The Geology of the Central Portion of Hokianga County, North Auckland.
[Read before the Auckland Institute, October 20, 1952; received by Editor, November 22, 1952.]
Most of the area discussed consists of Upper Cretaceous sedimentary locks which are an extension, both areally and stratigraphically, of similar locks described by Harrington (1944) from the district to the west. Whirinaki Range is composed of a complex of dolerites, basalts, variolites, and fragmental volcanic rocks, the product of submarine volcanic activity in Upper Cretaceous times. At the eastern fringe of the district are four district lava flows of Late Tertiary and Quaternary age.
The present topography of the land surrounding Hokianga Harbour was initiated in the Late Tertiary when a peneplaned surface was uplifted and dissected. During the course of this erosion there were interesting modifications of the drainage in the Taheke-Waima area. Submergence, within relatively recent time. of the valley system formed by this dissection has given rise to the present drowned topography of the area.
In minor detail, the structure of the Upper Cretaceous sedimentary rocks is confused, but there is evidence for Late Tertiary folding along a north-west axis and possibly Early Tertiary folding along an east-and-west one. The beds are crossed by many small-scale faults, and a major fault occurs along the steep northern face of Whirinaki Range.
The district considered in this paper comprises an area of approximately 100 square miles on the southern shore of Hokianga Harbour between the estuary of Whirinaki River and a point approximately 1 mile east of Horeke. Although the Hokianga area was one of the first parts of New Zealand to be settled by Europeans, its geology has received only cursory mention in the literature. McKay (1888) gives the most complete of the early accounts, though even his work was only of a reconnaissance nature. Harrington (1944) describes the western fringe of the area in his study of the district to the west and south-west. The present survey also includes a small portion of Omapere Survey District described by Bell and Clarke (1909).
On the eastern boundary of the area occurs the western margin of the “faulted tableland” described by Bell and Clarke (1909), bordered at various levels by lava flows that have exerted a protective influence against erosion. The western, dissected extension of this tableland forms a large portion of the district under discussion On the south-west fringe of the area is a rugged and heavily bushed range which Harrington (1944) refers to as “Whirinaki Range” The principal peak of this range, Ngapukehana (2,505 teet). is one of the highest in North Auckland. Hokianga Harbour owes its formation to recent submergence and the lower courses of the various tributaries of the drowned valley system are commonly tidal for 8 or 9 miles from their mouths.
Upper Cretaceous Sediments*
Upper Cretaceous sedimentary rocks cover over 90% of the area. They may be grouped in descending stratigraphic order as follows:—
|Grey-, green-, and buff-coloured claystones and siltstones.|
|4.||Shattered, siliceous mudstones.|
|Light-giey calcareous siltstones grading into angillaceous limestone.|
|(b) Concretionary sandstones with shaly bands and carbonaceous layers.|
|3.||(a) Sandstones. occasionally concietionary, with nbundant thin conglomneiate bands, hand calcareous shelly bands, and caibonaceous layers.|
|2.||Haid, caleaieous, dark-giey sandstones (commonly curr ent-bedded) alteinating with concietionary shales.|
|1.||Haid, shatteied, sihceous shales.|
1.—Hard, Shattered, Siliceous Shales
In the valleys of Okarari and Wairere Streams, west of Horeke, are isolated outerops of hard, shattered, siliceous shales which vary in colour from light-grey to light-green. East of the area, these siliceous shales appear better developed and are well exposed in two large, deep washouts alongside the Horeke-Rangiahua Road, where they form part of a series of hard shales which range widely in colour and are extremely fissile. Here they have a distinctive lithology not comparable with that of any other set of beds in the area at present considered. No direct evidence was obtained as to the age of the shales, but in view of their distinctive lithology they are not included in any of the other three groups, and are tentatively placed at the lower limit of the stratigraphic succession. Probably, detailed field work in the area to the east will provide more reliable information as to their correct position. Further east, McKay (1888, pp. 46, 49 and 50) records siliceous shales at several places near Lake Omapere. It is significant that at these localities he found the siliceous shales to occur at the base of the sequence, directly underlying sandstones which contain Inoceramus and other fossils and which in turn are overlain by greensands and hydraulic limestone, a succession comparable with that found by the writer.
2.—Alternating Sandstones and Concretionary Shales
In lithology these beds are similar to the hard, grey, Inoceramus-bearing sandstones which occur at Rout Point and other places on the Kaipara Harbour, usually in close association with soft sandstones that contain a rich ammonite fauna. In its typical development, this group consists of rapidly alternating bands, up to 2 feet in thickness, of hard, dark-grey sandstone and softer shaly sandstone. The hard, dark-grey sandstone bands are often current-bedded, whilst the shaly ones contain frequent, small, spherical or lenticular concretions from 1 to 4 inches in diameter. The beds, which have been strongly folded, have their most extensive exposures at the following localities on the shore of Hokianga Harbour:—
(a) 1/2 mile west of the mouth of Wairere Estuary where thin, green chert bands are sometimes interbedded with the typical sandstones.
(b) Immediately west of Horeke. At this locality the shaly bands contain abundant, small, barite concretions.
[Footnote] * In view of the fact that a New Zealand Geological Survey Bulletin, which includes the southern portion of the area at present described, is in prepaiation, the writer has not assigned a formation name to these beds.
(c) On the straight length of coast south of The Narrows, where concretionary buff-coloured claystones are interbedded with the normal sandstones and fragments of Inoceramus are common.
They also crop out beneath the overlying conglomerates and sandstones in two isolated outcrops on the western shore of Perunui Estuary, again with occasional bands of buff-coloured claystones and fragments of Inoceramus.
In a quarry on the west side of the mouth of Wairere Estuary is a striking outcrop of siliceous claystones and cherts with thin sandy bands (Fig. 1). They stand almost vertical in finely corrugated beds from 2 to 4 inches in thickness and exhibit a wide variety of colours, mainly reddish-brown, green, white and purple. The green bands are very similar to those mentioned above under (a) whilst on the shore 50 yards north-east of the quarry there is an outcrop of the typical alternating sandstones and shaly sandstones. On the west side of the exposure is a band, 3 feet wide, of very weathered dolerite which bears a concordant relationship to the country rock. Close to the quarry are several boulders, up to 8 feet in length, of variolite. Both these rocks are very similar to those which constitute Whirinaki Range and are discussed in a later section. Samples of the siliceous claystones and cherts were forwarded to Dr. H. J. Finlay, who reported the presence of abundant Radiolaria but no Foraminifera.
The sedimentary rocks exposed in this quarry are unlike any others in the area, and appear to be an “exotic facies” such as is commonly associated with variolitic submarine lavas throughout the world. Abundant Radiolaria are another characteristic feature of this association. The facts suggest that the emplacement of the dolerite exposed in the quarry was roughly contemporaneous with deposition of the associated sediments.
3.—Concretionary Sandstones, Conglomerates And Sandstones, Etc.
The most widespread bed of the Upper Cretaceous sediments is a coarse, massive sandstone with abundant, calcareous, sandstone concretions. It is best developed in the low hills between Omanaia and Whirinaki Estuaries whence it extends south and south-east to Whirinaki Range and westward beyond the limits of the survey. At The Narrows and on the shores of Waima Estuary a similar concretionary sandstone contains numerous bands and lenses of conglomerate and other varied phases. On the west coast of Perunui Estuary and on the coast below Oparia, south of The Narrows, this sandstone-conglomerate sequence occurs in close association with beds of Group 2 and appears to overlie them. To the south and south-west they grade into typical Group 3 concretionary sandstones and field relations indicate that they are a local basal phase of these beds.
In contrast with Group 2, the beds of Group 3 are less strongly folded and are cut by many small faults.
(b) Concretionary Sandstone.
The concretionary sandstone is moderately coarse in gram and varies in colour from bnght-green to grey. In coastal exposures between Omanaia and Whirinaki Estuaries it is commonly current-bedded and contains occasional thin shaly bands which contrast markedly with the massive sandstone. The spherical concretions that characterize the beds range from 4 to 12 feet in diameter, and are particularly abundant in the headwaters of One and Rurunga Streams and in the coastal exposures mentioned above, where they have been concentrated by
erosion of the containing sandstone (Fig. 2). Other beds interbedded with this concretionary sandstone include thin carbonaceous lenses, layers 2 or 3 feet thick of buff-coloured, concretionary claystones, and thin, gritty bands.
