; received by Editor, 31st December, 1919; issued separately, 10th, June 1920.]
In a small cove close to Rocky Point, which is the first headland on the south side of Taieri Mouth, and about a mile and a half from the Taieri jetty, there is a conspicuous bed of coarse red and green breccia. It is underlain by bluish-grey micaceous phyllites, and overlain by altered flaggy greywacke. The strike of the breccia and associated rocks is about N.N.E.-S.S.W., and the dip S.S.E. at angles ranging from 5° to 30°. Generally the inclination of the lowermost beds is flatter than that of the uppermost beds. At Taieri Mouth the dip of the grey micaceous slaty rocks ranges from 5° to 15°, and that of the greywacke south of Rocky Point from 15° to 30°.
The breccia is well exposed in the sea-cliffs near Rocky Point, and can be traced northward along the line of strike one-third of the distance to Taieri Island as a line of submerged reef that is in places awash at low water. It is not present on Taieri Island.
To the southward of Rocky Point the breccia ought to crop out on the ridge between that place and Akatore Inlet, but I failed to find it there. It is a rock not easily overlooked, and I am inclined to believe that it peters out before it reaches the crest of the ridge. It is probably a lensshaped mass with a maximum thickness of some 120 ft.
At Rocky Point the breccia resembles a consolidated rock-rubble, being mainly composed of a confused pile of angular and subangular fragments and blocks of red and green siliceous slaty shale. It also contains numerous masses of an excessively hard jasperoid and aphanitic breccia that appear to have been torn from some pre-existing breccia. The constituent fragments range in size from small grains to masses many feet in diameter.
The largest block in the breccia occurs in place, at the foot of the sea-cliff in the first sandy cove south of Rocky Point. It is an included breccia-boulder, partially rounded at the corners and sides, compact, and intensely hard. It measures some 7.5 ft. by 5.3 ft. by 4.5 ft. Its upper surface is fairly flat, and covered with distinct striae that, as a rule, run parallel with the longer axis of the block. The area of the striated surface is about 10 square feet. Several of the smaller included blocks on the south side of the cove are similarly striated.
This remarkable rock resembles the typical Te Anau breccia of Sir James Hector. It is underlain, apparently conformably, by the semi-metamorphic Kakanuian rocks of Hector, which everywhere in Otago overlie the mica-schists of Central Otago. There is no internal evidence to fix the age of the Kakanuian rocks. All that can be said is that they underlie the Mount St. Mary series, which is Triassic, and overlie the mica-schists of the interior. The Te Anau series of Hector, as identified by him in Nelson, conformably underlies the Maitai series, which is now known to be Upper Carboniferous or Permo-Carboniferous. I have always found it difficult to separate the Te Anau rocks from the Maitaian, and in 1910 grouped them as belonging to the same formation. Hector ascribed the Te Anau series to the Devonian period, the only evidence in favour of this being its inferior and conformable relationship to the Maitaian, which he placed in the Carboniferous.
In Nelson and Marlborough, the rocks identified by Hector and McKay as belonging to the Te Anau series are underlain by semi-metamorphic rocks of supposed Kakanuian age.
The Taieri Mouth breccia and overlying greywackes may very well belong to the Te Anau series of Hector, and the underlying grey silky micaceous phyllites to the Kakanuian. If this position can be established, the Taieri Mouth breccia may be placed in the Upper Carboniferous or Permo-Carboniferous.
This is the first discovery of striated boulders in the Palaeozoic formations of New Zealand, and the origin of the striae is certain to give rise to some diversity of opinion.
The striation may be glacial or dynamical. If the striated boulders occurred along the fracture of a shear-plane I should ascribe the striation to shearing. Though crushed and broken, the breccia shows no evidence of shearing along defined planes, and for this reason I am inclined to favour the glacial hypothesis.
If the glacial view be sustained we are at once confronted with the question—(a.) What relationship, if any, does the Taieri Mouth breccia bear to the glacial deposits reported in the Upper Palaeozoic formations of India,* Australia,† South Africa,‡ and Brazil§? (b.) Was the glaciation alpine or secular? (c.) Did the ancient Gondwana continent extend southward to the New Zealand area?
[Footnote] * H. B. Medlicott and W. T. Blanford, Manual of the Geology of India, pt. i, p. 110, 1879; and R. D. Oldham, Quart, Jour. Geol. Soc., p. 469, 1894.
[Footnote] † T. W. Edgeworth David, Geology of the Hunter River Coal-measures, Mem. Geol. Surv. N S. W. No. 4, p. 124, 1907.
[Footnote] ‡ E. T. Mellor, Study of the Glacial Conglomerate in the Transvaal, Quart. Jour. Geol. Soc., vol. 61, p. 682, 1905; W. M. Davis, Bull. Geo. Soc. Am., vol. 17, p. 413, 1906; and others.
[Footnote] § David White, Permo-Carboniferous Climatic Changes in South America, Am. Jour. Geol., vol. 15, p. 618, 1907.