From Castlecliff, at a point 124 chains north-west of North Mole at the mouth of the Wanganui River, to Nukumaru, 11 miles to the north-west, the coastline consists of youthful cliffs cut in gently-dipping marine Pliocene sediments, unconformably overlain by almost horizontal Late Pleistocene gravels and sands of the Rapanui formation of the Hawera Series (Fleming, 1946; see appendix to this paper). The cut surface at the base of the Pleistocene truncates the monocline of Pliocene beds at a height of 60–100ft above high-water level, which is the base on which the section (Fig. 1) is drawn. The section is interrupted by the mouths of the Mowhanau, Kai-Iwi,
Okehu, Ototoka, and several minor streams, but this interruption nowhere seriously affects the continuity of the beds.
The beds at Castlecliff strike about 80° east of north and dip to the south at 2° to 4°. Near Nukumaru the strike is more to the north-east and the dip locally approaches 8°. At the west end of Nukumaru Beach an important north-east-trending fault zone (the Nukumaru Fault of Superior Oil Company) upthrows resistant shelly conglomerates to the north-west some 80–100ft. against overlying Nukumaruan sands to the south-east. On either side of Ototoka Stream, faults with insignificant down-throw to the north-west are none the less important as indicating successive movements in at least three periods—pre-Castlecliffian, post-Castlecliffian, and post-Rapanui Pleistocene.
Nukumaruan: The oldest rocks in the section are the pebbly and sandy shell-limestones on the upthrow side of the Nukumaru Fault where they form a broad promontory between Nukumaru Beach and the mouth of the Waitotara River. These are included in the Nukumaruan as Nu 1, but on the coast their base is difficult to define and their subdivision into minor units is not possible at present. Inland exposures of this Nukumaru Limestone show it to consist of discrete lenses of “shell-rock” In loose, barren sands, with a basal pebbly sand overlying the more consolidated muddy-sandstones of the Upper Waitotaran. There is a suggestion of disconformity which is supported by the occurrence of Upper Waitotaran derived fossils in the lower shell reefs of the Nukumaru formation. On the coast, west of the plotted section, an extensive area of shell reefs represents these horizons and is quarried for agricultural lime.
Fig. 3. Nukumaru Beach. View looking west towards promontory showing the planes of two splinters of the Nukumaru fault. Resistant Nukumaru shell-limestone (Nu 1) forming the promontory is faulted against the Nukumaru brown sands (Nu 2) on right of sketch. Drawn from a photograph.
South-east of the fault, two splinters of which are beautifully exposed at the west end of Nukumaru Beach (Fig. 3), what appears to be the top of the shell-limestone is present a foot or so above beach level, and is overlain by the sands of the type section of the Nukumaruan
(here designated Nu 2). Estuarine and locally terrestrial sediments with intercalated marine phases, overlying to the southcast, comprise beds Nu 3 to Nu 8, and extend to a point 3 ¼ miles beyond Ototoka Stream, where they are overlain by the lowest Castlecliffian bed (Cl 1).
The Nukumaruan part of the section is the least satisfactory to map and subdivide. Featureless mudstones, siltstones and sandstones of paralic facies with no persistent or conspicuous marker-bands contain lignitic and marine (fossiliferous) horizons which are generally discontinuous. Also, the cliffs are not being actively eroded and are now more completely mantled with talus and vegetation than they were 20 years ago (Dr. J. Marwick, personal communication) and than those further south-east are now. The Nukumaru beds have been subdivided broadly into zones in which marine, estuarine, and terrestrial conditions appear to have been dominant. More detailed work, such as might be undertaken in the future for pollenanalysis studies of the lignitic bands, would result in finer subdivision.
Castlecliffian: The Castlecliffian beds have been divided into two series, an upper and a lower, with separate notation, Cl 1 to 11 for the lower and Cu 1 to 8 for the upper. The separation is made at the well-known stratigraphic break between Castleeliff and Kai Iwi described by Marwick (1931, p. 7). There is a number of faunal distinctions between the two subdivisions of the Castlecliffian (of which the upper includes the designated type locality) and a change in sedimentation. The Lower Castlecliffian contains rhyolitic pumice conglomerates and pumiceous sediment at a number of horizons (Cl 1, Cl, 5, Cl 9a, Cl 10), whereas fresh pumice is conspicuous only at the base of the upper beds. Andesitic pebbles were noted in the basal conglomerate of the upper beds, but not in the lower. Inland, the Lower Castlecliffian beds are dominated by pumiceous sediments and the upper by heavy greywacke and igneous conglomerates.
Though the stratigraphic break between the Upper and Lower Castlecliff beds is conspicuous, it is far from being the only (though it may be the most important) break in the section, which consists of a rapidly varying sequence of littoral and sublittoral silts and sands, shellbeds containing both biocoenotic and thanatocoenotic elements (Hesse, Allee, and Schmidt, 1937), beach deposits, and estuarine silts and sands. In general, a wave-cut or scoured surface is immediately followed by coarse and then fine sediment, and shellbeds may occur at the bottom, at the top, or within such a sequence. Strong local scouring has carved deep irregular channels in the beds underlying Cl 1, Cl 3, and Cu 8; wave-planed surfaces, locally bored by the intertidal mollusc Anchomasa and overlain by coarse beach or shallow-water conglomerates or shellbeds, occur below Cl 2, Cl 4, Cl 8, Cl 9, Cl 10, Cu 1, Cu 5, Cu 8. Such littoral and sublittoral horizons are separated by sandstones, mudstones, and siltstones of fine or medium texture, massive at Cl 6, Cl 11, Cu 3, Cu 4, and Cu 7, and laminated or flaggy, with alternating fine and coarse bedding, at Cl 2, Cl 4, Cl 8, Cl 9, and Cu 8. As a rule, massive (unbedded) beds are fossiliferous, and laminated, alternating beds almost barren. The shellbeds (Cl 7, Cl 10, Cu 2a, Cu 3, Cu 6) composed of concentrated biocoenotic faunal elements of off-shore
benthic facies present an unsolved ecological problem. Faunally they are more closely related to the generally finer textured massive and sparsely fossiliferous beds (Cl 6, etc.) than to coarser textured littoral shellbeds composed of thanatocoenotic elements.
The highest bed on the coast, Cu 8, disappears under a mantle of blown sands at Castlecliff and is believed to reappear as unfossiliérous siltstone on the north bank of the Wanganui River near Castlecliff wharf. Still higher Castlecliffian beds occur at Putiki and Landguard Bluff on the south bank of the Wanganui, and as these are classic fossil localities, brief descriptions are given of the beds there exposed and their probable stratigraphic relation to the coast section.