Art. LVIII.—On the Age of the Napier Limestones.
[Read before the Wellington Philosophical Society, 21st October, 1885.]
The late Dr. von Hochstetter, basing his expressed opinion upon material supplied him by Mr. Triphook and others, referred all the beds in Scinde Island to the upper part of his Hawke's Bay series.
This Hawke's Bay series of Hochstetter is by him referred to the upper division of tertiary deposits in New Zealand, as determined by him, and called “younger tertiary formations.” These embrace a triple series: the Awatere series; the Hawke's Bay series; and the Wanganui series. How these are related to each other we are not distinctly told; but it is evident that the terms are not geographical distinctions for equivalent formations in different districts, and that the Hawke's Bay series was considered intermediate in age between the Awatere and Wanganui series.
In the Geological Reports for the year 1868–69, Captain Hutton recognises the existence of the “Hawke's Bay series,” and refers to it the beds forming the Mahia Peninsula, and a large district N.E. and S.W. of Poverty Bay. Dr. von Hochstetter had previously referred the beds forming Mahia Peninsula to the Hawke's Bay series, so that there is no doubt that Captain Hutton meant the Hawke's Bay series of Hochstetter. His estimate on the age of these beds is expressed elsewhere, in a paper on “The Artesian Wells near Napier,” in which he describes the rocks forming Scinde Island as belonging to a formation “of late tertiary date.”
Dr. Hector, in the “Geological Reports” for 1870–71, describing the geology of the Hawke's Bay District, makes use of the term “Hawke's Bay series” for the same rocks as those described by Hochstetter, and quotes from Hochstetter to show that the beds belong to the “latest tertiary formation.” Dr. Hector traced the beds north and north-west from Petane to Pohui, and as outliers, further north, beyond the Mohaka River. He referred the Te Aute limestone to the Hawke's Bay series. This had been done by Captain Hutton on the 12th September, 1870.
Thus, to the middle of 1871, there appears to have been no question as to the “late tertiary age” of the Hawke's Bay series, nor as to the conformable. relations of all the beds ascribed to it. Next year (5th November, 1872) was published Captain Hutton's “Synopsis of the Younger Formations of New Zealand,” and of this, the “Hawke's Bay group” was referred to the oligocene period. This Hawke's Bay group is typically represented by the same locality as that of the “Hawke's Bay series” of Hochstetter, viz., “Napier,” but other and distant localities are added, and rock formations of a class and age never contemplated by Hochstetter. The result of this inclusion of strata older than the Awatere series of Hochstetter, was to lower the percentage of recent species found fossil in the beds to 20 per cent.; and, as a consequence, the reference of the Hawke's Bay group to a much earlier period than the Hawke's Bay series had been referred to. How far the two should be considered identical may be inferred, and remains to be seen.
In 1873, Captain Hutton suppressed the name “Hawke's Bay group,” and substituted “Ahuriri formation” in its place, and, describing its fossils, considered that 23 per cent. of its mollusca and echinodermata were recent species. The Hawke's Bay group or Ahuriri formation was now referred to the lower miocene period.
In June, 1875, the same beds were, by Captain Hutton, referred to the middle miocene period.
On the 4th of January, 1875, I divided the tertiary strata of Hawke's Bay District into Lower or Hawke's Bay series, and Upper or Wanganui series, including the shelly limestones of Napier with the lower group or series. During the early part of 1876, Mr. Cox examined the country between Poverty Bay and Napier, and determined the tertiary rocks of the district as a single sequence, to which, however, he assigned no particular period, eocene or pliocene. He considered the Napier limestone to be near the base of the series, although his statements are somewhat incompatible with the assumed conformity of the whole series of tertiary beds described by him. Dr. Hector at the time considered the higher beds described by Mr. Cox as belonging to the same horizon as the Upper Wanganui beds.
During November of the same year, Mr. S. Percy Smith read before the Auckland Institute a paper on the “Geology of the Northern portion of Hawke's Bay,” and, like Mr. Cox, describes an upper and a lower limestone, separated by a great thickness of sands, clays, and conglomerates; all presumably of tertiary date.