(a) Sandstone-Conglomerate Sequence.
The beds of this sequence are well exposed on the east coast of Rawene Peninsula, and on the southern shore of The Narrows. The prevailing sandstone is greyish-brown in colour and encloses concretions similar to those of the overlying concretionary sandstone. At the south end of Rawene Peninsula, east of Omanaia Estuary, it contains disseminated flakes of biotite. Interbedded with this sandstone are thin bands and lenses of diverse sediments, amongst which conglomerates predominate. The latter occur in layers from 6 mches to 3 feet in depth, and can readily be classified as follows:—
(a) Conglomerates containing sub-round pebbles, averaging 2 inches but up to 5 inches long, of quartzite, argillite, acid plutonic rocks and weathered volcanic rocks.
(b) Conglomerates characterised by abundant, small, waterworn pebbles, from 1/8 to 1/4 inch long, of hard argillite with occasional ones of quartz and igneous rock. These are the most general of the conglomerates. Associated with the pebbles are shell fragments which often have the prismatic structure characteristic of Inoceramus. At times, these shell fragments are extremely abundant and the resulting rock is probably McKay's (1888) “shelly limestone”.
(c) A most unusual conglomerate in which spherical, lenticular, or elongated concretions, up to 4 inches long, are associated with a smaller number of pebbles of igneous and sedimentary rocks In several outcrops, the concretions are fossiliferous, notably on the east shore of Omanaia Estuary, 2 miles south of Rawene.
Conglomerates (a) and (b) accord with types described by Bell and Clarke (1909, pp. 49, 50) from the Kaeo Series. Many of the concretions contained in conglomerate (c) are similar to those from the shaly sandstones of Group 2; others contain ammonites identical with those found at Kaipara and Whangaroa Harbours in close association with beds lithologically similar to those of Group 2 The excellent preservation of many of the ammonites shows that they must already have been protected by enclosure in the concretions when incorporated in the conglomerates, and there is little doubt that these ammonite-bearing concretions have been derived from the beds of Group 2. Thus, in certain localities, a slight erosional break must have intervened between the deposition of the beds of the second and third groups.
Other beds present in this conglomerate-sandstone sequence include thin carbonaceous bands, bands of shelly grit containing rare identifiable fossils, harder bands with indeterminable plant remains, shaly beds, and layers of soft, ftiable, dark-blue siltstone which enclose lenticular concretions covered by a layer of cone-in-cone limestone. These various bands range from 6 inches to 4 feet in thickness.
4.—Light-Coloured Claystones And Siltstones
These beds are lithologrcally similar to the Onerahi Beds (or Formation) of Ferrar (1925, 1934) which are widespread throughout North Auckland. In the
Fig. 1.-Thin-bedded siliceous claystones and cherts; west side of the mouth of Wairere Estuary. The dark rubble on the extreme right marks the position of a dolerite (?) flow.
Fig. 2.—Concretionary sandstone; inlet between Whirinaki and Omanaia Estuaries.
Fig. 3.—Margin of the Third Taheke Flow in the embayment near its north-west limit. Recent river alluvium in foreground.
Fig. 4.—North face of Whirinaki Range showing alignment of facetted spur ends. Kernbut at right.
(Photo. Prof. J. A. Bartum)
Fig. 5.—View looking north-east from Karakamatamata Trig. First Taheke Flow near skyline on left, Second Taheke Flow on right. The Third Taheke Flow occupies the valley just showing at the extreme right.
Fig. 6.—Panorama from near junction of Waoku Road and Main Highway. Earliest course of Waima River in foreground; second course passing from left to right in middle distance; the present course passes behind the bushy knoll in the centre middle distance.
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Fig. 7.—Panorama from 1 ½ miles north of junction of Waoku Road and Main Highway. Second course of Waima River in middle distance joining the present course at the right. Whirinaki Range in the background, with the Waipoua Plateau at the extreme left.
Fig. 8.—Coarse-gained dolerite. Whirinaki range. Plagioclase ilmenite (extreme left), interstitial weakly-birefringent zeolite (left-centre) and uralite (fibrous, centre). Crossed Nicols; magnification 38 dimeters.
Fig. 9.—Variolite; boulder on shore near mouth of Wairere Estuary. Ordinary light; magnification 37 diameters.
(Photo. Prof. J. A. Bantrum.)
Fig. 10.—Ordinary light, magnification 38 diameters.
Fig. 11.—Clossed Nicols; magnification 38 diameters.
Fig 10 and 11—Variolitic basalt from volcanic breccia, Whirinaki Range east of Pioitahi Stream. Large crystal of augite (top left) replaced by two forms of chlorite and a large feldspar pehnocryst partially replaced by a weakly-birefringent zeolite. At bottom, extreme left, a small portion of a feldspar crystal entirely replaced by opal.
Fig 12—Basalt of Second Taheke Flow. Augite enwrapping plagioclase (top right), olivine (top centre) and interstitial carbonate (dark, lower left) enclosing plagioclase Groundmass of feldspar laths, augite granules, and iron ore. Ordinary light; magnification 37 diameters.
Fig 13—Basalt of Horeke Flow (non-ophitic phase). Laths of feldspar, occasional granules of augite and olivine, and dark mesostasis. Large augite crystal at bottom. Ordinary light; magnification 37 diameters.
present area they are typically light in colour and fine in grain and, apart from a small outerop on the Main Highway east of Waima River Bridge, are limited to the hills on either side of Omanaia Estuary.
On the foreshore at Rawene extensive outcrops of slightly calcareous claystones and siltstones vary in colour from light-green to chocolate-brown and commonly exhibit polished, slickensided surfaces. They are veined with calcite and occasionally contain hard, ovoid, siliceous concretions, up to 9 inches in length, which, on being struck with a hammer, break up into a series of concentric shells. The highly variable disposition of the beds and the lack of outcrops in the critical area make it difficult to determine their relation to the concretionary sandstones and conglomerates to the south.
On the hill slopes west of Omanaia Estuary several isolated patches of white and light-grey calcareous siltstones grade in places into impure argillaceous limestone. Similar limestone occurs in a road cutting on the Main Highway immediately east of Waima River. These beds appear to cap the concretionary sandstones of the previous group, a relation borne out by the record in bores of “green sands” beneath the limestones and siltstones. The limestone differs from the typical argillaceous limestone of other North Auckland localities described by previous writers (e. g, Marshall, 1924) in that it only rarely contains foraminiferal tests.
Palaeontology of the Upper Cretaceous Sediments
[The writer is indebted to Dr. J. Marwick and the late Dr. H. J. Finlay for identification of the lamellibranchs and pelecypods and the Foraminifera respectively.]
Near The Narrows, beds of Group 2 contain occasional calcareous bands rich in Inoceramus fragments Amphicoelous centra were obtained from two buff-coloured concretions found in the bed of Wairere Stream, apparently derived from beds of Group 2 or Group 3
From the third group, a calcareous grit on the eastern shore of Kawitu Estuary (Grid reference N14/056313) afforded the following pelecypods.—
Neilo (Spineilo) sp
? Venericardia sp.
Nucula n.sp. aff. angulata (Sowby.)
Nuculana (Saccella) n.sp. aff. gaultina Gardn
Also from the third group, a fine sandstone from a head-water tributary of Pioitahi Stream at the edge of Whirinaki Range (Grid reference N14/067243) contained, in addition to abundant Radiolaria, the following Foraminifera:
Ammodiscus glabratus C. & J
Cyclammina incisa Stache
Dr. Finlay reported that this was “probably a Cretaceous fauna but too poor to tell.”
A boulder of similar sandstone in the bed of Pioitahi Stream and almost certainly derived from beds of the third group contaimed, in addition to the usual abundant Radiolaria,” Ammodiscus glabratus C. & J. and Rzehakina epigona
(Rzeh.). Dr. Finlay remarked that this fauna was “certainly Cretaceous, probably Teurian.”
Two samples of the calcareous siltstone of the fourth group from west of Omanaia Estuary were sent to Dr. Finlay, who reported as follows:—
“Both the samples had very poor faunas with only a few long-ranging arenaceous forms such as Rhabdammina tubes, Haplophragmoides sp., Cyclammina sp. etc., but both contained a few examples of Rzehakina epigona
which is not known in New Zealand after the Teurian nor below the Piripauan. On analogy with similar North Auckland faunas, I would say that the age is probably Teurian.”
