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Mount Curl Tephra, a 230000-year-old implications forquaternary chronology markerbed in New Zealand, and itsJ. D. G. Milne a ba Victoria University of Wellington , New Zealandb Soil Bureau , DSIR , New ZealandPublished online: 14 Feb 2012.

To cite this article: J. D. G. Milne (1973) Mount Curl Tephra, a 230000-year-old implications for quaternary chronology marker bed in New Zealand, andits, New Zealand Journal of Geology and Geophysics, 16:3, 519-532, DOI:10.1080/00288306.1973.10431375

To link to this article: http://dx.doi.org/10.1080/00288306.1973.10431375

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No.3 519

MOUNT CURL TEPHRA, A 230000-YEAR-OLD IMPLICATIONS FOR QUATERNARY CHRONOLOGY

MARKER BED IN NEW ZEALAND, AND ITS

J. D. G. MILNE

Victoria University of Wellington, New Zealand and Soil Bureau, DSIR, New Zealand*

ABSTRACT

The rhyolitic Mount Curl Tephra, probably erupted from the central region of the North Island of New Zealand, is inferred to be the product of a single major eruption, possibly associated with an ignimbrite. From potassium-argon and fission track age determinations, it is considered to be 2 30 000 years old.

It mantles ancient land surfaces that pre-date the three youngest raised marine benches of interglacial origin in the south-western North Island. From stratigraphic relations between the tephra and the loess and aggradation alluvium underlying it, and the dune sand, other tephra and loess overlying it, it is inferred that the tephra fell at the beginning of the penultimate interglacial-the interglacial during which the third youngest (Brunswick) marine bench was cut.

Using the age inferred for the Mount Curl Tephra together with overseas radio­metric dates, and assuming constant rates of uplift of the south-western North Island marine benches, it is considered that the Brunswick marine bench began forming at 230 000 years B.P., and stopped forming at or more recently than 190 000 years B.P., and that the second youngest (Ngarino) marine bench stopped forming at or more recently than 115 000 years B.P. The marine bench sequence can be correlated satis­factorily with raised coral reef sequences on Barbados and New Guinea.

The times of interglacials implied by the Wanganui marine bench sequence appear to correlate better with those implied by the Barbados and New Guinea sequences, than with those inferred by Emiliani & Rona (1969) from ocean core sequences.

INTRODUCTION

Marine benches extend along considerable distances of the coastline of the south-western part of the North Island of New Zealand. Three par­ticularly clearly developed benches (Brunswick, oldest; Ngarino, middle; Rapanui, youngest), occur to the west of the Wanganui valley (Grant-Taylor 1964). All three are considered to have been cut during times of inter­glacial high sea levels (Fleming 1959; Grant-Taylor 1964; Suggate 1965). From the relative heights of their cliff bases, and by analogy with similar marine benches in the north Westland area of the South Island, the Rapanui and Ngarino benches were considered to have been cut during the last (Oturi) Interglacial, and the Brunswick bench during the Penultimate (Terangi) Interglacial, (Suggate 1965).

Beds of tephra, loess, and dune sand cover. the marine benches. The beds are more numerous on older surfaces, and are a valuable means of correlating both the marine benches and the river terraoes throughout the area. Some of

*Now at Soil Bureau, Private Bag, Lower Hutt, New Zealand.

N.Z. Journal of Geology and Geophysics 16 (3): 519-32

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the tephra cover beds are especially valuable because they form marker beds and time planes, and because they contain minerals that can be radio-­metrically dated. This paper describes one such tephra marker bed, reports radiometric dat,es for its volcanic glass, and discusses the ;;ignificance of the dates with respect to the times of interglacial high sea levels.

MouNT CuRL TEPHRA FoRMATION (new name)

Type Section The type section is a road cutting on Mount Curl Road at gricl reference

Nl38/956822* (Fig. 11. The cutting is approximately 9:JO m distant on a bearing of 237° from Warring trigonometrical station.

