Geology of the West Coast of the Firth of Thames.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 72, 1942-43, Page 69

Geology of the West Coast of the Firth of Thames. By D. A. Brown [Read before the Wellington Branch, July 10, 1941; received by the Editor, February 1. 1942; issued scparately, June, 1942.] Introduction. The district discussed in this paper comprises a well-defined area of 140 square miles lying between the Firth of Thames on the east and Wairoa River on the west. Its northern boundary lies along the estuary of Wairoa River and Hauraki Gulf and its southern limit is the range of hills that trends south-west from Kaiaua (New Brighton). on the west of the Firth of Thames. The central portion of the area consists chiefly of heavily-wooded uplands, which rise in places to 2,000 ft. and more and on account of their rugged topography provide little opportunity for cultivation. In the lower parts of the region, however, river terraces are covered with rich soils and the staple industries are dairying and sheep-farming. The chief township of the area is Clevedon, situated on Wairoa River, about 30 miles south-east of Auckland. The chief means of communication are the roads; for the railway is eight miles distant and Wairoa River is navigable only by very small craft. Previous Work in the Area. Owing to the difficulty of access and the poor roads, little work has been done on the geology of this district. In 1864, von Hochstetter published a paper on the Geology of the Auckland Province, in which the present area was included. He described the oldest rocks, viz., the greywackes of this and adjoining areas, as of Silurian age, all other Palaeozoic and Mesozoic formations being absent. Overlying these basement rocks, he recognised two groups of Tertiary rocks, viz., the older Brown Coal formation and the Papakura Series which he correlated with the Waitemata Series of Auckland. Only small areas of the Papakura formation occur; the coal formation does not appear until the Lower Mangatawhiri Valley is reached in the south. The district has been touched on in various reports by Hutton, Cox, Park, McKay and Fox, and in some detail by Bartrum (1927) and Mead (1930). The adjoining areas on the west and south have been described by Laws (1931), Healy (1935) and Lyons (1932) and their accounts illustrate the close similarity of the several districts. Stratigraphy. The rocks of the district are separated into three distinct groups as follows: (a) Greywackes of the Hokonui series (? Mesozoic) (b) Waitemata sandstone (? Miocene) (c) Post-Tertiary deposits.

(a) Greywackes. The oldest and most abundant rocks outcropping over nearly three-quarters of the area described, are fine-to-medium-grained greywackes of uniform character and, with one possible exception, apparently unfossiliferous. Since they appear to be continuations of those on the flank of the great Mesozoic syncline that trends northeast from Kawhia, a Mesozoic age for these greywackes seems likely. The fresh, unweathered rock is bluish-grey in colour, very hard, and breaks with a conchoidal fracture. Microscopically, the rock is composed of an even-grained mosaic of feldspar and quartz in subequal amounts with minor quantities of ferromagnesian minerals and occasional flakes of biotite and chlorite. Fragments of a finegrained sandstone are found in the specimens from Otau, at the head of Ness Valley and nodules of chert up to an inch in diameter were observed in a sample from Kawakawa Bay. Everywhere except along the sea-coast and the more youthful stream valleys, the greywacke is weathered to a stiff creamy or iron-stained residual clay, the kaolinisation of the feldspars being greatly facilitated by the closeness of jointing which characterises these rocks. This regular spacing of joints produces cuboid blocks which soon give rise to spheroidally weathered cores that resemble boulders of Tertiary sandstone. In general, the jointing has destroyed all traces of the original bedding planes, but at Raukura Point, one mile northeast of Kawakawa Bay, there is a distinct trace of bedding with dips of 60 deg. west. The dominant greywacke is replaced locally by a slaty rock, as along the coast between Kawakawa Bay and Orere Point. In a section of highly siliceous greywacke from a hogback near the headwaters of the Parataki Stream, below Plow's farm at Otau, the writer noted a cylindrical portion 1/30 in. in diameter with a roughly crenulated end and filled with feldspar. This suggests the replacement of an echinoid spine and if this interpretation is correct the specimen is the only record of fossil remains in the greywacke of the district. (b) Tertiary Beds of the Waitemata Group. Upon the basement of greywacke, probably Mesozoic in age, lie, with strong unconformity, Tertiary beds of small extent. It is noteworthy that such beds as those of the Otamatea (Upper Cretaceous-Piripauan) and Onerahi (Mid-Eocene-Bortonian) formations, which are widespread in North Auckland, are absent from the present area, though fragments of the Onerahi limestone have been recognised by Healy (1935) in a volcanic deposit at Ramarama, 12 miles south-west of Clevedon. The absence of these beds and of the Tertiary coal-measures which occur farther west in the Hunua district shows that there are very large gaps in the post-Hokonui succession. Although, prior to the late Tertiary orogeny, the rocks of the Hokonui Series must have been covered with a considerable thickness of Tertiary strata, yet to-day only one small remnant survives on

