The Geology of the Brocken Range and the Kaiwhata Valley, East Wellington.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 72, 1942-43, Page 347

The Geology of the Brocken Range and the Kaiwhata Valley, East Wellington. By D. A. Brown, New Zealand Geological Survey. [Read before Wellington Branch, October 9, 1941; received by the Editor, November 24, 1942; issued separately, March, 1943.] Prior to his departure for England, the writer was enabled by the courtesy of Mr. A. W. Daysh, of Masterton, and Mr. and Mrs. Elliott, of Ngahape, to spend five days during August, 1941, near the Brocken Range, which lies about 15 miles south-east of Masterton. In the short time, no detailed work was possible, but, as the district is exceedingly interesting, the following notes may have value as a nucleus for later detailed surveys. During his visit, the writer made his headquarters at Ngahape, a settlement 19 miles (by road 28 miles) south-east of Masterton. The petrology of the igneous rocks collected is described by Dr. C. O. Hutton in the paper following this one (pp. 353–370). Previous Work in the Area. The district has been little studied in the past chiefly on account of the difficulty of access. The area here described forms part of four survey districts, Otahoua, Rewa, Wainuioru, and Kaiwhata in the Wellington Land District. The first record of geological investigation is that given in 1861 by James C. Crawford, the Wellington Provincial Government geologist. He described the precipitous and jagged hills “on a line parallel to the East Coast, and perhaps at a distance of ten miles from it” as composed of stratified and tilted sandstones. The Brocken Range is composed of a group of these “taipos,” so-called by the Maoris, who held them in superstitious dread as the dwellings of evil spirits. Crawford noted that they were intruded by igneous rocks, which he called “trachytes,” and also described pebbles of the same rock from the bed of the Upokongaruru Stream, which flows from the western slopes of the Brocken Range. He noted that this “trachytic” rock decomposed into an iron sand, and “in it Mr. Haast has discovered a speck of gold with the microscope.” In 1883, McKay visited the East Coast District of Wellington Province, and a year later published a short paper on the igneous rocks. In it he mentioned a later, fuller report; but this does not seem to have been published. He noted the frequent occurrence of igneous rocks on the coast from Castle Point southwards, and drew attention to the “syenitic and porphyritic” rocks brought down by the Kaiwhata River. He searched in vain for the source of these igneous boulders, but on information received from Mr. Beetham. M.H.R., he found a large outcrop of the rocks in the headwaters of the Upokongaruru Stream on the eastern part of the Brancepeth run. This, however, did not explain the presence of coarse, crystalline igneous rocks in the Kaiwhata River, for the Upokongaruru is a tributary of the Pahaoa River, a stream farther to the south. McKay recognized this fact, and added a note that Mr. Beetham had informed him that such igneous rocks traversed the old sandstones on the north-east. An examination of the Te Maire Stream or the Totara

Stream would have shown him that the igneous boulders do not come down either of these valleys, the only channels along which they could reach the Kaiwhata River from the Brocken Range. The explanation was partly given by McKay's son, W. A. McKay, in 1899, when he recorded the presence of igneous intrusions and tuffs first, in the Kaiwhata river-bed about a mile below its junction with Bismarck Creek and second, near the confluence of the Te Maire and Kaiwhata. W. A. McKay spent two or three days in the Kaiwhata Valley in very bad weather during the early part of 1899, and he has given a good account of his work there. The present writer found another completely independent intrusion of coarsely crystalline igneous rocks, which crosses the Kaiwhata Stream a quarter of a mile east of Ngahape. The regional examination by the officers of the N.Z. Geological Survey in Eketahuna Subdivision to the north has not yet been extended into the Brocken District; but much of the geology of the latter can be correlated with that of the former area. Geology. (a) The Brocken Range and Environs. The Brocken Range is an easterly offshoot of the Maungaraki Range, which runs south-westward, parallel with the east coast and ten to twelve miles distant from it. The Brocken Range splits off near Rewa Hill and lies about halfway between the coast and the Maungaraki Range. The portion investigated lies to the south of the Fernyhurst Road, and includes the prominent peaks Te Maipi, Pukekowai, and Puketeitei rising to elevations of 1500–1600 ft. above sea-level. These peaks are jagged and precipitous “taipos”; and Te Maipi is almost inaccessible except from the south side. They form open grass country with small patches of native bush near the tops. The range is composed of hard, moderately coarse, indurated and shattered greywacke (P.7599, 7604* The numbers prefixed by “P” refer to specimens in the rock and mineral collections of the N.Z. Geological Survey.) which Ongley (1935) has correlated on lithology with the Taitai formation of the Waiapu Subdivision. The line of peaks seems to follow strike ridges of harder components of the formation. This greywacke also forms the lower country on the east side of the range between it and the Kaiwhata and Te Maire Streams. Here again are the peculiar ridges and “castles” resembling basalt plugs or necks (see Fig. 3). The reason for their resistance to erosion is not known, as on examination their component material appears to be identical with that of the surrounding rocks. Such, for example, is the “Sugarloaf,” 780 ft. high on the north side of Totara Stream (P.7598). On the north bank, near the junction of Totara and Te Maire Streams, a few small, well-rounded pebbles of porphyritic igneous rock (P.7617) were discovered in the weathered sandstone, thus supporting, but not proving, the correlation with the Taitai formation, which farther to the north contains numerous bands of igneous conglomerate.