Nature and Distribution

At the type section (Fig. 2) the tephra is exposed along a total distance of some 150 m as a 0·9 m thick sub-horizontal layer which overlies loess and underlies dune-sands (Appendix 1). Although not very different in colour, it can be distinguished from the underlying loess because its basal portion is much coarser than the top of the loess, and at the eastern end of the section its base projects as a prominent ledge. It can be clearlv distinguished from the overlying sands because it is much light~r in colour and because a prominent ironpan has formed in the basal part of the sands.

At the type section, the sands overlying the Mount Curl Tephra are between 0·5 m and 3 m thick. Where they are thickest, they are moderately weathered in their uppermost horizons but they ar,e little weathered beneath. Elsewhere at the type section, they are little weathered throughout. Some 200 m to the west of the type section the sands are about 6 m thick, and they are strongly weathered in their uppermost horizons, being altered to a (2·5Y 6/4) clayey sand. The irregular thickness and lack of weathering of the upper horizons of the sands at the type section are due to erosion during or after the interval of strong weathering of the sands, recorded at the nearby exposure. The sands are absent at all other sections where the Mount Curl Tephra was seen, and the tephra at other sections is strongly weathered. The sands at the type section have thus protected the Mount Curl Tephra from post-depositional weathering, causing it to be exceptionally well preserved.

Mineralogical analysis (Appendix 2) shows that Mount Curl Tephra at the type section contains very abundant rhyolitic volcanic glass, very common sodic plagioclase, common hypersthene, scarce quartz and titanomagnetite and rare biotite, hornblende, augite, apatite, and zircon. From the presence of rhyolitic glass, and sodic plagioclase, and the abundance of hypersthene compared with augite, it is inferred that the tephra is a product of a rhyolitic volcanic eruption, probably from the central North Island region more than 100 km to the north of the type section.

*Grid reference based on the national thousand-yard grid of the 1: 63 360 topo­graphical map series (NZMS 1).

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No.3 MILNE -MouNT CuRL TEPHRA

0 5 Miles

5 0 5 Kil.ometres

UJ PQst-glacial dune sands

lrr.:":ii:: Ngarino and Rapanui ~ manne benches

[]!] Brunswick marine bench

~ ~~:~ig~ P,!;~~~~ue~tff:P~~~aces has not been identified

11m ~nre:~i~h ~o~~~uC'~~~t~~aces has been identified

X s Palmerston North o

FrG. 1-Known distribution of Mount Curl Tephra.

521

At the type section the tephra layer is a single graded bed, with fragments up to 3 mm in diameter in its basal part, and fragments up to 1 mm in diameter in its upper part. From its lack of internal layering, its thickness, and its probabl,e distance from source, it appears to be the product of a single very large eruptive event. For comparison, the most widespread upper Quaternary tephra marker bed in New Zealand, the Aokautere Ash (Cowie 1964a), is only 0·2 m thick in the exposure some 200m to the west of the Mount Curl type section, and it there consists of at least six depositional layers. Because of its thickness, the Mount Curl Tephra is thought to be the airfall equivalent of one of the younger of the central North Island ignim­brite sheets.

The tephra has been identified at five localities other than the type section, and tentatively identified at one other locality (Fig. 1). Field identification was based on the presenoe of coarse material in the basal portion of the tephra, and in a few places by the lighter colour of the tephra when compared with enclosing beds. In four of the five places where confidently identified, field identification was confirmed by analysis ·of trace elements contained in titanomagnetites (B. P. Kohn pers. comm.). The tephra has only been preserved where it mantles surfaces that have been immune from erosion for a long period of time, and at most such places it is buried more than 6 m beneath the ground surface by loess and younger tephra beds.