the highland block. Beds of the Waitemata Series (in modern geological classification, this would be called a group—see “Classification—Nomenclature of Rock Units,” Bull. Amer. Assoc. Petr. Geol., vol. 23, no. 7, pp. 1068–1088), a term introduced by Hochstetter (1864) for “the horizontal beds of sandstone and marls which form the cliffs of the Waitemata Harbour,” outcrop at Otau, at the head of the Ness Valley. Here, a moderately well-defined sequence of sandstones and calcareous grits underlie a portion of the uplifted remnant of a peneplain that lies west of Kohukohunui. The section is best seen along McKenzie Road, which passes westwards from the Ness Valley road at Trig Station 1946. As with most of the neighbouring Tertiary localities, the sequence is far from complete, so that correlation is difficult. Access to Otau from Clevedon is by way of the Kawakawa Bay road to Ness Valley. From the amphitheatre-like end of the valley, the road takes a sharp turn to the west and after a tortuous climb of some three miles reaches Otau at an elevation of about 1300 ft. Greywacke is prominent in all the roadside cuttings up to the height of 1040 ft., where the hard older rock gives place to a soft tuffaceous sandstone containing pellets of halloysite approximately 0·2 in. in diameter. As the road continues to ascend, the sandstone becomes markedly finer. Where the road gains the surface of the elevated peneplain, the turn-off to the west brings one into McKenzie Road, where an excellent vertical section of the undisturbed and approximately horizontal Tertiary cap is obtained. Here are exposed about 300 ft. of tuffaceous sandstones and grits with occasional thin limestone beds; these become noticeably coarser and in places conglomeratic towards the base, which is seen in the headwaters of Cossey Creek. The whole is overlain by a fine-grained white calcareous sandstone in which the only fossils seen were an internal cast of Turritella sp., Pecten williamsoni Zittel and a few Foraminifera. A notable feature of the sandstones is the presence in them of masses of grey-green kaolin up to 9 in. across. The green sandstones and sandy limestones which contain abundant Foraminifera and Bryozoa consist chiefly of quartz and calcite. In addition glauconite, chlorite, muscovite, hornblende, augite, halloysite, and occasional crystals of magnetite were recognised. Correlation and Palaeontology of the Beds. The presence of intercalated tuffaceous sandstones and grits throughout the succession immediately suggests correlation with the “Parnell Grit” formation of the Waitemata group at Auckland. The sandstones and grits under discussion closely resemble the normal facies of the “Grit” in exhibiting spheroidal weathering and in containing a number of well-preserved remains of Bryozoa and small Pectens, but are distinctly finer. In support of this correlation, there is also the evidence afforded by the presence of green sandstones in the section. In these latter are large numbers of Foraminifera, Bryozoa and small pelecypods.

The presence of this phase in company with the grits suggests correlation with the Turanga Greensand beds, which outcrop in numerous localities throughout the Whitford district north-west across the Wairoa estuary, and are believed by Firth (1930) to be a phase of the Parnell Grit. Another small area of Tertiary rocks occurs in the vicinity of Trig. 646, Wairoa South, about two miles south of Clevedon at an elevation of about 700 ft. above sea-level. Between Clevedon and Hunua, the Tertiary beds are first encountered as approximately horizontal, reddish sandstones which lie unconformably on greywacke in a road-cutting at an elevation of 480 ft. and attain a thickness of about 160 ft. Half-a-mile from the summit they give place to soft white sandstones which have been widely disclosed in slips and on examination are found to be slightly calcareous beds with rare fossils. Lower beds of the sequence are seen in the headwaters of the Taitaia, a small stream on the west, where the facies is a coarse sandstone with intercalated thin bands of impure limestone. The sandstone continues north-eastwards on the hills to within a quarter of a mile of Wairoa River and underlies swampy areas at the head of streams where the closely underlying greywacke has formed temporary base-levels. Small sink-holes are common in the white sandstone formation indicating that there are limestone beds beneath. Turner and Bartrum (1929) suggested that the beds of the Waitemata group represent the deltaic deposits of a large stream or streams draining a land-mass north of the present Waitemata area. From the more or less complete absence of fossil marine remains in great thicknesses of the beds it is apparent that the bulk of the sediments were laid down rapidly in an extensive, shallow sea during a period of progressive submergence. List of Foraminifera from Tertiary beds from Trig. Station 1946, E. of Hunua (identified by W. J. Parr). Nos. 1–9 on slide from Locality 1, light green sandstone. 1. Quinqueloculina sp. aff. lamarckiana d'Orb. 2. Nodosarian indet.; possibly Dentalina consobrina d'Orb. 3. Cassidulina subglobosa Brady. 4. Rotalia sp. aff. lessonii d'Orb. 5. Amphistegina sp. aff. lessonii d'Orb. 6. Globigerina bulloides d'Orb. 7. Globigerina sp. aff. inflata d'Orb. 8. Cibicides pseudoungerianus (Cushman). 9. Miogypsina sp. aff. irregularis (Mich.). Nos. 11–26 on slide from Loc. 2, dark-green sandstone. 11. Glomospira choroides (Jones and Parker). 12. Textularia sp. 13. Quinqueloculina sp. aff. lamarckiana d'Orb. 14. Dentalina consobrina d'Orb. 15. Nonion sp. aff. novozelandicus Cushman.

