The Geology of Whangarei Heads, Northland
[Read before the Auckland Institute, May 10, 1950; received by Editor. July 10, 1950]
General Description of Area
Outline of Stratigraphy
Ancient Buried Pre-Waipapa Basement
Earlier Tertiary Extrusions
Dacites of Parahaki Series
Wairakau Andesite Series
Magmatic History and Relationships
Clastic Dykes in Onerahi Limestones
Recent Coastal Uplift
The following paper deals with an area previously described as part of the Whangarei-Bay of Islands Subdivision by the late Dr. H. T. Ferrar (1925) of the New Zealand Geological Survey, but adds information not available in the Survey Bulletin and deals with certain problems not there discussed. The writer wishes to acknowledge the hospitality extended to him whilst in the field by Mr. and Mrs. N. Baker, of “Manaia Gardens.”
The area studied comprises part of the peninsula on the south-east coast of Whangarei Harbour; its northern limit is a line which runs north-east from a little north of Parua Bay to Taihururu Inlet, while to the south and east it is bounded by the open sea.
The area may conveniently be divided for purposes of description into two subequal portions, one north and the other south of an east-west fault along the northern shore of McLeods Bay, each with relatively distinctive geomorphic and geological characters. The northern half is essentially greywacke (? Trias-Jura) terrain with subdued topography seldom over 150 feet in elevation. The southern half is downthrown relatively to the other and is characterised by widespread steeply rising volcanic masses which reach an elevation of as much as 1610 feet (in Bream Head) and commonly are fringed by lowlands of soft Upper Cretaceous or Early Tertiary sediments. Andesitic agglomerates or breccias are prominent in these volcanic masses and have been eroded into bizarre bluffs and pinnacles which make elevations such as Mount Manaia individually recognizable from very far afield.
The shore lines of the area present highly contrasting elements; that which margins Whangarei Harbour shows all the characters
of fairly recent submergence followed by considerable infilling of embayments, amongst which Parua Bay is remarkable on account of the abnormal constriction of its entrance (Fig. 4). South of Urquharts Bay, near the entrance to Whangarei Harbour, bold bluffs fringe the shore and continue east for nearly four miles facing the open sea, unbroken but for two small, shallowly re-entrant bays, Smugglers Bay and Peach Cove, and at many places hundreds of feet high. At Bream Head they turn north-west for a little over one mile, to be replaced by the long sandspit of Ocean Beach, which continues north west for a little over four miles to where the slopes of Kauri Mount initiate a further stretch of steep sea-cliffs. Behind this sandspit there is a belt of sand-dunes about half a mile in maximum width and then a zone of variable width which earlier was swamp or lagoon, but now is largely drained and laid down in pasture. The intricate ramifications of the inland margin of this zone of “swamp” represent the initial shoreline of the sub-recent emergence.
Metalled roads of variable quality give access to the bases of most of the higher elevations. The various volcanic masses in particular are often crowned by scrub or native forest, this latter particularly extensive on Manaia and Bream Head Ranges.
The earlier work on the district, in particular from the time of Cox (1877) onwards to his own time, is dealt with by Ferrar (1925). Bartrum (1925, 1935, 1936, 1937, 1948) has subsequently written about various topics mentioned later in this paper.
Outline of Stratigraphy
The basement rocks are greatly disordered greywackes, with minor argillites, which have been referred by Ferrar (1925) to the Waipapa Series, established by Bell and Clarke (1909) at Whangaroa further north, and may tentatively be included in the Hokonui (Trias-Jura) System. Waipapa sedimentation was ended by vigorous folding and uplift in the early Cretaceous, which were followed by the extensive erosion that is well known to have reduced most of New Zealand to a mature land, if not a peneplain. Widely throughout North Auckland there was submergence after these events and, in our local area, blackish shales and concretionary greenish sandstones of the Upper Cretaceous Otamatea Series of Ferrar (1934) were deposited. Evidence is not conclusive, but it is reasonably certain that, near Whangarei, uplift quickly ended this period of sedimentation, for the next beds in succession are those of the Onerahi Series of Ferrar (1920), which include amongst less calcareous phases the well-known globigerinid “hydraulic” limestone of which upper horizons at least are Middle Bortonian (Middle Eocene) on foraminiferal evidence (Finlay and Marwick, 1947).
Ferrar (1925) has shown that Onerahi sediments were acutely folded and eroded before the next series of marine sediments was laid down, namely the Whangarei Series with varied sandstones and a strong limestone which usually is markedly echinodermal. This limestone has been accepted on foraminiferal evidence as Waitakian (Mid-Oligocene), but Mr. J. Healy, of the New Zealand Geological Survey, has kindly given information, not yet in print, that at Whangarei he has found beds beneath the limestone which are Whaingaroan (Lower Oligocene) or possibly even earlier.
No marine sediments, other than those of sub-Recent or Recent age, of later date than the Whangarei beds are yet known in the Whangarei Heads area.
This history of the main vulcanicity can be briefly given: small extrusions of limburgite separate the basement greywacke from the conglomeratic basal phase of the Whangarei limestone on the north shore of McLeods Bay and there are in addition extensive dacites (plagioclase-rich “rhyolites”) which also may be pre-Whangarei, though the evidence is not conclusive. Various dykes with the compositions of quartz andesites on the north shore of McLeods Bay are almost certainly pre-Whangarei, for they have not been seen to pass up into Whangarei beds in the adjacent sea-cliffs. The main mass of andesites presents no evidence suggestive of its age, though on lithology it has long been correlated with the andesitic fragmentals of Waitakere Hills, west of Auckland City, now known to be Altonian (Lower Miocene). Reasons are given later for doubting this correlation. A fairly large mass of granodiorite-porphyry at Big Point on the southern shores of Bream Head Range intrudes Onerahi beds and has been classed as pre-Whangarei by Bartrum (1925); and by Ferrar (1925), who regarded it as the intrusive equivalent of the dacites. Recent discoveries by the writer have shown, however, that near Peach Cove, east of Big Point, identical granodiorite-porphyry intrudes the fragmental andesites of Bream Head Range. In addition, the relations between dykes of highly acidic quartz andesites and normal andesites near the south end of Ocean Beach seem most reasonably to be interpreted as indicating that the acidic rocks pierce the others.
It is clear from these facts that the order of succession from acidic to basic of Whangarei igneous rocks given by Bartrum (1925) was not as uninterrupted as he had concluded from the evidence then available to him.
The various formations, series or beds* of the writer's area are described in the following order:
Ancient buried pre-Waipapa Basement.
Waipapa Series (? Trias-Jura).
Otamatea Series (Upper Cretaceous; Senonian).
Onerahi Series (Mid-Eocene; Mid-Bortonian).
Whangarei Series (Lower to Mid-Oligocene; ? Whaingaroan-Waitakian).
? Pliocene Conglomerates.
1. Ancient buried pre-Waipapa Basement
Bartrum (1937) has described different fairly high grade metamorphic rocks and associated plutonic igneous rocks which are ubiquitous as xenoliths in many of the andesitic dykes of Whangarei Heads and occur in especial abundance in garnetiferous intrusions on the north shore of McLeods Bay. He has recorded similar schists from
[Footnote] * No attempt has been made to distinguish between these terms in this paper.
Middle Jurassic conglomerates at Kawhia (Bartrum, 1935) and from xenoliths in serpentinites near Silverdale and Wellsford, north of Auckland (Bartrum, 1948).
The schists from “Whangarei Heads agree in grade of metamorphism, according to Bartrum, with the higher grade schists of southernmost Westland and Fiordland, so that they may be regarded as coming from an early Palaeozoic mass of considerable extent.