Fig. 1. East side, Brocken Range from Sugarloaf Te Maipi in centre, Pukekowai to left with Baneepeth between Te Stream in totoground Fig. 2. West side, Brocken Range, from Te Maipi. Pukekowai to left, Tertiary limestone scap running from middle distance to extreme right and Red Hill teschenite in low rounded hill near right-hand border. Maungaraki Range in the background. Fig. 3. Sugarloaf Hill, a prominent greywacke residual near Ngahape. Brocken Range in the background.

Text Fig. 1. Looking east up Kaiwhata Valley, The sill forms the prominent outcrop intersecting the opposite slope and forming a V pointing down-valley. The most interesting feature and the main purpose of the trip was to discover the source of the teschenite pebbles and boulders found by McKay (1884) in the Upokongaruru Stream near Red Hill, on the Brancepeth run, and briefly described by Sollas (1906). Between Pukekowai and Te Maipi is a low pass about 1200 ft., high over which runs a well-formed track. Passing from east to west, one descends into a hollow formed by the headwaters of the Upokongaruru Stream, evidently part of McKay's “crater” (1884), but in fact, merely the breached hollow between two greywacke strike ridges. Abutting sharply against the greywacke about half a mile northwest of Pukekowai and on the south side of the Upokongaruru is a ridge of dark-brown, moderately soft quartz sandstone (P.7611), capped by 4–5 ft. of extremely hard coquina (P.7600) (a detrital limestone composed chiefly of molluscan shells) dipping at 25° to the west. This latter rock closely resembles the Ugly Hill limestone (Ihungia) of the Dannevirke Subdivision. On the north side of the stream the igneous intrusion was discovered. This is evidently McKay's Red Hill; and it is composed entirely of coarsely crystalline basic igneous rock, a melanocratic teschenite (P.7616). From the scattered boulders of igneous rock aligned in a ridge on the south side of the Upokongaruru, it appears that here is a small dyke of teschenite which has altered sediments of the Taitai formation (P. 7606, 7613). The main intrusion appears to be roughly circular. The rock on decomposing gives rise to a green, sandy soil (P.7610), and the

mass extends for about 20 chains north of the Upokongaruru Stream to a point almost due west of Te Maipi, where it is seen in contact with the greywacke. The coarse texture of the igneous rock indicates a hypabyssal intrusion, the age of which is doubtful. The intrusion has certainly altered the older greywacke and argillite, but does not appear to have affected the younger (? Ihungia) sandstone and limestone, which seem to have attained their present position by fracturing along a line west of the Red Hill igneous mass. Hence the teschenite is post-Taitai (Upper Mesozoic) and probably pre-Ihungia (Lower Miocene) in age. (b) The Kaiwhata Valley. The Kaiwhata River drains the low country situated between the Brocken Range on the west and the coastal range on the east which runs south-south-west from the Mataikona River in Eketahuna Subdivision. The Kaiwhata Stream drains the western slopes of this latter range and flows westwards to Ngahape, where it is joined by the Kopi, Te Maire, and Totara Streams. From this point for a further six miles it travels south along the general strike of the country as a stream of considerable size; again it turns at right angles, and cutting through the coast range in a deep gorge, discharges on the east coast about eight miles south of Uruti Point. The drainage system is now deeply entrenched owing to uplift, whilst deforestation has greatly increased erosion. In the lower Kaiwhata Valley south of Ngahape there is a prominent, high-level terrace representing a former erosion level, protected now by the rock barrier of the coastal range, which is composed of white, pink, and grey fine argillites of the Whangai formation. Time permitted of only a rapid reconnaissance of the Kaiwhata Stream east of Ngahape; but the information gathered may be of value. At the stream junction about two miles east of Ngahape the stream cuts through well-bedded, fine, micaceous, black argillites and sandstones (P. 7612), which are twisted and shattered by faulting. The rocks strike about south-west and dip generally to the north-west but are often vertical. These beds are probably of Tapuwaeroa or even Raukumara age, and their attitude suggests a strong fault between this point and the coast range. Downstream the dips flatten somewhat, and the strike swings to the west. Half-a-mile west of the junction, near the old woolshed on the north bank, the first of a series of conglomerate bands appears with pebbles of greywacke up to 2 in. in diameter. No igneous pebbles were seen. This is followed by a set of fine bedded sandstones, 25 ft. thick, dipping 40° to the north and overlain in turn by another coarse conglomerate. A small fault here brings fine black mudstone into juxtaposition with the conglomerates. This mudstone contains Inoceramus (Callistoceramus) bicorrugatus Marwick (P.7603), the characteristic fossil of the Mangaotane mudstone (Raukumara Series). The succession continues downstream with alternating mudstones and sandstones and occasional thick conglomerate bands, the grade of the pebbles in the latter appearing to decrease as the beds become younger.

