(a) Trias-Jura: Hokonui System.
The basement rocks of the area described are greywackes which are microscopically similar to others general near Auckland, and contain a moderate quantity of feldspar (dominantly various plagio-clases), a variable amount of quartz in angular grains, tiny pellets of fine-grained andesitic lavas and fragments of pyroxenes and of hornblende showing different degrees of chloritisation. They vary in texture to a limited degree and invariably are massively bedded and traversed by numerous irregular joints which are generally occupied by thin seams of quartz, but occasionally, as in a small quarry about a mile north of the tennis courts at Happy Valley, by calcite with rare prehnite.
About 6 miles east of Bombay the greywackes contain a deposit of manganese oxide ore which, though small in extent, is probably worth commercial exploitation. Here and there, according to local report, they include minor quartz veins with small and unpayable content of gold.
The manganese ore is the product of oxidation of one or perhaps two manganiferous siliceous lodes outcropping about a mile south-west of Trig. 616 on the southern slope of a valley which drains south to the flats near Pokeno. When examined by the authors early in 1934, it appeared as if there were two intersecting veins, but the excavations made were too few to enable this point to be established, and one vein may merely be the fault-displaced continuation of the other. Near creek level a 10ft. body of ore is exposed and is largely hydrate or oxide of manganese with a considerable quantity of reddish haematite-stained siliceous material; it has a strike of 70° and dips at 65° to the north. About two chains further east, up the hill-slope, at an elevation of 700ft., a sub-vertical 10ft. vein, again with masses of reddish quartz included with the ore, is enclosed between walls of shattered greywacke and has a strike of 190°. The same lode is again visible in prospecting pits a chain and a-half further uphill, but is irregular and widens to 25ft. where
exposed in an open cut, whilst a little further on it appears to terminate against an oblique fracture with a strike of 158°, though additional prospecting might locate its continuation.
No internal evidence as to age is afforded by the greywackes, so that they are relegated to the comprehensive Trias-Jura Hokonui System on analogy with lithologically similar rocks elsewhere.
(b) Tertiary Beds.
In the Tertiary succession as determinable there are considerable variations in the nature of the beds pertaining to what appears to be the same horizon in neighbouring localities. Satisfactory palae-ontological evidence for correlation is absent, so that there is no present justification for any attempt to match the various strata with those of more northerly areas east of Auckland where some detailed study already has been made (see Laws, 1931; Firth, 1930), though they may well prove to be basal continuations of the Wai-temata Series.
As already shown earlier in this paper, the relief of the grey-wacke terrain when the basal Tertiary beds were being deposited was locally sufficient to give rise to the accumulation of coarse conglomeratic beds on the margins of enclosed basins, whilst in the deeper waters lacustrine shales were laid down, rich in dicotyledonous leaves* and occasionally yielding fresh-water molluscan shells.
The conglomerates outcrop in only two of the Tertiary areas described, one of which is east of Bombay, where they may be seen in two small quarries, one three hundred yards east of where Bombay-Paparata Road crosses Ingaia Stream, and the other a quarter of a mile south of this latter point. The other Tertiary area is the main central one, and the place of outcrop is near mid-Ingaia bridge south-west of Dome Hill. where these beds are represented by very coarse breccia-conglomerates, not exceeding 9ft. in maximum thickness, which rest on a greywacke surface inclined west at angle of from 20° to 25°. They grade upwards into a few feet of sandstones with broken plant remains, the dip flattening very rapidly westward; the sandstones are followed in turn by greyish fresh-water shales with well-preserved leaf impressions. In the area east of Bombay only a few feet of conglomerates are exposed, whilst neither upper nor lower surface is visible, so that their thickness is unknown. Neither is there any continuity of exposures, so that the exact sequence of the Tertiary succession there is a little doubtful.
