Art. XLVI.—On the Conformable Relations of the different Members of the Waitemata Series.
[Read before the Auckland Institute, 14th October, 1889.]
The beds forming the cliffs on the shores of the Waitemata generally occupy a more or less horizontal position, but at a few places they have been subjected to great local disturbances, often resulting in sharp contortions and faulting. Strangely enough, these disturbances have in many instances occurred at what may be termed critical points from a geological standpoint, and have thus caused obscurities which have led to much discussion and controversy, not only as to the age of the beds themselves, but also as to the relations which the different beds bear to each other.
The covering of Pleistocene lavas and scoria on the isthmus, and the numerous small bays or inlets which diversify the shores of the harbour, have rendered it difficult, if not quite impossible, to trace particular beds from place to place, while the absence of well-marked fossiliferous horizons has always been an obstacle to the safe correlation of distant beds of the series.
The Waitematas, with perhaps the exception of the Parnell grit or ash-bed, contain no beds possessing mineral characters sufficient to constitute chronological landmarks in the geological succession that would enable the field-work to confidently affirm that certain beds in one locality were the horizontal equivalents of other distant beds—as, for example, that the Fort Britomart beds were the same as the beds at the head of Hobson's Bay, or the North Shore beds the same as the beds north of Whangaparaoa Peninsula. The whole series consists chiefly of frequent alternations of blue clays and soft sandstones, sometimes succeeding each other rapidly as thin-bedded strata, sometimes as heavy bands varying from 2ft. to 10ft. in thickness.
The Fort Britomart beds, which may be taken as characteristic of the greater part of the series, consist of thin layers of blue crumbling clays alternating with layers of soft brownish-coloured sandstones. The clays vary from a few inches to a foot in thickness, and the sandstones from a few inches to 3ft. Fragments of carbonized wood, often laid in continuous layers, are not uncommon in the sandstone bands; and it is a noticeable feature that where the coaly matter is
abundant the adjoining beds are covered with an efflorescence of sulphur.
About 10ft. from the foot of the cliff there is a very characteristic band of gritty sandstone, speckled pretty evenly with small fragments of grey clay. This, or a similar bed, is seen at the top of the cliffs leading round the point from Mechanics' Bay to St. George's Bay, and again near the top of the cliffs behind the Calliope Dock.
The character of the sediments, together with their fossil contents, leave little room for doubt that the beds composing the Waitemata series are mostly of estuarine origin; while the marine forms in the Motutapu beds and Parnell grit, and in the calcareous cornstones at Orakei Bay, St. George's Bay, and Onehunga, point to several periods during which the conditions of deposition were truly marine.
Around Auckland there is no stratigraphical evidence to fix the age of this series. On the isthmus, and at the North Shore, they are overlain unconformably by stratified tuffs, solid lava-flows, and scoria-beds of probably Post-pliocene age; while at Motutapu Island they rest on, or, rather, lap on to, a highly-denuded rocky surface of the basement-rock of the district, of Palæozoic age, consisting of indurated sandstones and slaty shales.
The late Dr. von Hochstetter, in his lecture to the Auckland Institute, 24th June, 1859, speaking of these beds, says, “The horizontal beds of sandstone and marls which form the cliffs of the Waitemata, and extend in a northerly direction to Kawau, belong to a newer Tertiary formation, and, instead of coal, have only thin layers of lignite. A characteristic feature of this Auckland Tertiary formation is the existence of beds of volcanic ashes, which are here and there interstratified with the ordinary Tertiary layers.”* Subsequently he places the Waitemata series with the Aotea series, both being considered as Older Miocene.†
Professor Hutton, in a paper read before the Auckland Institute in 1870, showed that this series could be traced eastward beyond Tamaki and Howick to Turanga Creek, where, he says, “it rests unconformably on a dark-green or bluish sandstone, generally showing a concretionary structure.” ‡
In 1879 Mr. S. H. Cox, late Assistant Geologist, examined the country from Auckland northward to Cape Rodney and the Kaipara. During this survey I accompanied him as his assistant, and at Komiti Peninsula, opposite Batley, we made
[Footnote] * “Geology of New Zealand,” Hochstetter, 1864, p. 26.