16. Nonion stachei Cushman (sp. just been described from the shell-bed, Target Gully, Oamaru). 17. Elphidium advenum (Cushman). 18. Cassidulina subglobosa Brady. 19. Eponides umbonatus (Reuss). 20. Gyroidina soldanii (d'Orb.). 21. Rotalia sp. nov. aff. clathrata Brady. 22. Amphistegina sp. aff. lessonii d'Orb. 23. Globigerina bulloides d'Orb. 24. Globigerina sp. aff. inflata d'Orb. 25. Cibicides refulgens Montfort. 26. Miogypsina sp. aff. irregularis (Mich.). Parr remarks “The most important species is Miogypsina sp. aff. irregularis, which Chapman recorded from Pakaurangi and Hokianga South Head. It was also listed from the Maraetai-Whitford area in Mr. C. W. Firth's paper. A species which is new to the Waitemata beds is Glomospira charoides. You will notice also that a species of the Miliolidae, Quinqueloculina sp. aff. lamarckiana is present; the family has not been recorded previously from the Waitemata beds. I commented on this fact in my note on the Bombay forams.” (c) Post-Tertiary Deposits. The Waitemata period of sedimentation was brought to a close by the Kaikoura orogeny (Cotton, 1916); and, following the subsequent extensive erosion, varied deposits of both sedimentary and volcanic materials accumulated in the present area. The most outstanding accumulations of this type are the river terraces so conspicuous in the Clevedon and Orere districts. Their materials consist chiefly of greywacke boulders, pumiceous sands and silts, and occasional pebbles of andesite. After the Kaikoura orogeny, the mid-Auckland area was subjected to prolonged erosion; and the land was reduced substantially to a peneplain which at present is a dissected upland. This was followed by long-continued uplift so that the strand-line was lowered relatively 600 ft. The uplift, however, was interrupted by several periods of standstill of sufficient duration to allow the streams to become graded with respect to current sea-level and to develop benches, still partially preserved at elevations of approximately 350 ft., 100 ft. to 120 ft., 40 ft. to 60·ft., and 15 ft. to 20 ft. above present sea-level (Turner and Bartrum, 1929). A sharp uplift of the order of from 150 ft. to 200 ft., caused the re-invigorated streams to entrench themselves in narrow, gorge-like valleys in their flood-plains, where these had been carved or built. This was shortly followed by à widespread, probably eustatic movement of depression whereby the coastal lowlands were submerged and the characteristic embayed shorelines of many North Auckland harbours formed. The only movement subsequent to this drowning has been an uplift of from 5 ft. to 8ft. in Recent times.

The most noticeable erosion level is that of the terraces 135 ft. above sea-level which adjoin Orere Stream and are related to the 150 ft. terraces on the Coromandel Coast opposite, as recorded by McKay (1897) and Fraser (1910). Their construction apparently commenced when movement along the bounding faults of the Hauraki depression had caused the uprise of the land-mass west of the present Firth of Thames relative to that now submerged beneath this latter embayment (Bartrum, 1927). Fractures seem to have developed earliest in the north and to have crept slowly south, whilst the extent of movement has been greater on the north than in the south, so that, as pointed out by Cotton (1916), the northern end of Coromandel Peninsula shows a coastline of submergence which contrasts strikingly with the fault-coast farther south. As the land rose, the invigorated streams spread their alluvial fans of gravels at the foot of the growing fault-scarp, and later consolidation has given them the compaction that they now exhibit in the lower conglomerate in the Orere Valley. Shortly, however, cessation in the supply of fan gravels was occasioned by a somewhat spasmodic, slow, general submergence accompanied by temporary interruption in the differential movement of the adjacent blocks. As the trough of the graben subsided, the sea crept slowly southwards; and the occurrence of a sea-beach, a few feet above swamp level at Maukoro, 18 miles south of the present shore of the Firth of Thames, indicates that the former shore-line was many miles farther south than now. (See Bartrum, 1927.) There is no doubt that the sea would have extended much farther south, but for the fact that the ancient Waikato River then followed a northerly course through Hinuera Valley (Cussen, 1893) into the valleys of the present Piako and Waihou Rivers and thence into the Firth of Thames. Here in sheltered waters the fan gravels were replaced by delta deposits. The materials of this phase of deposition were chiefly pumice silts brought by the river from the great central rhyolite plateau near Taupo. At Orere Point, these silts and accompanying mudstones represent the top-set beds of such a delta which increased in thickness as sea-level gradually rose. During its passage northwards, the river left in its train deposits that are now represented by remnants of fairly well consolidated beds of pumice and other debris west of the railway at Walton and Waharoa which must be correlated with those at Orere (see Bartrum, 1927, p. 251). Progressive sinking of the region ultimately allowed a thickness of material well over 1200 ft. to accumulate, as proved by well-bores near the above-mentioned places. Marked changes in the process of filling the graben are shown throughout its length. At Clevedon, Mataitai and southwards from Orere Point are terraces constructed of pumice which reaches an elevation of not less than 100 ft. above sea-level in the highest of these benches. Many of them are small, however, and the most constant terrace is one about 35 ft. to 40 ft. above sea-level.