From the derived concretions contained in conglomerate (c) described above the following fauna was collected:
Boulder, east shore of Omanaia Estuary N14/013317.
Vertebrites murdochi Marshall
Baculites rectus Marshall
Tetragonites margaritatus Marshall
Several additional species of ammonites.
Boulder, east shore of Kawitu Estuary N14/054315
Baculites rectus Marshall
Shore of Waima Estuary, 300 yards east of the mouth of Kawitu Estuary N14/056318.
Nuculana (new sub-genus) n.sp.
Boulder in bed of Mangatete Stream N14/074314.
Wairere Stream N14/119377.
A curious feature of the ammonite fauna is the abundance of two forms, Vertebrites murdochi and Baculites rectus, there being fifteen specimens of the former and twenty of the latter in a total collection of fifty-six specimens According to Marshall (1926, p. 202) these two species indicate Maestrichtian age. Several of the ammonites collected appear to be distinct from any described by Marshall (1926). Ammonites have previously been recorded from the Hokianga area by McKay (1888) and Marshall (1926)
Age and Correlation
The beds of the fourth group are lithologically similar to some of those of the Onerahi Formation of Ferrar (1925) and of the Onerahi Beds of the same author (1934) whilst those of the second and third are similar to the Otamatea Beds of Ferrar (1934).* The character of certain of the beds also accords with that of the lower portion of the Kaeo Series of Whangaroa Subdivision (Bell and Clarke, 1909). Marshall (1926) concludes that the ammonite-bearing portions of his “Batley Series” and of the Kaeo Series are equivalent to the Upper Santonian or Lower Campanian of Europe However he states that certain anomalous species in the fauna indicate an apparent-mixture of zones which makes outside correlation rather uncertain. Macpherson (1948, pp. 290-1) discusses these anomalous forms particularly with reference to the strong Cenomanian element present in the fauna.† In the Hokianga area conglomerates which are not younger than Teurian in age have yielded a derived ammonite fauna characterized by abundant Vertebrites murdochi (previously recorded from the Hokianga area by Marshall, 1926, p. 132) and Baculites rectus, which are regarded as typical of the Maestrichtian Marshall found no evidence for the existence of definite stratigraphic zones in the Kaipara ammonite beds, but none of his specimens were found in situ. The evidence from Hokianga points strongly to the occurrence of a high Cretaceous horizon, probably Maestrichtian, from which the ammonites have been derived, and it is to be hoped that further investigation will locate this ammonite bed
[Footnote] * Finlay and Marwick (1947) have pointed out that in New Zealand the term “series” has been applied where a term designating a lithogenetic unit would have been more in order Ferrar possibly realized this, for nowhere in the literature does he use the term “series” in discussing the Upper Cretaceous and Lower Tertiary sedimentary rocks of North Auckland He refers to them as “Onerahi Formation” or “Onerahi Beds” and “Otamatea Beds” Unfortunately later writers have not adhered to the terminology instituted by Ferrar, and the terms “Onerahi Series” and “Otamatea Series” are now in almost universal application This usage is to a certain extent validated by the recent tendency to regard the terms “Oneiahi” and “Otamatea” as time-stratigiaphic in application rather than lithogenetic as originally defined by Ferrar (see, for example, Finlay, 1948, p. 296) Thus the light-coloured claystones and siltstones (Group 4) of the present area which hthologically belong to Ferrar's (1934) Onerahi Beds, would, on the basis of their Uppen Cretaceous age, be now referred to the “Otamatea Series” as defined by Finlay (loc cit)
[Footnote] † Macpherson places great emphasis on the identification of Acanthoceras ultimum in the Bull Point fauna He appears to have overlooked the fact that Marshall (1927). following consultations with Japanese specialists, found that this species really belonged to an undescribed genus of the Kossmaticeras group for which he then proposed the name Aucklandites.
From the palaeontological results, Groups 4 and 3 can be assigned to the Teurian stage. The alternating sandstones and shaly sandstones of the second group are lithologically comparable with portion of the Otamatea Beds which are correlated by Finlay and Marwick (1940) with the type Piripauan of Amuri Bluff. However, the discovery of an ammonite fauna of probable Maestrichtian age, apparently derived from beds of Group 2, suggests that they, also, should be referred to the Teurian. There is no evidence for the age of the siliceous shales which in this account are tentatively placed at the base of the sequence. They might well be considerably older than the other three groups.
No correlations can be made of the Hokianga rocks with other North Auckland areas, for very little information has been published on the Upper Cretaceous and Lower Tertiary rocks of North Auckland in the light of modern micro-palaeontological evidence. Owing to complexity of structure and lack of clear sections, the age and relationships of these rocks have long been a matter of contention and it is to be trusted, therefore, that the more definitive results that appear to have been obtained from foraminiferal study will soon be available.
Origin and Conditions of Deposition
The upward stratigraphic succession from fine-grained shales through hard, current-bedded sandstones to alternating conglomerates and concretionary sandstones (sometimes current-bedded) in the first three groups is suggestive of normal deltaic deposition in which fine-grained bottomset beds are progressively covered by coarser foreset and topset beds.
The pebbles in the conglomerates include both sedimentary and igneous rocks. The former are mainly quartzites, argillites and greywackes similar to those which characterize the “Waipapa Series throughout North Auckland. The igneous rocks, which are usually greatly altered, consist of altered basic flow rocks and crushed acid plutonic rocks. The few that can be identified with certainty include grano-diorite and granophyre and variolites similar to those which occur in Whirinaki Formation described below Granophyre pebbles occur in conglomerates at several localities in North Auckland, one of the most important discoveries being that of Battey (1950, p 52) of boulders up to 6 feet across, in conglomerates of doubtful age on Rangiawbia Peninsula. They have not been found in situ in the North Auckland Peninsula but from Great Barrier Island, Bartrum (1921, pp 124–5) records a pegmatite or granite-granophyre intrusion into shales and greywackes comparable with the Manaia Hill Series (Jurassic) of Coromandel Subdivision (Fraser and Adams, 1907) and thus with the Waipapa Series of North Auckland. Bell and Clarke (1909) found basic flow rocks interbedded with sediments of Waipapa Series in Whangaroa Subdivision Thus the character of the Upper Cretaceous sediments of the Hokianga area is quite compatible with the assumption that the parent terrain was composed largely of rocks of Waipapa Series.
In the vicinity of The Narrows, the beds of the second group are followed by an alternation of conglomerates and sandstones. To the south, these conglomerates and sandstones do not occur, but there is wide development of a soft micaceous sandstone which is possibly their lateral equivalent. There is, therefore, some indication that the parent land mass was situated to the north or northeast Battey (loc. cit) states that the large size of the boulders of graphic
granodiorite at Rangiawhia Peninsula (north of Hokianga) indicates that the terrain from which they came was close at hand.
Post-Tertiary rocks include lava flows, discussed in a later section, and deposits of river flats and terraces, estuaries, beaches and swamps. In certain areas, the estuarine mud flats have been reclaimed from the sea by the construction of dykes at their outer edges. Excavations on one of these reclaimed areas, at Rawene, have disclosed irregular, nodular concretions which contain Recent fossils and are similar to concretions from the Recent sediments of Waitemata Harbour described by Bartrum (1917).
Small diatomite deposits occur in shallow depressions on the surface of the lava flow that occupies the valley of Taheke River. The deposits appear to be fairly pure but of small extent, covering only a few square yards.
In the south-west corner of the area, the low hilly country immediately south of Hokianga Harbour is abruptly terminated by the rugged mountains of Whirinaki Range. This sudden change of relief is accompanied by an equally sudden change in lithology from soft Upper Cretaceous sediments to a complex of intrusive and extrusive igneous rocks, including fragmental members, which forms Whirinaki Range. To the rocks of this complex, together with similar rocks that occur in a number of small, isolated masses north of the range, Harrington (1944) gives the name Whirinaki Formation. As the slopes of Whirinaki Range are steep and heavily bushed, this account is based mainly on observations made during traverses along the northern margin of the range and up the valleys of several of the mountain torrents. In the area examined. Whirinaki Formation includes fragmental volcanic rocks and flows and dykes of dolerite, basalt, variolitic basalt and variolite.
