Geological History of Papakura-Hunua Area.
Beds of the Hokonui System form the oldest surface rocks yet discovered in the North Island, though there is evidence of a pressureaffected earlier terrain, for Bartrum (1921a, pp. 120–21) recorded the presence of granulites in conglomerates intercalated in the basement Mesozoic shales and greywackes of Great Barrier Island, whilst he found similar evidence in another conglomerate near Whangarei (1921b, p. 128). Later the same writer (1924a) stated his conclusions that the granulated and sheared plutonic rocks of various conglomerates represent a now-buried terrain existing before and during the deposition of Trias-Jura sediments, and that this terrain apparently persisted into Tertiary times, for mid-Tertiary conglomerates with similar material appear not to be a re-wash of mid-Mesozoic ones.
In the present area, however, the conglomerates of Tertiary age that have been described entirely lack material of the kind recorded by Bartrum.
Benson (1923, pp. 37, 41) mapped the coastline of the land-mass that supplied the Trias-Jura sediments as trending approximately north and south adjacent to the west coast of New Zealand, and stated (op. cit., p. 39) that this continent, which extended westwards over the present Tasman Sea and united New Zealand with Australia, remained above sea level until near the close of Jurassic times. At the close of the Trias-Neocomian period of sedimentation, however, the New Zealand area was elevated to form dry land, and the sediments were folded into mountain ranges, of which the “oldermass” of this paper forms the worn-down relics, for, as Cotton (1916) showed, long-continued erosion followed this orogeny.
Over the region now represented by North Auckland the truncated folds of this land-mass were next submerged beneath the Upper Cretaceous seas, where they were covered unconformably by ammonite-beds and the limestones, claystones and other beds of the Onerahi Series. In their turn, these Upper Cretaceous strata appear to have been uplifted and exposed to erosion and to have been removed from extensive areas. As a result of this emergence, and the longer one that preceded it, the land-mass had attained a very advanced stage in the cycle of erosion when depressional movements supervened, and it descended beneath the Tertiary seas to receive a cover of younger sediments. At first, basal conglomerates and associated marine beds were laid down, and then, during a temporary pause in the movement of depression, extensive swamps or lagoons came into existence near the coast. Over the present Lower Waikato area the coal-forming vegetation flourished more abundantly and the swamps assumed greater proportions than elsewhere, so that the seams of brown coal of that district attain considerable thickness and have considerable commercial importance. Near Papakura and Hunua, however, they are comparatively poorly developed and gradually thin out to the north, until near Auckland they have disappeared entirely. Subsequent progressive subsidence obliterated these lowlying swamps, and the coals became covered by the marine strata that now constitute the Papakura and Waitemata Series.
It is demonstrated by the characters of the sediments of the youngermass, and by the constant evidence of contemporaneous erosion they display, that the area was at no time deeply submerged and that the general movement of subsidence was of an oscillatory nature. Finally elevation ensued, and the compound mass became subject to normal subaerial denudation. Block-faulting, totally unaccompanied by folding so far as can be determined, then affected the area, and this was evidently a phase of the Kaikoura orogeny (Cotton, 1916, p. 248) which began probably in Pliocene times.
The subsequent history of the district is traceable only by reference to events near Auckland, which have been discussed by Bartrum (1922; 1929) and Turner and Bartrum (1929). Those of major importance include the filling of the downthrown block west of Hunua Range by the pumiceous silts brought by the Waikato River, and the subsequent sharp uplift by which deep trenches were excavated. These latter were later drowned by a submergence, sub-equal in amount to the uplift, which seems to have been the last diastrophic event of any real importance. Cotton (1916, p. 318) stated that these concluding movements, which he termed the post-Kaikoura movements, were epeirogenic rather than orogenic in character, and that there was evidence of a long period of rest between them and the earlier Kaikoura orogeny, during which the current cycle of erosion reached an advanced stage of development.
The upland block of Hunua Range has been sub-maturely dissected, but rejuvenation is shown in the fact that locally the streams have recently entrenched themselves to the extent of 6 ft. to 8 ft. below earlier flood-plains. Sub-Recent uplift has affected the whole Auckland area, for there are raised platforms and beaches, sometimes as much as 9 ft. above sea-level, around the shores of Hauraki Gulf, but it is doubtful if the rejuvenation can be a result of this movement, for it can scarcely have been reflected to so considerable an extent at parts of the stream courses that are relatively distant from the sea.