Greywacke pebbles in layers interbedded with the pumice of the terraces are so badly decomposed as to indicate that the terraces represent· relics of an early filling of the graben far more extensive than that of to-day and deposited when the land was probably considerably lower with respect to sea-level than now. An uplift of at least 150 ft. followed this period of extensive sedimentation and in consequence the Waikato River is believed by Bartrum (1927) to have excavated a wide trough, removing the northern portion of its earlier delta and leaving as relics of it only the fringe of terrace pumice that now exists along the western shore of the Firth. Considerable depression, indicated to-day in the topography of the North Auckland harbours, followed; and Henderson and Grange (1927) show that at about this period the Waikato appears to have become so overburdened with waste that it was forced to aggrade its bed and spill over from its course through Hinuera Valley into a new route through Mangatautari to Cambridge, whence it followed approximately its present course along the earlier valley of the Waipa. Bartrum (op. cit.) here suggests that in conformity with the course of events at Auckland, the beds of pumiceous debris were removed in large part by the work of rejuvenated rivers following an uplift which raised them considerably above their present position. This brings us now into comparatively Recent times; and here we must consider the terraces of the Wairoa, Orere, and neighbouring streams. A. Terraces of the Wairoa River near Clevedon Township. Probably as a result of slight Recent uplift the Wairoa River is entrenching itself in a small flood-plain carved in resistant greywacke a few feet above stream-level near Clevedon township. In the neighbourhood, more especially alongside the road to Kawakawa Bay two flights of prominent terraces occur with such marked regularity of surface as to suggest genetic relations to earlier sea-level. The lower terrace stands about 35 ft. and the upper about 50 ft. above sea-level. The former is particularly prominent on the west side of the river, whence it continues northward and also extends as a considerable flat in the Clevedon-Papakura valley. It is not prominent, however, east of Wairoa River although perhaps represented in extensive terraces of about equal elevation a little north of Kaiaua (New Brighton). The main terrace on the eastern bank of the river is the upper one that is about 50 ft. in elevation above sea-level. Distinct small remnants of the higher terraces occur at Kawakawa Bay, east of Clevedon. A succession of small terraces, the lowest of beach gravels, and only a few feet above storm-beach level, and the highest about 30 ft. above sea-level, are eroded in the delta gravels of a small stream entering the bay. Professor Bartrum informed the writer recently that he had discovered a beach terrace of fine greywacke pebbles reaching up to

80 ft. above sea-level about half-a-mile north of Kawakawa Bay. This sets a very definite limit to the north edge of the pumiceous deposits laid down in the Firth of Thames. B. Terraces of Orere Stream and of the Adjacent West Coast of the Firth of Thames. Seawards from its confluence with Paratahi Stream, Orere River has in its valley several prominent flights of terraces, one of the most persistent being a gravel-built bench 30 ft. above the level of the entrenched, swiftly-flowing stream. A few cuspate remnants of terraces stand out at various intervals above this bench; and in the neighbourhood of Orere School, about 3½ miles from the sea, an extensive ancient plain begins approximately 135 ft. above sea-level at the mouth of the stream, rises gently inland, and spreads out in delta fashion seawards. At the base there is a strongly-cemented coarse conglomerate made up of considerably weathered pebbles of greywacke averaging from 2 in. to 3 in. in diameter. The shape of the pebbles indicates that they are alluvial in origin; and the inference that they were laid down in an alluvial fan is confirmed by the intercalated, thin, discontinuous beds of mudstone or shale, with some layers rich in altered vegetation. In places, small trees in the position of growth are encountered. In several small sections, both north and south of Orere Stream, bands of deep blue clay are conspicuous and, at a corresponding level, there occur in the lower conglomerate about half-a-mile south of Orere Stream, concretionary spherules of vivianite that has crystallised in radial fibres. The 35 ft. terraces noted above near Clevedon have their counterparts in the valley at the mouth of the Tapapakanga Stream, approximately 1½ miles south-east of Orere Stream. The terraces are incised in a relatively compact silt overlain by a thin cap of stream-gravels in the shape of an alluvial fan. Igneous Rocks. The only igneous rocks known from the district are andesites and basalt of small extent. Basalts are absent from the main elevated block, but west of it have erupted in at least three places along the line of the Wairoa fault and are widespread farther west. 1. Otau and Uplands at the Head of Ness Valley. Scattered over the elevated peneplain and neighbouring ridges at Otau, south-east of Clevedon township, are numerous boulders of volcanic rock intermingled with boulders of greywacke that rest on the cap of Tertiary sediments. No outcrops were seen in place; but the deep red colour of the soil suggests the presence of a large igneous mass intruded into the greywacke. The boulders are found fresh as well as deeply weathered and form groups in scattered areas over the whole peneplain. Pebbles of similar andesites occur among the deposits of Aro Aro Stream. In hand specimen, the rock is a hard, dark, coarsely porphyritic andesite, the phenocrysts being chiefly feldspar and ranging in size up to ⅛ in. in length.

