Basal Waitemata Beds.
At distances varying from one-quarter to one-half of a mile east of Whitford-Maraetai road, in the beds of Bloomfield's Stream, South Branch, and Claude's Stream—the three largest streams draining into the head of Waikopua Creek—there are some very interesting but limited outcrops of impure limestone (Fig. 10), limecemented greywacke-breccias and calcareous green sandstones which
rest highly unconformably on Hokonui greywacke, and are followed conformably by normal beds of the Waitemata Series. These beds are the oldest exposed members of the “youngermass” in the present area, but in spite of their palaeontological and stratigraphical importance the only published account of them is a generalised description made by Park in 1886, who correlated them with the beds of the so-called Papakura Series of Hutton (1871, p. 246), developed at Hay's Creek, Papakura, which he showed to be the basal members of the Waitemata Series.
Considerable variation in facies occurs in the several outcrops at Waikopua, but nevertheless certain strata appear to be sufficiently constant throughout the main outcrops to suggest the following general succession, which varies in some respects from that given by Park:—
Normal Waitemata beds
Concretionary green sandstone
Impure limestone; coarse greywacke conglomerate
Concretionary green sandstone
The complete sequences exposed in the several streams are given in Text-Fig. 2. So far as is known that at Claude's Stream has not been previously recorded or described.
The main or downstream outcrops in all three streams display strata with a constant dip to the north-west of from 5° to 8°, which brings successively lower beds of the sequence into view for approximately 10 chains. They end abruptly against steep, narrow ridges of greywacke, upstream of which are further more. circumscribed outcrops which occupy pockets between similar greywacke barriers. The only connection between the beds of the main outcrops and these upstream outcrops lies in the fact that both are followed conformably by normal Waitemata beds which over-ride the intervening barriers of greywacke. The “basal” beds exposed in the upstream outcrop at Bloomfield's Stream are coarse, poorly-cemented conglomerates which grade upstream into gritty, impure limestone. Of two similar outcrops at South Branch, one shows a limited occurrence of gritty green sandstone and the other, 5 chains upstream from the first, a prominent band of limestone which is exceptionally pure in places and consists almost entirely of an open-textured mass of polyzoa, foraminifera, algae, brachiopods and echinoid fragments; above the limestone there is about 40 ft. of green sandstone. At Claude's Stream the only “upstream” representative is a band of poorly-cemented, roughly-sorted greywacke breccia, at least 8 ft. thick, which coarsens upstream until included fragments as much as 9 in. in diameter are common.
The green sandstone members of these basal beds are generally fine-grained and contain sporadic small greywacke pebbles. The limestones are much coarser in texture, exhibit cross-bedding on both fine and coarse scales, and thin out laterally in the form of lenses. Evidence of minor faulting or down-sagging at or adjacent to steep contacts with the greywacke of the “oldermass” is complete in
Fig. 5.—Typical by-filling between Whakakaiwhara and Te Kowai (right centre). The barrier-beach behind which this filling has taken place is clearly seen on its seaward margin. Remnants of an earlier barrier-beach are still preserved immediately to the right of this view.
Fig. 15.—Kiripaka Basin from the hills above Kiripaka scarp, looking south across the denuded greywacke floor to Waikopua divide (middle distance) and the main range of Maraetai Hills beyond. [Photo., R W Futh.
Fig. 17.—Material of the “30 ft. to 35 ft.” terrace at road from Clevedon to Duder's Beach. Grey muds at the base followed by greywacke conglomerate and then light-coloured pumiceous silts.
Fig. 19.—Main crater, Mount Mangere; a small scoria-cone rises from its floor with a tiny shallow crater at its left (west) margin and another (invisible) on the right. There is a smaller, deep crater in breached rim of main cone, near the middle group of trees.
Fig. 23.—Pukaki Basin from the west. Its caldera is occupied by a marine swamp with its outlet in distance to the right of centre.
Fig. 25.—Overlooking breached eastern lip of crater of Otara to scoria mounds beyond. Part of tuff-ring behind the mounds, and Tertiary sandstone hills of Maungamaungaroa Range in the distance.
Fig. 31.—Back-slope (distance) of tilted Hunua Range Block viewed from north-west across alluvium-filled fault-angle-depression of Papakura Valley (middle distance). Dissected scarp of Papakura Valley Fault in foreground.
Fig. 33.—Low scarp exposed by waves at faulted contact of Pleistocene beds (shore-platform in foreground) with greywacke (in scarp) at east end of Omana Beach. Elevated beach-deposits rest on wave-planed surface of greywacke. [Photo, R. W. Firth.
Fig. 35.—Coarse-grained greywacke from Trig. Station 648, Brookby Range. Ordinary light; magnification 46 diams. Quartz is sparse and weathered feldspar abundant. Grains of hornblende (h) and augite (a) are visible. [Photomicrograph by J. A. Bartrum.
Fig. 37.—Coarse-grained basic olivine basalt, Green Hill Quarry. Large idiomorphic phenocrysts of olivine appear in groundmass of augite, plagioclase and magnetite. Ordinary light; magnification, 46 diams. [Photomicrograph by J. A. Bartrum.
several cases, but is explicable merely by the shrinkage on consolidation and drying of a moderate thickness of beds.
The nature of the basal deposits described above suggests that the beds of the main outcrops were deposited in open water offshore from a rugged, youthful coastline, in a regular succession that suggests certain fluctuations of the strandline but the maintenance of moderately shallow-water and locally-sheltered conditions for some time prior to the deposition of the rapidly-accumulated sands and muds that characterize the main mass of the Waitemata Series. Highly-variable conditions of deposition must have held in embayments of this youthful coastline, in which the effects of minor fluctuations of the strand would be much less apparent than in open water; such fluctuations would give rise to the beds of the “upstream” outcrops and account for the lack of connection between these and the succession of the main outcrops.
The palacontology of these beds at Waikopua is as yet imperfectly known. The limestones are highly fossiliferous (Fig. 36), but unfortunately much of the material is fragmentary. Polyzoa, foraminifera, and algae (? Lithothamnion) abound, whilst fragments of lamellibranchs, echinoids, gasteropods, and brachiopods, and occasionally shark teeth, are plentiful. Portions of the shells of Ostrea are common in some coarse conglomerates. In the green sandstones foraminifera are particularly abundant, with less frequent, perfectly preserved brachiopods (Hemithyris antipoda), small Pectens (P. fischeri; P. polymorphoides; Amussium zitteli) and small gasteropods.
Through the kindness of Mr. W. J. Parr, of Caulfield, Australia, who most generously undertook the identification of the foraminiferal fauna both of limestone and green sandstone from Waikopua, the writer is able to supply lists of the foraminifera of these beds and thus bring this section of the palaeontology up to date. Mr. Parr also examined samples of the Turanga greensand and Parnell Grit facies. His determinations are tabulated at the conclusion of this paper.