Above the conglomerates, in all probability, come medium-grained white quartz-sandstones with plant-impressions; these outcrop in two places, one about half a mile north-west of the crossing of Ingaia Stream by Paparata Road and the other against steep and high greywacke slopes at the north-east corner of this Tertiary basin, about 1 ½ ml. south of the trig. station on Dome Hill. These beds are believed to represent about the same low horizon as coal and associated shales in a small mine on the north flank of Puketutu (see Map), which were also encountered, according to local report,
[Footnote] * Unger (see Hochstetter, 1864) describes several species from similar beds near Drury.
about 1 ¼ ml. north-north-east from there in a well sunk on the floor of the valley of the middle head-water tributary of Ingaia Stream. At the now-abandoned mine, the base of the coal seam was not exposed when visited by the authors, but 4ft. 6in. of lignite was visible dipping south-south-west at 7° beneath fissile grey shales with leaf-impressions. A few yards east of the collapsed drive of the mine, greywacke was encountered in a short prospecting drive, so that the extent of coal available at this locality must be very small, for the direction of dip will cause the seam soon to abut south against the greywacke of the northern lower slopes of Puketutu, whilst erosion will have removed any northern continuation unless carried to lower levels by change of dip or faulting, such as indeed appears actually to have occurred, if one may credit the reported discovery of coal in a well a mile and a half north-north-east from the abandoned mine.
Apparently succeeding the fresh-water shales in the Tertiary succession of Bombay Basin are greenish unfossiliferous sandstones, believed to be marine in origin, outcropping about 150yds. east of where Paparata Road crosses Ingaia Stream, which are poorly exposed for about 30ft. in depth, though their full thickness is not determinable. As their base is below the level of outcrop of the conglomerate 200yds. north of them (645ft.), it may be that, as in the section near mid-Ingaia bridge, this latter bed forms merely a veneer to an inclined surface of greywacke. Above the sandstone there is an eroded cap, not less than 30ft. thick, of foraminiferal shales conformably above the sandstone and appearing to dip with it to the south at 5°. They are well exposed at the road-junction about ¼ ml. east of Ingaia Stream and reappear about 300yds. further along Paparata Road, whilst the greenish sandstones outcrop again in a paddock about 150yds. east-south-east of the road-junction.
From the evidence of facts elsewhere it is clear that the marine beds described represent merely a remnant of an extensive sheet of such Tertiary beds now removed by erosion. There is no reason indeed for suspecting that, throughout the whole area described in this paper, there were not the same conditions as led to widespread marine deposition in the Tertiary in the Papakura-Ardmore-Hunua district further north, which was described by Laws (1931). If a cover of younger strata extended broadly over the whole area at the close of Tertiary sedimentation, the present limited extent of such beds is merely a reflection of the degree that the region has suffered faulting or other tectonic movements, which have caused relative uplift of certain areas thereby facilitating the removal from them by erosion of the weak younger beds.
In the main central area of Tertiary strata the succession is more complete than in Bombay Basin. As Ingaia Stream is followed from the western lowlands into the upland block studied, it first passes through a basaltic barrier with ejecta on its southern bank and lava on its northern. Very soon, however, fresh-water shales with the siderite concretions common locally in such beds and the usual plant-impressions, which are often wonderfully preserved in the concretions, appear in the stream bed, dipping very gently to the west.
After a few hundreds of yards, however, marine sandstones about 150ft. thick outcrop with a dip of 15° to the south and continue for about 200yds.; in them there are imperfect molluscan shells and occasional shark's teeth and rounded fragments of retinite. They are soon succeeded, apparently conformably, by puce-coloured or greyish foraminiferal shales similar to those of Bombay Basin and fairly well exposed on the south wall of the valley. After a short interval, fresh-water shales reappear upstream dipping south-west at about 5° and here in their true stratigraphic position, and not, as must be the case for their more westerly equivalents downstream, displaced in level by some dislocation. Such displacement is likely to be a consequence of the fault clearly demonstrated and mapped further north as separating the marine from fresh-water members of the Tertiary succession.
The full sequence of the beds at mid-Ingaia bridge may now be given, in spite of some repetition of facts already mentioned.