Isolated masses of the rocks also occur in areas of Group 3 of the Upper Cretaceous sediments, and have been found only in the district west of the Main Highway between Waima and Rawene. The two largest masses are aligned in an approximate east-and-west direction close to Whirinaki Range, but several smaller ones are situated on the shores of Karuhiruhi Estuary and at the south end of Rawene township. Occasional boulders of variolite, however, occur at rare intervals as far east as Horeke.
Fragmental volcanic rocks were found at the eastern end of Whirinaki Range and in two small isolated occurrences at the end of Karuhiruhi Road. At the eastern limit of Whirinaki Range, 1 mile south-east of Waima, is a large bluff composed of volcanic breccia consisting of subangular fragments of variolite, up to 18 inches in length, set in a greatly weathered matrix. Similar breccias occur on the northern fringe of the range, east of Pioitahi Stream, and in a small gully 100 yards west of the end of Karuhiruhi Road. At the northern edge of Whirinaki Range, east of Petaka Stream, is an outcrop of fragmental volcanic rock in which subangular fragments of variolite, up to 6 inches in length, are set in a shattered, black, glassy matrix. A boulder of similar material was found on the bank of Kauwati Stream at the edge of Whirinaki Range south of Waima. In thin section, the glassy matrix is light-brown in colour and contains a number of small crystals of plagioclase and augite scattered throughout. The nature of
both fragments and matrix indicates that the rock has been quickly quenched, and it appears to be a flow-breccia formed by sudden cooling of a lava in which crystallization had barely commenced. Bartrum and Turner (1928, p. 130) describe a similar rock from the North Cape area. On a small island in the estuary of Karuhiruhi Stream there is an agglomerate formed of subangular fragments of variolite and fine-grained dolerite embedded in a weathered tuffaceous matrix.
No sedimentary rocks, other than volcanic fragmentals, were found in situ in Whirinaki Formation, but in the valley immediately east of the headwaters of Oue Stream, several boulders of pink limestone were collected.
Age and Correlation of Whirinaki Formation
Pebbles of altered variolites similar to those of Whirinaki Formation occur in conglomerates of the third group, so that it is very probable that Whirinaki Formation is pre-Tertiary in age and at least older than the concretionary sandstones. If this is so, the isolated exposures of Whirinaki Formation north of the range are inliers, and must directly underlie the concretionary sandstones for, without exception, they are entirely surrounded by, and are apparently in contact with, them. Thus, either there has been considerable overlap of the concretionary sandstones on to a much older Whirinaki Formation, or the latter formation must be of Upper Cretaceous age. Bell and Clarke (1909) record basic lavas interbedded with sediments of Waipapa Series in Whangaroa Subdivision, and it would appear possible, therefore, that overlap has occurred and that Whirinaki Formation should be correlated with the lavas of Waipapa Series. Against this view, however, is the fact that neither Harrington nor the present writer has found outcrops of Waipapa sediments in the Hokianga district From the North Cape area, Bartrum and Turner (1928) describe an igneous complex similar to that of Whirinaki Formation and Bartrum (1934) assigns this complex, on revised foraminiferal evidence, to the Upper Cretaceous (Rahia Series of Bell and Clarke, 1910). Several similar masses occur in North Auckland, mainly in the western portion of the peninsula between Dargaville and Ahipara, although isolated occurrences are known as far south as Flat Top Hill in the Silverdale district. These masses are usually closely associated with Upper Cretaceous or Lower Tertiary beds. Thus Whirinaki Formation can be regarded as Upper Cretaceous in age.
Evidence which may further limit the age of Whirinaki Formation is provided by the dolerite band which occurs with siltstones and cherts of the second group at the mouth of Wairere Estuary. As has been mentioned previously, this dolerite is closely associated with boulders of variolite, and it is probable that both belong to Whirinaki Formation. If this view is correct, Whirinaki Formation is not older than the siltstones and cherts of Group 2 that are exposed in the quarry at the mouth of Wairere Stream and is not younger than the concretionary sandstones of Group 3.
Origin and Conditions of Deposition
In the area discussed in this paper, Whirinaki Formation consists of a series of fragmental volcanic rocks, dolerites, basalts, and variolites. True dolerites are confined to Whirinaki Range, usually in the central portion rather than the margin. Variolites are found at the northern margin of the range, but are best
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developed in the various inliers and as occasional boulders found north of the range; they form the most widespread group in Whirinaki Formation. This distribution, together with the nature and mode of occurrence of the constituent rocks, indicates that the volcanic activity that gave rise to Whirinaki Formation was in large part submarine. Doleritic complexes similar to Whirinaki Formation, with associated variolitic and fragmental phases, have often previously been recorded, particularly from the Palaeozoic of Great Britain, and are generally regarded as having been erupted under water. Pillow-lavas are another characteristic feature of such complexes. In the present survey doubtful pillow-lavas were observed only in a small inlier on the shores of the estuary of Karuhiruhi Stream, but Harrington (1944) reports pillow structure at several localities, notably in Whirinaki River, 1 3/4 miles upstream from its estuary. The abundance of Radiolaria in the associated sediments is almost certainly connected with the submarine origin of Whirinaki Formation.
The probable history of Whirinaki Range may now be summarized as follows:—Shortly prior to the cessation of deposition of Group 2 of the Upper Cretaceous sediments, volcanic activity broke out on the shallow sea floor. Previously deposited sediments were intruded by sills and dykes of dolerite, and submarine emission of lava gave rise to the variolites and fragmental volcanic rocks that are found in the several inliers and at the margin of Whirinaki Range. On the site of the range itself volcanic activity was more pronounced, and it is probable that a volcanic island (the ancestor of the present Whirinaki Range) was built.
Petrography of Whirinaki Formation
Approximately fifty thin-sections were made of rocks of Whirinaki Formation, and a brief description of the types found is given below. In general they are much altered, and their original character was often difficult to determine.
(a) Dolerites. Typically, the dolerites are coarse, even-grained rocks composed of plagioclase, clino-pyroxene, olivine and iron ore (Fig. 8). The dominant mineral is plagioclase (basic labradorite) usually in large, strongly-zoned crystals which have broad twin lamellae and constitute as much as 70% of the rock. It is difficult to estimate the proportion of ferromagnesian minerals as they are very prone to pull out of the mount during the grinding process. It appears, however, that the clino-pyroxene, which is a normal lime-rich augite, forms from 15% to 20% of the rock, usually in faintly-coloured, subhedral crystals. In one slide several crystals of almost non-pleochroic hypersthene occur. Olivine is present in rounded crystals and varies a great deal in abundance, whilst frameworks of iron ore form about 5% of the rock. The fine-grained varieties show a decrease in the amount of plagioclase and a tendency to porphyritic texture and are thus transitional to the basalts described below. Concomitant with these changes is the introduction of a chloritized interstitial material which probably represents an original mesostasis. Needles of apatite occur but are rare.
A characteristic feature of the dolerites is the ubiquitous presence of large interstitial masses of a very weakly-birefringent zeolite which often encloses small feldspar crystals in such a way as to suggest that it is the product of a late magmatic phase of crystallization. There are also occasional small spherulitic aggregates of a second zeolite with greater birefringence and slightly higher refractive index than the other. Similar spherulitic zeolite aggregates occur in the basalts and variolites.
Secondary chlorite and uralite are widespread, but never abundant; the latter mineral commonly forms a border to the pyroxene, whilst at times it veins the feldspar or occurs as minute needles in the interstitial zeolite mentioned above. The dolerites are usually less altered than the finer-grained basalts and variolites, but the presence of occasional bent crystals of feldspar indicates that they have been subjected to pressure
(b) Basalts. By increasing fineness of grain and the introduction of two generations of crystals, the dolerites described above grade into basalts. These, in turn, are often notably aphyric and sometimes exceedingly fine-grained, thus indicating a transition to the variolites and variolitic basalts. The most abundant mineral amongst the phenocrysts is plagioclase (basic labradorite), usually in large laths and forming from 15% to 40% of the rock. Augite constitutes from 10% to 30% and its phenocrysts, which are much less common than those of plagioclase sometimes ophitically enwrap the feldspar. The fine-grained basalts appear to contain only 5% of augite, but there is then a large amount of chloritized interstitial material much of which was probably original pyroxene. Small rounded granules of olivine occur in a few sections, but they are not common, whilst each section contains about 5% of iron ore. At times the phenocrysts tend to group themselves in a glomero-porphyritic manner, and in certain cases there is evidence of crushing.
