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Volume 76, 1946-47
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Maitai-Hokonui Unconformity.

[Read before the Southland Branch, September 23, 1943; received by the Editor, July 22, 1946.]

Abstract.

Erosion of Maitai (Carboniferous) sediments during Kaihiku (mid-Triassic) times is inferred from the presence of boulders of Maitai Limestone in Kaihiku conglomerate at the north end of the Takitimo Mountains, Southland. The distribution of the fossil Maitaia is discussed and three new localities mentioned.

Introduction.

Rocks of Maitai and Hokonui age form the younger part of the undermass in the South Island of New Zealand. These rocks were strongly folded and faulted during the post-Hokonui orogeny and are everywhere separated from the upper Cretaceous and Tertiary covering strata by a major unconformity. The intensity of the post-Hokonui orogeny makes it difficult to recognise unconformities in the older rocks, and the stratigraphic relation between the Mesozoic Hokonui Series and the upper Palaeozoic Maitai Series has never been clearly described. The succession of Hokonui beds is well shown in the type area, the Hokonui Hills of Southland, the area having been mapped in detail by Cox and McKay (1878). The fossils from this area have been described by Trechmann (1918, 1923), who showed that the beds range in age from mid-Triassic to upper Jurassic. Fossils of Maitai age have been collected from Clinton, two miles north of the oldest Hokonui beds (Kaihiku Series) by Ongley (1939), but the contact between the two series is obscured by recent gravels.

The type area of the Maitai rocks is the Maitai Valley, a few miles east of the town of Nelson; only a few poorly preserved fossils have been found (McKay, 1878; Treehmann, 1917), and although these fossils make it certain that the beds are upper Palaeozoic, and older than the oldest Hokonui beds, they are not well enough preserved or numerous enough to give a definite age determination. Hokonui beds are also represented near Nelson, but the contact with the Maitai beds is probably a fault contact and the stratigraphic relationship is consequently unknown.

Description of Area.

The northern end of the Takitimo mountains forms the northwest limit of the Southland area of Hokonui rocks; and is of particular importance, being one of the few places where the base of the Hokonui beds may be exposed. The district has not been geologically mapped in detail and the only description, is that given by

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Cox (1878). Cox divided the pre-Tertiary rocks into two groups; the older, his Takitimu Series, he considered the same age as either the Maitai or Te Anau series; the younger group, containing a heavy conglomerate at. Mount Hamilton, he correlated with the Kaihiku Series. Cox (1878, p. 113) states that “I have been unable to obtain any fossils from the beds which form the mass of Mount Hamilton except some very indistinct casts of Sprifers, but from their character and general position I have very little doubt that they are of the age just mentioned (Kaihiku). These beds consist of heavy coarse conglomerate with crystalline rocks, breccia ash beds, rotten sandstone, etc., and they lap round the base of the Takitimo Series and abut against them, which fact, when considered in conjunction with the occurrence of dykes of syenite, etc., in the Takitimus, which are reproduced in the conglomerates of the Kaihiku Series of Mount Hamilton last mentioned, is sufficient to establish an unconformity between the two.” Cox's correlation of the upper group with the Kaihiku Series of the Hokonui Range carries considerable weight for he was well acquainted with the rocks of the Hokonui Range, having just completed a detailed survey of that area.

It is not certain from the above description if Cox located the actual contact between the two groups of beds; it is more probable that the angular unconformity was inferred from regional observations, but as the structure is known to be complex, detailed mapping will be required before an angular unconformity can be considered proved. His reference, however, to derived pebbles in his upper group has been verified by the authors.

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Hutton (1872) noted the presence of heavy masses of conglomerate on the northern flanks of Mount Hamilton and the Takitimo Mountains, and although he was not able to place its age with satisfaction, he mapped it as his upper Palaeozoic formation, the Te Anau Series of Hector; but stated that it may belong to the lower or even to the upper Secondary formations.

Conglomerates belonging to the younger of the above two groups are well exposed about a mile south of a point on the Mossburn-Te Anau road three miles east of a place called the Key, and close to trigonometrical station Aa, Takitimo Survey District. The conglomerates are interbedded with sandstones, siltstones, and breccias, all of which closely resemble the type Kaihiku beds of the Hokonui Hills, with which they were correlated by Cox. The conglomerate bands are relatively resistant to erosion and form the conspicuous bare ridges so characteristic of the Hokonui Hills. These ridges enable the attitude of the beds to be observed from a distance, and it can be seen that they have been irregularly deformed and dip at moderately high angles to the south and south-west. Although more irregularly deformed than the type Kaihiku beds, they are no more indurated, for the siltstone bands break into irregular fragments on exposure and the pebbles in the conglomerates can be readily extracted from their matrix. The degree of induration lies between that of the comparatively well-indurated nearby Tertiary sediments and that of probable Maitai rocks near Mossburn, from which the component pebbles can be extracted only with difficulty. Hard, well-rounded, granitic pebbles form the whole of the fine conglomerate bands near trigonometrical station Aa, but the coarse conglomerates also contain softer sub-angular boulders of limestone and amygdaloidal basic volcanics. Neither the limestone nor the volcanic boulders are common, for only a dozen or so were found during a two-hours' search. The largest limestone boulder found was 2 ft. long. The limestone is a light grey rock, somewhat fetid when first broken, and shows fragments of a prismatic shell in relief on the weathered surfaces. The largest of these shell fragments (G.S.2927), about 2 in. long, has been examined by Dr. Marwick. who writes as follows:—

“The Maitai Limestone from Trig. Aa, Takitimo Survey District, is an interesting stone. Of course there is only the fibrous shell to go on, but it seems to be reliable. Sections of the rock show that it consists almost wholly of detached prisms. It is indeed a Maitaia limestone” (personal communication).