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~:L E 2

1

0 10 20 30 40 50 metres

1948

T1TT" paleosol

gravity fault plane trace

VoL. 16

FIG. 2-Mount Curl Tephra type section as exposed in 1948, 1970, and 1971, the 1948 exposure being modified from fig. 46 of Fleming (1953). The section has changed its appearance due to road realignment, the 1971 exposure being some 20m to the south of the 1948 exPosure. The line at the base of each of the exposures represents road level, ,;hich has remained at about the same height. Key to sections: 1 =Little weathered middle Quaternary (Castlecliffian Stage) bedded siltstone and sandstone with localised lenses of small well rounded pebbles. Shown as Kaiatea alluvium in fig. 46 of Fleming (1953). 2 =Slightly weathered loess. 3 = Almost unweathered loess. 4 = Mount Curl Tephra, shown as Fordell Ash in fig. 46 of Fleming (1953). 4A =Colluvium derived from Mount Curl Tephra. 5 = Brunswick Dune-sand moderately weathered in its upper part but only slightly weathered in its lower part. 6 = Rhyolitic tephra lenses, overlain by loess, and colluvium, all strongly weathered.

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No.3 MILNE - MouNT CuRL TEPHRA 523

The tephra layer at the type section was first recognised and illustrated by Fleming, who correlated it with Fordell Ash (Fleming 1953, pp. 251 and 252, and fig. 46.).

Tl:e name Fordell Ash was given by Fleming (1953) to tephras within a variable though n:ain: y terrestrial sequence that includes two or more beds of rhyolitic tephra, together with interbedded andesitic tephra, diato­mite, bedded silts, and aeoi:an and beach sands. Fleming (1953, p. 253) stated that in some exposures the Fordell Ash contained one or more paleosols.

Fleming designated road cuttings through the Brunswick marine bench on No. 2 Line Extension, east of Fordell Village, and some 20 km to the west of the Mount Curl Tephra type section, as the Fordell Ash type exposures, and figmed two of the type exposures (Fleming 1953, fig. 49 and 50). The Fordell Ash at the type exposure shown in fig. 49 of Fleming ( 195 3) appears to be an alluvial silt bed, quite unlike the Mount Curl Tephra. The Fordell Ash at the type exposure shown in fig. 50 of Fleming ( 195 3) is a white to light grey silt that infills a fossil gully. Although similar in colour to the Mount Curl Tephra, it is much finer grained and has a "smooth" feel characteristic of diatomaceous earths. The 20-200 p.m

sand fraction of the apparently most tephric part of the Fordell Ash in the fossil gully, a white (5Y 8/2) silt about 15 em from the base of the ash, consists of about 80% aggregates, probably clay, about 10% diatom skeletons, about 10% of quartz, and less than 0·05% of volcanic glass. No ferromagnesian minerals were recognised. The layer thus has a quite different mineralogy from that of the Mount Curl Tephra, and it appears to be a diatomaceous earth rather than a tephra.

Since the Fordell Ash at the type and other exposures on the Brunswick marine bench are directly overlain by dune sand, the stratigraphic positions of the Fordell Ash and Mount Curl Tephra appear to be superficially similar'. The sequence of cover beds overlying the dune sand on the Brunswick bench in the vicinity of Fordell is, however, quite different from that over­lying the dune sand at the Mount Curl Tephra type section. Although much of the cover bed sequence at the Fordell Ash type exposures has been eroded off the dune sands, a full sequence is clearly exposed in a cutting on No. 2 Line Extension 0·1 km to the 'east of Fordell Village (N138/757839), where three loess beds mantle the dune sands (Fig. 3). The sequence appears to be closely similar to that at Mount Stewart, some 40 km to the east, but to be much less complete than that overlying the dune sands at the Mount Curl Tephra type section (Fig. 3).

The dune sands at the Fordell Ash type exposures and at Mount Stewart are considered to have been deposited during the cutting of the Ngarino marine bench; they appear to be much younger than those at the Mount Curl type section and at Galpins Road, which are considered to have been deposited during the cutting of the Brunswick marine bench.

Because of the differences in mineralogy and stratigraphic position of the Fordell Ash, and the Mount Curl Tephra as here designat,ed, there is little doubt that they are indeed different beds.