2. Waipapa Series (? Trias-Jura)
As mentioned in an earlier section, these rocks may well be included provisionally in the Trias-Jura Hokonui System, which is so widespread farther south throughout New Zealand.
The typical local rock is a quartzo-feldspathic fine-grained greywacke with small included pellets of very fine-grained andesitic lava and crystals of more or less wholly chloritized ferromagnesian minerals. Professor Bartrum informed the writer that this phase of greywacke is that almost ubiquitous in his experience of Auckland and North Auckland. As mentioned by Ferrar (1925), small masses of manganese ore occasionally rest in pockets on the surface of the greywacke and were mined many years ago at Parua Bay.
On the north shore of McLeods Bay, the greywacke includes, in addition to the general characteristic stringers of quartz or rarer calcite, local enrichments in or veins of epidote and irregular lenses up to five inches deep of spherulitic red jasperoid quartz. In the same locality it also shows phases which, in thin-section, are seen to consist largely of chlorite, with grains of quartz and a little epidote, and appear to represent earlier, fairly basic, igneous tuff. Veins of epidote may reach as much as an inch across in some of these “greywackes” and are themselves transected by stringers of quartz. The host rock consists essentially of quartz and chlorite with very little epidote; in the veins this last mineral fills the median portions of the fissures with quartz on the margins, but in narrow veinlets these relative positions are reversed. The facts indicate that the beds suffered early fracturing which permitted, at all events at deeper levels, the formation of veins of epidote, probably largely at the expense of earlier plagioclase. Subsequently, there was again the formation of fissures which were infilled in the main by quartz.
Towards the eastern end of this same north shore of McLeods Bay, the greywacke includes a dislocated and much shattered dyke of dolerite, about ten feet in width, which so greatly exceeds any other igneous rock of the area—even including pre-Whangarei limburgite—in the degree of alteration of its minerals that it is thought likely substantially to pre-date other local igneous rocks. Bartrum (Ferrar, 1925) has recorded finding serpentine near the same locality when with the Geological Survey, but has informed the writer that he has been unable to re-locate it during many subsequent visits. Presumably, therefore, it has long been buried by the advance of bay-head filling.
3. Otamatea Series (Upper Cretaceous; Senonian)
In 1925 Ferrar did not separate from his comprehensive Onerahi Series, beds which later he placed in his Otamatea Series. This series included the Batley Beds in which Marshall (1926) found abundant ammonites, enabling him to fix the age as Senonian. Indeed, at
Urquharts Bay, Ferrar (1925) has mapped as Whangarei beds, sandstones from which Professor Bartrum* has since obtained fairly large fragments of Inoceramus. The present writer has not found this shell, but he has found other macro-shells in the same beds; these include Ostrea sp., Nuculana (Saccella) sp., and two indeterminate genera of gastropods.
Lithology is often a dangerous basis for correlation, but in failure, so far, of evidence from palaeontology except that just mentioned, it has been resorted to with confidence when there has been the association of blackish shaly mudstone with greensandstones characterised by major concretions, for this association is almost invariable in Otamatea beds from Orewa, about fifteen miles north of Auckland City, to Hokianga in the far north. (See, for example, Ferrar, 1934.)
As Ferrar (1925) did not recognize Otamatea beds at Whangarei Heads, and as many of the occurrences are so limited that they cannot readily be shown on a small-scale map, it has been considered advisable to list them approximately in order from north to south as follows:
At the most easterly part of the southern shore of Parua Bay.
In the water-table of the road to Whangarei about ½ mile from where it leaves McLeods Bay.
Greensandstones occasionally may be seen, swept free of shore debris, immediately north of the outcrop of Whangarei beds near the mid-east shore of McLeods Bay. They have shed a few concretions up to about 5 ft. in diameter.
Crushed black “shales” at Whangarei Heads wharf on the typical mudstones occur along with limestone of the Onerahi Series between the granodiorite-porphyry intrusion and the andesitie fragmentals of Bream Head Range, south shores of McLeods Bay.
Crushed black mudstone alongside the main road immediately north of High Is., Taurikura Bay. Greensandstones, though not exposed, are evidently present, for large characteristic concretions lie at the shore below.
Blackish mudstones in the shore-platform and at the roadside near “the natural jetty,” Taurikura, and south-east of there in McKenzies Bay.
A very small occurrence of mudstones on the southern fringe of the dacite between McKenzies and Urquharts Bays.
On the south shores of Urquharts Bay and thence towards the saddle leading to Smugglers Bay.
At Big Point on the southern shores of the area, where the
As blackish mudstones associated with a series of andesitic dykes a little north of the southern end of Ocean Beach.
As compact, very fine-textured conglomerate traversed by andesitic dykes about ¾ mile north-west of No. 10.
Highly glauconitic greensands in a small quarry at the end of the road about 1 ½ miles north-west of No. 11 probably belong to this series.
In addition to the occurrences of what are confidently regarded as Otamatea beds listed above, there are numerous places, especially
[Footnote] * Private communication.
along the north shore of Taurikura Bay, where black shales appear, intermixed often with material resembling the Onerahi limestone, as the crushed borders of intrusions.
At the Parua Bay occurrence (No. 1) black mudstones are associated with greensandstones from which concretions up to 6 ft. in diameter have been eroded, while not far distant is limestone of Onerahi type which is shortly overlain by “crystalline” limestone referable on lithology to the Whangarei Series.
The best section of Otamatea beds is that on the south shore of Urquharts Bay (No. 8) where steeply dipping greensandstones containing concretions which may reach 6 ft. in diameter appear to be folded in an anticline. Black mudstones appear on the slopes of the saddle leading to Smugglers Bay, though Onerahi limestone outcrops near the saddle itself. An approximate estimate shows that the Otamatea beds at this locality are not less than 220 ft. thick.
Concretions in Rocks of Otamatea Series
Highly characteristic, round concretions occur in greensandstones and range up to 8 ft. in diameter (Fig. 7). They largely consist of grains of quartz cemented by calcite and silica. Some show a central nucleus 8 in. or so across, while calcite-filled radial septarian cracks are not infrequent, the calcite sometimes in perfect rhombohedra. Occasionally imperfect cone-in-cone structure occurs in irregular bands within a concretion. Small concretions of barite found loose on shores may be shed from the Otamatea beds and not infrequently show cone-in-cone which sometimes simulates rosettes.
At McGregors Bay, Taurikura (opposite High Is.) and near Whangarei Heads wharf (east of Darch Point) there are a few irregular carbonate concretions, not over 4 ft. in maximum dimension, with solution-pitted, reddish-yellow surfaces.
Mr. R. N. Seelye, Chemistry Department, Auckland University College, very kindly carried out a partial chemical analysis of a sample from one of the concretions at Taurikura with the following results:
|A22O3 + Fe2O3||5·06|
|Residue insol. in H.C.2||4·62|
Abundant CO2 was given off during digestion in acid.
A thin section of the material shows that the various bases are combined to give the one mineral, which constitutes a fine-grained holocrystalline rock.
Relation of Otamatea Series to other Series
Although no contact with the Waipapa greywacke is visible in the Whangarei Heads area, the Otamatea beds followed the epi-Hokonuian orogeny and subsequent peneplanation in the earlier Cretaceous and thus rest highly unconformably upon the older rocks.
Ferrar (1934) in the Rodney-Dargaville Subdivision and Turner and Bartrum (1928) in the Takapuna-Silverdale area, found little evidence of physical unconformity between the Otamatea and succeeding Onerahi Series. Harrington (MS.) also failed to distinguish a break between beds equivalent in age to those of the Otamatea Series
and those of Onerahi facies in the South Hokianga district. Yet Finlay and Marwick (1947) have referred the “hydraulic limestone,” more particularly as developed in the Kaipara region, to the Mid-Bortonian stage (Mid-Eocene), so that, unless there are hydraulic limestones developed at two different horizons. Onerahi beds must be unconformable to the Otamatea ones.