In general, the rocks of this group have been much altered, and all sections contain from 10% to 40% of interstitial chloritic material. The feldspars, in particular, show very characteristic alterations and replacements, at times being replaced wholly or in part by opal, at other times by a weakly birefringent zeolite similar to that occurring as interstitial masses in the dolerites (Figs. 10 and 11). In both types of replacement, the altered feldspars are often crossed by thin veins of chloritic material. Each slide contains three or four veins in which occur opal or zeolites, or less commonly chlorite and limonite, or sometimes several of them together. In certain cases the zeolites of such veins form a mosaic of irregular plates, whilst in others they occur as short interlocking prisms which project inward from the side of the vein into another of the vein' minerals Similar veins also occur in the rocks of the next group.
(c) Variolites and Variolitic Basalts. In certain of the fine-grained basalts, both pyroxene and plagioclase show a slight tendency to form sheaf-like aggregates of rods and microlites and thus show a transition to the variolites. In the variolitic basalts, phenocrysts and laths of plagioclase (basic labradorite) form from 5% to 40% of the rock, but only rarely does pyroxene form large crystals. The groundmass, which is usually strongly chloritized, consists of microlites and skeletal crystals of plagioclase, grains of pyroxene and iron ore, and arborescent growths of pyroxene and plagioclase. By reduction in the number of phenocrysts, the variolitic basalts grade into true variolites (Fig. 9). The arborescent growths of pyroxene and plagioclase exhibit a wide diversity of form They may have the appearance of miniature fir trees, they may occur as plume-like aggregates, or they may take the form of true varioles in which case they are sometimes surrounded by a border of limonite. These aggregates are usually speckled with minute grains of iron ore, so that they have a dusty-brown appearance in ordinary light. Associated with the aggregates are small skeletal crystals and microlites of feldspar with tufted or fork-shaped terminations. Occasionally the feldspar forms minute stubby prisms arranged in lines in single
or multiple series. An interesting feature of these variolitic rocks is the manner in which fibres of feldspar and augite are often closely set at right angles to the margins of the feldspar phenocrysts.
Large irregular masses of limonite occur in some sections, and the same mineral often veins feldspar laths. The feldspars may alter in a similar manner to those of the basalts, but in addition they are sometimes mottled by patches of calcite and, in certain cases, are in the process of being replaced by a fibrous zeolite. Definite olivine was not found in any of these rocks, but in one or two sections there are pseudomorphs after it consisting mainly of carbonate with limonite and pale-green chlorite aggregated in the vicinity of margins and cracks. Augite is commonly replaced by chlorite and, in one specimen two types of chlorite, one colourless and the other pale-green, occur in the same pseudomorph (Figs. 10 and 11) At other times the augite is replaced by green chlorite at the margins and opal at the centre.
In the typical variolites occasional amygdules, bordered by hmonite in certain cases, contain pale-green chlorite, but in others are formed of minute threads of a dusty-brown mineral which radiate inward from the margin of the amygdule into a colourless fibrous zeolite. Hutton (1943) describes similar amygdules in variolitic pillow-lavas from the Brocken Range-Ngahape area, near Wellington.
The variolites are far more susceptible to alteration than the dolerites or basalts and megascopically the more altered rocks are seen to contain abundant small spheroids of calcite which represent original varioles. This calcite is not filling original vesicles for, in a boulder found on the western shore of Waima Estuary, the rock had been so completely calcified as to resemble a hard limestone; its true nature was only evident in thin-section, where ghosts of original feldspar crystals could be seen.
Pliocene-Recent Lava Flows
In the valley of Taheke River occur portions of three separate lava flows in varying stages of preservation, while there are remnants of a fourth flow south of Horeke. All are products of the intermittent volcanic activity that occurred in the North Auckland Peninsula in Pliocene, Pleistocene and Recent times. For the purpose of this discussion they may be designated as follows, but it should be emphasized that detailed study in the area to the east may reveal the sources of the various flows and permit adoption of a more suitable nomenclature.
|Third Taheke Flow||Covers a large part of the floor of Taheke Valley|
|Second Taheke Flow||Represented by structural benches on the eastern side of Taheke Valley.|
|First Taheke Flow|
|Horeke Flow||Occurs at approximately 600 feet above sea level south-east of Horeke.|
Third Taheke Flow (Fig. 3)
This flow occupies Taheke Valley east of the river, and can be traced for a distance of 10 miles east of Taheke to the well-preserved cone of Tauanui, about 6 miles south of Kaikohe. The undissected nature of the flow, the paucity of its soil cover, and the perfect preservation of the cone indicate emplacement within comparatively recent times. The thickness of the flow cannot be directly estimated, but in the vicinity of Taheke its upper surface is 100 feet above bordering alluvium, whilst near its north-eastern limit the corresponding height is 30 feet.
The surface of the flow is dimpled by numerous shallow depressions up to 100 yards in diameter Many of them are occupied by swamps and near Taheke frequently contain small deposits of diatomite. Bartrum (1925A) describes the lava mould of a fallen tree trunk overwhelmed by this flow.
Second Taheke Flow
Remnants of this lava flow are preserved on the eastern slopes of Taheke Valley as benches approximately 350 feet above sea-level, or 250 feet above the surface of the Third Taheke Flow. The presence of a line of cliffs allows the outer margin of the flow to be mapped with a reasonable degree of accuracy, but, owing to lack of outcrops, the inner margin is much more difficult to distinguish. The maximum thickness at the outer margin is over 100 feet. To the east, this lava terrace can be traced up the valley of Punakitere River (a tributary of Taheke River) to a position 3 miles south of Kaikohe.
First Taheke Flow
2 miles east of Taheke, a remnant of still another lava flow occurs as a structural bench with its upper surface approximately 420 feet above the Third Taheke Flow. It has only minor development in the area, but occasional small blocks loose on the surface testify to its former extension to the north-west
Fig 5 shows the relationship between the three Taheke Flows. They have been emplaced at three different periods during the excavation of Taheke Valley, the Third Taheke Flow being the most recent.
Immediately south-east of Horeke the land rises steeply from sea-level to an upland of rolling country which represents a western extension of the southern block of the “faulted tableland” of Whangaroa Subdivision. In the Horeke-Taheke area, this upland owes its preservation to the protective influence exerted by a series of basie lava flows, of which three occur in the valley of Taheke River and have been described above Remnants of the fourth, the Horeke Flow, occur immediately south-east of Horeke and at the head of Wairere Stream. They represent the last remaining portions of a sheet which once extended to the north and westward as far as Perunui Estuary. The outer edges of these remnants are marked by prominent escarpments which may be as much as 60 to 70 feet high. South-east of Horeke, the upper surface of the basalt is over 700 feet above sea-level, but in its southern portion it falls to 600 feet or less Former important extension of this flow to the west is shown by the presence of abundant gravity-transported blocks on hill slopes as far west as Perunui Estuary. These blocks range up to 30 feet in maximum dimension and in the valleys of Okarari and Wairere Streams commonly exhibit an excellent lapiez-like fluting which is discussed by Bartrum and Mason (1949). In thin-section the rock is similar to the basalt near Okaihau, north of Lake Omapere, included by Bell and Clarke (1909) in their Kerikeri Series Lava remnants similar to those of the Horeke Flow occur between Okaihau and Horeke, and there is little doubt that the Horeke Flow represents an extension down the valley of the ancient Hokianga River of a lava flow from the Okaihan area.
Age and Correlation of the Pliocene-Recent Lava Flows
Stratigraphic evidence obtained during the present survey merely serves to indicate that the lava flows are post-Cretaceous in age, but they can be dated in reference to Pliocene, Pleistocene and Recent strand-line movements. These will be discussed in detail in a later section, and it is sufficient to state here that, following Late Tertiary peneplanation set in hand by an orogenic movement broadly corresponding to the Kaikoura orogeny in the south, the peneplaned surface was uplifted to a height of approximately 900 feet above present sea-level. Remnants of this uplifted peneplain are preserved as a western extension of the “faulted tableland” of Whangaroa Subdivision (Bell and Clarke, 1909). There then followed a period of vigorous erosion, after which there was submergence of the order of 200 feet.
It has been stated above that the Horeke Flow probably followed the valley of the ancient Hokianga River. The general level of this flow is only slightly less than that of the uplifted peneplain, so that it was probably outpoured shortly after the development of the Hokianga River and, if not prior to the uplift mentioned above, at least very soon after its commencement. Bell and Clarke (1909) found that the Kerikeri Series had been involved in the differential faulting that accompanied this uplift and therefore conclude that the lava outpourings took place prior to uplift. Harrington (1944) comes to a similar conclusion in discussing his Waipoua Formation.
