Tuatara: Volume 19, Issue 2, May 1972
The Contribution of C14 dates To the Quaternary Geology of the ‘Golden Coast’, Western Wellington*
The Contribution of C14 dates To the Quaternary Geology of the ‘Golden Coast’, Western Wellington*
In a Continuing Study of the geology near Waikanae 14C dates have contributed to the geochronology of the last 35,000 years, when climate, vegetation, and sea-level fluctuated dramatically in response to the last two discernable cold stadials of the Last Glaciation (Otiran) and to post-glacial warming.
The time-span covered by 14C is small compared with the total spectrum of geological history, but disproportionately significant because of the importance of late events to the understanding of present geography and vegetation. What can be dated depends more on the limited occurrence of datable material (lignite, peat, wood fragments) than on the sediments one wants to date; many sequences spanning parts of the 14C range remain undatable because they contain no carbon.
The early temptation to submit samples on the chance of them being datable resulted in dates beyond the limit of the method. With more knowledge of the geochronology, later samples have all produced positive dates. In an unknown area, it is most profitable to work back in a stratigraphic framework, from known post-glacial samples towards Late Glacial, awaiting results from younger samples before asking for older ones, but this assumes a leisurely timetable for the project. Palynological examination provides so much more significant history to date than a mere lithological sequence that it is perhaps unjustified to submit 14C samples from a carbonaceous sequence unless pollen analysis has already determined its vegetational and climatic history.
The geological interest in testing the contemporaneity of coolings in different parts of New Zealand and the biological interest in vegetation history during the Otiran (‘Glacial’) Stage gives dates in the range 20 to 40 thousand years a particular importance so that improvement of the reliability of the method in this range is amply justified.
The ‘Golden Coast’
The area called the Golden Coast by land-agents is the coastal lowland between Paekakariki and Otaki, a triangle with its apex in the south, lying between the west coast and the mountains.page 62
Fig. 1: Diagram showing topographic expression of the main formations of the Paekakariki-Otaki district. I Greywacke suite. II Oligocene. III Reikorangi Gravels. IV Penult. glacial stage terraces. V Otaki Formation. VI Parata Gravels. VII Matenga Fanglomerate. VIII Glacial valley in Tararua Range. IX Kenakena Formation. X Post-glacial cliff. XI Foxton Dunesand. XII Taupo Dunesand. XIII Motuiti and Waitarere Dunesand.
The total span of time represented by the deposits of the district covers about 200 million years, back to the Triassic greywacke suite forming the Tararua Range and its foothills; 14C dating covers at most 40,000 years. But because we look back to the past through ‘perspective spectacles’, the period to which 14C can contribute is far more important to the geologist than any 40,000-year period in the last 199 m. years.
We can skip over the deposition of the Triassic greywackes (Fig. 1 I), their deformation and elevation in the Rangitata Orogeny (L. Cretaceous), subsequent peneplanation and transgression by Oligocene seas (40 m. years ago), and the deformation that resulted in a narrow infaulted strip of Oligocene greensand behind Otaihanga (Fig. 1, II). Later in the Tertiary, about 10 m. years ago, the deformation of the Kaikoura Orogeny began to raise the Tararua Range as a complex of high blocks (and Kapiti is a smaller one); these anticlinals were progressively elevated in relation to the synclinal or relatively subsiding area of Cook Strait and the narrower synclinal between Kapiti and the mainland.
Older Pleistocene (Wanganui Series)
The Quaternary Era is first represented by a formation of ancient coarse gravels (Reikorangi Gravels, III in Fig. 1) that are strongly deformed and weathered. They probably represent the early Pleistocene Wanganui Series and are in part equivalent to the Kaitoke Gravels of Hutt, the Moutere Gravels of Nelson, and the marine Wanganui Series of the Palmerston-Wanganui Basin to the north-west. At some later Quaternary date the land surface was deeply weathered page 63 to a bright rosy red colour, probably in an interglacial age when climate was unlike any modern New Zealand climate. From experience, we know that deposits older than the period or periods of red weathering are well outside the period of 14C dating; 20 years ago geologists were less experienced and wood or lignite from such beds could still be submitted in error (but would probably not get past the screening process).
Younger Pleistocene (Hawera Series)
Before the later ice-ages, the land gained more or less its present outline, although tilting and faulting movements have continued. Interpretation of younger Pleistocene deposits is within the framework of several theoretical generalisations, some of which are still being tested:
Climatic fluctuations (and the deposits they controlled) were contemporaneous, in a general way, through New Zealand if not throughout the world.