Suggestion of such unconformity is, indeed, presented in the writer's area at a small headland ¾ mile north of the southern end of Ocean Beach, for a matrix of argillaceous limestone indistinguishable from that typical of the Onerahi beds includes well-rounded but unsorted pebbles and boulders up to 2 ft. 6 in. in diameter of a sandstone indistinguishable from the concretionary greensandstones of Otamatea Series. (Fig. 8.)
4. Onerahi Series (? Mid-Bortonian–Mid-Eocene)
The beds referred to this series by the present writer are mainly an argillaceous limestone (the “hydraulic limestone” of local usage) and occasional siliceous or argillaceous phases associated with the more calcareous rock. A greensand exposed in a small quarry at the end of Robinsons Road, which runs from Taurikura towards Ocean Beach, may also belong to this series instead of to the Otamatea Series to which it has provisionally been referred above.
The inclusion of these beds with the Onerahi beds of Ferrar (1934) is based purely on lithology; as already mentioned, Finlay and Marwick (1947) have placed them in the Mid-Bortonian, but this correlation is doubtful in view of recent discoveries of the Geological Survey.*
Though invariably greatly disordered by acute compressive forces, the Onerahi beds fail to show any decipherable structure in the writer's area, for bedding lamination is not present.
A little north of Calliope wharf, near Urquharts Bay, and, to a less extent a few chains south of the outcrop of Whangarei beds towards the middle of McLeods Bay, the Onerahi beds include interesting sedimentary dykes which are dealt with in detail in a later section.
5. Whangarei Series. (Lower to Mid-Oligocene. ? Whaingaroan to Waitakian)
This series includes basal conglomerates, which generally are fine in texture, argillaceous or glauconitic sandstones and limestone of varied purity which is crystalline in appearance owing to the abundance of broken echinodermal remains that it contains. The beds outcrop at Whangarei Heads wharf at the south shore of McLeods Bay, near Darch Point south-west of this, at the middle and northern shores of McLeods Bay (Fig. 10) and at various localities on the shores of Parua Bay. In contrast with the preceding Onerahi beds, they are relatively little disturbed except by faults.
Ferrar (1925; 1934) has shown that folding and erosion followed the deposition of the beds of Onerahi Series, so that Whangarei strata rest unconformably either on these latter or on the Trias-Jura greywacke basement.
[Footnote] * Personal communication from Mr. B. F. Hay.
Towards the east end of the southern shore of Parua Bay, Otamatea beds are followed first by those of Onerahi Series and these latter by 8 feet of coarse basal conglomerate of Whangarei Series with greywacke pebbles between 1 in. and 2 in. in diameter set in a plentiful matrix of echinodermal limestone. This is followed upward by grey sandstone which dips at 20° to the south-west and, in thin section, shows, in addition to grains of quartz, a little glauconite and abundant broken tests of Foraminifera. About 100 yards to the west of this section, the series abuts on greywacke and shows a sinuous strike with a dip of 70° to a little east of north. In conjunction with the steep rise of the greywacke in adjacent sea-cliffs and hill slopes to a height of over 300 ft., this disposition of the Tertiary beds strongly suggests that the beds are involved in a fault which strikes approximately parallel to the contact between the two series.
Near Whangarei Heads wharf, the Whangarei beds rest on black shales which appear to belong to Otamatea Series, but on their eastern margin they are succeeded by “hydraulic” limestone of Onerahi Series. They comprise about 150 ft. of beds which are undisturbed but for minor folding and associated faulting and consist of sandstones which become increasingly calcareous from the base up until they include a fairly pure flaggy limestone. In thin section this latter shows a varied assortment of Foraminifera, broken echinodermal remains, Polyzoa and occasional fragments of corals, calcareous algae and lamellibranch shells, along with grains of quartz and a little glauconite. Occasional thin layers are greatly enriched in a species of Amphistegina.
In their exposure near the mid-east shore of McLeods Bay the Whangarei beds show an alternation of sandstone layers in a sea-cliff about 18 ft. in height; some of the bands are fairly richly calcareous and they show in thin section numerous broken tests of Foraminifera. The writer failed to find, however, any horizon that afforded a micro-fauna that was of any use for determining the age of the beds; occasional broken macrofossils were found, especially in conglomeratic calcareous phases on the north shore of McLeods Bay, where the beds have been downthrown to the south along an east-west fault. At this last locality the basal Whangarei beds overlie a flow of limburgite and include boulders as much as 2ft. 6 in. across of this latter rock.
6. ? Pliocene Conglomerates
Ferrar (1925) has recorded stream gravels in a terrace 40 ft. above sea-level at the eastern base of Mt. Manaia and has suggested that they may be Pliocene. They contain no internal evidence of age, so that they may well be of the age to which Ferrar has assigned them.
7. Pleistocene Beds
Amongst the beds included here are early stream fans which flank the steep slopes of Mt. Manaia Range at the southern part of the eastern shore of McLeods Bay. They have been cut back into low terraces by wave action in the past, but shallowing of embayments of the initial shore-line of submergence has led to reversal of type of wave activity and there has been recent substantial progradation in front of the terraces.
Amongst other Pleistocene deposits there are elevated beaches which will be described later, and somewhat consolidated dune sands which appear from beneath modern dune sands at Smugglers Bay and Ocean Beach, often showing an earlier soil with plant roots around which concretions of impure limonite may occur. These consolidated dune sands appear here and there over a very extensive area at Ocean Beach and, particularly towards the southern end of this latter, are locally surfaced by very numerous pebbles averaging about 1 ½ in. in diameter and often carved into ventifacts. These stones cover areas several acres in extent at heights reaching over 60 ft. above sea-level and include representatives of almost every local type of rock. They contrast in size and lack of sign of having been heated by fire with the firestones common around Maori middens of the locality and are believed by Professor Bartrum* to represent stones originally cast up on the adjacent sand beach by waves and gradually blown up the slopes of modern sand-hills by the winds of powerful gales. Subsequent deflation has removed the loose sand from them and caused them to be aggregated on the surface of older consolidated sands which have resisted deflation.* It may be mentioned that some countenance is given to Professor Bartrum's views by the presence of sporadic well-rounded pebbles up to 2 in. in diameter, without any indication of sorting, in early partly-consolidated dune sands at Smugglers Bay. These sands show characteristic large-scale cross-bedding and in thin section are seen to be well-rounded. About 80 per cent. of the grains are of plagioclase along with augite, hornblende, magnetite, a little quartz and tiny pellets of fine-grained andesite.
Such consolidation as is shown by the sands of these Pleistocene dunes is generally due to the oxidation of grains of magnetite.
It is obvious that some changes of conditions must have occurred to permit first of all the fixation of these early dunes and development upon them of a substantial soil horizon, and then later their covering by readvance of sand in recent times. The cause of the early cessation of supply of sand may well have been slight submergence, which perhaps completed the movement of depression that gave rise to the embayed shore-lines of North Auckland. Sub-Recent elevation up to a maximum of 15 ft. is shown freely around the local area by uplifted beaches; this may have inaugurated the phase of sand-dune formation that is now more or less in progress.
8. Recent Deposits
Recent deposits include narrow beaches and other bay-head fillings in suitably sheltered bays and the modern dune sands of Smugglers Bay and Ocean Beach.
Some comparison was made by the writer between the modern beach sands of these localities and the sands of the modern dunes. The grains of the beach sands are often well rounded, though not to so high a degree as those of the dunes. Mechanical analyses showed, as was to be expected, that the sands from the western portions of the dunes at Ocean Beach contain a much higher proportion of smaller grains than the beach sands. Near an outcrop of Onerahi limestone the dune sands contain grains of this limestone to the extent of as much as
[Footnote] * Personal communication.
