Art. XXXV.—The Conglomerate at Albany, Lucas Creek, Waitemata Harbour.
[Read before the Auckland Institute, 22nd December, 1919; received by Editor, 31st December, 1919; issued separately, 16th July, 1920.]
About 200 yards up-stream from the lower wharf at Albany (see locality map), conglomerate bands outcrop which have given rise to abundant cobbles strewing the banks of Lucas Creek in the vicinity. They are without doubt members of the local Tertiary sequence known as the Waitemata beds, but their horizon therein is doubtful, for folding and faulting are common in the sandstones on the shores of the upper Waitemata Harbour (into which Lucas Creek flows) and obscure the sequence. The writer is inclined to place the conglomerate bands amongst the lowest of the Waitemata beds in the near vicinity of Auckland, but there is at present no means of fixing their absolute horizon, and, as the purpose of this paper lies rather in the direction of describing the interesting assortment of rock-varieties in the bands than in discussing their stratigraphy, this aspect will not receive further attention herein.
The main band of conglomerate near Albany is about 12 ft. thick, and has a strike that is approximately north-east by east and south-west by west, and a dip of 45° up-stream to the north-west by north. About 20 ft. beneath it is a similar band of smaller cobbles 2 ft. in thickness. Not many yards down-stream the direction of dip is reversed, the sandstones dipping gently in an approximately south-east direction. The actual reason for the reversal is obscure: it may be anticlinal structure, for there is evidence at Riverhead favouring this explanation, although the conglomerate was not found to reoccur in the down-stream section.*
[Footnote] * A much-weathered coarse conglomerate is exposed at Cut Hill, about two miles south-south-east of the Albany outcrop, but it appears that this band is not the same as the Albany one.
The material of the bands consists of very-well-rounded pebbles and boulders usually a few inches in diameter, but reaching as much as 1½ft., set in fine mortar; they appear typical sea-beach drift, a conclusion borne out by the discovery of shell-fragments in a somewhat calcareous fine grit-conglomerate about 20ft. to 25ft. in depth overlying the upper conglomerate band, and outcropping, therefore, up-stream from it. In facies these boulders consist mainly (perhaps 90 per cent.) of rocks of holocrystalline igneous character, many distinctly gneissic in structure, along with grey-wackes, andesites, and occasional trachytes and cherts.
No attempt was made to trace these conglomerate bands across the low hills, rising to about 400 ft. above sea-level, immediately west of Lucas Creek, because of the heavy overburden of residual clay general upon the hills of the district; they were looked for and picked up again in the low-lying wide basin adjoining the Rangitopuni and Mahoenu Streams (see locality map), into which the roads from Albany to Riverhead descend, and where one can find not only distinctive shoading but also several outcrops adjacent to the main road to Riverhead. About two miles east of this latter place, just above the conglomerate is a quartz-rich grit which passes into a fine grit-breccia about 1 ft. in depth, containing fragments of wood, pumice, and a very dense felsitic mica-rhyolite in fragments ranging up to ½in. in diameter The dip is about 80° to the south-south-west. In the main conglomerate band of this outcrop jasperoid argillite, greywacke, and a good deal of andesite, some of it very coarsely porphyritic and very strongly oxidized, are frequent, in addition to the dioritic types common at Albany. Towards Riverhead the band of grit-breccia characterized by the pumice and other rhyolite thickens considerably, and the material also is coarser, forming a very curious firmly cemented breccia where it is exposed in the tideway near the paper-mills at Riverhead. It is not far above the main conglomerate band—here of much smaller pebbles than elsewhere; below this latter also are several distinct bands, each about 2 ft. in depth, of fairly coarse conglomerate. The tide was unsuitable for close investigation of these bands during the writer's visit, but they did not appear to be of special interest.
Petrography of the Conglomerate Bands.