[Footnote] † “Reise der ‘Novara:’ Geology,” i., p. 34.
[Footnote] ‡ “Trans. N.Z. Inst.,” vol. xvii., p. 307.
a large collection of marine fossils from certain blue sandy marls and marly greensands which rest on the denuded and upturned edges of the “Chalk-marls.” On examination at the Colonial Museum these fossils were found to include numerous Lower Miocene forms, together with Pecten zittelli, Hutton, Pecten fischeri, Zittel, and many corals and Foraminifera which also occurred in the Orakei Bay beds.*
For this reason Mr. Cox in his report correlates the Komiti beds with the Orakei Bay beds, which he thought should now be regarded as Lower Miocene. Sir James Hector, in his progress report for the same year, dissents from this view, and considers that the Waitematas should be divided at the Parnell grit, all the beds below this horizon, including the Orakei Bay beds, being still retained as belonging to the Grey Marls series of the Cretaceo-tertiary formation.
In 1880, and again in 1881, Mr. Cox re-examined this point, and on both occasions reported that he was fully convinced of the correctness of his former work, although he now considered it possible that the Waitemata series might be of Eocene age.† On the latter occasion he followed these beds eastward to the Maraetai Range, and at Turanga Creek found them resting unconformably, as he thought, upon a concretionary tufaeeous sandstone, thus agreeing with Professor Hutton, who had previously examined and described this line of section.‡
In October, 1883, Mr. A. McKay, F.G.S., Assistant Geologist, examined the Orakei Bay section, and the coastline from Lake Takapuna northward to Wade. In his report he considers the Fort Britomart beds the horizontal equivalents of the Orakei Bay beds, and in his section showing the general structure of the country from Wade to Auckland he shows the Parnell grit and associated beds resting highly unconformably on the Orakei Bay beds at Hobson's Bay, and on the hydraulic limestone at Wade.§
He also correlates the Takapuna breccia with the Parnell grit and the Cape Rodney slaty breccia—with the former on stratigraphical, and the latter on palæontological grounds; and concludes his report by stating that he supports Dr. Hector's conclusion that the Waitematas should be divided at the Parnell grit, the lithological characters of which he thinks must mark an unconformity or stratigraphical break.
Professor Hutton, on the 27th November, 1884, read a paper before the Philosophical Institute of Canterbury “On the Age of the Orakei Bay Beds, near Auckland,” in which he
[Footnote] * “Geological Reports,” 1879–80, p. 37.
[Footnote] † “Geological Reports,” 1881, pp. 28 and 95.
[Footnote] ‡ “Trans. N.Z. Inst.,” vol. iii., p. 244.
[Footnote] § “Geological Reports,” 1883–84, p. 103.
reviews the whole history of the Waiternata beds from the time of Hochstetter up to that date. He criticizes Mr. McKay's work at some length, and disposes of the supposed unconformity at the Parnell grit. He also shows that no evidence had been adduced to prove that the Parnell grit was superior to the Orakei Bay beds, or that the latter were the horizontal equivalents of the Fort Britomart beds.*
In the beginning of 1885 I was directed to revise Mr. Cox's work around Auckland and at the Kaipara, and commenced by making a detailed examination of the fine natural section exposed on the coast from Auckland to the Maraetai Range. Previous to this date fossils had only been found at Orakei Bay and Lake Takapuna; but during the progress of this survey I succeeded in discovering eleven new localities, where exhaustive collections were made, and from the facts disclosed by these, and a large amount of new stratigraphical evidence, I arrived at the following conclusions:—
That the Manukau volcanic breccias and tuffs are unconformable to the Waitemata series.
That the Parnell grits are under the Fort Britomart beds.