Microscopically, the rock is a strongly porphyritic hyalopilitic pyroxene andesite with extremely abundant, strongly zoned plagioclase approximating to basic labradorite. The pyroxene is chiefly a strongly pleochroic hypersthene, though occasional large crystals of pale green augite also occur. The groundmass is microlitic, in some rocks being hyalopilitic and in others pilotaxitic. Accessory minerals are sphene and apatite. An analysis made by Mr. F. T. Seelye, of the Dominion Laboratory, through the kindness of Dr. J. Henderson, Director of the Geological Survey, proves the rock to be a normal andesite with dacitic affinities. It is interesting to note that the norm shows 17·38 per cent. of free quartz, though the microscopic examination failed to reveal any. Most of it, however, is probably distributed through the glassy base. Analysis B3150—Hypersthene andesite from Trig. 1946, Otau. % SiO2 59.67 Al2O3 17.10 Fe2O3 2.42 FeO 3.46 MgO 3.32 CaO 6.25 Na2 3.29 K2O 1.05 H2O < 105°C. 1.24 H2O > 105°C. 0.97 CO2 0.02 TiO2 0.79 ZrO2 nt. fd. P2O5 0.15 S tr. Cr2O3 nt. fd. MnO 0.12 BaO 0.04 SrO tr. NiO tr. — 99.89 Norm. Q. 17.38 Or. 6.23 Ab. 27.84 An. 28.79 Di. 0.96 Hy. 11.07 Mt. 3.52 Il. 1.51 Ap. 0.37 “II.4.3 (4).4” Tonalose. 2. Kawakawa Bay. Along the foreshore below the point at which the road from Clevedon descends into Kawakawa Bay, a line of some 20 or 30 large boulders, 3 ft. and more across, runs north-west for approximately 100 yards. The rock is similar to that of the boulders near Otau, but differs slightly in the greater proportion of hypersthene to augite, and these ferromagnesian minerals are now markedly more abundant. In respect of age, these andesites may, with confidence, be correlated with similar rocks forming prominent terrains at Whangarei Heads, North Auckland, and Coromandel Peninsula. In the last area, the andesites belong to what is termed the Beeson's Island Series, regarded as of Miocene age; and it appears safe to correlate the present rocks with this series, especially as both occurrences are closely related petrologically and occur in similar country.

3. Hunua Falls. The only other known igneous rocks in the district occur at Hunua Falls and Paparimu on the Wairoa River, the latter forming the boundary line between the Clevedon area and the block farther west. The rocks at both localities are Pleistocene basaltic flows and were described in detail by Healy (1935), who stated that they are porphyritic and vesicular basalts rich in olivine. At Hunua Falls, the rocks form a crater, over the southern lip of which the waters of Wairoa River tumble about 70 ft. Boulders of vesicular basalt are common throughout the river gravels between this locality and Wairoa South. Physiography. The district represents the most easterly of a series of earth-blocks which are bounded by major fractures trending north-north-west. This block, which is the most elevated of all, is divided into two, and consists of uplands rising to as much as 2,200 ft. east of the Kohukohunui fault. An interesting feature of the block, in common with neighbouring blocks, is the notable absence of folding or flexure; the Tertiary beds wherever visible are only very gently inclined. It is thus evident that faulting unaccompanied by folding was responsible for the movement of the blocks. The disturbances apparently took the form of normal block-faulting accompanied by subsidiary tilting, the latter continuing after major movements along the fractures had ceased. In 1929, Henderson envisaged the North Auckland Peninsula as a large arcuate block, broken up into many fragments, but with a pronounced south-westerly slope, the Firth of Thames and the Hauraki Plains being regarded as a foundered crest of the south-eastern extension of the main crustal arch, of which the uplands of the present area form a part, the arch having been forced upwards by lateral pressure. Description of Faults. (1) The Miranda Fault-Complex. The steep scarp of this fault may be followed southwards along the coast of the Firth of Thames, and is continued by the hill margin on the western side of the extensive plains that partially fill the Hauraki Graben. Henderson and Bartrum (1913) were unable to find any direct evidence of the fault in this southern continuation, but they stated that “if a line be drawn tangent to the eastern tips of the spurs of the Hangawera Hills, its extension northward will trace the boundary between the Paturoa Range and the plain and gulf further north.” It is possible, however, that in the south the fracture has given place to a monoclinal flexure such as Bartrum (1930) believes forms the extension of the Thames Fault-Complex on the east side of the graben. The occurrence of hot springs at Miranda is evidence supporting the conclusion that the Hauraki depression is bounded by a fault on the west side, just as on the east by the well-demonstrated Thames Fault-Complex.