In a recent volume of these Transactions the writer (Bartrum, 1917) described gneissic diorites discovered in loose boulders at Albany, and suggested that they had come from a boulder-bed known to outcrop near by in the Waitemata beds. Upon visiting the locality he soon found that his surmise was correct, and recognized several other rock-types in addition to the two represented in the specimens first given to him. There are several plutonic types, which nearly all show the effects of considerable pressure, notably granulation in various stages of intensity. The complete list is as follows—
(A.) Rocks of sedimentary facies.
Greywackes (fairly common).
Jasperoid argillite (rare).
No particular attention was accorded the pebbles of sedimentary facies, as the writer's interest lay chiefly with the igneous types, so that the above list-may be very incomplete. The greywackes and argillite are obviously derived from the “Maitai” rocks, which from the basement of the Waitemata beds. The chert resembles rocks of similar appearance which
Fig 1.—Granodiorite. The lower half of the photomicrograph illustrates a large crystal of amphibole ophitically enclosing plagioclase. Above this, granulation and shadow extinction are just detectable in the quartz. Crossed nicols. Magnification, 20 diameters.
Fig. 2.—Banded dioritic gneiss. The granulitic structure is well exhibited. Crossed nicols. Magnification, 21 diameters. Fig. 3.—Dolerite showing the lath-like form of the feldspars, which are ophitically enwrapped by amphibole. Crossed nicols. Magnification, 21 diameters.
Fig. 4.—Trachyte. The upper portion of the coarse white mineral is plagioclase, the lower samdme, the intervening dark area being a gap in the section. Crossed nicols. Magnification, 21 diameters.
are common in the hydraulic limestone member of the Oamaru system of Marshall (1911, pp. 22 et seq.); such an origin would raise no fresh difficulty, for Hector (Cox, 1881, p. 29) and Henderson (1914, p. 157) have observed pebbles of the hydraulic limestone itself in Tertiary conglomerates in various places in the Warkworth-Mahurangi district and elsewhere, from which Henderson has deduced the discontinuity of sequence of the rocks of the Oamaru system in that district, which is not far distant from Albany.
(B.) Rocks of igneous origin.
Banded dioritic gneiss.
Andesites of varied type.
Rhyolite (not in the main boulder-band itself).
The granulated diorites preponderate amongst the igneous types, and must constitute 80 per cent. of the pebbles in the outcrop of the conglomerate at Albany.
Description of Types.
1. Granodiorite (granulated).
A fairly common type. A fresh coarse-grained rock, outwardly dioritic and rich in hornblende. In section notably and coarsely ophitic (see Plate XXIX, fig. 1). The light-coloured minerals comprise nearly two-thirds of the rock: basic andesine is the most abundant of them, and next in order come orthoclase and quartz, this last interstitial and abundant. The darker minerals consist of a very little coarse iron-ore and large hornblende crystals enclosing smaller ones of andesine in perfect ophitic manner. A little apatite furnishes the only accessory besides the iron-ore. The effects of severe pressure are very obvious—shadow extinction and granulation of quartz and sometimes orthoclase, and bending of the twinning lamellae of the plagioclase.
2. Quartz-diorite (granulated).
This type was described in an earlier paper by the writer (Bartrum, 1917, p. 423); it approaches the granodiorite mineralogically, but differs from it structurally.
3. Banded Dioritic Gneiss.
The banded structure is quite obvious in hand-specimen. In section it is somewhat lost sight of owing to the mosaic-like structure resulting from complete granulation (see Plate XXIX, fig. 2). In composition it is practically the same as the general granulated diorite next to be described, and there is no need to supplement the brief description of this type published in the earlier paper just referred to.
4. Diorite (granulated).
Judging by macroscopic examination, the vast majority of the pebbles and boulders of the conglomerate belong to this type—a moderately fine even-grained dioritic rock, showing numerous glistening feldspars and darker amphiboles. Usually under the microscope there is sufficient granulation
of the borders of the crystals to give an appearance approaching that of a mosaic, owing to the rather equidimensional rounded crystals of green amphibole and plagioclase in approximately equal proportions. A little iron-ore is present, and usually sphene, the latter sometimes quite plentiful. Pyrite is a common secondary adjunct. Much of the amphibole is fibrous, and in cases is developed as uralite from central kernels of unchanged augite. In one or two sections the feldspars have a distinct broad lath-like form.