In 1873, Captain Hutton rejected, as not belonging to the Ahuriri formation, the conglomerate sands and clays in the Cape Kidnappers section, which are described by Hochstetter as the base of the Hawke's Bay series, the higher beds appearing at Scinde Island and at Petane. These beds were considered by Dr. Hector, when he examined the district in 1871, as occupying the position assigned them by Hochstetter. Captain Hutton considered them pleistocene, and later I spoke of them as belonging to the Wanganui series. The position of similar rocks, described by Mr. Cox and Mr. Percy Smith as underlying the limestones of Scinde Island and the coast to the northward, and the reference of these with the overlying shelly limestones, seemed to call for a revision of the Ahuriri series of Hutton. Other causes, however, brought this about at an earlier date than the publication of Mr. Cox's report, which did not appear till 1877.
In a paper read before the Otago Institute on the 24th of October, 1876, Captain Hutton discusses the relation between the Pareora and Ahuriri formations; in which, referring to the classification of the tertiary formations of New Zealand in his “Catalogue of the Tertiary Mollusca and Echinodermata,” speaking of the beds separated and grouped under one or other of these formations, he says: “I have been gradually led to doubt the correctness of this division, and to consider it probable that both ought to be regarded as one and the same formation.” He now gives the proportion of recent species found in the Ahuriri formation as being 35 per cent., or, with the same number of species, 12 per cent. more than in 1873.
Early in 1877 I examined the country between Masterton and Napier; and in reporting on the geology of this district, I divided the tertiary rocks as I had previously done in 1875, referring the beds overlying the Te Aute limestone to the pliocene period, and the limestones W. and N.W. of the Ahuriri Plain, and in Scinde Island, to the upper part of this higher series; and later, in August, 1878, I pointed out that these were unconformable to the Te Aute limestones. This had already been indicated by Dr. Hector as their probable relation, in his Progress Report for 1876–77.
On the 14th of January, 1885, there was read before the Geological Society of London “A Sketch of the Geology of New Zealand,” by Captain Hutton. In this the author states that the grouping of the tertiary rocks is founded on that given in a former
communication to that Society, which was the same as the synopsis of the younger formations of New Zealand, published in the “Reports of the Geological Survey” for the year 1871–72. Captain Hutton, however, adds that the new classification includes modifications subsequently made. These, however, cannot affect the chronological arrangement of the different series or groups of strata, without at once destroying all semblance which the latter might have to the former classification; and we are compelled to take the different series included under the Pareora system as a chronological arrangement, and in the order in which they are stated. Those series, in descending order, are:—
1. Awatere series.
2. Kanieri series.
3. Tawhiti series.
4. Ahuriri series.
5. Waitemata series; and
6. The brown coals of the Pomahaka, etc.
The Awatere and Kanieri series, or groups, formerly constituted the Pareora formation, the Ahuriri formation being the next underlying. Now, however, we have between these the Tawhiti series; and it is manifest that Captain Hutton has abandoned the idea that the Ahuriri and Pareora formations are the same. If it is otherwise, he makes no distinction (stratigraphical or palæontological) between the Scinde Island limestones and the rocks forming the Taipos, on the east coast of Wellington, or the brown coal beds of the Pomahaka, in Otago; all the divisions being referred, not to the relative parts of a system of rock-formations, but to a single series, having strict equivalents in all the localities where rocks belonging to the Pareora system are present.
On the 2nd July last, Captain Hutton read, before the Philosophical Institute of Canterbury, a paper on the “Geology of Scinde Island,” in which, for the first time, he describes the limestones present in Scinde Island, the lower of which he refers to the Ahuriri series of his last classification, and the upper to the Wanganui system and Petane series of the same. He says that the upper limestone, with the accompanying underlying sandy beds, is unconformable to the lower limestones, and shows them highly so in the section which accompanies his paper. It is further said that the lower limestone is the equivalent of the Te Aute limestone, which is also stated to be the equivalent of the Pohui limestone of Te Waka. Of the 24 species of fossil shells collected from the lower or Ahuriri limestone, 15, or 61 per cent., are noted as recent species; and we must remember that the original Hawke's Bay group was supposed to contain no more than 20 per cent. of recent shells.
During the 5th, 6th, and 7th September last I examined Scinde Island, and agree with Captain Hutton that there is an upper and a lower limestone in Scinde Island, but saw no reason to suppose that these were unconformable to each other. To me, the evidence was quite clear that the lower limestones and overlying sands are connected by passage beds, and shade into one another. I further found that, not the northern, but the western side of Scinde Island showed the presence of the younger series; and I could not arrive at the conclusion that the lower beds are the equivalents of the Te Aute limestone, nor of any formation containing no more than 35 per cent. of recent species. The upper beds, I admit, resemble the shell limestones of the mainland to the W. and N.W. of the Ahuriri Plain, but I was forced to the conclusion that either the upper limestones are not the same as those on the mainland already mentioned, or that the lower limestone was not the Te Aute limestone, and in all these conclusions differ from the opinions of Captain Hutton.
