The Western Coast of the Firth of Thames.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 57, 1927, Page 245

The Western Coast of the Firth of Thames. By J. A. Bartrum. Auckland University College. [Read before the Third Science Congress of the New Zealand Institute, Dunedin, 28th January, 1926; received by Editor, 9th February, 1926; issued separately, 7th December, 1926.] The writer recently travelled from Clevedon, a township situated at the upper tidal limits of Wairoa River, which flows into the enclosed harbour waters south of Waiheke Island near Auckland, eastwards by road to the west shore of the Firth of Thames and then south along that shore to New Brighton. From this point almost to the head of the Firth there is a sweeping shell beach backed inland for some five or six miles further south as far as Miranda by wide swampy flats. From New Brighton return was made by way of a road which, after crossing the main divide, follows down the broad valley of the middle portion of the Mangatangi Stream until it joins the Miranda-Pokeno road. Although the area of which a glimpse was thus obtained is correctly shewn on geological maps of New Zealand as constituted by greywackes and associated sediments now assigned to the Hokonui System of Trias-Jura age, the writer has been unable to find any descriptive account of its geology or any reference implying knowledge of certain interesting features it possesses. Fault Systems of the Area. In conjunction with Mr. C. R. Laws the writer recently closely studied an area between Papakura and Hunua west of that shewn on the present map, and found that there are two systems of faults blocking out the district, and that these are the same as those recently mapped by Henderson (1924a) which originate in the central or Taupo region of the North Island. Their directions in the present area are respectively approximately N.E.-S.W. and N.W.-S.E. It will be remembered that N.N.W.-S.S.E. faults are believed to be responsible for the development of the graben-like depression occupied by the Firth of Thames and the Hauraki Plains, and that the existence of a great fracture on its eastern margin has long been clearly demonstrated. On its western margin, however, the evidence so far adduced has not been at all conclusive, apparently on account of inadequate knowledge of the west coast of the Firth of Thames; for, in actual fact, this coast furnishes topographic and other evidence so indicative of the presence of a great fracture that its existence can scarcely be questioned. Another member of this N.W.-S.E. system of faults follows the middle course of Wairoa River and then, after crossing the lowlands connecting Papakura with Clevedon, which are themselves the filling of a fault-angle depression due to a fracture of the N.E.-S.W. series, is traceable by means of a very distinct fault and fault-line scarp

north-west towards the Waikohu Estuary on the outer harbour coast east of Auckland. The N.E.-S.W. fault responsible for the Papakura-Clevedon depression has been named the Papakura Valley fault*Laws, C. R., and Bartrum, J. A, The Geology of the Papakura—Hunua Area. M.S. in possession of Auckland University College.; it is marked by an unmistakeable fault-scarp on the north-west border of the depression towards which the earth-block represented by the Hunua Range is sharply tilted. At Clevedon the fracture appears to bend more to the north and thence to follow closely the coast trending north from the mouth of Wairoa River. A most instructive idea of the structure of the area shewn in the accompanying map can be obtained from distant points such as Waiheke Island or Motutapu. There are two erosion levels clearly recognizable in relics constituting broad flat interfluves, which are seen to rise successively in steps from south-west to north-east above the lowlands bordering adjacent arms of the Manukau Harbour. The first of these levels is constituted by the area west of the Wairoa River fault, and the second by the elevated block north-east of the middle course of the Wairoa, which has especially well-preserved plain-remnants at a maximum elevation of between 1,300 ft. and 1500 ft. This latter block, in common with neighbouring earth-blocks, appears to have a tilt towards the north-west; its back-slope on the south-east is deeply eroded by the Waiora, Mangatawhiri, Mangatangi and other streams, and the topography suggests strongly that there is considerable relative downthrow of the area south-east of this slope along a N.E.–S.W. fault which passes to the Firth of Thames near New Brighton. Study of the map will shew that the elevation called Kohukohunui (2,145 ft.) rises very definitely as an elongated dome above the 1,300 ft. to 1,500 ft. erosion surface just mentioned; the disparity of heights is very striking in distant views, and it is not improbable that Kohukohunui represents a block elevated above that west of it along a N.W.–S.E. fracture. Such a fracture may well have topographic expression in the broad straight valley of the middle portion of Mangatangi Stream. On the eastern flank of Kohukohunui there is a particularly rapid descent to coastal lowlands built in part of fan-débris, and for many miles this steep slope is an almost rectilinear scarp which, in conjunction with the presence of hot springs at Miranda, is strong evidence in support of the existence of a faulted western margin to the Hauraki depression, as was surmised by Lindgren (1905) when he termed this latter a graben. Though failing to find evidence of this western fault in the Aroha Subdivision, Henderson (1913, p. 51) accepted Lindgren's views as to its presence and gave it the name of the Miranda Fault. He further pointed out that in all probability it actually is a fault-complex and not a simple fracture. Terraces near Clevedon. The Wairoa River is entrenching itself near Clevedon township in a small flood-plain carved in resistant greywacke at a height of a few feet above stream level. Nearby there rise two flights of very