A boulder of this rock about 18 in. in diameter was found in the debris of an outcrop of conglomerate alongside the Albany-Riverhead Road. Macroscopically it is a greyish or bluish-white crystalline rock resembling marble, and weathering to a white kaolinitic product. It has small patches of green chlorite, and others of a reddish-brown mineral which apparently is a chloritized mica allied to biotite, and which shows distinct pearly cleavage-faces.
Seen in section, almost the whole of the rock appears as a highly refractive colourless mineral. Its allotriomorphic equidimensional crystals are very strongly cleaved, are fractured, and are separated by zones of a weakly refractive substance which also penetrates the fractures in the main mineral and has every appearance of being a derivative from it. A little chlorite (perhaps derived from biotite) and zoisite are also present.
The mineral constituting the mass of the rock shows some perthitic inter-growths, much irregular twin-lamination, and occasional definite twin-lamellae giving extinction angles of 45° on either side of the composition plane. It was tentatively identified by the writer as a basic plagioclase, and Dr. J. Allan Thomson has been kind enough to confirm this identification. The residuum of feebly refractive mineral was regarded by the writer as a zeolite, but Dr. Thomson suggests that it is largely a more acid feldspar.
No exact quantitative chemical analysis of the rock was made, but Mr. A. H. Bowell, of Auckland University College, performed tests which showed that it is essentially a silicate rich in alumina and lime, with only traces of iron and potassium, and very little magnesia. No attempt was made to ascertain the sodium content.
The writer is convinced that this rock is a plutonic type, for he considers that the finely crystalline material is secondary in origin; he therefore classes it as an anorthosite, a rock of somewhat rare occurrence.
Three specimens of dolerites were collected showing slight differences in the hand-specimen, and only with difficulty separable from the diorites. In mineralogical character they are very similar to the diorites, for they consist of amphibole and basic andesine, the former slightly in excess of the latter. Coarse irregular ilmenite is general, and is sometimes associated with sphene. There is usually a little secondary pyrite and epidote. There are two important differences from the diorites: (a) absence of granulation or other signs of intense pressure, (b) structure. It is on the basis of the structure that they are here classed as dolerites; the texture is coarse and even-grained, and the amphibole has markedly ophitic relations to the long irregular laths of feldspar, as Plate XXIX, fig. 3, well shows.
Some of the greenish amphibole is fibrous uralite obviously derived from augite, as in the diorites, for cores of unreplaced earlier mineral survive. Generally the amphibole is a coarse green hornblende, but often it is a finely bladed, or even granular, apparently secondary variety.
In general the andesitic material is much weathered and altered, the ferro-magnesian minerals in particular having been affected. It was not to be expected, considering the wealth of variety of our Auckland andesites, that any new types would be revealed. Several varieties, however, were discovered, some with plagioclase as the chief phenocryst, others with some or other ferro-magnesian mineral more prominent in the first generation; but as a rule chlorite and other secondary aggregates greatly mask the original nature of the rocks. Mention may be made of the following varieties:—
Hypersthene-andesite.—Macroscopically a particularly coarsely porphyritic type with coarse phenocrysts of pyroxene and feldspar. The hyper-sthene is accompanied by some augite and is largely replaced by calcite. Some small serpentine pseudomorphs suggest olivine. The groundmass is hyalopilitic.
Pyroxene-andesite.—Another very coarsely porphyritic andesite, rather light grey in colour. In section phenocrysts of colourless hypersthene are both abundant and coarse, along with large equally colourless augite crystals. Most of the rest of the rock is composed of much smaller phenocrysts and intermediate lath-like crystals of plagioclase, there being quite a minor amount of the microcrystalline residuum of feldspar, pyroxene, and a little magnetite. There is some coarse totally resorbed hornblende.
