Art. LVII.—Geology of Scinde Island, and the Relation of the Napier Limestones to others in the surrounding District.
[Read before the Hawke's Bay Philosophical Institute, 11th October, 1886.]
In the last volume of the “Transactions,” two very interesting and, so far as they relate to the geology of this district, two important papers, appear on the geological structure of the Napier hills. One of the papers is by Captain F. W. Hutton, F.G.S., Professor of Geology at the Canterbury College, and it is headed “On the Geology of Scinde Island.” The other paper is by Mr. A. McKay, of the Government Geological Department, and bears the title “On the Geology of the Napier Limestones.”
Napier, or, more properly, that portion of it which is known as Scinde Island, has formed for years past a kind of battleground for the geologists; and, if we may judge from the two papers referred to, it is likely to remain so for some time to come. The questions to be decided are: 1st, As to the age and conformity of the Napier limestones; and, 2nd, As to the relation they bear to the other limestones in the surrounding district.
I cannot do better than state in their own words the conclusions arrived at by the authors of the above-named papers, after paying special visits to this district to prosecute their inquiries.
Captain Hutton says (“Transactions,” vol. xviii., p. 329): “The result of my examination is to show that the northern end of the island is formed by the Petane series. This series rests unconformably on the Scinde Island limestone, which forms, with the underlying sandstone, all the southern part of the island.” On page 371 of the same volume, Mr. McKay, after an examination of the Napier beds extending over three days, concludes that “there is an upper and a lower limestone in Scinde Island,” but he sees no reason to suppose that these are unconformable to each other. “To me,” continues Mr. McKay, “the evidence was quite clear that the lower limestones and overlying sands are connected by passage-beds, and shade into one another;” and, further, “that not the northern, but the western side of Scinde Island shows the presence of the younger series.” Nor could Mr. McKay” arrive at the conclusion that the lower beds [of Scinde Island] are the equivalents of the Te Aute limestones, nor of any formation containing no more than 35 per cent of recent species.” I am informed that Dr. Hector agrees entirely with the conclusions arrived at by Mr. McKay, as here quoted.
It is useless to point out how entirely different are the opinions of these geological experts, and it seems to me that this Society, or at least those members who take an interest in geology, should endeavour to clear up the points of difference as soon as possible.
It is a curious circumstance that each geologist who has written about Scinde Island differs as to the dip of the beds. Mr. Cox* says: “At Scinde Island, Napier, where the typical development of these beds occurs, they are forming a low anticline, dipping on the sea face S. 10°, but on the harbour side N.W. 25°.” Mr. McKay† says: “These marls form the west side of the Napier Harbour…the lowest beds exposed on the south-west side of the island …they dip N.E., bringing the limestones to the sea level at the north end of Shakespeare Road.” As Mr. McKay says in his recent paper that there is no unconformability between the upper and lower Napier limestones, and as the marls are certainly not the lowest beds, but are above the lower limestones, I infer that he wishes it to be understood that the general dip of the Napier rocks is to the north-east.
On the other hand, Captain Hutton, in the paper from which I have already quoted,‡ says: “On the south-east side of the island this series [i.e. the Ahuriri series] dips about S.E. 5°. To the northward it gets horizontal, and then dips to the northwest. On the east side, at Curling's Gully, the dip is N.W. 20°, and on the west side, at Taradale Bridge, it is N.N.W. 10°.”
These quotations will serve to show how wide are the differences of opinion between the geologists on a question of fundamental importance, and to me they constitute strong presumptive evidence in favour of unconformability between the Napier series.