The Kaiwhata Sill (See Sketch Map). In the Kaiwhata Stream, about 20 chains upstream from the Ngahape bridge, the stream intersects a large sill of hard, crystalline, dolerites weathering in cuboidal blocks (P.7602). The sill appears here to be about 20 ft. thick, though contacts with the neighbouring rocks were not seen. The reason for calling this igneous mass a sill is due to its attitude seen from the hills above the stream. It stands out as a hard erosion remnant amongst the softer sandstones and mudstones, and is seen to conform closely with the bedding of the latter, and follows the inclined V-shape of the valley. That it is not a flow is indicated by outliers above the next succeeding sedimentary rocks. The writer followed the sill exposures for about a mile and a half to the north-east, but owing to the limited time did not go further, though the indications were that the sill is exposed for a long distance farther north-east. McKay (1899) noted that an intrusion crossed the Kaiwhata River a mile below the junction with Bismarck Creek; this is on a projection of the line of the Kaiwhata sill and two miles south of its intersection with Kaiwhata Stream. The igneous rock forms prominent buttresses, and in exposures to the north of Kaiwhata Stream is extremely coarse-grained with long fibrous crystals of feldspar and clinopyroxene (P.7607, 7615). Downstream from the sill the mudstones and sandstones continue, and at the junction of the Kaiwhata and Kopi Streams there is a thick bed of soft greensand (P.7609). Westward to Ngahape the beds are highly glauconitic. West of the Kaiwhata and Te Maire, greywacke and indurated sandstone form prominent hills such as the Sugarloaf. This is strong evidence for a fault running approximately north and south along the line of the Kaiwhata Valley; but owing to the lack of data its position is ill-defined. Ngahape Intrusion. On the Kaiwhata Valley road about 200 yards north-west of Ngahape School is a large outcrop, 100 yards long and 30 ft. high, of black, zeolitic, moderately fine-grained variolite displaying pillow forms in one or two places (P.4886–4893; P.7608). It intrudes sandstone and argillite (? Tapuwaeroa), pieces of which form large horn-felsed xenoliths in the cliff exposure (P. 7614). The rock is deeply weathered and is traversed by thick veins of calcite (P.7605). This mass was first noted by W. A. McKay (1899), and was recently visited and sampled by Mr. I. J. Pohlen, of the Soil Survey Department. The upper part of Cemetery Hill, of which the igneous rock forms the lower slopes, is composed of pink and yellow fine argillite. Literature. Crawford, J. C., 1861. N.Z. Government Gazette (Province of Wellington). Vol. 8, pp. 239–42. McKay, A., 1884. On the Igneous Rocks of the East Coast of Wellington. Rep. Geol. Explor., No. 16, pp. 71–5. McKay, W. A., 1899. Report on the Geology of the East Coast from the Kaiwhata River to Glenburn, East Coast of Wellington. N.Z Parliamentary Paper C–9, pp. 36–43. Ongley, M., 1935. Eketahuna Subdivision. Ann. Rep. N.Z.G.S., 1934–5, pp. 1–6. Ongley, M., 1936. Blairlogie Gas-vent. Ann. Rep. N.G.S., 1935–6, pp. 9–12. Sollas, W. J., and McKay, A., 1906. The Rocks of Cape Colville Peninsula, Auckland, N.Z. (Addendum). Vol. 2, pp. 155–7, Govt. Printer, Wellington.

Geological Sketch Map —Brocken District— Otahoua, Wainuioru, Rewa & Kaiwhata, S.Ds. Wellington Land District