prominent terraces with regularity of surface which is suggestive of genetic relations to earlier sea-level. The lower terrace rises about 35 ft. and the upper approximately 80 ft. to 100 ft. above sea-level. The lower continues north on the west bank of the Wairoa River and also extends as a great plain in the Clevedon-Papakura lowland, but is not at all prominent east of Wairoa River, though there are extensive terraces of about equal elevation a little north of New Brighton. The 80 ft. to 100 ft. terrace, on the other hand, has important development only east of the Wairoa River. A short distance north-east of Clevedon it encircles in interesting manner an island of more ancient greywacke 278 ft. high, on the summit of which is trig. station 646. As will shortly be shewn, the terraces are built of material deposited from still waters, so that depression, which caused the submergence of valleys substantially the same as the present ones of the Wairoa, Ness and adjacent streams, began before and, as we shall see later. continued during the building of the terraces. The lower or 35 ft. terrace appears to have been carved by the Wairoa and other streams, but the higher one skirts around the ends of divides separating adjacent streams and so passes from the Wairoa into the Ness valley. The road from Clevedon follows close to the northern margin of this terrace, which is succeeded seawards by an extensive modern delta-flat built in the enclosed sea-waters by the Wairoa and Ness Streams. Near the mouth of the Wairoa estuary, however, an island of greywacke rises prominently above the low surface of the delta and the adjacent sea. In such exposures as were examined the material of the terraces was found to be a fine-grained pumice-silt in which there are occasional beds of fine pumice-conglomerate with pebbles about ¼inch in diameter. Near the bridge over the lower Ness Stream, the silt is laminated in a manner exactly reminiscent of varve shales; the laminae separate readily one from another and yield impressions of leaves and other plant-remains. It is obvious that the material has been deposited in very sheltered still water. Near Papakura there are similar deposits of pumice-silt, though they do not rise so high above sea-level, and it is probable that similar silts underlie the sub-recent swamps and buried forests of the Papakura-Clevedon depression. The south-eastern fringes of the lowlands of that area, which rise considerably above the swamps, are carved, however, in soft Tertiary (Papakura Series) sandstones. At Kawakawa Bay east of Clevedon, the higher terraces, though inextensive, are distinctly exhibited, whilst 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. Terraces of Orere Stream and of the Adjacent Western Coast of Firth of Thames. Seawards from its confluence with Paratahi Stream, Orere Stream exhibits in its valley several flights of terraces. One of the most persistent is a gravel-built bench rising about 30 feet above the gravel-strewn bed of the entrenched swift-flowing stream. Slightly above

