Art. XLIV.—Notes on the Geology of the Country between Dannevirke and Wainui, Hawke's Bay.
[Read before the Hawke's Bay Philosophical Institute, 12th June, 1893.]
The formation of a new road between Wainui, near Cape Turnagain, and Dannevirke, in the Seventy-mile Bush, is of much interest from a geological standpoint. The road runs in a north-west and south-east direction, and passes over the Puketoi limestone range at a point where the rocks are much fractured, or where they are so denuded as to show but small remnants or outlines. The distance between Wainui and Dannevirke is about forty-six miles, and from the latter town to the Ruahine Mountains the distance is about ten miles, so that a complete section is now obtainable of all exposed rocks from the Ruahine to the coast between the 40th and 41st parallels of south latitude. The road in question passes
through three river-basins—viz., the Wainui, the Akiteo, and the Manawatu—the two former emptying their waters into the ocean on the East Coast to the south of Cape Turnagain, whilst the Manawatu discharges its waters into the ocean in the South Taranaki Bight, a few miles to the west of the Foxton Township. There is a great difference in the character of the country drained by these rivers. The Manawatu drains an area which is essentially Pliocene and Post-pliocene, whilst the country drained by the Akiteo and Wainui Rivers belongs either to the Older Tertiary or to the Younger Tertiary series. After quitting the Ruahine Mountains the Manawatu passes through a valley made up entirely of young deposits, except in the single instance of the Manawatu Gorge, below Woodville, which separates the Tararua from the Ruahine Mountains. The valley extends from the Ruahine to the Puketoi Ranges, and is important as being, as far as the gorge at least, the remains of an enormous rift or subsidence which has been filled from the products of streams derived in great part from the volcanic district which is now separated from the area known as Hawke's Bay district by the Ruahine Range, which was slowly rising as the rift took place, and was no doubt the primary cause of the rift.
The Ruahine rocks towards the south are made up mainly of sandstones, having a great similarity to the New Red Sandstone of England. When climbing to the trig, station known as Wharati, at the south end of the range, a short time ago, I noticed that the Maitai slates, which are exposed at Maharahara, and which thicken out further north ward, were but slightly exposed here. Large boulders of jasperoid quartz overlying conglomerates were met with about 1,200ft. above sea-level; and at the highest elevation where there are traces of settlement blue fossiliferous clays were exposed resting against the slates unconformably. I have seen them in a number of cases elsewhere. Beyond this point the sandstones appeared, and no other kind of rock was seen up to the trig. station, where every exposure shows the fine-grained compact sandstone. In the Ruahine, immediately opposite Dannevirke, the sandstone appears in connection with splintery or drossy slates, but the upper rocks are sandstone, with here and there traces of a compact conglomerate of the millstonegrit type. The same kind of grit appears in the Whakarara Mountains, between Hampden and Kereru, and it may be that the latter range was connected at one time with the Ruahine in the direction indicated by the grit stone. The whole of the valley between the Ruahine and the Puketoi Mountains is made up mainly of Post-tertiary deposits. They belong to what may best be described as the Kidnapper pumice and conglomerate series. These beds, which are very thick in places,
rest on the blue-clay marls, which were once overtopped by limestones.
Along the banks of the Manawatu River, which passes down the valley two miles or so to the eastward of Dannevirke, there are exposures of these blue Younger Tertiary beds similar to what are seen in the Tukituki, just above the crossing between the Ongaonga and Ashcott, and also at the Kidnappers, immediately above the limestones, at the place known as the Black Reef. In several places a little further to the eastward of the river the limestones are seen similar to those at Heretaunga and at Ashcott, between the Tukituki and Tukipo Rivers. This, it seems to me, is important, because it connects all the limestones on the western flank of the Cretaceo-tertiary rocks into one great and continuous whole. These limestones are met with as outliers in a number of places flanking the Ruahine Mountain between Woodville and Kereru. The Mangatoro Stream, near to what is known as Hamilton's Homestead, appears to separate the younger from the older blue-clay marls. The latter marls are similar to those exposed in the cliffs between Waikare and Wairoa, but the fossils are too fragile to make a satisfactory collection. Between the younger and older series of blue-clay marls a calcareous sandstone is met with in places, which passes sometimes into limestone of fairly compact texture.
The limestone is finely exposed in a scarp some six miles or so beyond the Mangatoro Stream, and, as there is no stone suitable for road-metal between this place and Weber, or, indeed, between here and Wimbledon, no doubt it will be largely used for road purposes in the near future. This limestone is similar to that found in the gorge of the Tukituki River, on the side towards Waipukurau, and the same fossils, T. angulare and O. ingens, are common to both. The limestone found in the Wanstead Gorge appears to belong to the same series, and as an impure sandy calcareous deposit it is seen to pass between the two clay-marls midway between Patangata and Tamumu, on the right bank of the Tukituki River. The hills, which are topped by the limestone scarp, form the dividing-range between the east coast and the Manawatu basin, and from this place there are no rocks younger than these, which are, in reality, the youngest of the Puketoi rocks exposed in this direction. Proceeding further towards the coast the blue clays, wherever exposed on the roadside, are somewhat indurated, but they seem to be acted upon by strains at right-angles to the pressure, and they break from the rock in sheets with a kind of conchoidal fracture. These rocks have a great tendency to move glacier-like into the valleys; and the whole country can be read in the peculiar appearance presented by the moving masses towards the streams,
which they sometimes dam back until a new channel has been cut.