Field relations show that the First Taheke Flow is the oldest of the lava flows in Taheke Valley and the Third Taheke Flow the youngest. The three flows follow a valley which has been excavated below the level of the “faulted tableland” so that all are later than commencement of the uplift, though the emplacement of the First Taheke Flow possibly may have been coeval with that of the Horeke Flow if the latter was outpoured after uplift had begun.
The Second Taheke Flow can be traced as a distinct bench to within 3 miles of Kaikohe, where it connects with the Later Basic Volcanics of Bell and Clarke (1909) with which, along with the Third Taheke Flow, it may be correlated, whilst the First Taheke Flow and the Horeke Flow belong to the same eruptive period as the Kerikeri Series of the same authors. The Horeke Flow may possibly be correlated with the Waipoua Formation of Harrington (1944) whilst the Third Taheke Flow is comparable in its youthful appearance with flows in the neighbourhood of Auckland. Collectively, the flows are correlated with the extended Kerikeri Series of Ferrar (1925) in Whangarei-Bay of Islands Subdivision and with the Recent Basalts and the younger of the Earlier Basalts of Ferrar (1934) in Dargaville-Rodney Subdivision.
Petrography of the Lava Flows
The rocks of the Pliocene-Recent Lava Flows show no marked differences from those of similar lava flows further east as described by Bell and Clarke (1909) and Bartrum (in Ferrar, 1925). They are olivine basalts or andesitic basalts in which the essential constituents, in order of abundance are plagioclase (labradorite), clino-pyroxene, olivine and iron ore. Each of the first three may occur in two generations. The clino-pyroxene is a normal lime-rich variety and is only faintly-coloured, if at all. The olivine is generally remarkably fresh, and, when the optical character is determinable, is found to be negative, so that it must contain more than 13% of FeO (Winchell, 1927, pp. 166, 167). In all
except the Third Taheke Flow radially-fibrous carbonate is fairly common, often in rocks that are quite unweathered; it usually occupies vesicles, but may occur interstitially to the feldspar laths of the groundmass and sometimes enwraps them (Fig. 12). A similar feature is noted by Laws (1931) and Battey (1949) in basalts south of Auckland City and by Bartrum (1925, and in Ferrar, 1925) in the Late-Tertiary to Recent basalts of North Auckland Peninsula. In the Hokianga basalts the carbonate is commonly associated with limonite, which is probably an alteration product, for chemical tests carried out on samples from the First Taheke Flow indicate that, in this flow at least, the carbonate is an iron-bearing variety, probably the “?sphaerosiderite” mentioned by Bartrum (in Ferrar, 1925, pp. 69, 70). In common with the above authors, the writer believes that the manner in which the carbonate enwraps crystals of plagioclase indicates that it is a product of the final phases of consolidation of the lavas.
(a) Petrography of the Horeke Flow (Fig. 13). Macroscopically, the basalt of the Horeke Flow is a dense, even-grained rock which ranges from light- to dark-grey in colour and is usually remarkably fresh. Microscopically it consists of small feldspar laths (medium to acid labradorite), irregular masses and granules of augite, and a smaller amount of olivine and iron ore. The texture varies from ophitic to more or less aphyric and large phenocrysts are rare. The feldspar generally constitutes about 40% of the rock and occurs in short laths which sometimes show a slight fluxional arrangement. In ophitic types, the pyroxene occurs mainly as large irregular masses enwrapping the feldspar laths but also as small rounded granules between the latter, whilst in less ophitic types it forms occasional subhedral phenocrysts. Olivine is in small rounded crystals; it is much less abundant than augite and never forms more than 5% of the rock. There is a variable amount of a dark-brown mesostasis which is most prominent in the non-ophitic types, where it sometimes forms over 40% of the rock. Under high power it is found to consist of a light-brown glass crowded with abundant rods and specks of iron ore and very minute rods of pyroxene. A curious feature of this interstitial material is the tendency of the iron ore and pyroxene to cling to the margins of the feldspar laths. In some slides small patches of deep-red hematite are scattered throughout the rock, and in one case this mineral forms a conspicuous vein crossing the full width of the section.
(b) Petrography of the Third Taheke Flow. In hand-specimen the basalt of the Third Taheke Flow is distinguished from those of the other flows by abundance of vesicles and the presence of large, translucent, iron-stained phenocrysts of feldspar (medium labradorite). Under the microscope it is seen to be markedly porphyritic, with feldspar constituting about 60% of the rock; plagioclase phenocrysts are abundant, augite ones much less common, and those of olivine rather rare. The groundmass consists of abundant feldspar, in crystals of varied size and shape, ranging from stubby prisms to long laths, and less common augite and olivine. Rarely, there is a small amount of a dark-brown mesostasis which consists largely of rods of pyroxene and iron ore, whilst scattered throughout the rock are irregular frameworks of ilmenite. The augite is faintly coloured and tends to form subhedral crystals which are sometimes twinned and usually have a well-developed cleavage. Olivine is subsidiary to augite in amount and forms both small rounded granules and anhedral phenoerysts. Occasionally it is seen altering to ?iddingsite.
(c) Petrography of the Second Taheke Flow (Fig. 12). In baud-specimen the basalt of the Second Taheke Flow typically is a light-grey rock with conspicuous olivine phenocrysts. In thin section, plagioclase is seen to make up about 60% of the rock and ferromagnesian minerals about 30% with augite usually more abundant than olivine, though the relative proportions of the two vary considerably. The texture in most sections is ophitic and phenocrysts of olivine and less commonly plagioclase occur in addition to the large, irregular plates of augite that show ophitic relation to the feldspar. The groundmass consists of feldspar laths, granules of augite and olivine in varying proportions, and grains of iron ore. In the typical ophitic types, plagioclase (basic labradorite) occurs mainly as small laths enwrapped by augite but in one section ophitic texture is lacking and plagioclase forms abundant phenocrysts which are about basic labradorite in composition whilst the groundmass laths are acid labradorite. The olivine commonly forms rounded phenocrysts, and in the less ophitic types is sub-equal to augite in amount. In most sections there is a small amount of chlorite apparently derived from the augite.
(d) Petrography of the First Taheke Flow. Macroscopically, the basalt of the First Taheke Flow is a dark, fine-grained rock which often contains vesicles occupied entirely or in part by ?sphaerosiderite. Microscopically, the rock is most markedly ophitic and rather altered, and in each slide there is approximately 20% of radiating and fibrous aggregates of chlorite derived from the augite. Plagioclase (medium labradorite) makes up about 30% of the rock, and occurs mainly as small laths enclosed by the augite, though in each section there are four or five phenocrysts. Augite is sub-equal to plagioclase in quantity, whilst olivine occurs only in minor amount, usually in small rounded phenocrysts. Scattered throughout the rock are small rods of ilmenite and rare needles of apatite.
Harrington (1944) points out that the accordance of summit level of the divides of the lowlands that surround Hokianga Harbour is so striking “that the area may be regarded as a peneplain dissected in consequence of later uplift” (Fig. 6). On the eastern fringe of the area discussed in this account, this “peneplain” has been protected from dissection by the Horeke and Taheke Lava Flows, and is preserved as a western extension of the southern block of the “faulted tableland” of Whangaroa Subdivision, at a general height of 600 to 800 feet. Evidence of similar peneplanation and uplift has been found elsewhere in North Auckland (e.g., Bartrum and Turner, 1928; Turner and Bartrum, 1929). Cotton (1938, p. 7B) believes that this peneplanation postdates the Kaikoura orogeny “if, as is not quite certain, the deformation of North Auckland was contemporaneous with, and not earlier than, the Kaikoura orogeny in the south.”
The modern drainage was initiated by uplift of this peneplain and in the vicinity of Hokianga Harbour, the peneplaned surface was dissected to a stage of maturity by the Hokianga River and its tributaries. The reason for the constriction of the channel of this river (now represented by The Narrows) is not obvious in adjacent exposures, for on both sides of the harbour the rocks consist of sandstones and conglomerates of the Upper Cretaceous sequence. However, in this area there is strong deflection of the compass needle from its true position, so that it is probable that at The Narrows the beds are underlain by basic igneous rock (? Whirinaki Formation) on to which the Hokianga River was superposed.