(2) Lower Wairoa Fault. The presence of this fault was inferred by Firth (1930), although no direct evidence of movement was given. According to him, the abrupt slope that separates the bush-clad Maraetai Hills from the alluvial flat across which the Wairoa River meanders for two miles north of Clevedon, undoubtedly represents a maturely dissected fault-line scarp. Bartrum (1937) mapped this fracture as a curved continuation of the north-east striking Papakura Valley Fault of Laws (1931). Although this fault-line scarp bears away to the north, it is possible that a sub-parallel fault—a continuation of the Papakura Valley Fault—bounds the south side of Wairoa Estuary, where a similar clear differentiation with occasional shatter belts is shown. (3) Wairoa Fault. This very prominent fracture, first described by Laws (1931) determines the course along which Wairoa River makes its way north from Paparimu to Clevedon. Bartrum (1927) mentioned that the fracture extends north to the shore of Waiheke channel in Hauraki Gulf; and Firth (1930) has described details of its scarp. An extension farther north would intersect Motutapu Island along the contact between the Waitemata sandstones and the Mesozoic greywacke (Mead, 1930). At the junction of Cossey Creek and Wairoa River, just north of Hunua Falls, an excellent section of the fault is displayed. The plane of the fault is approximately vertical; and the Tertiary beds show prominent drag-folding. A shatter-belt nearly 10 ft. wide occurs along the fault-plane and consists chiefly of slickensided material (Healy, 1935). This fault-section, however, is of little value in computing the relative throw of the opposing blocks along the fault, as the beds involved cannot be matched on both sides. It is evident from the paucity of Tertiary beds both on the uplands east of the fracture and on the Hunua Depression immediately to the west, that erosion has played a prominent part in determining the present appearance of the scarp, which is probably a resequent fault-line scarp, or “fault-line erosion scarp” (Johnson, 1929). This fact makes it difficult to estimate the actual throw of the fault, but since areas on opposite sides of the fault have summit levels strongly suggesting that they represent a resurrected late mature or possibly peneplained surface, it is permissible to use their difference in height for calculating the relative movement. Using this assumption, Healy (1935) computed a downthrow to the west of approximately 600 ft. near Hunua, and Laws (1931) estimated the movement a little further north as of the order of 400 ft. (4) Mangatangi Fault. The upland block which forms the major portion of the district is bounded sharply in the south by a fracture which trends approximately north-east and produces a steep descent to the Pokeno-New Brighton lowland along which the Mangatangi and Mangatawhiri Rivers have their courses. These streams have cut deep gorges in the

elevated block and on reaching the lowland have extensively aggraded their courses with the formation of prominent terraces (Lyons, 1932). The throw, as computed by Lyons, is of the order of 600 ft. In the fault angle is a small area of Tertiary sandstones and brown coal of poor quality, the coal having been worked both here and more extensively at Kopuku some miles distant. where the Bridge-water Colliery was opened before 1877. Further indications of faulting are mentioned by Lyons (op. cit.), who noted the presence of slickensides in a tributary of the Mangatawhiri River and the steep abutment of Tertiary beds and coals against the scarp. This fault is believed by Bartrum (see Lyons, op. cit.) to unite with the Waikato Fault of Gilbert (1921); and in a recent map, Bartrum (1937) showed the Mangatangi Fault as part of the same fracture-system as the Pokeno Fault of Bartrum and Branch (1936) and the Waikato Fault. (5) Other Faults. Curious topographical features in two areas within the district seem to indicate faulting. The first is in the upland block at the head of Ness Valley at Otau. Here, an uplifted remnant of a peneplain about a mile and a-half square is bounded sharply on the east by a meridional, sharp-backed ridge about three miles long, which includes Kohukohunui or Mount London (2146 ft.). The discordance in summit level between this ridge and the undulating tableland to the west is about 800 ft. and from distant vantage points such as Waiheke and Motuihi Islands is striking. Between the two blocks, the headwaters of Paratahi Stream, a tributary of Orere Stream and the Mangatawhiri River are eroding in opposite directions. Two hypotheses are possible to account for this feature. First, faulting may be supposed; the presence of a fault along the line that separates Kohukohunui ridge from the lower plateau west of it would account not only for the wall-like appearance of the ridge, but also for the straight courses of the streams entrenched along that line, and, in the case of the Paratahi, the remarkable right-angle bend that it makes on reaching the Orere Valley. On the other hand, the eastern block, composed wholly of greywacke, may merely represent a monadnock left during the peneplanation of the remainder of the block (Mead, 1930). The second region that exhibits abnormal topographic features is the Ness Valley itself. This valley is remarkable on account of its marked asymmetry which impresses itself on the observer as soon as he enters the valley from the Clevedon end of the road that follows the valley. On the north-east side, the Aro Aro Stream follows closely at the base of steep, almost rectilinear, slopes towards which incline long, gentle, dissected slopes from the other side of the valley. Near the floor of the valley, the slopes are composed of Pleistocene and Recent river gravels and silts.