During each glacial period, when glaciers advanced in the South Island mountains, the Tararua Range suffered a periglacial climate even if it did not support glaciers. Mountain vegetation was reduced to sub-alpine grassland and herb field, solifluction was widespread, and streams and rivers, overloaded with waste, aggraded their beds to a steep gradient with coarse ill-sorted gravels derived by even steeper fans from tributary streams. Small glaciers (VIII in Fig. 1) were certainly present in the highest Tararuas in the Last Ice Age (Adkin, 1910).
World-wide withdrawal of ocean water to form ice sheets and glaciers lowered ocean level to depths of some 120 to 200 m. below its present level. Because deep water is quite close to the Golden Coast (south of Kapiti), ice-age gravel plains plunged steeply to coastlines at no great distance from the present shores.
During interglacial periods, melting of ice raised sea levels generally to heights above present sea-level. The land was fully forested; rivers cut down in their inland courses. Gravels in rivers and on coasts tended to be better sorted.
Tectonic movements, continuing through the Pleistocene, have caused irregularities in the levels of glacial and interglacial shorelines but were not sufficiently strong to obliterate completely the cycle of relatively low levels for glacial and high levels for interglacial ages. Therefore marine deposits on land are almost certainly interglacial, coarse ill-sorted gravels probably glacial.
Sand dunes characterise stable or prograding coasts (not advancing seas), so that most dune-sand deposits, especially those containing mica derived by long-shore drift from Tertiary beds further north, probably date from a period of sea retreat page 64 from a high interglacial level. There are, in fact, some dune sands (Koputaroa and Te Whaka Dunesand) of glacial age, derived not from beaches but from glacial riverbeds, but they can be distinguished from beach-derived interglacial dunesands by absence of mica.
In the valleys of the Waikanae and other rivers terrace remnants of the penultimate and probably anti-penultimate glacial gravels (Fig. 1, IV) can be recognised as such by their altitude, dissection and strongly weathered loess soils that are developed on them. Remnants of the deposits of the Antipenultimate Interglacial are also suspected in several places. These have to be recognised and classified by geomorphological, pedological, and lithologic criteria; they are all beyond the range of 14C. In practice, so little carbonaceous material occurs in gravels formed by cold-climate aggradation that there has been little temptation to submit material from such deposits.
Last Interglacial (Oturian Stage)
The most conspicuous formation of the coastal lowland has long been known as the Otaki Sandstone or Otaki Formation (Fig. 1, V) and was studied by Adkin, Oliver, and Cotton before the development of 14C techniques. Te Punga (1962) described a well-exposed section near Te Horo and collected a sample (N.Z. 65) dated as greater than 45,000 years from a lignite band. By the criteria listed above, the Otaki Formation is Interglacial, consisting of the marine beach gravels and sands of a transgressive high-level sea, the beach-derived micaceous dune sands that advanced as the sea retreated, and lignite deposited in swamps ponded by the dunes. It is backed by an old cliff-line (Fig. 2, Line A-A). As it is the latest interglacial deposit recognised on the coast, the Otaki is attributed to the Last Interglacial Age, which from isotope datings overseas is thought to be about 80,000 to 120,000 years old and thus beyond the range of 14C.
Last Glacial (Otiran Stage)
Overlying the Otaki Formation are a variety of deposits attributed to the last glacial stage: coarse ill-sorted river gravels in terraces, sometimes with erect pebbles indicating frost climate near the surface (Parata Gravels, VI in Fig. 1); mantling loess deposits with fossil soils; gravel fans of angular Matenga Fanglomerate sloping from the hills to the lowland (VII in Fig. 1; see also Fig. 2); their stratigraphic relations have to be pieced together, partly from their geomorphic relations, as exposures showing them in sequence are rare. The gravels, like most cold climate gravels, lack wood or carbonaceous deposits, probably because vegetation was sparse and not woody. The loesses were mainly deposited under oxidising conditions that burnt out their carbon content. 14C dates would be very valuable for correlation with other New Zealand sequences.page 65
Fig. 2: Diagram illustrating relationship of Matenga Fanglomerate (M) laid down in fans during the Otiran glacial stage, to the deposits of the preceding interglacial (Otaki Sandstone, O) and to the Foxton Dunesand (F) and Paraparaumu Peat (P), deposited on the coastal plain during the Holocene. R: Ruahine Greywacke Suite. A-A: Interglacial sea cliff. B-B: Post-glacial sea cliff. (From Fleming, 1970)
The fan gravels occasionally preserve thin lenses of carbonaceous silt, representing swampy deposits later buried by the growing fan (Fleming, 1970). One such deposit yielded a sequence of pollen samples indicating cold wet grasslands — shrubland vegetation with a temporary silver beech phase, dated (N.Z. 573) as 19,200 years B.P. (± 560 years), and thus falling in the period of the Later Kumara-2 glacial advance of Westland (Suggate, 1965) and (less precisely) of the Takapu Stadial of Wellington Peninsula (Brodie, 1957). The fans appear to slope concordantly to the main terrace of the Parata Gravels, which is thus correlated on indirect evidence. This 14C date has been extremely valuable.