8·6 per cent. by weight. This has led to cementing and partial local fixation of such dunes. Elsewhere, where temporary pools of water have collected in hollows after rain, sheets of moderately coherent sandstone have formed, either as a result of cementing by the calcium carbonate of included shell fragments (which, however, were not observed), or, more likely, by the settling of colloidal and other particles of dust which would have adhesive properties
Apart from igneous rocks already described by Bartrum (1937) as represented along with varied metamorphic types amongst xenoliths that occur widely in andesitic intrusions, the igneous rocks of the Whangarei Heads area fall into the following groups:
Pre-Onerahi intrusions in the Waipapa greywacke series.
Early Tertiary (pre-Whangarei) limburgitic extrusions.
Dacites of Parahaki Series (Ferrar, 1925).
Wairakau Series (Ferrar, 1925) of varied andesitic intrusions, agglomerates and minor flows.
Granodiorite-porphyry of Big Point and Peach Cove on the southern shore of Bream Head Range.
The present writer made thin-sections of practically all of the different igneous rocks that he encountered, but found in most cases little if anything to add to the descriptions given already by Bartrum (Ferrar, 1925).
1. Pre-Onerahi Intrusions in the Waipapa Greywacke Series
When at work for the Geological Survey, Bartrum (see Ferrar, 1925) collected a specimen of dunite-serpentine from the east end of the north shore of McLeods Bay. He has informed the writer that he has not been able to re-locate it in the course of at least twenty subsequent visits. The writer made diligent but unsuccesful search, and it is to be presumed that the outcrop found early by Bartrum has subsequently been covered by beach debris.
Not far distant to the west of the locality mentioned, there are two greatly shattered outcrops of what appear to be dissevered portions of one and the same 10 ft. dyke of dolerite intrusive into greywackes. It has shared in the folding of these latter rocks and, therefore, precedes the epi-Hokonuian (Earlier Cretaceous) orogeny.
2. Earlier Tertiary (pre-Whangarei) Limburgitic Extrusions
As mentioned on an earlier page, a flow of limburgite underlies the basal conglomerate of the Whangarei Series at several points on the north shore of McLeods Bay. Its depth is unknown, for it is found only as boulders up to 3 ft. in diameter uncovered on the shore platform by the erosion of the overlying sediments. Identical rock also occurs as boulders in a cut of the Parua Bay-Pataua Road about ½ mile south-west of Pukenamu. Hutton (1943) stated that this latter rock is identical with a limburgite at Kawarau Gorge, Central Otago. Apart from lacking phenocrystic augite, it is precisely the same as the rock from McLeods Bay. This latter has numerous phenocrysts of augite and of olivine almost wholly replaced by yellowish-green serpentine and carbonate in a fine-grained matrix which consists essentially of augite and iron ore, with minor plagioclase
and rare brownish hornblende, enwrapped by a small amount of colourless isotropic residuum which, judging by the norm from analysis, appears to be analcite. Bartrum (Ferrar, 1925) overestimated the proportion of plagioclase in this rock, for he classed it as a basalt, though recognizing its equivalent near Pukenamu as a limburgite.
Mr. M. Ongley, Director of the New Zealand Geological Survey, very courteously arranged for analyses of both rocks at the Dominion Laboratory. These were carried out by Mr. F. T. Seelye, who kindly also worked out norms and classification according to the C.I.P.W. system. They are appended below along with others for comparison.
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
Limburgite (N.1382), north shore of McLeods Bay, Whangarei Heads. IV.”2.2(3).2.2—Montrealose. Analyst: F. T. Seelye.
Limburgite (N.1383), ½ mile S.W. of Pukenamu, Whangarei Heads. IV.”2.2(3).2.2—Montrealose. Analyst: F. T. Seelye.
Limburgite, Riquewihr, Vosges. (Friedlander and Niggli, 1931, p. 399.)
Limburgite (Rosenbusch). (Daly, 1933, p. 21.)
3. Dacites of Parahaki Series (Ferrar, 1925)
These rocks have fairly extensive outcrop on the eastern shore of McLeods Bay, whence they extend inland at least as far as the nearby
road. They next occur farther south between McKenzies and Urquharts Bay and then again on much of the ridge that runs north from Busby Point and east of there on the western and southern flanks of Busby Head. There is also a further occurrence of unknown extent on the south-east flank of Mount Stewart.
These dacites are acidic rocks and range in silica from 70·89% to 67·98%, in potash from 4·26% to 2·19%, and in soda, conversely to the potash, from 2·62% to 4·31% (Ferrar, 1925). The proportion of plagioclase to potassic feldspar increases with the percentage of soda. Bartrum (Ferrar, 1925) has adequately described the petrography of the rocks and has mentioned the remarkable fluxional banding shown by those at McLeods Bay. The only information that the writer would add is that the dacite of the wave-cut shore platforms at Smugglers Bay contain not infrequent xenoliths of an earlier more acidic dacite.
The dacites at McLeods Bay have weathered very deeply to a clay which is being used extensively in Auckland in the ceramic industry. Usually this clay is white, but locally it shows vivid pink tints. The ferromagnesian mineral of the parent rock is biotite, which has become bleached during late stages of weathering to a pearly product which closely resembles white mica and in thin section is seen often to be closely crowded by sagenitic clusters of needles of rutile. Yellowish grey to greenish nodules of a clay mineral which closely resembles halloysite in macroscopic properties are abundant in the clay. Mr. I. McDowall, of the New Zealand Pottery and Ceramic Research Association, Wellington, has made full investigation of this clay and very kindly furnished the writer with his unpublished results. The analyses that he forwarded are interesting in that they show that the final product of weathering contains decidedly more silica and less alumina than are present in the intermediate product in which “mica” persists. The alumina has perhaps been carried away in colloidal form by downward percolating waters.
Ferrar (1925) regarded the dacites of Whangarei-Bay of Islands Subdivision as Eocene, but later, in dealing with the Dargaville-Rodney Subdivision, he stated (Ferrar, 1935) that this is incorrect and that they probably are Pliocene, for he found (Ferrar, loc. cit., p. 58) that near Waipu they have been extruded by way of a fault plane which he believed was formed during the Pliocene Kaikoura orogeny.
Doubtless, certain of the North Auckland dacites may well be Pliocene, but others definitely are Lower Miocene or earlier, for they occur plentifully in Altonian (Lower Miocene) conglomerates near Auckland.*
In a discussion later in this paper on clastic dykes (p. 26) it is shown that there is strong suggestion that certain, if not all, of the dacites at Whangarei Heads are post-Onerahi and pre-Whangarei. If this is correct, they were extruded between the Middle Eocene and the Lower Oligocene.
[Footnote] * Professor Bartrum informed the writer that rocks from these Albany Conglomerates which were described by him as rhyolites almost certainly are dacites.
4. Wairakau Series (Ferrar, 1925) of Andesitic Intrusions, Agglomerates and Minor Flows
Analyses published by Bartrum (1925) and his descriptions in the Geological Survey Bulletin (Ferrar, 1925) have shown that the rocks included here range from varied, relatively acidic, quartz andesites to normal types. Small intrusions abound, radiating irregularly from the many centres of andesitic eruption and normally of rock indistinguishable from extrusive phases. In addition, there are coarsely porphyritic intrusive types which are best described as porphyrites. They are usually present in bodies of considerably greater size than those of normal andesitic appearance. The considerable mass of Mount Stewart consists of such porphyrite and it is very doubtful if it is truly an intrusion; it is not unlikely to be the core, uncovered by erosion, of an early large tholoid.