this there are occasional cuspate remnants of terraces and then at the distance of about three miles from the sea, there begins an abrupt rise to the top of an extensive ancient plain, approximately 135 ft. above sea-level near the shore, but rising gently inland, which spreads out in delta-fashion at the mouth of the stream. At the shore-line the sea-cliffs yield a splendid section of this lofty terrace. At the base there is a strongly cemented coarse conglomerate exposed for a depth of 25 ft. and passing down below sea-level, which is made of considerably-weathered pebbles of greywacke averaging 2 ins. or 3 ins. in diameter. The shape of the pebbles shews that they are ancient alluvial fan-gravels, and further evidence of such origin exists in the presence of intercalated thin discontinuous lensoid beds of mudstone or shale, which, with certain layers rich in altered vegetation, including small trees in the position of growth, evidently represent the filling of sheltered pools. Next above the gravels there are very fine-grained whitish pumice silts and greyish muds bedded horizontally and containing many carbonaceous bands rich in plant-remains, some of which are partially carbonised and some not. One such band in particular contains stumps of trees six inches and more in diameter, with horizontal radial roots exactly in the position of growth. Above the silts and muds there are further coarse conglomerates about 30 ft. in depth where exposed in the sea-cliffs, but doubtless increasing in thickness inland. As with the lower conglomerate the material appears to be wholly greywacke with argillite and allied sedimentary facies derived from Hokonui strata; it is very firmly consolidated and shews fairly general weathering, so that locally it is converted wholly to blotchy clays. About half a mile or more south of Orere Stream the lower conglomerate locally contains abundant concretionary spherules of vivianite which have crystallized in radial fibres in the interstices between the pebbles. At the mouth of Tapakanga Stream, the first important stream south of Orere Stream, there is reappearance of the 35 ft. terraces. These are carved in very pure relatively compact pumice-silt which is capped by a thin veneer of stream-gravels spread out like an alluvial fan. Shortly, however, these terraces are interrupted by a tongue of greywacke pushing out to form the sea-cliffs for perhaps half or three-quarters of a mile, but beyond this they reappear at Waimangu Stream, where they form a distinct salient of the coast. Near their seaward margin they are approximately 35 feet above sea-level, but this height increases upstream towards the head of the ancient alluvial fan. Below their surface the Waimangu Stream has cut down a narrow valley with flights of small terraces on its sides. Terraces of similar height have very extensive development several miles further south, though it was not ascertained whether or not pumice silts underlie the surface gravels. They have been considerably dissected by streams which, like the Waimangu, have often developed flights of terraces in the somewhat flaring valleys carved in their material. From the westward margin of this extensive zone of terraces the mountain slopes rise steeply in the scarp of the earlier-mentioned Miranda fault. Seaward there are either

low sea-cliffs now more or less protected by a fairly broad beach of cobbles as at the mouth of Waimangu Stream, or else, as is more general, there are forelands built almost wholly of a succession of cobble-beaches, although here and there, as near Colonel Adams's property, small streams have infilled lagoons enclosed behind cobble loops. No fewer than five or six successive beach-ridges can be counted on parts of the extensive foreland crossed by the Waihopuhopu, Puwhenua, and neighbouring streams; it is difficult without accurate means of obtaining heights to be sure if the earlier of these are or are not substantially above the level of the modern storm-beach, though they appear to be so. At Waihihi, however, there are remnants of beaches uplifted at least five or six feet, whilst near Ohinemahoi a low sea-cliff now retrogressing under wave-attack shews a beach-deposit of typical lensoid pebbles resting at a height of six or seven feet above the highest cobbles of the modern beach upon a platform of greywacke. A glance at the map shews how considerable is the progradation of the shore-line near and north of New Brighton; its explanation is to be found in the abundant supply of pebbles and boulders of greywacke by wave-erosion on sea-cliffs of greywacke further north and by vigorous streams draining the western mountain slopes. Near the mouth of Orere Stream the supply must be especially abundant, since the Orere is a particularly vigorous stream and, in addition, destruction of the conglomerates in the sea-cliffs must yield plentiful pebbles and boulders. Origin of the Terraces and History of the Area. In a recent paper on the post-Tertiary history of New Zealand Dr. J. Henderson has summarized our knowledge regarding evidences of uplift gained from elevated wave-cut benches and other coastal terraces, and has attempted the almost impossible task of evolving order out of a chaotic list of facts. The Tertiary closed with the earth-movements of the Kaikoura orogeny (Cotton, 1916) which dislocated the Hokonui (Trias-Jura) strata and their covering of younger rocks along fault-lines such as those traced in this paper. Henderson (1924b) shews how the land was at one time 1000 ft. or more higher than now, but was later depressed until the old strand-line was submerged to 1000 ft. or more below its present level. The elevation began during what he calls younger Pleistocene time, and “high-level terraces bordering river-valleys, and littoral deposits forming coastal platforms or veneering wave-cut benches” were brought into existence. A general movement of elevation continued until the land was several hundred feet higher than now, but was succeeded by depression which raised the strand-line about 120 ft. above that of to-day and so developed a very persistent erosion and deposition-level at that height. The last considerable movement he believes to have been an uplift bringing such terraces to their present elevation. There can be no denial of the fact that oscillations of level have occurred which roughly have the order set forth by Dr. Henderson, but analysis of evidence from different areas leads to apparent contradictions, which may be illustrated by citation of the apparent