In two exposures near the crossing of the Waikanae River by Main Highway No. 1, Parata Gravels overlie a sequence of older rocks. In the railway underpass, Parata Gravel overlies a silt deposit with most of the characteristics of a buried loess but with carbon preserved by burial in reducing conditions, overlying a weathered gravel. The buried loess (Tini Loess) contains pollen of grassland-shrubland vegetation (cold climate) and branchlets near its top have given a 14C age (N.Z. 700) of 35,400 ± 900 years. This result, the cold climate vegetation and the stratigraphic relations with the overlying Parata Gravels (correlated with Later Kumara-2, say 22 to 18 thousand years B.P.) suggest that the Tini Loess and underlying gravel date from an early Otiran glacial, probably the undated Earlier Kumara-2 of Westland, and the Opunake glacial cooling of Taranaki (Grant-Taylor, 1964), dated as 30,000-38,000 B.P.
In the second exposure, 0.75 km. downstream, a relatively complete section of the Otiran Stage overlies Otaki Formation (Oturian Stage) (Fig. 3)page 66
|(7 m.)||Parata Gravel (correlated with Later Kumara-2, 18,000-20,000 B.P.) (F)|
|(1 m.)||Tini Loess (35,400 B.P. at top) (E)|
|(1 m.)||Unnamed gravel, weathered (D)||Earlier Kumara-2|
|(1.5 m.)||Waimahoe Lignite (C)||Pollen Zones:
Grassland (cold) Silver beech (cooling)
|(0.5 m.)||Strong paleosol, with rhizomorphs of clay mineral, (B) developed on|
|(2-5 m.)||Otaki Dunesand — Last Interglacial (Oturian) (A)|
The Waimahoe Lignite (C) records a cool-warm fluctuation before the cooling that presumably led to the ‘Earlier Kumara-2 = Opunake’ glacial (D-E). The only 14C sample, collected by Te Punga from an uncertain horizon, is N.Z. 22 (more than 35,000 B.P.). If confident dating in the range 35,000-40,000 becomes possible the section is worth re-sampling for 14C, as it is a rare example of three cold stadials in stratigraphic sequence in a single exposure.
Twenty-seven miles north of Waikanae on broad surfaces above the aggrading riverbeds, Koputaroa Dunesand (a cold climate dunesand) accumulated over a span of time that included a climatic cooling indicated by pollen in peat (McIntyre, in Cowie, 1963) since dated as 35,000 ± 1700 years B.P. (N.Z. 522) (i.e. equivalent to Early Kumara-2) and also the Oruanui Ash (dated as about 20,000 B.P. by correlation and thus Later Kumara-2). Recognition of this ash shower, widespread throughout the North Island (Vucetich and Pullar, 1969), allows a valuable link with the history (and 14C dates) of other districts. A similar but younger cold climate dunesand (Te Whaka Dunesand) overlies the Parata Gravels at Otaki but carbon for dating it has not yet been found. Such glacial-stage dunesands seem to have accumulated when streams began to incise below an aggraded terrace, lowering the water-table, i.e. after the peak of a cooling had passed.page 67
Fig. 3: Section exposed in south bank, Waikanae River, 0.75 km. downstream from main road bridge. A: Otaki Dunesand (Last Interglacial). B: Interglacial Paleosol. C: Waimahoe Lignite (< 35,000 years B.P.). D: Weathered gravel. E: Tini Loess (35,400 ± 900 years B.P. at top). F: Parata Gravel (? c. 20,000 years B.P.). (N.Z. Geological Survey, photo)
Younger Quaternary Deposits Near Waikanae
|Waitarere Dunesand||European Period, 0-150 B.P.|
|Motuiti Dunesand||Polynesian Period 150-1000 B.P. (includes N.Z. 293: 855 ± 50 B.P.)|
|Taupo Dunesand||c. 1800 B.P.|
|Foxton Dunesand||1800-c. 5500 B.P.|
|Paraparaumu Peat (interfingering with above)||0-5500 B.P.|
|Paripari Formation||5140 B.P. ± 90 (N.Z. 519)|
|Kenakena Formation||5500-?10,000 B.P.|
|Last Glaciation||Otiran Stage|
|Te Waka Dunesand||? c. 16,000 B.P.|
|‘Judgeford Loess’||(contemp. with next two)|
|Parata Gravels||c. 18-23,000 B.P.|
|Matenga Fanglomerate||19,200 ± 560 B.P. (N.Z. 573)|
|Tini Loess||35,400 ± 900 B.P. (N.Z. 700)|
|(older gravels)||? c. 38,000 B.P.|
|Waimahoe Lignite||‘Early Otiran’ < 35,000 B.P. (N.Z. 22)|
|Awatea Lignite (< 45,000 B.P. N.Z. 65)||Otaki Formation (c. 80,000-120,000 B.P.)|
|Otaki Beach Sand|
|Penultimate Glacial Stage||Deposits not detailed|
|Penultimate Interglacial Stage|
Post-Glacial (Aranuian Stage)
During the Otiran, the sea is assumed to have been at low levels (c. 120-200 m.) and the coast to have been somewhere south of the Kapiti Strait synclinal, which was a deep depression receiving the gravels from Waikanae district.