The porphyrites and the quartz andesites universally have characters that distinguish them, apart from texture, from the normal andesites, for they invariably contain garnet and also numerous xenoliths of the igneous and metamorphic rocks that have been described by Bartrum (1937). The quartz andesites in addition are usually characterised by biotite, which does not appear in the normal andesites. It appears from these facts that the latter must originate from different magmatic sources from the others; it is difficult to understand why almost without exception they lack the xenoliths which are so prominent in members of the other group, for obviously the magma that gave rise to them must have arisen through the same basement rocks as the others. It possibly is the case that, as the normal andesites are associated with eruptive centres of considerable size, the uprising magma had sufficient volume and heat to assimilate any fragments broken from basement rocks through which it passed.
The normal andesites form agglomerates varied by occasional flows in the elongated Manaia Range, in Mount Aubrey southwest of Mount Manaia, in Bream Head Range and in Lions Head on the southwest margin of Urquharts Bay. In Manaia Range the agglomerates show good bedding with an easterly dip of about 25°; this appears to indicate that the fissure from which issued the eruptions that built the range was located west of the present crest of this latter, and that the western half of the original volcanic edifice has been removed by erosion, the debris being deposited in an early substantial valley which is now represented by McLeods Bay. Such erosion would be facilitated owing to the fact that soft Onerahi sediments, which rise to over 200 feet above modern sea-level, outcrop almost to the base of the precipitous western slopes of the range.
Near where the road to Parua Bay from Whangarei Heads turns east away from the shore of McLeods Bay, there is a conical mound of andesitic lava isolated amid sediments which appear to be mainly those of the Onerahi Series. Its time-relation to the eruptions that built the nearby Manaia Range is obscure, for its rock is petrographically distinct from that of the range.
A distinctive feature of the various accumulations of andesitic agglomerate is the manner in which they frequently are cut by wide-spaced prominent joints which in turn have controlled the topographic
expression of the agglomerate masses, so that bizarre pinnacles such as those of Mount Manaia are common.
Towards the eastern extremity of Bream Head Range a number of vertical holes 2 ft. to 3 ft. across at the surface, but widening downwards, and of unknown but very considerable depth, occur in andesitic agglomerate in a narrow belt about 100 yards in length. About six of these holes were seen by the writer, but a number of others had been closed by Mr. Crook, the owner of the property on which the holes occur. They are at a height of several hundred feet above sea-level but are not far distant from the shore-line and appear to be due to collapse at points of intersection of major joints along which waves have excavated tunnels at sea-level. Unfortunately, the precipitous coast at this locality is practically inaccessible, so that the writer was unable to see if such tunnels exist.
Amongst the numerous dykes of normal andesite in the Whangarei Heads area there are one or two that deserve special mention. One that arouses a good deal of popular interest is the “natural jetty” at the shore at Taurikura (Fig. 12). This is a vertical dyke about 6 feet in width and jointed in horizontal columns about 9 inches across. It has been intruded into blackish-grey mudstone of the Otamatea Series and now forms a miniature hogback which runs out like a jetty for perhaps three chains into the sea at high tide with its sides plastered by a thin selvedge of “baked” mudstone adhering to the igneous rock.
Another intrusion with interesting features is one about 60 feet in width which is intruded into blackish-grey Otamatea mudstone and forms a small rocky headland near the south end of Ocean Beach. An early dyke of andesite has been intruded into the mudstones, baking them and attaching them to its sides. Subsequently the fissure has been reopened and a second intrusion of andesite practically identical with that of the first has arisen, finding its way between the earlier dyke and the invaded mudstone. The skin of baked mudstone has, on the whole, adhered to the wall of the earlier dye and the result is that a relatively straight vein of black argillite can be traced more or less continuously in the igneous rock for over 100 yards, averaging perhaps 4 in. in width, but sometimes thinning to a mere film and at others thickening considerably and sending out narrow tongues for several inches into what is taken to be the earlier intrusion (Fig. 11). In one wave-eroded chasm a considerable wedge of argillite, 3 feet or more across at the base, penetrates the invading andesite.
The metamorphic effects of the various intrusions upon invaded sediments seldom exceed induration, while adjacent to the walls of the dykes the sediments very commonly show shearing and brecciation which have intermixed varied facies of sediment. It is clear that the exposures in the main are superficial portions of the dykes. On the north-west slope of Busby Head, which is a western part of Bream Head Range, however, Bartrum (Ferrar, 1925) found that Onerahi limestone in contact with andesite had been sufficiently metamorphosed to contain numerous very minute garnets. Also, on the western slopes of Mount Stewart, adjacent to the road between Taurikura and McKenzies Bay, “baked” limestone of the Onerahi Series exhibits prominent spherulites a little over ⅛ in. across and includes rare
crystals of sphalerite.* Similar but smaller spherulites appear occasionally in the same type of rock a little west of Whangarei Heads wharf in McLeods Bay.
Age of Wairakau Series
The age of the Wairakau andesites is uncertain and, further, it is probable, from considerations of magmatic history, as already stated, that the quartz andesites and porphyrites have a magmatic source distinct from that of the normal andesites and need not, therefore, have been contemporaneous with these latter.
All these intermediate intrusions penetrate both Otamatea and Onerahi beds. The normal andesites have not been found locally in association with Whangarei strata, so that their relation to these latter is not shown. They have long been correlated on lithology with the Manukau Breccia Series of Waitakere Hills, Auckland, which are now found by Dr. H. J. Finlay† on foraminiferal evidence to belong to the Altonian (Lower Miocene) stage.
For certain of the quartz andesites the position, however, is different, for on the northern shores of McLeods Bay several very large intrusions of these rocks, intersecting greywacke on the shore, almost certainly fail to pass up into Whangarei beds which rise for 80 feet or more above the greywacke. Talus has obscured actual contacts, but it is reasonably certain that, had the dykes invaded the Whangarei strata, their passage would have been indicated by blocks of their rock in talus and other signs. Such indications, however, are absent, so that early members of these quartz andesites almost positively are pre-Whangarei. Yet other highly acidic quartz andesites have relations to the normal andesites of a small headland near the south end of Ocean Beach which are best interpreted as indicating that a series of these light-coloured acidic dvkes there penetrates the normal andesites.
If these latter are correctly assigned to the Altonian, it is clear that intrusions of quartz andesites have occurred over a very long period of time, namely from a time in advance of local Whangarei sedimentation, which at latest is Waitakian (Mid-Oligocene), to the Altonian (Lower Miocene).
An alternative to acceptance of this conclusion, and it would appear to be a preferable one, is to discard the previously accepted correlation of local fragmental andesites with the Manukau Breccia Series and instead to correlate them with similar “First Period” rocks of Coromandel Peninsula. These latter overlie sediments of the Torehine Series (? Lower Eocene) from which Dr. Brian Mason has recently collected Foraminifera regarded as Waitakian by Dr. H. J. Finlay.†.
5. Granodiorite-porphyry of Big Point and Peach Cove on the Southern Shores of Bream Head Range
A little west of Big Point there is a large intrusion of granodiorite-porphyry, shown by analysis to be the analogue of local dacites, which
[Footnote] * Personal communication from Professor Bartrum.
[Footnote] † Personal communication.
Fig. 11—Argillite “Dyke” in Anuesite, so. Photo.? Prof. J. A. Bartrum.
Fig. 12—“Natural Jetty,” Taurikura Bay—dyke of augite-andesite intrusive into Onerahi sediments. Photo.: Prof. J. A. Bartrum.
Fig. 13—Photomicrograpn of Globigerinal lime-stone (Whangarei), Whangarei Heads Wharf. Little glauconite. Ordinary light. Magnification: 55 diams. Photo.: Prof. J. A. Bartrum.