At the stream known as Kereru, five miles or so from Weber, the rocks belonging to the upper greensands appear. These sands contain concretionary bands of impure limestone, which are fossiliferous, but exposure to the weather rapidly acts upon them, and they quickly decompose. The greensands are filled with black flakes resembling lamellar hornblende, but whether they are hornblende or not I cannot say. These greensands seem to me to belong to the same series which are so largely developed along the coast-hills between Porangahau and Wainui—Cook's Tooth being the central height—and are first met with on the top of the hills overlooking Porangahau on the Wallingford side. In neither of the latter places, however, have I observed a ferruginous limestone band, but, as traces of this greenstone extend to the hills overlooking the Roan Creek, a mile or so from the Weber Township, there can be but little doubt that they are the representative beds of the coast greensands, seeing that the Waipawa chalkmarls make their appearance in this creek. Between Roan Creek and the Akiteo Stream, where it crosses the road, the whole area is made up of chalk-marls. In some places these marls have a conchoidal fracture, but near their junction with the blue clay they weather into small cubical pieces, and the whole exposed surface has what may be termed cleavageplanes not unlike the splintery slates at the base of the Ruahine, except in the matter of hardness. The blue marls continue from the Akiteo Stream past Tea-tree Point, and thence onward to the top of the hill leading to the Wainui Stream. No fossils of any kind were observed, in these blue marls.
At the bottom of the hill leading to the Wainui Stream the black and brown oil-shales are well exposed. They have been used here for road purposes, as they are the hardest rocks in the district. These shales have a very wide distribution along the east coast of the Island. Their most northern locality is at Port Awanui, a few miles to the south of East Cape. There they are largely exposed in connection with the greensands and the blue-clay marls. The next place where they appear as surface-rocks is near the Waipaoa homestead, on the Waipaoa River, thirty-five miles to the north-west of Gisborne, in the vicinity of the once much-talked-of Poverty Bay oil-springs. They are next seen well exposed near- Baker's Brewery, at Waipawa, within half a mile of the town, and again on the top of the hills overlooking Porangahau. In each place named they are met with in connection with greensands and chalks marls. Southward from Porangahau no trace of the rocks is again met with until reaching the Wainui Stream, when they are exposed three times along the high banks in a distance of
six miles. Thus in a distance of 350 miles the black oil-shales are met with five times under almost identical circumstances. These shales contain a large percentage of vegetable matter. Numerous fish-scales have also been found, and peculiar tubes like compressed pipe-stems are very common throughout the beds. The Wainui Stream, between Wimbledon and Speedy's homestead, shows several fine sections where the black, red, and grey shales merge into each other, proving that the socalled oil-shales are not an isolated deposit, but are a part of a widely-distributed series of beds, which, though deposited at the same time, were deposited under different conditions. From Speedy's to the coast the rocks are either the green-sands or the blue-clay marls, which are interbedded with them.
Up to the present time no minerals of commercial value have been found in the district under notice. Some years ago, I remember that a Mr. Wilie, who was postman and telegraphist at Wainui, showed me some fine specimens of agate which he had collected in various parts of the Akiteo river-bed. I have no specimens of the stone he collected, but similar unpolished specimens were lately sent to me by that hard-working member Mr. Taylor White, who found them in the vicinity of the same river. Quartzose boulders, but of an orange-yellow, with agate bands, are found over the hills between Wallingford and Porangahau—the last remnants of the greensands in these places. I have not yet had an opportunity of visiting the lower portion of the Akiteo River, but, from information, it appears there are some old rocks to be seen in that district. An old Maori legend says that it was near Akiteo that the Natives made their meres, or obtained stone for sharpening purposes; and Mr. Tone, a surveyor, who is a careful observer, tells me there are rocks—black rocks—which he thinks must be volcanic. When the bush about Wimbledon was being surveyed previous to being thrown open for selection I visited the place with the then Chief Surveyor, and in the bed of the stream thin bands of bright coal were seen interbedded with a greyish-blue sandstone. I have not visited the place since—now some eight years ago—but it would be well if the banks of the stream near the Wimbledon sawmill were carefully explored. The district is situated within a Cretaceo-tertiary area, and it is among the rocks of this period that the best coal deposits of New Zealand are obtained; nor need one be surprised to learn at any time that a coal deposit has been met with. Certainly the rocks in the district are favourable to such a discovery being made.