In sub-Recent times, North Auckland Peninsula was lowered relative to sea-level and the valley system of the Hokianga River and its tributaries was partly drowned to form the present harbour. Turner and Bartrum (1929, p. 891) believe that this submergence was of a eustatic kind, for it has affected the greater part of New Zealand, and suggest that the vertical movement involved “may well be of the order of one hundred and fifty or two hundred feet, a figure which Henderson (1924) shews is the minimum demanded by facts in other parts of New Zealand”.
During the present survey, bedding was observed in the Upper Cretaceous rocks at many localities, but the strikes observed are so diversely oriented that they serve only to indicate that the structure is confused. Similar complexity of structure characterizes the Otamatea and Onerahi Beds and their correlatives throughout North Auckland.
Although the detailed structure is confused, evidence of regional structure is provided by the distribution of Cretaceous and Tertiary rocks along the southern shores of Hokianga Harbour. The youngest beds occur at Hokianga South Head and successively older beds outcrop in fairly regular succession north-north-east and then north-east along the coast (Text-fig. 2).
Text-Fig. 2.—Geological Sketch-map of the Southern Shore of Hokianga Harbour (Geology in part after Harrington, 1944).
As shown by Text-fig. 2, there is evidence in this distribution of a north-west structural trend. Harrington (1944) found that the dominating structure in Hokianga Formation (Middle Tertiary) was a broad anticline striking in a northwest or north-west by west direction. Little is known of the geology of the northern shore of the harbour, but beds of Hokianga Formation occur at Mitimiti on the outer coast (Laws, 1947) indicating a considerable extension of the northwest trend noted by Harrington. The sub-parallel arrangement of streams and divides in a north-west or north-north-west direction on either side of Hokianga Harbour is possibly a reflection of this trend.
Further evidence of structure is provided by the distinctive conglomerate of Group 3 that contains abundant derived concretions, for its known outcrops are located along two eastward-trending lines, 3 miles apart. From a similar study of distinctive beds in Whangaroa Subdivision, Bell and Clarke (1909) conclude that the strike of the Kaeo Series in the southern part of the subdivision is east-north-east and in the northern section, meridional. It is reasonable to assume that the former strike is continued in the Hokianga area as the eastward trend suggested above. Support is added to this view by the alignment of Whirinaki Range, and also of inliers of Whirinaki Formation immediately north of the range, in an approximate east-and-west direction.
Thus in the Hokianga area, Lower Miocene rocks (Hokianga Formation) have been folded about a north-west axis, whilst Upper Cretaceous rocks, in addition to this north-west folding show evidence of a roughly east-and-west folding. Battey (1950, pp. 19, 50) adduces evidence for the existence of post-Cretaceous east-and-west folding in the Far North and Healy (1949, p. 283), in discussing the distribution of Waipapa Series in North Auckland, states that younger rocks often erroneously included in the series usually strike a little north of east. This easterly trend is apparently limited to Cretaceous and Lower Tertiary rocks, and is therefore probably due to an Early Tertiary deformation. Ferrar (1925, 1934) presents evidence for Early Cretaceous folding along north-west lines in North Auckland, and Bartrum and Turner (1928) and Turner and Bartrum (1929) believe that the Late Tertiary orogenic movements were largely a recurrence of activity along this north-west trend. Evidence from the Hokianga area supports the existence of this Late Tertiary north-west folding and, indeed, such folding appears to be the logical explanation for the trend of the North Auckland Peninsula and the distribution of the various rock groups. The Lower Tertiary east-and-west folding cannot be regarded as of equal importance for in the present state of knowledge it seems to be limited to the far northern area.
Studies in other parts of the world have shown that there exists a constant relationship between the tectonic and igneous histories of geosynclinal areas. Turner and Verhoogen (1951, p. 200 et seq.) discuss this relationship by reference to four successive stages of development:
“(1) Eruption of dommantly basic (including spilitic) lavas, during the geosynclinal phase of the tectonic cycle.
(2) Injection of ultrabasic and basic plutonic intrusions during the early stages of folding; in some cases this overlaps phase 1 above.
(3) Development of granodiontic and granitic batholiths during and following the main period of folding.
(4) Surface eruption of basalts, andesites, and rhyolites during and following elevation of the folded mass. This phase is typically separated by a lengthy period of time from the main phase of folding and plutonic activity.”
Of these, stages 1, 3 and 4, at least, can be recognized in North Auckland as follows:—
Stages 1 and 2: The Upper Cretaceous and Lower Tertiary rocks of North Auckland consist of greatly disturbed sediments associated with basic lavas (often submarine) and basic and ultrabasic intrusives. The sediments (Otamatea and Onerahi Beds) contain many cherts and other fine-grained siliceous phases and
commonly contain abundant Radiolaria. The associated igneous rocks are best developed in the western portion of the peninsula north of Dargaville, an area which is almost unknown geologically although officers of the Geological Survey are at present investigating the district. However, the rocks already described include peridotiles, serpentinites (North Cape, Silverdale), gabbros and dolerites (including variolites). In the North Cape and Hokianga areas, at least, the lavas are Upper Cretaceous but little is known of their age in other districts. The serpentinites and other ultrabasic and basic intrusives are, as is to be expected, slightly younger in age. In the Silverdale area they intrude sediments belonging to the Arnold Series, whilst at North Cape they intrude the lavas of the Rahia Series (Upper Cretaceous) mentioned above.
Stage 3: Acid plutonic rocks have not been found in situ in North Auckland, although intermediate plutonic rocks (diorites) are known at several places in the Hokianga-Mongonui area However, they possibly exist, buried beneath the surface, in the district.
Stage 4: There was practically no igneous activity in North Auckland from the Mid-Eocene (Bortonian) to the Lower Miocene when the deposition of the Waitemata Beds (Altonian) and their correlatives was associated with andesitic eruptions Following deposition of the Altonian sediments, the North Auckland area seems to have been raised above sea-level and in the Late Tertiary and Quaternary there were extensive outpourings of basalt flows.
A further working hypothesis which may prove of value in North Auckland is Stille's concept of orthogeosynclines as outlined by Umbgrove (1947, p. 342). Orthogeosynclines are divided into eugeosynclines which are the more mobile tectonic belts associated with basic and ultrabasic rocks, and miogeosynelines which are less active belts usually originating at a later stage alongside the eugeosynelines. In view of the present stage of our knowledge of the area, it is, perhaps, extremely speculative to apply this concept to the North Auckland Peninsula, but it does seem capable of explaining the concentration of Upper Cretaceous and, to a lesser extent, Lower Tertiary sediments on the western side of the peninsula, in association with basic igneous rocks. These beds are typically strongly deformed, and their basement is nowhere exposed. The younger beds to the east are less deformed and rest on a basement of rocke of Waipapa Series with only minor igneous associations. These younger beds do occur in the western area, but in this connection it is worthy of note that Healy (1949, p. 284) in discussing the Whangarei Beds, states that “although they are little disturbed in the eastern area where they rest on a greywacke basement, they appear to be well folded farther west.” Beds of the overlying Southland Series (the Pareora Series appears to be absent in North Auckland) which are widespread in the southern portion of the peninsula and occur at isolated localities further north are comparatively undisturbed.
The idea that emerges is that, in North Auckland, the distribution and character of the sedimentary rock groups together with the nature of the associated igneous activity point strongly to some form of orogenic control. The largely siliceous sediments of the Otamatea and Onerali groups, together with the associated basic lavas, represent the geosynelinal phase of the orogenic cycle. In the Middle or Upper Eocene these beds were intruded by basic and ultrabasic rocks during the early phases of the period of folding, the culmination of which may well be represented by the absence of the Pareora Series. To speculate
further, it is possible that about the Upper Eocene or Lower Oligocene a new depression (miogeosyncline) developed east of the original geosyncline, and in this depression accumulated the dominantly shallow water deposits of the Whangarei group (Landon and Arnold Series) with a marked absence of volcanic activity. The tuffaceous sandstones and andesitic eruptives of the Waitemata Group (Altonian) have not been subjected to strong folding, and their distribution shows little evidence of any form of erogenic control similar to that shown by the older beds. They are the youngest marine sediments in the North Auckland Peninsula. The ensuing uplift was followed by the eruption of basalt lavas which continued almost to the present day, marking the final stage of the orogenic cycle.