That the Parnell grits are the equivalents of the Takapuna breccias and grits.
That the Orakei Bay beds are equivalent to the Turanga greensands.
That the Turanga greensands are equivalent to the concretionary sandstone at Tamahua, near Cape Rodney.
That there is a direct sequence from the Fort Britomart beds to the Papakura limestone.
That the Papakura limestone is the base of the Waitemata series, and equivalent to the Cape Rodney calcareous slate-grit.
That the Waitemata series is unconformable to the chalk-marls and coal-measures.†
I will briefly refer to the above points in detail.
With regard to the Manukau breccias, I am now inclined to think that I was wrong in separating them from the Waitemata series, my work at Komiti Point in 1886 tending to show that they originated during submarine volcanic outbursts of an intense character, some time during the deposition of the Orakei Bay beds, most probably at the horizon of the Parnell grit and Takapuna ash-bed. At Komiti Peninsula, and further north, on the Wairoa, marine beds, containing the characteristic fossils of the Orakei Bay horizon, are interbedded with heavy deposits of volcanic breccias, tufas, and agglo-
[Footnote] * “Trans. N.Z. Inst.,” vol. xvii., p. 313.
[Footnote] † “Geological Reports,” 1885, p. 158.
merates, and occasionally sheets of solid lava, consisting of dolerites rich in olivine, hornblende, and augite-andesites. These can be traced southward to the Hoteo and Kaukapakapa, and an examination of the bush country south of the latter will probably show that they are connected with the breccias of the Waitakerei Range and Manukau Heads. The fossils in the Komiti beds indicate a range from the Awamoa series to the Grey Marls series,* which would thus fix the period of volcanic activity in the Kaipara district somewhere about the horizon of the Hutchinson Quarry beds at Oamaru, the deposition of which was attended by violent volcanic displays.
In his paper on the age of the Orakei Bay beds† Professor Hutton states that the Parnell grit either passes below the Fort Britomart beds or thins out in that direction. In 1885 I showed in the report already referred to that at dead low water it can be seen distinctly passing below the Mechanics' Bay beds, and, as its inferior position has now been admitted, nothing more remains to be said except to point out that if the Fort Britomart beds are the equivalents of the Orakei Bay beds, as maintained by Mr. McKay, then it follows that the Parnell grit must also underlie the latter, as it has been proved to pass below the former.
With regard to this point, I shall have something to say later on.
In 1885 I obtained sufficient evidence at Howick and Turanga Creek to clearly demonstrate that there was complete conformity between the Waitematas and concretionary greensand. I showed that the stratigraphical break contended for by Professor Hutton and Mr. Cox at Turanga Creek was only apparent, resulting from the peculiar contours produced by the unequal erosion of hard and soft strata, this view being supported by the discovery of Pecten zittelli and Pecten fischeri and other Orakei Bay fossils in the concretionary sandstone.‡
No fresh evidence has yet been discovered to lead me in any way to modify this conclusion.
This point will be dealt with later on.
With regard to this, it should be observed that on the slopes of the Maraetai Range the Papakura limestone rests hard upon the old rocks. It does not, however, by any means follow that conformably lower beds may not be developed towards Hunua and Papakura, and I believe it quite possible that detailed survey in that direction may show that the
[Footnote] * “Geological Reports,” 1886–87, p. 228.
[Footnote] † “Trans. N.Z. Inst.,” vol. xvii., p. 311.
[Footnote] ‡ Geological Reports, 1885–86, p. 151.
Papakura beds rest conformably on the brown-coal measures.
The unconformity between the Waitematas and the hydraulic limestone at the Wade has been pointed out both by Mr. McKay and myself.*
In 1885 I contended for an unconformity between the Waitematas and the brown-coal measures, not because I had any stratigraphical evidence to prove this, but only on account of the conformable relations supposed by the Geological Survey to exist between the brown-coal measures and the chalkmarls, which I had shown to be unconformably overlain by the Waitematas. The unconformity I placed at this point was only a conditional one.