Another interesting andesite, in which plagioclase is the only important phenocryst, shows an abundance of branching growths of iron-ore in the groundmass. The ferro-magnesian phenocrysts are small, infrequent, and greatly altered. There are common inclusions of aggregated quartz grains, often with epidote and chlorite, around which the iron-ore growths form a conspicuously dense halo-like border. Coarse secondary epidote is frequent.
These are rare; only two types were found, both typical trachytes, so far as macroscopical examination is concerned, but one characterized by a little biotite and the other by a little hornblende.* In section both types show many idiomorphic plagioclase phenocrysts, with more numerous but smaller sanidine ones.
In the hornblende-bearing type the amphibole is the green variety, and is in scattered idiomorphic crystals. The groundmass is almost entirely feldspathic: a few plagioclase laths are clear-cut and determinable, but the main mass is of small ill-defined crystals lacking the albite lamination, and presumably alkali-feldspar. Plate XXIX, fig. 4, shows the general structure fairly well.
Though the biotite of the biotite variety is apparent in the hand-specimen, the only section cut shows no phenocrysts of that mineral, but numerous small shreds of it in a groundmass built up almost wholly of narrow laths of feldspar which lack the albite twinning and often show good fluxional arrangement.†
Fragments of a curious white porcellanous felsitic rhyolite are frequent, along with pumice and other material, in a thin band of grit-breccia
[Footnote] * A third variety has been found since the above was written; it is a silky rock with phenocrysts of conspicuous greenish-brown hornblende.
[Footnote] † A third type has been found recently; it has fairly conspicuous phenocrysts of brown amphibole.
already described as exposed about two miles from Riverhead alongside the Albany-Riverhead Road, and again near the paper-mills at Riverhead itself, where the fragments are much coarser. The finer portions of the grit-breccia are largely angular quartz grains with flakes of biotite, small grains of the whitish rhyolite, and some of fine argillite.
The rhyolite exhibits a few corroded quartz phenocrysts of moderate size, and plentiful small rather rod-like flakes of brown biotite in a dense ground-mass of minute microlites of feldspar, which are arranged more or less in parallelism with the biotite flakes, and are enwrapped by a small amount of irresolvable base from which they have only imperfectly separated.
Significance of the Material of the Albany Conglomerate.
The probable origin of the sedimentary material included in the conglomerate has already been sufficiently considered (see p. 424), for it raises no point of especial interest. The igneous constituents, however, present a very different case.
There is very general agreement amongst New Zealand geologists (Fraser and Adams, 1907, table facing p. 22) that the eruptions of andesite which have contributed so largely to the building of Coromandel Peninsula and other northern parts of Auckland Province probably began before the Miocene, and thus before the period of formation of the Waitemata beds, which are commonly regarded as Upper Miocene in age. Andesitic eruptions have evidently been common since pre-Jurassic times, for andesitic pebbles are found in Jurassic rocks in the Cape Colville Peninsula (Fraser and Adams, 1907, p. 52), at Port Waikato (Bartrum, 1917, p. 422), and elsewhere. The writer has observed andesitic debris capping hills adjacent to the hill route between Riverhead and Helensville, not many miles from Riverhead, which may represent an extrusion of pre-Waitemata time.