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No.2 Line Extension

Ford ell Village

CJ . Dune Sand

ltij Tephra

Mount Stewart

Scale 0

1m

2m

lliii Wanganui Series Sediments

- Unconformity

l_.l Abbreviated section n

Galpins Road

VoL. 16

Mount Curl

FrG. 3-Correlation columns between the Fordell Ash type exposure shewn in fig. 50 of Fleming ( 195 3), and the Mount Curl Tephra type section. For localities of the columns see Fig. 1. The Mount Curl column is a composite of the type section and the section some 200 m to the west of the type section. The loess beds have been given the same names as the aggradation terrace flocd plains from which they were largely derived (see Milne J 973). DS = Dune sand; Oh = Ohakea loess; Aok = Aokautere Ash; Rt = Rata loess; Po = Po rewa loess; G = Greatford loess; M = Marton loess; Bu = Bur nand loess; Fa= Fordell Ash; R =Rhyolitic tephra lens; MCT = Mount Curl Tephra; A= Aldworth loess; W = Waituna loess.

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No. 3 MrLNE -MouNT CuRL TEPHRA 525

The Aokautere Ash (Cowie 1964a), forms a prominent marker bed throughout the area where Mount Curl Tephra has been recognised. It is much younger than either Mount Curl Tephra or Fordell Ash, and can be distinguished from them at almost all places by its conspicuous basal layers of fine ash.

Age Volcanic glass was separated from samples of a fresh exposure of Mount

Curl Tephra some 50 m to the west of the type section. Samples of the unweathered glass were dated using the potassium-argon and fission track methods (Table 1).

Both ages aDe in agreement, and because the fission track dating method is more reliable for such young material, an age of 230 000 -+- 30 000 years B.P. is preferred for the Mount Curl Tephra.

Stratigraphic Significance The stratigraphic significance of Mount Curl Tephra has been interpreted

from its relationship to marine benches, coastal dune sands, river terraces, and loess.

The marine benches are considered to have been cut during times of interglacial high sea levels.

The coastal dune sands within the area are considered to have been deposited during the high sea levels of the interglacials. During the present interglacial high sea level stand, there have been three well marked dune sand depositional phases (Cowie 1963 ), during which coastal dune sands have been deposited up to 12 km from the coast. Older coastal dune sands ar'e also extensively preserved on marine benches and on coastal extremities of old river terraces. There is no evidence to suggest that any of the older coastal dune sands in the area were deposited during the last glaciation and it has been therefore assumed that all coastal dune sands are likely to be of interglacial origin.

Major river terraces in the Rangitikei-Manawatu districts are considered to have formed during cold-climate episodes. Reoent work by the writer

TABLE !-Radiometric ages obtained for Mount Curl Tephra glass.

Method

Fission track

Potassium-argon

Geology-14

Apparent age

(years B.P.)

230 000

250 000

Analytical error

(years B.P.)

± 30 oou

± 120 000

Source

B. P. Kohn D. Seward

(pers. comm.) C. J. Adams

(pers. comm.)

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Brunswick Marine Bench

VoL. 16

FIG. 4-Diagrammatic cross-section showing stratigraphic relationships between cover beds on the Brunswick marine bench, and older landforms. 1 = Middle Quatern­ary ( Castlec!iffian Stage) sediments; 2A = aggradation alluvium; 2B = loess derived from 2A; 3 = Mount Curl Tephra; 4A = Brunswick marine bench deposits; 4B = Brunswick Dune-sand; 5 = loess and tephra.

has shown that the major river terraces are in part aggradational, and probably formed by the mechanism proposed by Vella (1963): partial devegetation and accelerated erosion of the high steep headwater areas of the major rivers.

During the cold-climate episodes loess was blown from the aggrading flood plains (Cowie 1964b; Milne 1968). The loess forms prominent layers ov,er large areas of flat and rolling land in Rangitikei and Manawatu districts. River aggradation and loess deposition did not occur continuously during the intervals between interglacial high sea level stands. Rather, they occurred during fairly short episodes of relatively intense cold. River degradation, and occasionally quite intense weathering and soil formation, recorded in paleosols, occurred during intervening episodes of cool climate.