Fig. 14—Photomicrograph of Pleistocene Sandstone, Smugglers Bay, showing plagioclase (white) with ferromagnesian minerals and fragments of andesite. Ordinary light. Magnification: 38 diams. Photo.: Prof. J. A. Bartrum.
Fig. 15—Photomicrograph of limburgite from boulders near Pukenamu Hill, showing phenocrysts of serpentinized olivine set in a matrix of augite and iron-ore with minor hornblende, feldspar and glass. Ordinary light. Magnification: 55 diams. Photo.: Prof. J. A. Bartrum.
Fig. 16—Photomicrograph of granodiorite-porphyry, from near Peach Cove, showing phenocrysts of plagioclase, quartz and biotite set in a groundmass of equidimensional orthoclase, quartz and plagioclase. Ordinary light. Magnification: 38 diams. Photo.: Prof. J. A. Bartrum.
has been described by Bartrum (Ferrar, 1925). It invades and “bakes” Onerahi limestones which are excellently exposed in a neighbouring rill, but not more than 400 yards to the north-east there are blackish shales of the Otamatea Series which intervene between the mass of granodiorite-porphyry and the steep slopes of the andesitic mass of Bream Head Range. Bartrum (loc. cit.) regarded this intrusive rock as the magmatic equivalent of the dacites, which Ferrar (1925) believed on such field evidence as was then available to be earlier than the andesites of his Wairakau Series. The granodiorite-porphyry, therefore, was considered post-Onerahi and pre-Wairakau.
A recent discovery by the present writer shows, however, that this is not correct, for he found on the south-west shore of Peach Cove a flat-lying dyke of granodiorite-porphyry, identical petrographically with that at Big Point and varying in width from 8 feet to 25 feet, which clearly intrudes into andesitic fragmental rocks of the Wairakau Series. The two intrusions of this acidic rock are so close together that it is unlikely that they are other than contemporaneous, and they must be regarded as post-Wairakau in date. This being so, it is clear that the order of appearance of the igneous rocks of the Whangarei area given by Bartrum (1925) in proffering variation diagrams needs emendation in the light of present knowledge.
Magmatic History and Relationship
In describing the igneous rocks of the Whangarei-Bay of Islands Subdivision, Bartrum (Ferrar, 1925) accepted Ferrar's belief, based on the field evidence then available, that, apart from the pre-Whangarei limburgite on the north shore of McLeods Bay, the dacites were the earliest of the igneous rocks of the subdivision, classing as contemporaneous with them the granodiorite-porphyry of Big Point. Variation diagrams based on an excellent series of analyses showed that the rocks dealt with were members of a differentiation series. These were believed to maintain a regular order of decreasing acidity from the acidic dacites to sub-Recent olivine basalts, omitting from the series, however, the pre-Whangarei limburgite.
It has been shown above that this regularity of succession does not in fact exist; granodiorite-porphyry and some highly acidic quartz andesites (at Ocean Beach) alike have been found to post-date normal andesites. There thus has been alternation in time of injection or extrusion of more acidic and less acidic rocks in the early stages of the magmatic cycle of the Whangarei region; this cycle has closed with the emission of widespread basalts.
From his work on Keweenawan lavas, Broderick (1935) concluded that the presence of oxidized minerals at the upper levels of the flows indicated that volatile transfer played an important part in the differentiation of these lavas. Plotting the percentages of alkalies against the silica, he found that the curve cut across similar curves for the classical Katmai and Lassen Peak Series at the lower silica end and that only later did it turn and parallel these latter for the higher silica ranges. He believed, therefore, that the differentiation of the Keweenawan rocks was not wholly by differentiation as Bowen (1928) believes was definitely the case for the Katmai and Lasaen Peak rocks.
The present writer plotted a corresponding curve for the Whangarei Heads rocks, using the analyses published by Bartrum (1925; Ferrar, 1925) and found that it parallels those for Lassen Peak and Katmai. There is thus no need to invoke any other agency for the origin of the local rocks than crystallisation-differentiation.
The alkali-lime index of the Whangarei Heads rocks on the method of Peacock (1931) is approximately 62. This means that, at a point on the variation diagram where the silica percentage is 62, the sum of the alkalies equals the percentage of lime. In this respect also there is correspondence with the Lassen Peak series, where the alkali-lime index is similarly 62.
If the analyses of the Whangarei Heads limburgite published herein are included with those of the other rocks of the region, the curves of the various oxides in the variation diagram depart from the relatively smooth curves obtained when these analyses are omitted. This applies particularly to the curves for alumina, magnesia and ferrous oxide and results from the high proportions of augite and olivine in these limburgites. Bowen (1928) has shown that, if we assume that basalt is the parent magma, the composition of rocks such as these limburgites must be determined by that of a mesh of accumulated down-sunken crystals. In the case of the local limburgites, however, it is clear from the texture, which is characterised by an abundant fine-grained mesostasis enwrapping phenocrysts of olivine and augite, that crystal-settling had only limited importance in the genesis of the rock. A far more probable explanation is that involved in the reaction upon early separated hornblende which has sunk into a hotter liquid (Bowen, 1928, p. 270), whereby olivine and pyroxene, with perhaps some calcic plagioclase, are precipitated together with nepheline and probably leucite. The magma must then have been extruded before these reactions could be reversed.
Clastic Dykes in Onerahi Limestones
Two series of interesting clastic dykes enclosed in limestone of the Onerahi Series occur on the shore, one about 80 yards and other about 160 yards north of Calliope (Urquharts) wharf (Fig. 9). The first consists of highly angular material, averaging perhaps ¾ in. in average dimension, which is almost wholly derived from the Mesozoic greywacke series, and is present in irregular flat-lying seams not exceeding about 8 inches in depth. The other, more northerly, dykes contain in addition to the greywacke a large proportion of the limestone host in small fragments and a moderate number of fragments of dacite, some of them as much as 8 inches in average dimension and a few fairly well rounded by attrition. Dacite microscopically identical with that of these included blocks occurs in bluffs at the back of the shore, although any possible contact is obscured by debris dumped during construction of the adjoining coastal road.
At this northerly occurrence of the dykes, there is a central mass of breccia about 2 feet across and 4 feet in length, with a visible depth of 1 foot, from which two main subsidiary dykes, one 15 feet and the other 25 feet in length, come off in a meridional direction. These branch dykes vary from as little as an inch in breadth to as much as 1 foot at various parts of their length and in turn give rise to
branching and re-branching stringers. Excavations made by the writer suggest that all are practically vertical.
The rock fragments in the dykes average under 1 inch in maximum dimension and tend to have their long axes parallel to the dykes, though there is no sign of any assorting. They are tightly interlocked and cemented one to another and to their walls by calcium carbonate. There is no sign of the dykes having been affected by earth movements since they were formed.
Similar but shorter dykes also occur close to the south side of Hollingworth's boat-slip near the middle of McLeods Bay.
Origin of Constituent Materials of the Clastic Dykes
The hydraulic limestone and dacite of the dykes have their parent masses close at hand, as mentioned already, but the presence of rocks of the greywacke series cannot be so easily explained, for, although these latter rocks undoubtedly underlie the area concerned, they do not outcrop nearer than the north shore of McLeods Bay, 4 ½ miles to the north-west, where they appear in an earth-block which has been upfaulted with reference to the adjacent southern block not less than 200 feet.
It is clear from the marked angularity of the greywacke fragments in the dykes that they cannot have been transported by water from so far afield as these north-western outcrops of the parent rock. There are, however, three other possibilities:
They may have been eroded from some bed of breccia in yet undiscovered post-Waipapa rocks near at hand.