Rhyolites are first known in the Auckland Province from the supposedly pre-Jurassic sediments (Tokatea Hill series) of Coromandel Peninsula (Fraser and Adams, 1907, p. 43), and have considerable importance from early in the Tertiary (Fraser and Adams, 1907. table facing p. 22). Their only special interest as concerns this paper is that they have not been recorded previously from any of the other conglomerate or grit bands in the Waitematas.*
The presence in abundance of gneissic rocks in the Albany conglomerate raises an interesting question regarding the earlier geological history of the North Island of New Zealand, but so large a one that it is inadvisable to deal with it at all fully in a paper such as this, devoted mainly to petrographic description. Briefly, the facts are these:—
1. Gneissic plutonic rocks occur in the North Island of New Zealand in conglomerates at Alexandra, in the King-country (Park, 1893); at the gorge of the Waipaoa River, Poverty Bay (Sollas and McKay, 1906, pp. 175 et seq.); in Cretaceous or early Tertiary beds in the Whangaroa district (Bell and Clarke, 1909, p. 50); in” Maitai” conglomerates in the Hautotara Mountains of south-east Wellington (Sollas and McKay, 1906, p. 185); and, as now recorded, at Albany.
[Footnote] * This statement requires some modification: C. E. Fox (1902, p. 462) records fragments of pumice. Their presence is, however, no necessary indication of the vicinity of a rhyohtic terrain, for pumice can be naturally transported immense distances by water. An interesting example of this fact is furnished by the presence of abundant pumice in sub-recent shore-deposits at the Big Omaha, south of Cape Rodney, which has probably been carried by the coastal drift around Cape Colville from the east coast.
2. The basement rocks* of the North Island consist of shales and greywackes which are largely unfossiliferous and therefore of uncertain age, though in part mid-Mesozoic (Marshall, 1911, p. 20). These rocks were subjected to compression in the later Mesozoic which locally was moderately intense, but caused no noteworthy metamorphism in the North Island area. In the Whangaroa district, it is true, schistosity is locally developed in altered igneous rocks associated with the basement strata of that district (Bell and Clarke, 1909, p. 44), though it is still by no means certain that folding movements earlier than the late Mesozoic may not have affected them. In the South Island metamorphism is very general in the members of the oldermass associated with rocks lithologically similar to those of the basement rocks of the north, but it is probable that this metamorphism long antedated the late Mesozoic orogenic movements (Morgan and Bartrum, 1915, pp. 67–71).†
Plutonic intrusions are rare in the North Island, but such as there are perhaps synchronize with the vastly greater ones of the west coast of the South Island. Fraser and Adams (1907) assign a pre-Jurassic age to the Moehau intrusion of Coromandel Peninsula, whilst Marshall inclines to the belief that the olivine-norite at Ahipara, in North Auckland, underlies the Mesozoic (“Maitai”) beds of that district—an important conclusion—and considers the schillerization of the augite of the norite evidence of intense pressure. It is necessary to add that it is obvious from his paper that Marshall (1908) believes that this pressure was associated with the late Mesozoic orogenic movements. In the South Island some at least of the granite and other batholithic intrusions, if not Palaeozoic, were certainly very early Mesozoic in age, for pebbles of granite and other plutonic types are found in a conglomerate near Nelson (Marshall, 1904).
Having regard, then, to the probable early date of the plutonic intrusions of the North Island, and to the lack of noteworthy metamorphism throughout the sediments of the oldermass, there is surely justification for more than a suspicion that the gneissic constituents of the conglomerates already mentioned are vestiges of a land area which antedated the period of deposition of the Mesozoic (“Maitai”) sediments, and which suffered in turn folding and severe erosion so that intrusive batholiths were uncovered and caused to protrude. Support is afforded this idea by the presence of dioritic material in Jurassic shales in Coromandel Peninsula (Fraser and Adams, 1907), of granitic pebbles in Jurassic sand stones at Kawhia (McKay, 1884), and of a foliated granite in a conglomerate in the “Maitai” rocks of the Hautotara Mountains of south-east Wellington (Sollas and McKay, 1906, p. 185); but it must be admitted that these plutonic rocks have not been found in the lowest rocks of the sequence either at Coromandel or Kawhia, whilst they are absent from many other known conglomerate bands in the basement rocks. Much can be argued both for and against this view that the writer is inclined to favour with regard to the significance of the gneissic boulders, but one cannot ignore the possibility that these rocks disclose a glimpse of the early geological history of the North Island of which we are ignorant In the present state of our knowledge it is impossible to come to any conclusions as to the exact date of the abysmal injections, beyond that
[Footnote] * The “oldermass” of Cotton (Cotton, 1916).