In his progress report for 1885 Sir James Hector dissents from my view that the Parnell grit is the horizontal equivalent of the Takapuna breccia, which he considers of Pliocene age and quite unconformable to the Waitemata series.† In support of this he shows two views of a section on the coast north of Lake Takapuna, which he believes favours his unconformity. It should, however, be pointed out that wherever the strata occupy a horizontal or undulating position the breccia is seen to be interbedded with and quite conformable to the adjacent beds, and at its base is frequently more or less false-bedded with the underlying clays and sandstones. On the other hand, at points of severe local disturbance, where the breccia is present, the softer and more yielding clays and soft sandstones have in many instances been crushed and contorted and often turned over the more compact, heavy, and unyielding ash-bed, thus giving rise to apparent unconformity.
Sir James advances a rather curious theory for the origin of this fossiliferous breccia. He supposes that it accumulated in great pot-holes during volcanic eruptions in Pliocene times. It would be interesting to know what formed these holes, also what geological agencies were in operation at that time to limit the distribution of the breccia and its imbedded life to these repositories.
As a matter of fact the breccia has a linear extension of many miles, extending from the Waitemata to Whangaparaoa Peninsula, and varying in thickness from 10ft. to 20ft. As to its age, it contains Terebratella dorsata, Pecten polymorphoides, Celleporina, and many net and branching corals and cupshaped Bryozoa, which led Mr. McKay to correlate it with the slaty breccia at Cape Rodney, which has always been considered as Lower Miocene.‡
[Footnote] * “Geological Reports,” 1883–84, p. 103; 1885–86, p. 162.
[Footnote] † “Geological Reports,” 1885–86, p. xxxviii.
[Footnote] ‡ “Geological Reports,” 1883–84, p. 101.
In January of 1888 Mr. McKay was directed to examine the principal sections around Auckland and at Howick, on which I had based my conclusions in 1885. He agrees with me on all the main stratigraphic points, and now admits that the Parnell grit passes below the Fort Britomart beds.* He is also agreeable to my correlation of the Fort Britomart and Calliope Dock beds, the latter of which he thinks must also be underlain by the Parnell grit.
Speaking of the Cheltenham Beach breccia, he says it dips to the eastward, and must therefore pass over and be superior to the Calliope Dock beds, which dip N.E. or N. at a very low angle; and in consequence of this he renounces his former opinion that the Parnell grit is the southern extension of the Cheltenham breccia, his reason for adopting this new view being based on his admission that the Parnell grit passes below the Fort Britomart. In his own words, he states, “As a consequence of my admission that the Parnell grit does or should pass under the Fort Britomart and Calliope Dock beds, and of the observed fact that the breccias north of Cheltenham Beach overlie there, I am forced to agree with Sir James Hector that the Parnell grit and the Cheltenham Beach and Takapuna breccias do not occupy the same horizon, and that the Parnell grit is the older deposit.”†
On the 29th August last I examined the line of section between Cheltenham Beach and the Calliope Dock, believing that the stratigraphical evidence relied on by Mr. McKay was not sufficient to prove that the fossiliferous breccia at the former place passed over the almost horizontal strata at the latter.
At Cheltenham Beach the dip of the strata forming the cliffs at the north end of the sandy beach is to the east or seaward at an angle of 35°, as ascertained by myself in 1885, and subsequently by Mr. McKay in 1888. The cross-section of the end of the cliff is illustrated by the annexed sketch (Plate XXX., fig. 1), where—
is blue clays;
bed of coarse grit, showing false bedding at base;
bed of brown sandstone speckled with white grits (pass insensibly upward into the overlying bed);
greenish tufaceous grits;
soft rusty-coloured sandstone, probably the upper decomposed surface of 4.