The above model is used to interpret the climate at the time when the Mount Curl Tephra was erupted. At Griffins Road (Fig. 4), the tephra rests on aggradation gravels. At the type section, it rests on loess that is assumed to have been blown from the aggradation gravels during the aggrada­tion phase. The tephra thus rests on cold climate episode deposits at both sections.

The sands that overlie the tephra at the type s,ection were mapped as Brunswick Dunesand by Fleming (1953). They mantle definite terrestrial beds, contain lenses of peat, and closely resemble other nearby upper Quaternary dune sands; there is thus little doubt that they are indeed dune sands, not marine or fluvial sands. They contain a high percentage of

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ferromagnesian minerals (a characteristic feature of coastally derived sands in the area) and they are inferred to be coastal dune sands deposited during an interglacial episode.

At the section to the west of the type section the sands are mantled by apparently the same loess sequence that mantles the Brunswick marine bench, some 7 km to the south-west. Because of the similarities of the loess sequence, and because of the proximity of the Brunswick marine bench to the type section, it is reasonably certain that the dune sands at the type section were derived from the coastline during the time when the Brunswick marine bench was being cut, that is, during the Terangi Interglacial.

The Mount Curl Tephra thus vests on cold climate deposits that predate the Terangi Interglacial and it underlies dune sands deposited during the Terangi Interglacial. Because of the excellent preservation of the Mount Curl Tephra at the type section, it is estimated that the tephra fell at the beginning of the T,erangi Interglacial.

The loess beneath the Mount Curl Tephra is only slightly weathered, and the top of the tephra is likewise only slightly weathered. In contrast the top of the overlying dune sands is strongly weathered. Compared with the range of weathering displayed in pr,esent day soils that range in age from about 12 000 years to less than 50 years, formed in tephra, dune sands, and loess, it is considered that both Mount Curl Tephra and the loess underlying it were weathered for less than 3000 years before burial. In contrast, it is estimated that the dune sands were weathered for 40 000 or more years before burial. It is thus estimated that the Mount Curl Tephra was deposited within 3000 years of the end of the cold climate episode pre­dating the Terangi Interglacial, that it was buried by Terangian age dune sands within another 3000 years, and that deposition of the dune sands was followed by a prolonged episode of weathering during the remainder of the Terangi Interglacial. The beginning of the Terangi Interglacial, is therefore here assigned an age of 230 000 -+- 30 000 Y'ears B.P.

CHRONOLOGY OF MARINE BENCHES IN W ANGANUI BASIN

In order to estimate a more complete chronology for the marine benches, the following assumptions are made. ( 1) Mean sea levels during the inter­glacials were the same as that at the present day. (2) The elevations of each of the cliff bases were within 2 or 3 m of the mean high sea level at the time when they were last trimmed, (the cliff base as here defined is the top of the marine deposits veneering the undermass rock, not the cut surface across the undermass rock). ( 3) The rate of uplift along any particular section at right angles to the coastline has remained constant over the last 250 000 years. ( 4) For the benches to the west of the Wanganui River, the final cliff trimming was at the end of a major interglacial high sea level episode, not at the beginning. The last assumption is made because at the present day the coastline to the west of the W anganui River is retrograding over much of its length, and progradation appears to be a temporary phenomenon where it is occurring.

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The heights of the Brunswick and Ngarino cliff bases at the three different localities to the west of the Wanganui River have been calculated from topographic heights given in Fleming (1953, pp. 42 and 47), by subtracting 10 m from each of the topographic heights to approximately compensate for the thickness of the dune sands and other cover beds that mantle the marine deposits themselves (Table 2). Since the marine benches have been deformed by tilting and faulting (Fleming 1953, pp. 46, 48, and 50), the estimated heights of the cliff bases are different at the three different localities. Never­theless, the ratios of heights of the Brunswick and Ngarino cliff bases at each of the localities are remarkably similar (Table 2), the Ngarino cliff base averaging 0·6 times the height of the Brunswick cliff base. Accepting the assumptions made earlier, it would appear that the Ngarino cliff base is 0·6 times as old as the Brunswick cliff base.