Geological Map of Orere and parts Wairoa, Opaheke, Wharekawa Survey Districts.

The most likely explanation, to account for the straightness of the stream and the asymmetry of the valley, is the presence of a small north-west trending fault. Consideration has, however, to be given to the probability that an abundant supply of waste brought by streamlets from the south-western greywacke slopes may have pushed the stream against the north-eastern wall. This does not explain, however, the asymmetry and straightness of the valley. It is indeed probable that there has been some effect as a result of the process mentioned, but it could merely be consequential on the earlier results of faulting. The Drainage of the Area. The upland block is drained by three systems of streams, viz. (a) the short, swift streams draining east down the scarp of the Miranda Fault System; (b) the larger, slower streams draining the western portion of the area; and (c) those draining the northern parts of the region. The Tapapakanga, Waimangu, Waihopuhopu and Puwhenua Streams are the main members of the first system, and none exceeds three miles in length. All are consequent and characteristically have built considerable alluvial fans of large boulders of greywacke. In their upper portions, they are deeply incised in the greywacke, and have produced close-textured topography with deep gullies separated by steep, narrow divides. The second group comprises four main streams, divisible into two sub-groups. The first includes the two largest streams of the area, viz., the Mangatangi and Mangatawhiri, which flow south-west over the greater part of the block. They commence as rivulets in deep gullies and, in a few miles, become graded streams with built-up extensive flood-plains; at still lower parts of their courses they enter steep, narrow gorges through which they are almost but not quite graded, having, particularly near the upper ends of the gorges, minor falls and rapids. The extensive aggradation of the Mangatangi and Mangatawhiri Valleys above their gorges is evidently a result of reversal of earlier gradient caused, in the main, by slow, progressive, northward tilting of the block during its uplift, a movement which has given rise to several other curious features in the drainage system. The headwaters of the Mangatangi are particularly interesting in this way. They drain eastwards from Kohukohunui towards the coast, later turning to the south-west in common with the other streams of the area. Orere Stream, a member of the third group, the upper reaches of which drain the western slopes of Kohukohunui, after entering a widely open hollow at lower levels, bends sharply and escapes through a narrow gorge to the east coast. The cause of these abrupt deviations of drainage is not quite clear, but, in the main, may be more or less satisfactorily explained by the above-mentioned tilting movement of the block and the formation of consequent streams as postulated by Bartrum (1927). This tilting is further borne out by the general lower summit levels from the south northwards, with the exception of Kohukohunui.

From a study of the map, it is evident that the Mangatangi River is in danger of becoming beheaded by the small swift streams that head inland on the eastern scarp. Appearances suggest, however, that the headwaters of the Mangatangi may represent an original east-flowing stream captured by a vigorous, consequent stream flowing to the south-west on the back-slope of the tilted block during its uplift. This river and its western companion, the Mangatawhiri, flow south-west in deep gorges until they reach the lowlands beyond the bounding faults and finally join the Waikato River. The second subgroup includes two smaller stream systems, viz. Upper Wairoa River and Cossey Creek, which, like the members of the first subgroup, also drain south-west. They differ from the above, however, in that they have not attained grade, so that they lack the wide open valleys and extensive flood-plains of the Mangatangi and Mangatawhiri. The final group of streams includes the Lower Wairoa, Orere, Aro Aro, etc. These streams drain the north and north-eastern part of the district and exhibit beautiful terrace formations all along their margins. In connexion with the Orere Stream, a curious feature is developed at the Orere Settlement sawmill. This consists of a wide basin or depression flanked by steep hills; and in this depression the stream has developed a right-angle bend from north to east, the cause of which is rather obscure. It may be suggested first that the Upper Orere Stream followed a fault, here postulated as defining the west flank of Kohukohunui, and that it was captured later (still at an early stage in the stream's history) by a stream working headward from the east, and that the broad open valley or basin at the settlement represents a result of some greater ease of weathering of the underlying rock. The small outlet gorge east of this hollow is partly, of course, a result of rejuvenation by uplift that has left its mark as terraces along the Lower Orere Stream. Apart from their terraces, the Aro Aro and Lower Wairoa Streams have no extraordinary features. Economic Geology. The Clevedon district has few deposits of economic value, and the only ones put to any use at the present time may be dealt with under two headings: 1. Greywacke. This rock covering the major portion of the district is used extensively for road-metal, being very hard and wearing moderately well. It is commonly termed “blue metal” and is not used otherwise than as roading material. 2. Manganese. At the head of Mangatawhiri Valley, near Otau, are two deposits of manganese, one of which may prove of some value. The first, worked by Mirandite Products Ltd., of Christchurch, is situated about 1½ miles south of Plow's Farm, and the other, on Piggot's Farm, a further 2 miles to the south.