order of events in the vicinity of Auckland, disregarding vulcanism. A prolonged post-Miocene period of erosion with ensuing interrupted uplift, led to the development of a broad peneplain, now a dissected upland, with two lesser erosion-levels at about 90 ft. to 100 ft. and 25 ft. to 35 ft. or more respectively. A sharp uplift, probably in the vicinity of 200 ft. in amount, caused the reinvigorated streams to entrench themselves in their flood-plains where these had been carved or built, but was shortly followed by a widespread probably eustatic movement of depression which gave rise to the submergence of coastal lowlands and to the embayed coastlines characteristic of so many of the harbours of North Auckland, and which quantitatively seems to have been nearly equal in amount to the preceding uplift. Since then there has been sub-recent minor uplift testified to by raised beaches and wave-cut benches in numerous localities. The explanation of these apparent inconsistencies in the evidence perhaps lies partly in errors of interpretation, yet it is more probable that it is in the fact that the full history of any area is likely to be more complicated than we realize. Further, it must not be forgotten that the differential movements of fault-blocks by which New Zealand reached approximately its modern outlines during the Kaikoura orogeny may have continued practically to sub-recent times; the coastal outlines of adjacent blocks could then expectably shew very different sequences of events except in so far as they were affected by movements of a eustatic character. The higher terraces of approximately 135 ft. in height on the west coast of the Firth of Thames are related to those about 150 ft. in height on the other side of the Firth near Thames, which have been recorded by McKay (1897) and Fraser (1910), but supply greater detail of their history than these latter. The evidence points to the probability that their building began when movement along the bounding faults of the Hauraki graben had caused the elevation of the land-mass west of the present Firth of Thames relative to that now submerged beneath this latter embayment. Swift streams spread fans of gravels at the base of the developing fault-scarp and these have been consolidated to form the lower conglomerate at Orere. Shortly, however, a somewhat intermittent slow general submergence began, whilst differential movement of adjacent blocks temporarily ceased, so that the supply of fangravels was suspended and instead there was the building in sheltered waters of the delta of some large river—the Waikato in one of its early phases—which brought pumice from the central volcanic area near Taupo. The pumice-silts and the mudstones of Orere represent the topset beds of such a delta which increased in thickness as sea-level gradually rose. Between the Arapuni and Maungatautari gorges of the Waikato River, Henderson (1918, p. 58) has described deposits ascribable to the Waikato during this early phase, and even enclosing buried forests which have recently been disclosed in excavations for a deviation-channel in connection with the power scheme at Arapuni, and which indicate temporary cessation in deposition in conformity with the evidence at Orere. From

the gorge at Arapuni the early course of the Waikato followed north down the Hinuera Valley (Cussen, 1888; 1894) into what are now the valleys of the Piako and Waihou Rivers, where the river laid down deposits of which remnants exist in moderately well consolidated beds of pumiceous and other débris constituting the downs west of the railway at Walton and Waharoa. These beds, thus, must presumably be correlated with those at Orere and mark the deposits of a period when the strand-line, at the time of the maximum subsidence, was at least 100 ft. higher than now; their extent must have been very great, for they seem to have occupied almost the whole of the Firth of Thames south of the mouth of Wairoa River. Whilst recognising the fact that his evidence is far from conclusive, the writer would hesitate to synchronise this period of negative movement of the land with the period of depression given by Henderson (loc. cit.) as the penultimate diastrophic movement of post-Tertiary time. He would suggest that, in conformity with the course of events at Auckland, after the deposition of the beds of pumiceous débris, an uplift took place which raised them well above their present levels, and which enabled the rejuvenated rivers to remove the greater part of these deposits. Close upon this there followed widespread depression of the land which had as its result the embayed coast-lines of Auckland, and which caused an advance of the waters of the Firth of Thames far south of their present southern shores. It is difficult to understand how, under the conditions of level of land and sea that obtain to-day, the removal of such vast quantities of earlier deposits, as undoubtedly have been removed, could have been effected. The very fact that the sub-recent and modern delta of the Piako, Waihou, and other streams has advanced 18 miles from former beaches at Maukoro, shews that such removal could not have occurred unless the erosive processes were stimulated by uplift. A further argument in favour of regarding the terrace-deposits as more ancient than those laid down during the last period of depression is based on the degree of consolidation and of weathering of the conglomerates at Orere. All deposits known definitely to have accumulated since this last submergence began are poorly consolidated and practically unweathered. The presence of a conglomerate composed of coarse stream-gravels above the pumiceous silts and muds near the mouth of Orere Stream has yet to be explained, and implies a return of conditions similar to those which previously allowed the building of the lower conglomerate. Presumably there was renewed uplift of the earth-block west of Miranda fault with reference to that east of it at the conclusion of the period of subsidence already discussed. The differential uplift cannot have continued long, for general uplift of the whole region commenced after a comparatively small depth of fan-gravels had been deposited by streams upon the earlier silts and muds. This uplift was shortly interrupted for a sufficiently long period to permit the development of the extensive series of terraces here called the 35 ft. terraces, and was then continued until these latter stood far higher than at present, as has already been suggested.