Distribution of Maitaia.

The relative importance of the limestone boulders can best be appreciated if a short account is first given of the distribution of the fossil Maitaia. Although Maitaia is not a very impressive fossil, and one which can be easily missed if not known to the geologist, it has proved to be important, being apparently confined to beds of Maitai age. It is not known outside New Zealand, but is far more common than the rare fossils of known upper Palaeozoic age. Maitaia was first found in the Maitai Limestone at the Dun Mountain tramway by McKay (1878). Its organic origin was first doubted, but McKay's

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later discovery near the Wairoa Gorge of the fossils which fixed an upper Palaeozoic age for the Maitai Limestone together with better specimens of Maitaia removed this doubt. Maitaia was later discovered, together with other Palaeozoic fossils, at Clinton in South Otago by Ongley (1939) and was found in the same beds unassociated with other fossils 33 miles to the north-west at Pyramid Hill by Macpherson (1935). Mr. Ongley informs us that he recently found Maitaia in limestone associated with basalt at Sugar Loaf Hill, Marlborough, although this is not mentioned in his report on the limestone (1945). North of Nelson, Maitaia was found by one of the authors to be common in the limestone of D'Urville Island. In the southern end of the South Island the present authors recently found scattered fragments of Maitaia in hard tuffaceous beds on the west coast of Howell Point near Colac Bay. This locality is at the east end of Colac Bay, at the most westerly exposure of the basement rocks of Howell Hills. It lies 345 chains at 283° from Riverton Trig. Station, Howell Hills, Jacob's River Survey District, Southland.

A single fragment in greywacke (G.S.2928) associated with typical red and green Maitai slate was found on the eastern shore of Lake Te Anau in shoad material derived from the Livingstone Range and only 50 miles north of the Takitimo Range. The above occurrences are all from rocks of known or suspected Maitai age and there is no evidence of the fossil having been found in other rocks. It appears to be more common than is usually realised, and there is little doubt but that a careful search would reveal its presence in other areas of suspected Maitai sediments.

Evidence for Unconformity.

The limestone boulders closely resemble the limestone of the Nelson district, the presence of Maitaia showing that the limestone is almost certainly of Maitai age. The associated basic volcanic pebbles provide contributory evidence; for the fossils found at the Wairoa Gorge, Marlborough, Colac Bay, and Clinton are associated with either basalts or basic tuffs.

There is no difficulty in finding a probable source for the limestone boulders, for Maitai Limestone has been noted at several places not more than 50 miles to the north. Maitai Limestone has been reported from the Harris Saddle by McKay (1881) and Hector (1892), from the Brynera Range by Park (1887), and from the north end of the Livingstone Range by Park (1921). Although these reports do not mention Maitaia, a pebble containing Maitaia, almost certainly derived from the Livingstone Range, has already been described. It is probable that the Maitai Limestone once extended farther to the south, and, indeed, it is not impossible that it may extend south to the Takitimo Range.

The presence of an unconformity below the Kaihiku beds depends not only upon the limestone boulders having been derived from the Maitai Limestone, but also upon it being proved that the conglomerate in which they were found is really of Kaihiku age. Cox's correlation is good evidence, for he was well acquainted with the type Hokonui beds, but the correlation will not be certain until better fossils are found.

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In conclusion, it appears that although there is good indirect evidence for an unconformity between the Hokonui and Maitai beds in the Takitimo Mountains, sufficient work has not yet been done to show the true nature of the unconformity, and further work in this critical area is much to be desired.

References.

Cox, S. H., 1878. Report on the Geology of the Hokanui Ranges, Southland. Rept. Geol. Explorations during 1877–78, No. 11, pp. 2548.

—— 1878. Report on the Geology of the Te Anau District. Rept. Gcol. Explorations during 1877–78, No. 11, pp. 11018.

Hutton, F. W., 1872. Reports on the Geology of Southland. Rept. Geol. Explorations for 1871–72, No. 7, pp. 89112.

Hector, J., 1891. Progress Report, 1890–91. Rept. Geol. Expl. during 1890–91, No. 21, p. xlv.

McKay, A., 1878. Report on the Wairoa and Dun Mountain Districts. Rept. Geol. Explorations during 1877–78, No. 11, pp. 11959.

—— 1881. District West and North of Lake Wakatipu. Rept. Geol. Explorations during 1879–80, No. 13, pp. 11847.

Macpherson, E. O., 1935. Wakaia Subdivision. 29th Ann. Rept. Geol. Survey Branch, Dept. Sci. and Ind. Res., pp. 68.

Ongley, M., 1939. The Geology of the Kaitangata-Green Island Subdivision. N.Z. Geological Survey Bulletin No. 38, pp. 3234.

—— 1945. Matai Limestone in Marlborough. Jnl. Science and Technology, Vol. 26, pp. 3034.

Park, J., 1887. On the Country between the Dart River and Big Bay. Rept. Geol. Explorations during 1886–87, No. 18, pp. 12137.

—— 1921. Geology and Mineral Resources of Western Southland. N.Z. Geol. Surv. Bull. No. 23.

Trenchmann, C. T., 1917. The Age of the Maitai Series of New Zealand. Geological Magazine (N.S.), Dec. 5, Vol. 4, pp. 5364.

—— 1918. The Trias of New Zealand. Q.J.G.S., Vol. 73, pp. 105246.

—— 1922. The Jurassic Rocks of New Zealand. Q.J.G.S., Vol. 19, pp. 246312.