[Footnote] † Mr. Morgan favours an Aorere age (Ordovician) for several great rock-units in Westland and south-west Nelson usually grouped in the Maitai system of early to mid-Mesozoic age.
it was pre-Jurassic, but the conviction grows upon the writer that Professor Park's (Park, 1893, pp. 358–59) early view of the great scientific importance of the discovery made nearly thirty years ago of gneissic rocks in the King-country was not an exaggerated one.*
Summary and Conclusion.
In the Albany-Riverhead district conglomerate bands occur in the Waitemata (probably Upper Miocene) beds, two of which are particularly well exposed near Albany, and contain material of very varied petrographic nature. The various igneous types are described above, and it is pointed out that the gneissic rocks there and in other conglomerates elsewhere in the North Island perhaps furnish evidence of a terrain injected by batholithic intrusions, subjected to compressional stresses and eroded before the deposition of the main mid-Mesozoic sequence of the North Island.
The occurrence of anorthosite is particularly interesting in view of the limited distribution of this type of rock.
In conclusion, the author would like to thank Mr. A. H. Bowell, of Auckland University College, for performing chemical tests upon the anorthosite, and Dr. J. A. Thomson, Director of the Dominion Museum, for most valuable help in the determination of this same rock.
List of Papers cited.
Bartrum, J. A., 1917. Additional Facts concerning the Distribution of Igneous Rocks in New Zealand, Trans. N.Z. Inst., vol. 49, pp. 418–24.
Bell, J. M., and Clarke, E. de C., 1909. The Geology of the Whangaroa Subdivision, Hokianga Division, N.Z. Geol. Surv. Bull. No. 8 (n.s.).
Cotton, C. A., 1916. The Structure and Later Geological History of New Zealand, Geol. Mag. (n.s.), dec. 6, vol. 3, pp. 243–49 and 314–20.
Cox, S. H., 1881. Geology of the Rodney and Marsden Counties, Rep. Geol. Explor. during 1879–80, pp. 13–39.
Fox, C. E., 1902. The Volcanic Beds of the Waitemata Series, Trans. N.Z. Inst., vol. 34, pp. 452–93.
Fraser, C., and Adams, J. H., 1907. The Geology of the Coromandel Subdivision, Hauraki, Auckland, N.Z. Geol. Bull. No. 4 (n.s.).
Henderson, J., 1914. Coal Possibilities of the Warkworth District, 8th Ann. Rep. (n.s.) N.Z. Geol. Surv.
McKay, A., 1884. On the Geology of the Kawhia District, Rep. Geol. Explor. during 1883–84, pp. 140–48.
Marshall, P., 1904. Boulders in a Triassic Conglomerate, Nelson, Trans. N.Z. Inst., vol. 36, pp. 467–71.
—— 1908. Geology of Centre and North of North Island, Trans. N.Z. Inst., vol. 40, pp. 79–98.
—— 1911. New Zealand and Adjacent Islands, Handbuch der regionalen Geologie, pp. 20 et seq.
Morgan, P. G., and Bartrum, J. A., 1915. The Geology and Mineral Resources of the Buller-Mokihinui Subdivision, Westport Division, N.Z. Geol. Surv. Bull. No. 17 (n.s.).
Park, J., 1893. On the Occurrence of Granite and Gneissic Rocks in the King-country, Trans. N.Z. Inst., vol. 25, pp. 353–62.
Sollas, W. J., and McKay, A., 1906. The Rocks of Cape Colville Peninsula, vol. 2.
[Footnote] * The writer would add that since writing the above he has been fortunate enough to find several bands of conglomerate in the “Maitai” rocks outcropping in the north of Great Barrier Island, and that in these bands there are abundant granite boulders and occasional garnet-granulites, if macroscopic appearances are not deceptive.