The conclusions at which I have arrived with respect to the Napier series are that, exclusive of the comparatively recent surface-deposits of brick and pumiceous clays and sands and ordinary soils, there are three distinct series of rocks forming the Napier hills. These series are unconformable to one another, the lower limestones being succeeded by marls, and the marls by limestones, which in this paper are termed the upper Napier limestones. My reason for arriving at these conclusions will be found in the following evidence:—
In a journey round the base of the Napier hills the following principal alterations in the dip of the beds will be seen:—
Commencing at the junction of Byron Street with Beach Road, there is at this point an important exposure of what I
venture to say are the lowest of the lower characteristic Napier limestones, in bands of a steel-grey colour and interbedded with yellow and grey calcareous sands and breccia. These beds dip N. by W. at an angle of about 5°. At the junction of Coote Road with Beach Road the rocks forming the bold cliffs along the seaward side of the island are seen to dip to the S.E. at an angle varying from 5° to 10°. Thus, at the point where the prisoners from the gaol carry on their work of stone-breaking, a syncline is observable in the lower beds. Further on, along the beach, the rocks of the lower series dip to the N. by W. at a low angle, in no case exceeding 12°. On the Ahuriri side of the island, at the junction of Hospital and Battery Roads, an anticline is formed by the lower limestones, where they are to be seen dipping N.E. and N.W., at varying angles from 10° to 25°. Along the S. and S.S.E. sides of the hills, extending from the recreation-ground to the starting-point on Beach Road, none but the lower limestones are to be seen—overtopped here and there by marls—and these dip to the N.W. at slightly varying angles, but in no case exceeding 15°. At the places known as Battery Point and Pandora Point, on the west side of the hills, the limestones and sands overlying marls are seen dipping W. and S.W. at an angle of 10°; but near to the large exposure of marls, limestones, and sands belonging to the Railway Department, and locally-known as Scandinavian Point, the lower limestones are just exposed, and are seen to dip to the N.W., or N. by W., at a low angle, whilst the upper limestones have a similar dip to those exposed at Pandora Point.
My own opinion is that the general dip of the lower Napier limestones is N.W., at angles varying from 5° to 25°, and that the oldest rocks exposed in the Napier hills are those seen between the Napier public school and the quarry at the junction of Byron Street and the Marine Parade.
1st. Now, as to unconformability or otherwise of the Napier series.
Captain Hutton says: “The upper limestones in Scinde Island are unconformable to the lower;” whilst Mr. McKay says “there is no unconformability between the upper and lower limestones.” After a detailed examination of the numerous exposures to be seen on and around the island, I agree with Captain Hutton as to unconformability between the limestones; but I am prepared to go a little further by stating that there is unconformability between the lower limestones and the marls which rest upon them, except where denudation has taken place, and between the marls and the upper limestones.
My reasons for holding this opinion are to be found in the following evidence:—
Along the east side of the island, extending from Beach Road on the south to Lyndon's corner, at the Ahuriri end of
Shakespeare Road on the north, all the principal exposures of the Napier series are to be found. I have already pointed out the existence of a syncline at the junction of Coote Road with Beach Road in connection with the lower limestones. If these limestones are followed along the ocean side from Coote Road in a northerly direction, a marlbed will be seen to make its appearance about half-way between Coote Road and what is locally known as the First Bluff. This marl is exposed about 100 feet above high water-mark, and where first seen is only a few feet in thickness. It is readily distinguished from the overlying beds and from the limestones by its yellowish straw-colour. A little further on the marl thickens out rapidly, but at the point the marl seemingly disappears, and the limestones are overlaid by the reddish-coloured pumiceous clay sands—the loëss, or brickearth, of Hutton. A little further to the north the marl again reappears, and at the highest point in the island, immediately above where the breakwater operations are being carried on, the marl is seen to thicken out, in a distance of not more than 120 yards, from about 15 feet to more than 60 feet, and the upper series of Napier limestones make their appearance, resting, as they do, unconformably upon the marls, and being in their turn overlaid by extensive deposits of brick-earth, pumiceous sands, and black soils composed of vegetable matter, volcanic dust, scoria, and pumice grit. Structurally, the upper Napier limestones are quite unlike the lower ones, and, once seen, their peculiar compact and dark shelly structure is readily distinguishable. At the time when the pumiceous clays, sands, and grits were deposited, it would appear that denudation had washed away a large proportion of the upper limestones and the underlying marls, and that the lower limestones, equally with the marls and upper limestones, had become surface-rocks.