The thin basal breccia-conglomerate described earlier, which rests on a steeply plunging surface of greywacke, contains angular blocks of rock as much as 2ft. or more in diameter intermixed with finer material. Its uppermost portion shows a greater proportion of water-rolled pebbles than other parts and is covered by soft argillaceous sandstones with imperfect plant-impressions; these are followed by a 2ft. layer of fairly resistant sandstone with abundant small concretions of marcasite, and these in turn by fissile shales with abundant good carbonaceous imprints of dicotyledonous leaves. Away from the steeply inclined basal conglomerate, which is well exposed adjacent to the road in a tiny rill on the north side of Ingaia Stream, the dip of the series speedily flattens to about 10° or 12° to the north-west. As the road is followed north from the bridge, the fresh-water shales can be traced up to a height of 500ft., their thickness being about 100ft., and they then appear to be overlain by a few feet of glauconitic sandstone, with occasional casts of marine molluscs, which appears in some slipped blocks at the road. The next bed consists of about 100ft. of fine-grained greyish foraminiferal shales with rare molluscan shells. Towards their upper limit these shales become glauconitic and are succeeded at an elevation of 610ft. by poorly consolidated greyish-black somewhat glauconitic sandstones containing occasional thin layers of shale interbedded with them and unfossiliferous, but for a few imperfect casts of molluscan shells. This sandstone is widely exposed at higher levels north of Ingaia Stream and gives rise to highly characteristic breakaway scarps at times as much as 30ft. in height.
Attention may be drawn to the noteworthy thinning eastward of the marine sandstone that lies beneath the foraminiferal shales. It is 150ft. thick in the lower reaches of Ingaia Stream and not more than a few feet deep at the road north of the bridge, whilst south of this latter it appears to be absent, for the marine shales there seem directly to overlie the fresh-water ones at an elevation of 470ft.
It is to be noted also that the foraminiferal shales are one of the most persistent beds of the area, for they occur not only in the
Ingaia Creek section, but also in Bombay Basin and on Ramarama-Ararimu Road a few hundreds of yards east of the line of the fault that separates the marine from the fresh-water facies of Tertiary beds.
Before passing from the Ingaia Stream section, mention should be made of a small isolated outcrop of the white fresh-water sandstone at an elevation of about 550ft. in a small gully about a mile, or less, north-east from the bridge. It shows layers of sandstone about 18in in depth interbedded with more argillaceous layers of the same thickness, and must lie very close to the greywacke basement.
Not far distant the uppermost marine sandstones outcrop at little higher elevation, so that, when due consideration is given to the general gentle dip of the series, the mutual relations of the two sets of beds are somewhat corroborative of the suggestion made earlier that the greywacke-Tertiary junction on the west of Dome Hill lies near a fault-line.
White quartz-sandstone similar to that just described is exposed for a depth of 20ft. in a small quarry exposed on the scarp of the fault shown on the map as dividing marine Tertiary beds from fresh-water ones. It dips gently to the west and is in layers 2ft. thick alternating with slightly thinner argillaceous ones. A little south of the quarry, greywacke can be detected high up on the scarp a relatively few feet below the level of the floor of the quarry, but, about a hundred yards north of this latter, the sandstone outcrops well below this level, so that its thickness is evidently dependent on the rise or fall of the irregular basement of greywacke on which it has been deposited, and must reach a maximum of about 80ft. West of it fresh-water shales with plant-remains appear in road cuttings at intervals, forming the summit of the high ridge followed by Ramarama Road until superseded by basaltic débris. Similar beds outcrop south of this road in the westward-flowing stream which roughly follows the contact between greywacke and Tertiary strata and have given rise, near where the tongue of basalt marks the debouchure of this stream, to concretions of impure siderite scattered on the stream bed, which, with blocks of the shale, have yielded some excellent valves of large fresh-water molluscs in addition to plants. The molluscs have not been submitted to an expert for identification, though Professor Berry, of John Hopkins University, states in a letter that one closely resembles Anodonta.*
In the small Tertiary area north of Whisky Creek, the fresh-water shales are well exposed in the bed of the small stream draining the cup-like hollow there present, and dip west at about 20°, their attitude thus being in conformity with the existence of a fault determining their western boundary, as has been suggested in an earlier section. North of the area studied similar shales are locally conspicuous, especially near the western margin of the upland block constituting what may be called the Hunua Hills structural unit, and in places contain workable coals.