So far as the writer is aware there are no accumulations of pumiceous material in the Lower Waikato—Manukau Harbour area reaching so high an elevation as the surface of the high-level terraces (80 ft. to 100 ft.) near Clevedon and at Kawakawa Bay, so that deposition of the material of these terraces probably was synchronous with that of the Orere Terraces. The pumice-silts of the Lower Waikato—Manukau area, on the other hand, are later deposits which were laid down in an estuary formed at the mouth of the Waikato River after it had been diverted to the west from its earlier course by way of Hinuera Valley to the Firth of Thames. Igneous Intrusions in the Hokonui Sediments. A careful search was maintained in the gravels of streams, draining the greywacke terrain of the area described in this paper for pebbles of igneous rock, but none were found apart from basaltic ones in the gravels of Wairoa River which intersects basaltic flows giving rise to the falls near Hunua. On the shore of Kawakawa Bay, however, near where the road from Clevedon comes to the beach, there are about twenty large scattered masses up to 3 ft. and more in diameter of an andesitic rock, which have probably been shed from some intrusion which the writer was not able to locate in the short time at his disposal. The rock itself is a strongly porphyritic hyalopilitic pyroxene andesite. with very abundant plagioclase (acid labradorite) in both generations and a small amount of glass in the groundmass. The pyroxene is chiefly in phenocrysts and includes both augite and hypersthene. Contrary to their abundance at Coromandel Peninsula, Whangarei Heads, and Great Barrier Island, intrusions of igneous rock are rare in the sediments of the Hokonui System nearest to Auckland. Besides the present example the writer knows of only one other, namely an outcrop of what appears macroscopically to be a porphyrite, which forms a knoll near Cowes Bay at the east end of Waiheke Island. Conclusion. The western coast of the Firth of Thames is defined by a fault (the Miranda fault of Henderson) which forms the western boundary of the Hauraki graben. On the east of the fault-line there is a discontinuous zone of uplifted ancient gravels and fine-grained pumiceous sediments representing early deposits upon the floor of the graben, which are now raised into conspicuous terraces fringed along a considerable portion of their seaward margin by beach-ridges and other deposits characteristic of a prograded shore-line. The pumice is believed to have been transported by the Waikato River when the latter followed an earlier course down the Hinuera Valley into the Hauraki graben, and an attempt has been made to trace the succession of diastrophic events by which the development of the topography of the area under consideration has been controlled. High-level terraces at Kawakawa Bay and near Clevedon are considered to have been built, like those east of the Miranda fault, during the period prior to the diversion of the Waikato River to its present course.

Map of an area west of the Firth of Thames shewing the Distribution of Post-Tertiary Sediments. List of Papers Referred to Cotton, C. A., 1916. The Structure and Later Geological History of New Zealand. Geol. Mag. (n.s.), dec. 6, vol. 3, pp. 243–249 and 314–320. Cussen, L., 1889. Notes on the Waikato River-Basins. Trans. N.Z. Inst., vol. 21, pp. 406–416. —1894. Notes on the Piako and Waikato River-Basins. Trans. N.Z. Inst., vol. 26, pp. 398–407. Fraser, C., 1910. The Geology of the Thames Subdivision. N.Z. Geol. Surv. Bull., No. 10 (n.s.). Henderson, J., and Bartrum, J. A., 1913. The Geology of the Aroha Subdivision. N.Z. Geol. Surv. Bull. No. 16 (n.s.). Henderson, J., 1918. Notes on the Geology of the Waikato Valley near Maungatautari. N.Z. Journ. Sc. & Techn., vol. 1, pp. 56–60. —1924(a). The Structure of the Taupo–Rotorua Region. N.Z. Journ. Sc. & Techn., vol. 6, pp. 270–274. —1924(b). The Post-Tertiary History of New Zealand. Trans. N.Z. Inst., vol. 55, pp. 580–599. Lindgren, W., 1905. The Hauraki Goldfield, New Zealand. Eng. & Min. Journ., New York, 2nd Feb. See also N.Z. Mines Record, vol. 8, p. 370. McKay. A., 1897. Report on the Geology of the Cape Colville Peninsula, Auckland, N.Z. Parliam. Paper C.—9.