Between Coote Road on the south-east and Taradale Bridge on the south-west the lower limestones have undergone a large amount of denudation, and in one place only is the marl to be found, this being on the town side from the residence occupied by Dr. Hitchings, and nearly opposite Holt's sawmill. Near the Taradale Bridge there is a large exposure of the marls, and the unconformity between the lower limestones and the marls and between the latter and the youngest beds of the upper (?) limestones is well defined. Near Mr. Glendinning's brickyard the upper or higher marls become somewhat sandy in character, as compared with those seen on the east and north sides of the island, and in one place they are overlaid unconformably by a remarkable bed of pure pumice, dipping to the S.S.E. at an angle of about 40°, and occupying the place of the otherwise denuded fossiliferous sands and craggy limestones. This pumice-bed is the one, I imagine, referred to by Mr. McKay, in one of his reports, as underlying the limestones. An inspection of the
sections a little further to the westward shows that Mr. McKay was in error in this surmise. At the quarry used by the Railway Department, at Scandinavian Point, the marl is at least 50 feet thick; and when the exposure is viewed from the second bridge along the Taradale new road, the unconformability between the different series can be readily distinguished. At this point several new overlying beds make their appearance, being similar to the upper beds at what is known as Battery Point, some half-mile further to the north-west in the direction of Ahuriri. I am doubtful, however, as to whether these new beds belong to the same horizon as the upper Napier limestones, as seen at the Bluff Point, or whether they are the representatives of the limestones as seen on the Pukekuri Hill and the other hills lying between Napier and Puketapu. I am inclined to the latter opinion, because behind Mr. Glendinning's brickyard, immediately E. by N. of the craggy limestones containing pebbles, and which are the highest limestone beds at Scandinavian Point, the dark compact shelly limestones are met with, dipping S. by W. at an angle of about 15°. Where the compact limestone is found, the sequence of the beds in ascending order is—
Craggy limestone, with nests of broken and loose shells.
Coarse and impure limestone.
At Scandinavian Point the sequence is:—
Cretaceous sands, with thin beds of coarse nodular sandstone.
Craggy limestones with pebbles.
Fossiliferous sands, with thin chert bands.
At Pandora Point, which is about midway between Scandinavian and Battery Points, the marl appears to be the only exposed rock, but this is true only of the south side of the point. On the north side the craggy limestone is seen to rest unconformably upon the marls, the evidence being quite clear.
At Battery Point the sequence of the rocks exposed is:—
Hard compact limestone.
Brecciated limestone with pebbles.
Black pebble bed, 12 inches thick.
Sand beds (fossiliferous), with thin chert bands.
Here the marls are largely developed, and unconformability clearly exists between the marls and overlying beds. None of the lower limestones are seen at this point. On the Ahuriri or Port side of the island the marls are exposed in one or two places only, one being near the junction of the Battery and Lighthouse Roads, where the anticline appears to which reference has already been made. In several places on the N.E. side of the island, between Curling's Gully and Breakwater Point, the marls are exposed. Near Sturm's Gully they are interbedded with the pale blue sandy clays, similar to the rocks on the western side of the Napier harbour. From their position in the cliffs, I have been unable to obtain good sections at this point, but I hope to do so shortly, Mr. Goodall, C.E., the harbour board's engineer, having promised to render me some assistance in this matter.
Summarizing the foregoing, it appears to me that the lower Napier limestones, if denuded of the marls, upper limestones, and overlying beds, would resemble a wedge in appearance, having the thicker beds facing S.E. and slanting off in a N.W. direction. Upon the irregular surface of this imaginary inclined plane come the marls, of varying thickness, being somewhat sandy above, earthy below, and having their chief development along the east and west sides of the island. The upper Napier limestones have their chief exposures on the east and west. They dip to the south-west, and near Mr. Glendinning's these limestones must be at least 100 feet thick. On the denuded surfaces of the three series come the pumiceous clays, with grits, pumice sands, brick earth, and black soils, which are to be found more or less over the island, and which, I am inclined to think, will be found the Napier equivalents of the Redcliffe and Kidnapper pumice and conglomerate beds.
2nd. As to the relation of the Napier limestones with those of the surrounding district:
With a single exception, the Napier lower limestones are not represented, as far as I can find, among the rocks to the west and north-west of Napier within a radius of fifteen miles. This exception is to be found in the hills on the west side of the inner harbour and lagoon, having Pukekuri, the hill at the back of Greenmeadows Station, near Taradale, on the southern boundary, and the island known as Quarantine Island as the northern. Considered in connection with the limestones covering the hills between Napier and Tiwhinui Hill, a few miles to the south of Lower Mohaka, these limestones form an important link. Pukekuri is a hill 472 feet high, and consequently 140 feet higher than the highest point on the Napier hills. It is mostly composed of marls similar to those exposed on the saddle at the back of Taradale, on the road to Puketapu. Its summit, however, is covered with limestone similar to the
upper limestones found at Battery Point and Scandinavian Point, containing well-worn pebbles. These limestones do not appear on the old coast hills between Pukekuri and Petane, but they top the hills further to the west as far as Puketapu, and they are seen to overlie the Napier marls which are exposed in a small cutting on the Petane-Puketapu Road, near Alexander's pleasure-gardens. The beds exposed at the place known as Quarantine Island belong to the lower Napier limestones, and are similar to those seen near Mr. Dolbel's brickyard; and it would seem as if Scinde Island were once joined to the mainland in this direction. Between Napier and Lower Mohaka, along what is known as the Napier-Wairoa Road, the whole of the country as far as Waikaari River is covered with limestone. At Tiwhinui (1,289 feet), which is the highest point reached on the Napier-Wairoa Road, the limestones and sands similar to those seen at Battery Point are exposed as the highest beds in the perpendicular cliffs. Underlying them unconformably are light sands and marls similar to the Napier marls, which are here interbedded with the pale blue-clay bands. These are followed by the leda marls (fault?), which rocks, Mr. Cox, in his report upon the country between Poverty Bay and Napier,* places among the cretaceo-tertiaries. The leda marls at Tiwhinui are similar to those that are exposed near the mouth of the Mohaka River, and which are seen dipping S.S.E. at an angle varying from 10° to 20°.