Judged by the degree of weathering of the top of the Brunswick Dune­sand near the Mount Curl type section, and assuming that the loess on top of the dune sand was deposited during the succeeding glacial stage, the Terangi Interglacial is inferred to have ended at or more recently than 190 000 years B.P. It is thus inferred that the Brunswick cliff base was last trimmed at or more recently than 190 000 years B.P., and that the Ngarino cliff base was last trimmed at or more recently than 115 000 years B.P.

CoMPARISON OF THE W ANGANUI MARINE BENCH

SEQUENCE WITH BARBADOS AND NEW GUINEA

CORAL REEF SEQUENCES

Elevated coral reefs on the coastlines of Barbados and New Guinea have been described and the younger reefs have been radiometrically dated (see Mesolella et al. 1969; James et al. 1971; Veeh & Chappell 1970).

TABLE 2-Altitudes of Brunswick and Ngarino marine bench cliff heights on the Wanganui coast.

------ -------------- ---

Locality

Immediately West of Waitotara Valley Immediately West of the Nukumaru Fault Westmer~ district

Height of cliff base (a.m.s.l.)

Brunswick bench Ngarino bench

2201

1541

Ratio Height Ngarino/ Height Brunswick

0·63

0· 58

0· 56

Notes: lValues calculated by subtracting 10m from heights quoted by Fleming (1953, pp. 42, 47).

2Value calculated by adding 10m on to height quoted by Fleming (1953, p. 42), since height quoted was for the erosion surface cut into the undermass rock.

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No. 3 MILNE- MouNT CuRL TEPHRA 529

These authors have all concluded that the reefs were built during high sea level stands, but that their present elevation is largely due to subsequent tectonic uplift. From the physical stratigraphy of the Barbados reefs together with the radiometric dates, Mesolella et al. (1969) concluded that the three lowest major coral reefs were built at c. 82 000, c. 105 000, and c. 125 000 years B.P., and that the three next highest major reefs were built between c. 170 000 and c. 230 000 years Il.P. (Fig. 5).

A 60 000-year-old discontinuous reef within 4·5 m of present sea level was subsequently recognised by James et al. ( 1971), but was considered to have been built during an interstadial high sea level stand (lower than at present), not durit~g an interglacial high sea level stand.

The New Guinea sequence described by Veeh & Chappell (1970) is in an area that has undergone rapid tectonic uplift (c. 3 mm year-1 ) for at least the last 200 000 years. As a result, reefs built during both interstadials and interglacials are preserved (Fig. 5). The ages of the post-200 000-year­old reefs are those suggested by Veeh & Chappell (1970) from radiometric dates and stratigraphic positions. The ages of the two lowest of the pre-200 000-year-old reefs have been estimated by the writer from the radio­metric dates given in table 1 of Veeh & Chappell (1970). Veeh &

Chappell (1970) considered that the 30 000, 35 000, and 60 000-year-old reefs were built during relatively high interstadial high sea level stands, but that all the older reefs were built during full interglacial high sea level stands. Like the 60 000-year-old reef, the 140 000-year-old reef has formed at the base of a high cliff formed by the outer edge of the next oldest major reef, and, if it is assumed that the rate of uplift of the New Guinea reefs has remained constant on any segment of the coastline over the last 140 000 years, then the J"eef appears to have been built during an interstadial stand of sea level, lower than those of the full interglacials. Thus of the post-250 000-year-old reefs, only the 74 000, c. 105 000 and 118 000-year-old reefs, together with the c. 185 000 and c. 215 000-year-old reefs were apparently built during full interglacial high sea level stands. As here interpreted, the major reef building phases, and therefore the inter­glacial high sea level stands, are in good agreement with those from the Barbados sequence.

The Barbados and New Guinea interglacial coral reefs thus apparently fall into two groups (Fig. 5). The Wanganui district Brunswick bench falls into the older o'f the groups, while the Ngarino bench apparently falls into the younger. The Rapanui bench may also fall into the younger group, although lack of height information makes its age difficult to determine. The two groups probably span the last, and the penultimate interglacials r<espectively, although Emiliani & Rona (1969), consider from ocean core evidence that the last interglacial was from 65 000 to 100 000 years B.P.,

and the penultimate interglacial was from 120 000 to 175 000 years B.P.