Passing northward from the end of the cliffs, the grit-beds (Cheltenham Beach fossiliferous breccia) run parallel with the shore a distance of 10 or 12 chains, beyond which they are
[Footnote] * “Geological Reports,” 1888, p. 40.
[Footnote] † Ibid.
followed by a great thickness of banded clays, which form the headland here, where the dip changes to westward. The section along line C-D (Plate XXX., fig. 2) shows the arrangement of the beds at the point as follows:—
1, false-bedded sandstones, blue clays, and grits;
2, tufaceous grit (fossiliferous);
3, banded clays and ferruginous sandstones, alternating as thin layers.
Proceeding along the beach from C (the west end of the section), the dip rapidly increases from 35° to 45°, and then to 60°, and at the extreme point the direction of the dip changes from E. to W. at angles between 45° and 50° (Plate XXX., fig. 3);
1 being blue clays;
2, grits and tufaceous sandstones;
3, banded sandstones and clays;
H, Cheltenham Beach.
At the Calliope Dock the strata are lying almost horizontal, or dip at an angle of 2° or 3° to the northward. From the top of the cliffs behind the dock the surface of the ground slopes rapidly to the northward, and at the road-line to Lake Takapuna is only a foot or two above high-water mark. From this point towards Cheltenham Beach there is a wide stretch of low swamp-land where it is impossible to make strati-graphical observations of any kind whatever. But, besides this break, it is necessary to point out that the foot of Mount Victoria lies almost, if not right, in the line of section, and I think it more than probable that the easterly dip of the beds at Cheltenham Beach is due to the eruption of the lavas and ejecta which compose that hill.
As there seemed to be little prospect of obtaining satisfactory evidence in this direction, the next day I made a close examination of the Parnell grit at Judge's Bay and Parnell Point, and at the former was fortunate enough to discover a number of marine fossils near the base of the cliff, at the end of the long flat reef which extends from the east side of the bay far into the harbour in the direction of the North Head. They occur mostly in the lower 2ft. of the grit, which is generally more or less calcareous; but they are by no means plentiful, and from their dirty-yellowish colour, which closely resembles that of the matrix, they can rarely be seen without close inspection of the weathered surfaces. They are mostly testiferous, but brittle and fragmentary, and good specimens are scarce.
Among the forms collected were a Cerithium, Pecten polymorphoides, a Teredo, several small corals, and fragments of a
long thin shell which at first I thought might be Scalpellum aucklandicum, Hector, of which I made a large collection at Motutapu Island in 1887; but better examples will have to be obtained before this can be definitely determined.
The Cerithium, Pecten, and corals are the same as those found in the breccia at Cheltenham Beach, thus proving conclusively that the Parnell grit is the southern extension of that stratum, deposited at the same time and under the same geological conditions. It also shows that the Calliope Dock beds, if related to those at Fort Britomart, must be superior to the breccia at Cheltenham Beach and Lake Takapuna.
As bearing upon the relation of the Parnell grit to the Orakei Bay beds, I may mention that during my last visit to St. George's Bay I found a number of Orakei fossils in the flat, irregular, calcareous gritty cornstones at the foot of the cliff on the west side of the bay. These cornstones are only exposed at low water, and occupy a position some 15ft. or 20ft. above the Parnell grit. Their exact position is shown in Plate XXX., fig. 4:—
A. St. George's Bay;
B. Parnell Point;
C. Mechanics' Point;
D. Fossiliferous cornstones;
Mechanics' Point beds;
Layer of cornstones;
The fossils collected at point marked D in Plate XXX., fig. 4, were Pecten fischeri, Vaginella, Orbitolites, and a number of small corals.
The occurrence of Orakei Bay fossils in this position would tend to show that the Parnell grit is inferior to the Orakei Bay beds; but, if the evidence is not sufficient to prove this, it shows that these two horizons are at least not far separated from each other.
At the east side of Judge's Bay the Parnell grit rests on an irregular thin layer of blue clay containing minute Foraminifera. An examination of these might throw some light on this point.