As the sea level rose it flooded the Kapiti Strait depression which later was partly filled by long-shore drift and prograding deposits.page 69
At Foxton, sea level had risen to −50m. by 9900 (± 150) years B.P. (N.Z. 81). Thick beach sands with shells (e.g. at −63m. at Paraparaumu Aerodrome) presumably represent this period of rising sea level, but have not been dated. They are represented by IX on Fig. 1, and have been named Kenakena Formation. Post-glacial sea level rose to its maximum at about 5140 ± 90 years B.P., a date (N.Z. 519) from the beach deposits (Paripari Formation) near Centennial Inn, Paekakariki (Fleming, 1966). At about this date, the sea cut a prominent low cliff (Te Punga, 1962) truncating the older formations of the coastal lowland (Fig. 1, X; Fig. 2, line B-B). Sea level may have been higher than now by a few metres, but part of this elevation is due to subsequent tectonic uplift. From this time onward, the coast prograded and fed a succession of dunes which now form a coastal belt of variable width, separated by peaty lowlands.
The oldest dune group (Foxton Dunesand, see Cowie, 1963) laps on the post-glacial cliff and probably began to accumulate soon after the sea retreated from it as progradation began. 14C dates for woody peats interfingering with the oldest Foxton dunes at Waikanae are 4730 and 5290 years B.P. (N.Z. 2973) suggesting that the Waikanae coast was already prograding before the sea retreated from the cliff behind Centennial Inn, a situation comparable with the present. The next formation, Taupo Dunesand, formed largely from pumice granules derived from drift pumice piled on the beaches after the Taupo Pumice eruption (c. 1800 B.P.), is a prominent marker from Paekakariki to Te Horo; its date has not been independently tested. Cowie attributes the next younger Motuiti Dunesand to early human disturbance, citing a date of 855 ± 50 B.P. (N.Z. 293) from a forest tree overwhelmed by Motuiti dunesand at Taikorea, Manawatu. Finally, Cowie's Waitarere Dunesand is attributed to the European period when fire and grazing animals reactivated the dunes.
The table of formations (Table 1) summarises the stratigraphy and dates.
Adkin, G. L., 1951: Trans. R. Soc. N.Z. 79: 157; 1910: Trans. N.Z. Inst. 43: 496; 1919: Trans. N.Z. Inst. 51: 108.
Brodie, J. W., 1957: N.Z. Jl. Sci. Tech. B38: 623.
Cotton, C. A., 1918: Trans. N.Z. Inst. 50: 212.
Cowie, J. D., 1963: N.Z. Jl. Geol. Geophys, 6: 268.
Fleming, C. A., 1966: N.Z. Jl. Geol. Geophys. 8: 1222; 1970: Trans. R. Soc. N.Z. (Earth Sci.) 7 (11): 197.
Grant-Taylor, T. L., 1964: Geology, in Egmont National Park (Egmont Nat. Park Board).
Oliver, R. L., 1948: N.Z. D.S.I.R. Geol. Mem. 7.
Suggate, R. P., 1965: N.Z. Geol. Surv. Bull. n.s. 77.
Te Punga, M. T., 1962: N.Z. Jl. Geol. Geophys. 5: 517.
Vucetich, C. G., and Pullar, W. A., 1969: N.Z. J. Geol. Geophys. 12 (4): 784.
* A paper presented to the Radiocarbon Users' Conference, Lower Hutt, August, 1971.