(Fig. 5). Judged by the date for the Brunswick marine bench, the Barbados and New Guinea datings of the interglacials seem the more r'easonable.

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Ol 0 N TIME OF <D ['.

~ c) ~ z "'

<D < z EVENT c: (/) ~ [;: UJ a; iii

0 0 0 ~ z c. < Q; 0:: c. ...I 0 Cij 5 I...«< Ill c. o::· "0 < a; c;;. <!J .,-'= "' (Millions of 0 c: Ill -u ::;) c. ~ "' 0:: E a; ~ 'lii"O z "' years before.

·;:: i]§· a; "' UJ c: < :.c ~ 0 "' z "' <!J !:::

present) 'E= (/) E .<:: z Q) ~ (]) <

!:':! :::;: (])

~ ~. G.

··············· ·::.·.·::::::.·.·.·.·.·:

r- 0·050 -

r- o·2oo

-r- 0·300 -

FIG. s-Estimates by different authors of times of high sea levels and interglacials. Ages of radiometrically dated features indicating high sea levels shown by solid horizontal lines. Ages of stratigraphically dated features indicating high sea levels shown by dashed horizontal lines. Inferred· 'interglacial episodes shown by dotted areas. Ages for the two youngest world interglacials are those stated by Emiliani & Rona ( 1969); ages for the two older are estimated by the writer from fig. 2 of Emiliani & Rona ( 1969). Ages from the Barbados sequence are those estimated by Mesolella et al. ( 1969) and Broecker & Ku ( 1969). Ages from the New Guinea sequence are those estimated by the writer from data in Veeh & Chappell (1970).

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No.3 MILNE -MOUNT CURL TEPHRA 531

ACKNOWLEDGMENTS

This study forms part of a Ph.D. project done at Victoria University of Wellington. I wish to thank Mr B. P. Kohn and Mrs D. Seward, for permission to publish

radiometric dates obtained by them, and also Mr A. V. Weatherhead for identifying the minerals in the sand fraction of the Mount Curl Tephra. Dr R. P. Suggate, Professor H. W. Wellman, Mr C. G. Vucetich, fellow graduate students, and c?lleagues at Soil Bureau made constructive suggestions concerning the text and diagrams.

The potassium-argon date was obtained as part of a research programme by the Institute of Nuclear Sciences New Zealand and I am grateful for permission to publish ;t. '

REFERENCES

BROECKER, W. S.; Ku, T. K. 1969: Caribbean cores P 6304 and P 6304-9; new analyses of absolute chronology. Science 166: 404-6.

CoWIE, ]. D. 1963: Dune-building phases in the Manawatu District, New Zealand. N.Z. Journal of Geology and Geophysics 6: 268-80.

1964a: Aokautere Ash in the Manawatu district, New Zealand. N.Z. Journal of Geology and Geophysics 7: 67-77.

1964b: Loess in the Manawatu district, New Zealand. N.Z. Journal of Geology and Geophysics 7: 389-96.

EMILIANI, C.; RoNA, E. 1969: Caribbean cores P 6304-8 and P 6304-9: New analyses of absolute chronology. A reply. Science 166: 1551-2.

FLEMING, C. A. 1953: The geology of the Wanganui subdivision. N.Z. Geological Surz:ey Bulletin 52.

1959: "New Zealand." Lexique Stratigraphic 11Ziernational 6, (Oceania), Fasicule 4: 528 p.

GRANT-TAYLOR, T. L. 1964: Volcanic history of western Taranaki. N.Z. Journal of Geology and Geophysics 7: 78-86.

]AMES, N. P.; MouNTJOY, E. W.; AKIO, 0. 1971: An early Wisconsin reef terrace at Barbados, West Indies, and its climatic implications. Geologica! Society" of America Bulletin 82: 2011-8.