They may have come from an early outcrop of greywacke which now is hidden beneath volcanic rocks or sea-waters.
The fragments may have been brought up from the buried greywacke basement along planes of shearing and represent, therefore, components of a friction-breccia.
Before discussing these possibilities, it is necessary to decide, if possible, whether the dykes were filled by injection from below or from above. Lahee (1931) lists as evidence of infilling from below such features as upturned strata in wall-rock; upward thinning and final upward termination of the dykes; flow structure parallel to the walls in the filling; inclusion of wall-rocks in the dykes; considerable size and continuity of some of the dykes and similarity of their material to underlying rocks. Admittedly, several of these criteria apply to the dykes in question, but they could equally well support the hypothesis of filling from above.
Objections to the theory of filling by injection from below in consequence of pressure include the absence of fine-grained interstitial matrix that would appear essential for mobility of the mass, and the omission, so far as could be determined, of fragments of Otamatea beds, which are stratigraphically below those of the Onerahi Series. Yet these objections are not insuperable, for earlier finegrained matrix may have been strained off from the mesh of coarser fragments by pressure, whilst it is known that an erosional interval separated Otamatea from Onerahi sedimentation. Otamatea strata may well have been removed from the area concerned prior to the emplacement of the Onerahi beds and, therefore, of the dykes.
With so much uncertainty as to the interpretation of the facts, the writer is inclined to prefer the hypothesis that early dyke fissures were infilled from above, being led to do so because of the incorporation in the dykes of blocks of dacite, which occasionally are well rounded, for dacite overlies the limestone that is the host-rock of the bodies discussed.
If this conclusion be accepted, the source of the fragments of greywacke must be sought either in rock of this kind which outcropped at the time of formation of the dykes, though not now visible, or in a breccia of post-Waipapa age. The process of formation of the dykes conceivably could have been more or less as follows:
Onerahi limestone fissured by some means, whether by tension resulting from broad up-arching or by earthquakes or other cause, formed either the shore-platform of an area or the bed of a stream and was covered by a sheet of drift which included all the components of the dyke-filling; the fissures were filled by such drift and the constituent fragments of the filling were cemented firmly by calcium carbonate available in the limestone. Subsequently the balance of this sheet of debris was removed before the area in question was deeply buried by sheets of dacite later than the early flows of that rock that supplied the dacitic material found in the dykes.
Some support for this particular theory of origin is given by the fact that the clastic dykes nearer to Calliope (Urquharts) wharf, and those near Hollingworths at McLeods Bay, appear to be merely plastered on the Onerahi limestone and may well represent remnants of the sheet of drift invoked.
Age of Clastic Dykes
These dykes do not include fragments of Whangarei strata, so that they appear to be earlier than these latter which, on unpublished information from Dr. H. J. Finlay, contain basal members at least Whaingaroan (Lower Oligocene) in age, although the limestone, which is practically the lowest member at Whangarei Heads, is about Waitakian (Middle Oligocene). Since the dykes transect Onerahi beds which are regarded as Bortonian (Middle Eocene), their age, therefore, is between the Middle Eocene and the Lower Oligocene.
The dykes in their turn may be taken as throwing light on the question of the date of the nearby dacites, for, on the facts given above, these latter appear to be pre-Whangarei and post-Onerahi. This conclusion, however, is only tentative, for it is possible that these dacites belong to a very much later period and that any early cover of Whangarei beds in the area concerned was removed by erosion prior to the emission of the dacites and to the formation of the clastic dykes. This question is also discussed on an earlier page.
Recent Coastal Uplift*
S. Percy Smith (1881) noted the occurrence of beaches raised about 15 feet above modern sea-level at many parts of the coastline of North Auckland. In the writer's area there is only one example of fairly
[Footnote] * No attempt has been made to distinguish between eustatic and diastrophic movements,
recent uplift to a height as much as 15 feet, but there are several which show it to the extent of 4 feet or 5 feet (Fig. 6).
The 15-foot uplift is shown at Smugglers Bay on the southern shores of the area described in this paper and Professor Bartrum has pointed out to the writer that the beach concerned appears to date from an earlier period than those at lesser heights, for in the past there has been substantial recession of the low bluff of weathered dacite which underlies the beach, although to-day its base normally is protected from waves by a narrow fringe of dune sand.
It is by no means improbable, according to Professor Bartrum, that this “uplifted” beach actually represents a feature developed prior to the sub-Recent submergence that has affected North Auckland. If this be so, it probably was uplifted during a phase of fairly rapid and considerable elevation which, as Turner and Bartrum (1928) have shown, occurred near Auckland City immediately before the sub-Recent submergence to which it was sub-equal in amount. This uplift may well have extended to the Whangarei area and, if so, the “uplifted” beach now discussed may be regarded as a “palimpsest” feature which was not covered by sea waters when the submergence took place.
This beach averages about 3 feet in depth and contains rare leached shells amid well-rounded boulders averaging about 4 inches, but up to 9 inches in diameter, which are mainly of dacite, which occurs in adjacent hill-slopes and sea-cliffs, but also includes occasional granodiorite-porphyry brought by waves, as is the case also with modern beach boulders, from Big Point, 1 ½ miles to the east. No boulders of andesite were found with the others, possibly because andesites not far distant to the east are closely jointed and have been broken sufficiently small to be carried off-shore by waves. Above the boulder beach there is talus of angular blocks of dacite fallen from adjacent hill slopes, while below it there is a surprisingly plane surface, eroded in dacite and dislocated 18 inches by a small sub-vertical fault, which is seen to represent the plane of a sub-horizontal joint or shear traceable clearly in bluffs to the west.
Sub-Recent Uplift of 4 feet to 5 feet
This is demonstrated by an interesting rock-platform of the storm-wave type of Bartrum (1926; 1935), which is now undergoing reduction of its surface to bring it into conformity with modern sea-level, and by raised beaches in the following localities.
In a small cove ¼ mile west of Smugglers Bay.
A little south-east of Home Point and north of Busby Point.
About 1 mile west of McGregors (High) Is., Taurikura, in the banks of a small stream.
The storm-wave platform is a horizontal bench of dacite about 80 yards in maximum width, with its surface about 2 feet above high-water level and remarkably plane except where interrupted by gashes worn by waves along major joints. Professor Bartrum has informed the writer that, when he first saw the platform in 1919, its surface did not appear to be inundated by other than exceptional storm-waves and was surmounted by numerous honeycombed pinnacles of rock up to 3 feet in height. Many of these show in a photograph taken as
late as 1930, though all are now obliterated (see Fig. 5) and storm-waves beat freely upon most of the platform.
The first of the raised beaches in the list given above is a storm-beach of fairly recent date, for, although its surface is well grassed, there are fragments of shells preserved in its highly pervious material. This latter ranges from boulders about 8 inches across to small pebbles and extends as a strip 30 feet or less in width which hugs the adjoining hill-slopes for about 80 yards. About 4 feet below its low cliffed face there is a modern storm-beach of similar coarse material. This higher beach is not merely one built with sea-level as to-day before off-shore waters were shallowed by deposition, but is due to uplift, for the adjacent shore is an irregular hard-rock platform which passes into waters which appear to deepen rapidly, if one may judge by the growth of giant kelp.
The second on the list of uplifted beaches is very similar to the last. The third, however, has very limited exposure; above a planed surface of weathered andesite 2 feet above high-water level there is a 2 foot deposit of water-worn pebbles about 1 ½ inches across followed by finer drift and then a layer of shells of a few inches in depth immediately below a mask of soil.