These leda marls form, so it appears to me, the northern bend of a syncline which extends to Patangata, near Kaikora, on the Tukituki River, where the leda marls are seen on the right bank of the river, nearly opposite the hotel, dipping to the N.E. at an angle of about 15°. It is at Tiwhinui, to the north of Napier, and at Patangata to the south, where the limestones are met with resting unconformably upon the lower tertiaries, and it would seem that within this syncline all the limestones, marls, sands, and conglomerates found between Patangata and Tiwhinui must be classed. They rest within the syncline as in a basin, and the Napier limestones occupy almost the central position in the trough of the syncline. The limestones, marls, and sands which are so largely developed on the Tiwhinui, Moaeangiangi, Arapanui, and Tongoio Hills, to the north of Napier, undoubtedly belong to the Napier upper limestones only, as seen at Battery Point and Scandinavian Point. There is no trace whatever of the lower Napier limestones north of Tongoio; but on a small rise about midway between the Maori pahs at Petane and Tongoio traces of the lower Napier limestones are seen, overlaid by marls, followed by a conglomerate bed.
Between Napier and Patangata, viâ Havelock, through what
[Footnote] * “Geol. Report,” 1874–76, p. 97.
is known as the Middle Track, the marls similar to those at Napier are to be met with among the higher rocks only. The lower rocks are classed by Mr. McKay as belonging to the Te Aute limestones. If such is the case, I venture to disagree with Mr. McKay in his conclusions that “the Napier lower limestones are not the equivalents of the Te Aute limestones.” There is no doubt in my own mind that the limestones behind Havelock correspond stratigraphically with the Napier lower limestones; and I believe that palæontological evidence will shortly be forthcoming to prove the correctness of this statement.
I have purposely omitted all reference to the fossils collected in the different beds to which reference has been made, my aim having been to show, as far as I could, (1) that the Scinde Island rocks are made up of three distinct series, which are unconformable to one other; and (2) that the upper Napier limestones are related to the limestones to the N. and N.W. of Napier, whilst the lower limestones have their equivalents in what have been termed the Te Aute limestones.
Description of plate XXVII.
Fig. 1. Ideal section of Scinde Island, from east by north to west by south:—a. Pandora Point.—1. Lower limestones. 2. Marls and clays. b. Breakwater Bluff.—3. Upper Napier limestones. 4. Pumiceous clays and sands.
Fig. 2. Scandinavian Point.—1. Pumiceous sands and clays. 2. Brecciated limestones. 3. Calcareous sands. 4. Limestone and pebbles. 5. Sands and marls. 6. Compact limestone. 7. Marls and sands (blue paper).
Fig. 3. Breakwater Point.—1. Pumiceous sands and clays. 2. Compact limestone. 3. Marls. 4. Lower Napier limestones. 5. Fault (downthrow). 6. Blue sands (fossiliferous).
Fig. 4. Junction of Byron Street with Marine Parade.—a. Lowest exposed Napier beds, dipping N.N.W. b. (see description fig. 5.)
Fig. 5. Junction of Marine Parade with Coote Road.—b. Showing syncline; c. marls; d. clays and pumiceous sands.
Fig. 6. Battery Point, West of Scinde Island.—1 and 2. Pumiceous clays and sands. 3. Fossiliferous sands (calcareous). 4. Limestone (brecciated). 5. Calcareous sands with nodular chest-band (fossiliferous). 5′. Black pebble bed. 6. Calcareous sands (fossiliferous). 7. Limestone.—compact, similar to 2, Breakwater Point, Fig. 3. 8. Marls and sands.