[Footnote] * Hochstetter (1864) records the occurrence of this genus in similar beds near Drury.
(c) Basaltic Lavas and Ejecta (? Pliocene and Pleistocene).
A number of centres of basaltic eruption occur either actually along or near the line of Papakura-Drury Fault from Papakura south, two at least coming within the area now described. The most northerly is near the debouchure of Whisky Creek, and is best shown on the north side of the creek by a lava-flow 150ft. or more in depth opened up in a quarry, 200ft. above the level of the adjacent lowlands, which shows remarkably good columns over 80ft. in length.
In the small gorge of Whisky Creek nearby there are bedded deposits resting on Tertiary beds and containing pebbles of greywacke intermixed with far more abundant angular fragments of basaltic ejecta; these deposits evidently largely represent stream-assorted eruptive material.
On the south side of the creek there is a cone-like mass built of tuff and consolidated lapilli; a great part of the original cone has, however, been removed by erosion. From this centre of eruption probably came also the small isolated tongue of basalt that outcrops a little over half a mile further south.
Basaltic tuffs and coarser ejecta exposed in a fairly narrow gorge near where Ingaia Stream leaves its Tertiary terrain, and columnar lava nearby on the northern side of the valley, are possibly the products of a small local eruption rather than those of the major Bombay Hill centre, which has given rise in the main to flows.
Puketutu (1229ft.) is capped by 100ft. or more of basaltic lava, whilst adjacent to it there is the great dome-like mass of Bombay Hill (1000ft.) from which, and possibly other sources south-east of it, considerable floods of basalt have poured westward across the line of Papakura-Drury Fault, completely masking its scarp and burying the underlying terrain whether Tertiary or Mesozoic. It appears to be unlikely that the cap of Puketutu was derived from the same source as gave rise to the Bombay Hill lavas; the date of eruption, however, certainly is so distant from the present that it would be impossible to discuss with any prospect of finality the explanation of this high-level lava-remnant as also of that on the top of Trig. 619 (886ft.). Many considerations make it unlikely that this last cap emanated from the cone-like basaltic mass a little over a mile west of it. This latter does not rise to so great a height, and its lava streams found their way south down the Paraureroa Valley and to-day form a mesa-like divide between two head-water tributaries of Paraureroa Stream.
The only other eruptive foci not yet mentioned include a small one marked by coarse ejecta about 5 miles east of Bombay on the south side of Bombay-Paparata Road and another, indicated by ash deposits, about a mile west of Trig. 616. This last probably was the source of lavas which poured south down an adjacent stream valley on to the flats near Pokeno, where they left a fan-like accumulation of lava.
A thin sheet of weathered volcanic ash, recognisable in a few road-cuttings near Paparata, has undoubtedly been derived from one or other of the adjacent eruptive centres.
Fig. 2.—Tertiary basin north of Whisky Creek, Ramarama. Tertiary fresh-water shales underlie grassed slopes bordered by greywacke which is capped in middle background by basalt. Drainage gap on left, with probable resequent fault-line scarp beyond it.
Fig. 6.—The valley by which Happy Valley drainage joins Paraureroa Stream, which trends from right to left on the near side of distant lofty hills (greywacke). Foreground and bushed terrace across stream, with flat-topped hill on right, are basalt; steeper slopes on left are greywacke.
Fig. 7.—Outlet gorge of Paraureroa Stream, looking south. High greywacke hills in distance; basalt platforms astride of stream in middle distance.
Fig. 8—Air-Gap Valley from the south. Watei-paiting on broad flat in middle distance. High hills on sky-line form scarp of Wairoa Fault, Note, just below them, the regularity of the sloping greywacke surface (? a resurrected surface).