Adjacent to the south-east shore of McLeod's Bay there are stream fans which have been cliffed by waves and could easily be mistaken for raised beaches. It is clear, however, from the nature of their material, which locally shows excellent large-scale lens-and-pocket bedding, that they are not such beaches. Bartrum (1948) has recently drawn attention to the fact that the pebbles and cobbles in these fans show a degree of rounding which is surprising in view of the insignificant length of the streams responsible for their deposition.
Early in this paper it was noted that the area described was roughly divisible into northern and southern halves by a fault trend ing approximately east and west which followed the northern shore of McLeods Bay and caused downthrow of the southern region relative to the northern. This fault is demonstrated by considerable crushing, even to the extent of developing friction-breccia, on the north-east shore of McLeods Bay and by downthrow of Whangarei beds to levels far below the surface of greywacke of the upthrow block on which undoubtedly they once rested. Towards its western end this fault curves from an earlier N.E.–S.W. course to turn to the west and cause the downthrow both of basement greywacke and its partial cover of Tertiary beds in the low peninsula that forms the southern head of the entrance to Parua Bay.
Another fault trends north-west from the north-east shore of McLeods Bay, intersecting that just described, to pass to Parua Bay by way of a low saddle between the volcanic mass of Trig. station M and a greywacke ridge which rises abruptly on the north-east side of the saddle to about 200 feet above this latter. There is an inlier of Onerahi limestone well exposed in the valley of a small stream at the north-west end of this fault and another small one of Onerahi or
[Footnote] * Addendum by Professor J. A. Bartrum.
Otamatea beds on the line of the fault near where it intersects the shore of McLeods Bay.
From suitable viewpoints to the south it can be seen that the surface of the greywacke, overlain by the volcanic rocks of Trig. station M, declines markedly to the north-east towards the line of the N.W.–S.E. fault described and thus indicates that the earth-block of which it is in part the surface has been down-tilted in that direction.
Taking into account the displacements suffered by greywacke surfaces, both faults described above have throws which approximate 200 feet in the vicinity of McLeods Bay, but it is likely that this figure increases very materially eastwards, in the case of the east-west fault, particularly where it separates the elevated Kauri Mt. block of grey-wacke from much younger beds to the south.
The only other fault of note for which definite evidence exists is one on the south-east shore of Parua Bay which has downthrown Whangarei and underlying Onerahi and Otamatea beds to the north. Its trend is not clearly defined and the amount of its downthrow is not determinable for the thicknesses of the downthrown sediments is not known. Trig. stations M and N are conical elevations of finegrained andesitic rocks which appear to represent lavas. If it is safe to assume that the period of andesitic eruption is not later than Altonian,* (Lower Miocene), the andesites of Trig. stations M and N throw light on the date of adjacent faulting. Whangarei beds of probable Waitakian age (Middle Oligocene—Finlay and Marwick, 1947) have been displaced by the fault and had been eroded from the greywacke basement before these andesitic masses were erupted. If these assumptions are correct, faulting must have occurred subsequent to the Middle Oligocene and before the Lower Miocene, that is in Upper Oligocene times.
If it be claimed that the faults are to be regarded as a phase of the Pliocene Kaikoura orogeny, the andesites discussed cannot be earlier than later Pliocene. This appears to be most unlikely in view of the evidence of similar rocks from Coromandel Peninsula, from the Waitakere Hills and in the Parnell Grit near Auckland, where they are known with certainty to be Altonian (Lower Miocene) in age, and from their free incorporation in other Altonian beds known as the Albany Conglomerates which occur in a fairly extensive area a little north of Auckland.
It has been mentioned on earlier pages that there is evidence of the existence beneath the Mid-Mesozoic greywackes, which are the oldest rocks actually exposed in Whangarei Heads area, of metamorphic rocks probably of early Palaeozoic age and of plutonic igneous rocks not known in situ in Northland. The deposition of the greywackes and associated argillites was terminated in the early Cretaceous by the epi-Hokonuian orogeny by which these sediments were complexly folded. After long continued erosion, which in places attained peneplanation (Cotton, 1916). our area was submerged in the Upper Cretaceous (Senonian) and received the near-shore and
[Footnote] * Ferrar (1934) suggested, though not on secure evidence, that andesites of the Dargaville-Rodney Subdivision are Pliocene.
somewhat deeper water sediments of the Otamatea Series. From as yet incomplete evidence from the Hokianga and other regions further north, it appears likely that in those northern regions deposition continued with almost negligible interruption until at least Bortonian (Middle Eocene) times, but in our area uplift and erosion appear to have separated the Otamatea and succeeding Onerahi beds of Bortonian (Mid-Eocene) age. These latter locally are mainly argillaceous globigerinid limestones deposited probably in warm sea waters of only moderate depth.* About the Upper Eocene these beds were strongly folded with the accompaniment nearer Auckland and at North Cape of injection of ultrabasic rocks now represented mainly by serpentines.
After this post-Onerahi orogeny there was substantial erosion and there then came depression of the land relative to sea-level, at first at almost inappreciable rate, so that in favourable basins or other areas freshwater coal measures accumulated such as those near Kamo and Hikurangi, a little north of Whangarei. Soon, however, the rate increased and marine transgression began in the Whaingaroan (Lower Oligocene), or possibly a little earlier, allowing the deposition of the lowest beds of Whangarei Series upon less-elevated portions of the subsiding land-mass. In our local area Upper Eocene erosion had left the surface of this land-mass constituted here and there by Otamatea or Onerahi strata and elsewhere by Mid-Mesozoic grey-wackes from which these later rocks had been stripped. There is reason to believe that at Whangarei Heads the resurrected greywacke surface, which probably constituted higher portions of the subsiding land, was not submerged until Waitakian (Mid-Oligocene) times, when conglomeratic phases of the well-known Whangarei limestone accumulated, locally covering limburgitic lavas which had been erupted in the meantime. In addition, intrusions of quartz-mica andesite and, it is believed, extrusions of dacites had begun prior to the deposition of local Whangarei sediments. Soon after this latter, major faulting appears to have taken place, at an earlier date, therefore, than that of the late Pliocene Kaikoura orogeny of Cotton (1916). This conclusion, however, hinges on the date of eruption of the considerable bodies of andesitic rock which are so prominent a local feature.
There is reason for believing that these andesites may not correlate, as has earlier been taken to be the case, with those of Altonian (Lower Miocene) date in the Waitakere Hills near Auckland, but with the earlier, “First Period” similar rocks of Coromandel Peninsula.
Subsequent to Whangarei (Mid-Oligocene) times and quite apart from the uncertainty about the time of outbreak of andesitic vulcanism, there is a long unbridged gap in the local geological record, for no marine beds of post- Whangarei date are known from Whangarei Heads. From evidence at Dairy Flat, near Auckland, however, it has recently become known that near Auckland there was a considerable time-interval after the Waitakian (Mid-Oligocene) before marine sedimentation recommenced and the Waitemata beds (Lower Miocene) of the southern region were laid down.
[Footnote] * On account of the lengthy discussion that would be required, no attempt has been made to co-ordinate events with Macpherson's (1946) structural scheme, which incidentally appears to face many difficulties in its detailed application to North Auckland geology.
At Whangarei Heads, of post-Tertiary events, only those of sub-Recent time are decipherable. These include substantial uplift, prior to the widespread submergence that has affected Northland, which resulted in deepening of stream valleys, then the submergence as the penultimate event and finally uplift of a few feet.
The writer wishes to acknowledge his great indebtedness to the late Professor J. A. Bartrum for his work in preparation of this manuscript for publication. Although in failing health during his last year at the University, the Professor's thoughts were continually with his students and an amazing amount of work was accomplished, at the expense, finally, of his life. Professor Bartrum was an able and conscientious teacher and a sincere friend; and he will long be remembered with endearment by those who were fortunate enough to study under him.
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