Both these deposits have been examined by officers of the New Zealand Geological Survey, Mr. J. Healy, in 1935, and Mr. E. O. Macpherson, in 1940, and accounts of their investigations published. (Healy, 1937, and Macpherson, 1941.) Mr. Healy estimated that the trenching carried out at Mirandite Product Ltd's claim proved about 9,000 tons of high-grade ore (oral communication). Conclusion. This paper offers a unified description of the geology of an area that has received little attention from geologists, probably on account of the difficulties of access. Although the major portion of the area is constructed of somewhat unvaried greywacke, yet the topographical features and fluviatile deposits present many topics of interest. The writer is deeply indebted to Professor J. A. Bartrum, of Auckland University College, for his invaluable aid and advice in the compilation of this paper. Literature. Bartrum, J. A., 1927. The Western Coast of the Firth of Thames. Trans. N.Z. Inst., vol. 57, pp. 245–53. —– 1937. Notes on Excursion to Hunua, Aust. and N.Z. Assoc. Adv. Sci., Auckland Meeting. —– and Branch, W. J., 1936. Geology of Bombay-Happy Valley Area, Franklin County, Auckland, Trans. Roy. Soc. N.Z., vol. 65, pp. 386–404. Cotton, C. A., 1916. Structure and Later Geological History of New Zealand, Geol. Mag., Dec. 6, vol. 3, pp. 243–9, 314–20. Cussen, L., 1893. Notes on Piako and Waikato River Basins, Trans. N.Z. Inst., vol. 26, pp. 398–407. Firth, C. W., 1930. The Geology of the North-west Portion of Manukau County, Auckland, Trans. N.Z. Inst., vol. 61, pp. 85–137. Fraser, C., 1910. Geology of Thames Subdivision, Hauraki, Auckland, N.Z. Geol. Surv. Bull., No. 10. Gilbert, M. J., 1921. Geology of Waikato Heads District and the Kaawa Unconformity, Trans. N.Z. Inst., vol. 53, pp. 97–114. Healy, J., 1935. Geology of Hunua-Ramarama Area, Franklin County, Auckland (MS. unpublished). —– 1937. Manganese Deposits at Otau, Block XIV, Waimea S.D., Ann. Rep. N.Z.G.S., p. 22. Henderson, J., 1929. Late Cretaceous and Tertiary Rocks of New Zealand, Trans. N.Z. Inst., vol. 60, pp. 271–99. —– and Bartrum, J. A., 1913. Geology of Aroha Subdivision, Hauraki, Auckland, N.Z. Geol. Surv. Bull., No. 16. —– and Grange, L., 1927. Geology of Huntly-Kawhia Subdivision, N.Z. Geol. Surv. Bull, No. 28. Hochstetter, F. von, 1864. Geologie von Neu Seeland, Reise der Novara, Geol. Theil, Bd. 1, Ab. 1. —– and Petermann, A., 1864. Geology of New Zealand and Atlas, T. Delattre, Auckland.

Johns, D., 1929. Geomorphological Aspects of Rift Valleys, Compte Rendu. XV Session, South Africa, vol. 2, Scientific Communications, pp. 354–73. Laws, C. R., 1931. Geology of Papakura-Hunua District, Franklin County, Auckland, Trans. N.Z. Inst., vol. 62, pp. 37–66. Lyons, R. R., 1932. Notes on Geology of Mangatangi-Mangatawhiri District, Auckland, N.Z. Journ. Sci. and Tech., vol. 13, pp. 268–77. McKay, A., 1897. Geology of Cape Colville Peninsula, N.Z. Parl. Pap.—C3 (1897). Macpherson, E. O., 1941. Manganese Deposits in Franklin County, N.Z. Journ. Sci. and Tech., vol. 22, pp. 185B–192B. Mead, A. D., 1930. Physiography and Geology of the Uplands West of Firth of Thames, N.Z. Journ. Sci. and Tech., vol. 11, pp. 311–8. Turner, F. J., and Bartrum, J. A., 1929. Geology of Takapuna-Silverdale District, Waitemata County, Auckland, Trans. N.Z. Inst., vol. 59, pp. 864–902.