To try to solve the various problems thus requiring to be considered, I went to Petane, and thence by coach to the Mohaka Valley, spending two days to the west and north of the river-crossing, and the other available day on the Te Waka Range; the sequence to the eastward I but partly observed. I traced the tertiary sequence, as here represented, to its base in the Kiwi Range, and further to the north along the Taupo Road. I found strata rich in fossils in this direction, on the north-western side of this part of the Mohaka Valley, and was able thus to refer nearly a thousand feet of strata to the Pareora series of the Geological Survey classification.
The fossils of this part of the tertiary sequence are abundant in the Mohaka river-bed, near the bridge and crossing of the Taupo Road; but I did not content myself with these, but sought out the fossiliferous beds in section.
These lower beds are characteristic, and not difficult to be distinguished from those that over-lie on the south-east side of the valley. They are brown, green, or grey sands, or fine grit, with concretions or beds of harder and more calcareous material full of shells. In their upper part, the brown sands alternate with lighter-coloured sandy clays. They dip a little to the S. of E. at moderate angles, 20° to 25°. Eastward of the Mohaka these are followed conformably by a great thickness, more than 1,000 feet, of light-grey sandy beds of a more argillaceous type than the last brown sand bed appearing in the Mohaka east bank, at the crossing. This series is closed by a bed of brown sand of considerable thickness, which shows on the western brow of the Titiokura saddle, by which the Napier-Taupo Road reaches the Mohaka. These appear to be the gritty sandstones, “No. 9,” of Mr. Percy Smith's map, and the “grey and brown sandstones” of Captain Hutton. Fossils are rare, and I collected
none; but Captain Hutton mentions four species occurring in them, only one of which, Struthiolaria tuberculata, I found in the lower beds. I would refer these rocks to the Awatere series of the Geological Survey. They are conformably overlaid by the Pohui limestone. This is a thick band of coarse, shelly limestone, often loose calcareous sand, with harder bands at irregular distances, and not continuous, at least where the Taupo Road crosses it: Further to the S.W., on the Te Waka Range, it is perhaps 100 feet thick; and further to the S.W. appears to be much thicker. It abounds with fossil shells in the lower part; and in the upper part, not far from the Taupo Road, it is full of small corals. Further to the S.W., and S.E. of its disappearance on the road-line before reaching Pohui, this lower limestone is followed by argillaceous sands of about the same thickness as those seen between the two limestones in Scinde Island, and these are followed by a second limestone, as in Scinde Island. It is this upper limestone that forms the peculiar cubical, castellated feature of Te Waka itself, the inferior limestone forming the western scarp of the range, and separated from the upper as already stated.
The lower limestone, however, shows in the scarp, running nearly N. and S. at the back of Pohui, and finally disappears at Pohui Lake. The upper limestone, yet separated by the argillaceous sands spoken of, disappears half a mile to the S.E. at the first bridge on the road to Napier. This represents the section in Scinde Island; that is, the succession and character of the rocks are the same. The fossils have yet to be exhaustively collected before this can be finally determined.
The fossils I obtained are chiefly the larger Pectens found in the lower limestone in Scinde Island. They were specially sought for, as I was under the impression these would determine the age of the beds; but Voluta pacifica, Pinna neozealanica, Pecten radiatus, Modiola areolata, and Waldheimia lenticularis, were also collected, shells yet living, and not found lower than the limestone in Scinde Island. It further seemed to me that had an exhaustive collection been made, it would have been characterised by a very large percentage of recent species. It would, however, be unfair to add these recent species as occurring in the lower of the Scinde Island beds, and thus raise yet higher the percentage of recent species found in that limestone. This is already sufficiently high; but there seems some reason to suppose that, contrary to what Captain Hutton says is probable, further research will add to and not diminish this percentage; and I believe that two of the species mentioned do occur in the lower beds in Scinde Island.
I have not time, nor is it my present purpose, to discuss the probabilities of an unconformity in this line of section east of Pohui. I could not determine any such to be present. I
observed that the next rocks seen on the road-line, to the S.E. of the disappearance of the Upper Pohui limestone, were light grey sandy beds, very much resembling those seen to the N.W. of the Lower Pohui limestone on the fall from the Saddle into the Mohaka Valley, and these might be brought into this position by a fault or unconformity; but, were this so, the overlying brown sands and conglomerate would yet have to show evidence of unconformity, and I could discover none.