MESOLELLA, K. ]. ; MATTHEWS, R. K.; BROECKER, W. A.; THURBER, D. L. 1969: The astronomical theory of climatic change: Barbados data. Journal of Geology 77: 250--74.

MILNE, ]. D. G. 1968: Geology and soils of the Apiti District. (Unpublished M.Sc. thesis, lodged in the library, Victoria University of Wellington New Zealand.) 159 p.

MILNE, ]. D. G. 1973: River terraces in the Rangitikei basin. N.Z. Soil Bureau Maps 142/1, 142/2, 142/3, 142/4.

SuGGATE, R. P. 1965: Late Pleistocene geology of the northern part of the South Island, New Zealand. N.Z. Geological SuJTey Bulletin 77: 90 p.

VEEH, H. H.; CHAPPELL, ]. 1970: Astronomical theory of climatic change: Support from New Guinea. Science 167: 862-65.

VELLA, P. 1963: Upper Pleistocene succession in the inland part of Wairarapa valley, New Zealand. Transactions, Royal Society of N.Z., Geology 2: 63-78.

APPENDIX 1

STRATIGRAPHIC DESCRIPTION OF MOUNT CURL TEPHRA TYPE SECTION

(Numbers refer to beds shown in Fig. 2. Description is of the eastern and of the type section.) Unit 6: Loess

Light olive grey (5Y 6/2) clay -sharp contact-

Thickness (m)

1+

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532 N.Z. JoURNAL OF GEOLOGY AND GEOPHYSICS

Unit 5: Brunswick Dune-sand

Olive grer (5Y 5/2) sand; bedded in 0· 5 m to 1· 5 m thick layers, locally w1th large scale cross-bedding. Boundaries of sand layers defined in most places by silt laminae up to 3 em thick, and in remaining places by oxidised peat lenses. About 25% of the sands consist of ferromagnesian minerals. Basal 0 · 2 m to 1 m of the sands are cemented into a massive oxide pan

-sharp contact-

Unit 4: Mount Curl Tephra

Pale brown ( 1 OYR 6/3) fine sandy loam; contammg rhizomorphs with thin and discontinuous colloidal coatings

-contact gradational over 3 em-Light olive brown (2 · 5Y 5/4) medium sandy loam

-contact gradational over 3 em-Yellowish brown (10YR 5/4) sand; with fine lapilli up to 3 mm

in diameter in basal 0 · 1 m -sharp contact-

Unit 3: Loess Pale brown ( 10YR 6/3) silt loam; contammg rhizomorphs with thin and discontinuous colloidal coatings

-contact gradational over 5 em-Brown (10YR 5/3) fine sandy loam; rhizomorphs as above

-contact gradational over 5 em-

Unit 2: Loess

Brown (10YR 5/3) silt loam; thicker colloidal coatings than those in loess unit )

-sharp contact-Yellowish brown (10YR 5/4) sandy clay loam;

-sharp contact marked locally by an oxide pan-

Unit 1: Middle Quatemary ( Castlecli.ffian Stage) Sediments

Pale brown (10YR 6/3) siltstone; thin bedded; rare layers and lenses of well rounded pebbles up to 3 em in diameter

APPENDIX 2

MINERALOGY OF THE MOUNT CURL TEPHRA

VoL. 16

Thickness (m)

3

0·25

0· 28

0· 35

0·2

0·2

0.35

0·35

2+

Titanomagnetite was magnetically separated from a 95 g split of the sand fraction of the same bulk sample from which the glass for radiometric dating was obtained. Ferromagnesian minerals, zircon, and apatite were separated from the same split using bromoform. Weight precentages for the titanomagnetite, and the light and heavy mineral fractions were determined.

Individual minerals were identified using a petrographic microscope and relative abundances of the minerals were determined visually. The refractive index of the volcanic glass, determined using white light without temperature control was 1 · 496. Mineral abundances are set out below.

Volcanic glass 77% Hornblende trace Sodic plagioclase 12% Biotite 0·5% Ouartz 2% Titanomagnetite 1% Hypersthene 7% Apatite 0·5% Augite trace Zircon trace

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