The above notes, based on the as yet incomplete knowledge of the area, are put forward with some diffidence, but they seem to justify the conclusion that there exists a broad structural and orogenic plan for the North Auckland Peninsula, the geological structure of which has been most difficult to decipher. Admittedly, some of the observations made are obvious, but up to the present no attempt has been made to integrate them into an orderly account, and it is to be hoped that the investigations at present being carried out by the Geological Survey will allow the formulation of a systematic structural scheme, even if not along the lines here indicated.
The sudden change in relief and lithology, and the alignment of facetted spurends at the northern face of Whirinaki Range indicate the presence of a fault flanked by a kernbut (Fig. 4). Thus Whirinaki Range probably owes its present elevation partly to faulting as well as to superior hardness of its constituent materials. This fault may extend eastward beyond the area for Waipoua Plateau, further south, is bounded on the north by scarps which are possibly the result of faulting. Apart from a probable fault separating the beds of Groups 3 and 4 south of Rawene, no other large displacements occur, although many small scale faults occur in the Upper Cretaceous sedimentary beds.
The History of Waima River
The most striking physiographic feature in the tract of country between Taheke and Waima Rivers is the presence of two broad abandoned valleys which represent former courses of Waima River (Figs. 6 and 7). Since the conditions leading to the formation of these abandoned valleys have not previously been recorded from New Zealand, they will be dealt with in some detail.
The older of the valleys, 2 miles in length, follows a north-east course from the junction of the Main Highway and Waoku Road to the valley of Taheke River. At its north-eastern end it is approximately 100 feet above the level of Taheke River, which is tidal at this point, but rises to over 300 feet above sea-level at its south-western end, where this is a drop of 100 feet to the second and younger of the two abandoned valleys. This second valley trends slightly west of north from the junction of the Main Highway and Waima Valley Road, and is terminated abruptly at each end by the present valley of Waima River. At its southern end, a few hundred yards west of the road junction mentioned above, it is 270 feet above sea-level, or 40 feet above the level of Waima River, whilst the corresponding heights at its other limit, 13/4 miles to the north, are 130 feet and 30 feet. On the floors of both valleys there are abundant boulders up to 18 inches
in maximum dimension which consist almost entirely of basalt derived from Waipoua Plateau to the south.
The dominant factor in the changes of course of Waima River has been its gradient, which has been (and is) much steeper than that of adjacent much smaller streams. The various heights given above show that during the formation of these valleys, Waima River had a gradient of 80 to 100 feet per mile. The differences in the levels of the two previous courses and the present course of the river indicate that, throughout the time interval involved in the diversions. Waima River was engaged in down-cutting its bed. This, together with the superficial nature of the river boulders on the floors of the two abandoned valleys precludes any possibility of “diversion by alluviation”.
Waima River rises on the northern fringe of the Waipoua Basalt Plateau (Harrington, 1944) at a height of 2,000 feet or more. Owing to the abundance and coarse nature of the debris supplied from this high plateau, the early Waima River was unable to reduce the steep gradient of its longitudinal profile. Adjacent much smaller streams, however, adjusted to the transport of fine waste derived from soft sediments (concretionary sandstone and soft micaceous sandstone), would be able to flow on a much lesser gradient and would lower their beds below that of Waima River. Thus conditions would greatly favour diversion as shown by Rich (1935, pp. 1008–1010).
The probable history of Waima River can now be outlined, beginning at the time the ancestral river was flowing along the broad valley which trends northeast from the junction of the Main Highway and Waoku Road. A much smaller stream, flowing approximately along the line of the present Waima Valley Road and with its valley entirely in soft Upper Cretaceous sediments, was at a lower level than the main river, and by its own headward erosion or by that of a tributary, captured the headwaters of the larger river. There probably then followed a short period of aggradation in which this second course was raised to a gradient sufficient for the transport of coarse debris supplied from Waipoua Plateau, after which the river resumed its work of downcutting. Meanwhile a binall downstream tributary of the river was cutting rapidly headward in soft Upper Cretaceous sediments and adopted a course along the present valley of Waima River, guided by the presence of the resistant rocks of Whirinaki Range on its south-west side. Eventually conditions were suitable for a second diversion, and the river was captured by one of its own tributaries. As the final stage in its history, Waima River has aggraded the lower part of its valley in response to recent submergence and now deposits the coarser part of its load in the vicinity of Waima Settlement.
A particularly interesting feature of this history of Waima River is that at one stage its headwaters were captured by one of its own tributaries despite the fact that this tributary followed a longer course to the stream junction—a complete reversal of the normal conditions of “domestic piracy”.
The Origin of the Present Valley of Taheke River
The valley of Taheke River shows many features which indicate that it is the result of a complicated sequence of events. These features are as follows:—
(a) The greater part of the floor of the valley is occupied by a lava flow, whilst remnants of two earlier flows are preserved as benches on the eastern side of the valley, though never found west of the river.
(b) On the western side of the valley there are numerous boulders of basalt of the type characterizing Waipoua Plateau and similar to those found on the two abandoned valleys of Waima River; these do not occur on the eastern side of the valley.
(c) On the east bank of Taheke River, between the river and the west margin of the Third Taheke Flow, there are four “islands” of Upper Cretaceous sedimentary rocks, the highest of which is 80 to 90 feet above the level of Taheke River. The three northern “islands” are separated from the lava flow by “channels” of alluvium, but the southernmost one, on the Main Highway 100 yards east of Taheke River, has its eastern margin everywhere in contact with the lava flow.* On the two central “islands” are occasional boulders similar to those mentioned above under (b). In general, the western margin of the flow shows very few signs of erosion.
(d) Boulders of Waipoua Basalt also occur on saddles in the divide between Taheke Valley and the adjacent valley to the west.
As very little is known of the valley features south of Taheke the origin of the above features cannot be discussed in detail. However, it appears probable that the present Taheke Valley is a composite feature consisting of the remnants of two earlier valleys. Under the influence of successive lava flows, there was a lateral westward migration of the stream in the eastern valley until the two valleys coalesced, the “islands” described above representing remnants of the original divide.
The sequence of events in the Hokianga area may be summarized as follows:—In the Middle and Late Cretaceous, the area lay submerged beneath seas in which accumulated deltaic deposits derived from a landmass lying to the north or northeast. Deposition was not entirely uninterrupted, for slight local warping took place soon after the deposition of the hard sandstones of Group 2. Roughly eoeval with this warping, volcanic activity broke out on the shallow sea floor and previously deposited sediments were intruded by dykes and sills of dolerite whilst submarine emission of lava gave rise to variolitess and fragrmental volcanic rocks. From the district that he describes, Harrington (1944) records beds belonging to the Altonian and to the Whaingaroan-Waitakian, but finds little evidence of the stratigraphic relations of the beds. Tertiary rocks do not occur in the central Hokianga area, but some, at least, of the beds noted by Harrington were probably once present. In the Early Tertiary there was folding, in part, at least, along an east-and-west axis.
In the Late Tertiary, North Auckland was uplifted as a result of a movement which was rather earlier than the Kaikoura orogeny in the south, with broad folding along north-west lines. Following this emergence, a lengthy period of erosion ensued, during which the Hokianga region, apart from areas of resistant rock such as Whirinaki Range, was reduced to a peneplain. Uplift then recommenced and was accompanied by differential faulting, so that certain blocks were raised to a greater extent than others. In the district under discussion the peneplaned surface was uplifted to a position approximately 900 feet above
[Footnote] * The lava mould of a fallen tree trunk described by Bartrum (1925A) was collected from the contact between this “island” and the lava flow.
present sea-level. Contemporaneous with this uplift were the first of several volcanic eruptions which continued into sub-Recent times. Except where protected by lava flows, the uplifted peneplain was maturely eroded and then submerged in Late Pleistocene or Recent times to the extent of 150 to 200 feet.
The above account is based on work carried out under the direction of the late Professor J. A. Bartrum and submitted as a thesis for the Master's Degree. The writer gratefully acknowledges the practical assistance and advice received from the Professor, but far more than this, wishes to record his appreciation of the kindly influence exerted by a thorough and sympathetic teacher and a generous friend. Thanks are also due to Dr. J. Marwick and the late Dr. H. J. Finlay, who kindly identified most of the fossils collected during the course of the work, and to Mr. E. J. Searle for helpful criticism of this manuscript. The writer is also most grateful for hospitality received from settlers in the district.
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