Further to the E. and S.E. the section has already been described by previous observers, and I need not here detail it.
Grey and brown sands and coarse sandstone conglomerates, pupa rock, and tufaceous sands, form a great series of strata before reaching the overlying shelly-limestones of Petane and the coast range to the N.E. of the Lower Esk. Between Pohui Lake and the coast there may be 2,000, 3,000, or even 4,000 feet of strata; its exact measure is not at present of importance, it being admitted on all hands that collectively there is a great thickness of strata, amounting to some thousands of feet. This, in some way, we have to consider represented in Scinde Island, and by not more than some 120 to 150 feet of strata. This is possible, but, considering the distance between the Esk Valley and Napier, barely probable.
Next we have to consider that the section from Puketapu, on the Tutaekuri River, back to the S.W. continuation of the Pohui limestone, shows no diminished thickness of the beds overlying the latter and underlying the Petane limestone, rendering it yet less probable that this great series can be represented a few miles off by so small a thickness as that of their supposed representatives in Scinde Island. Farther to the S.W., along the Ngaruroro River, from the limestone hills on the western border of the Ahuriri Plain to the lower end of the Ngaruroro Gorge, a yet greater thickness of these beds is developed; and let any one look from the offing in Hawke's Bay at the immense development of conglomerates, sands, and clays, that between Cape Kidnappers and the mouth of the Tukituki are present, and then consider that these must be fully represented in Scinde Island–if we are to regard the upper shelly limestones there the same as that found on the mainland at Petane; or, as an alternative, the lower limestones the same as the Te Aute limestone. And, in spite of liberal allowance in the way of thickening and thinning of the strata, the reasonable probabilities of the case will be, with most observers, that either the Petane limestones are not present, or, that the Te Aute limestones are absent. There is, however, a third possibility: but this has never yet suggested itself to any observer of the geology of the district, and I dare say will not now be entertained. This is: there may be a double unconformity in Scinde Island. Firstly, between the lower or supposed Te Aute limestones of Hutton; and, secondly,
between the Petane sands and the overlying shelly limestones, thus admitting of the reduction by denudation of the intervening beds down to the meagre thickness which they now present.
What may be the final conclusions respecting this stratigraphical difficulty I am not prepared to hazard an opinion. Meanwhile, I do not consider the upper miocene Te Aute limestone present in Scinde Island. The palæontological evidence brought forward by Captain Hutton is against this, and the evidence, as collections are added to, is likely to be strengthened rather than weakened; that is, if the Pohui limestone be the same as the lower limestone in Scinde Island, which it is asserted to be.
One difficulty in the way of regarding these rocks as of pliocene age has been the number and remarkable size of the extinct forms of Pecten found in them, which are not supposed to occur in the upper shelly limestones of admittedly pliocene age, and which occur also abundantly in the Te Aute limestone. This is by no means an insuperable difficulty, and we have only to consider them as exceptional, and in reality belonging to an older period. Looked at in that light, they would have to be excluded in arriving at the age of the beds, as determined by the percentage of living species; and were this done, all doubt of the pliocene age of the beds would be removed. There would then be 71 per cent. of recent species found in the beds. Were these Pectens retained, and the five recent species found at Pohui added to the 15 occurring in Scinde Island, we should have a like result—viz., nearly 70 per cent. of recent species from the limestones of this horizon.
From the Lower Wairarapa Valley, N.E., to the northern part of Hawke's Bay Provincial District, the Te Aute limestones everywhere close the middle tertiary sequence, as seen in this part of the North Island. The Pareora formation of Hutton, characterised by a proportion of recent species equal to 37 per cent., should, one would think, underlie the Te Aute limestones, at least ought to underlie its supposed representative in Scinde Island, with 61 or 70 per cent. of its species recent. And yet, if we accept Captain Hutton's latest classification, we are required to suppose that the Pareora beds, containing little more than half the number of living species, are actually the older series. The Te Aute limestone cannot be made to occupy this position relative to the Pareora series, without setting aside all the evidence obtainable, both palæontological and stratigraphical; but if regarded as the highest member of that series, this would be more in accord with what is known as to its actual position. It may be the lowest member of the young tertiary sequence; more probably, along the East Coast of the North Island it closes the middle tertiary series.