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Aokautere ash, loess,and river terraces in theDannevirke District, NewZealandK. P. Rhea aa Geology Department , Victoria University ofWellington , New ZealandPublished online: 21 Dec 2011.

To cite this article: K. P. Rhea (1968) Aokautere ash, loess, and river terracesin the Dannevirke District, New Zealand, New Zealand Journal of Geology andGeophysics, 11:3, 685-692, DOI: 10.1080/00288306.1968.10420278

To link to this article: http://dx.doi.org/10.1080/00288306.1968.10420278

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No.3

AOKAUTERE ASH, LOESS, AND RIVER TERRACES IN THE DANNEVIRKE DISTRICT,

NEW ZEALAND

K. P. RHEA*

Geology Department, Victoria University of Wellington

(FIrst submitted tor publication 25 June 1964; accepted in relJisWn form 30 January 1968)

ABSTRACT

685

The Aokautere Ash is described from the Dannevirke district 30 miles north-east of its type locality near Palmerston North. In conjunction with loess thicknesses the ash is used to correlate the main terraces of the Dannevirke district with those of the Palmers ton North district. The oldest post-ash terrace is the most extensive terrace. It is correlated with the Ohakea Terrace of the Palmers ton district and is considered to have been formed by aggradation towards the end of the Last Glaciation.

INTRODUCTION

The Aokautere Ash Formation was first recognised and named near Palmerston North by Cowie ( 1964a). The ash was later found within loess on the eastern side of the Ruahine Range about 30 miles north-east of Palmerston North near Ormondville and on the Dannevirke-Weber Road about 8 miles south-east of Dannevirke.

An area of about 200 square miles that includes the town of Dannevirke was examined to determine the stratigraphy and areal distribution of the Aokautere Ash and its relation to river terraces.

The principal terraces of the upper Manawatu River and the Makaretu River were mapped from aerial photographs and were correlated by com­parison . of loess thicknesses, terrace characteristics and the presence or absence of the ash. The main terraces are shown on Fig. 1 and the ash localities are listed in Table l.

AOKUATERE ASH

The Aokautere Ash was first identified in the Dannevirke district as a conspicuous ledge projecting from a road cutting (Fig. 2) and was later found at many localities as a 7- to 14-in. thick layer usually recognisable only because it is stained by iron oxide. No other deposits with which the ash could be confused exist in the Dannevirke district.

The ash consists of five separate members (Table 2) of which numbers 2, 3, and 4 show grain size increasing with depth. The coarse layers are usually stained reddish brown and in many sections are represented by

*Present address: c/o AMAX Exploration, Inc., Belmar Sta., Box C 12620 West Cedar Drive, Denver, Colorado 80226, U.S.A.

N.Z. JI Geol. Geophys. 11: 685-92

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686 N.Z. JOURNAL OF GEOLOGY AND GEOPHYSICS VOL. 11

TABLE l-Aokautere Ash Localities in the Dannevirke District. Thicknesses of ash and loess shown in inches where reasonably representative

Overlying Locality Loess (some Ash Grid* Locality

No. eroded) Thickness Ref. (in.) (in.)

32 12 N145/648514 Cowper Rd 2 32 12 565568 Umutaoroa Rd

3 18 11 571506 Tiratu Rd 4 36 11 554621 Dittmers Rd 5 38 552602 Umutaroa Rd 6 30 603552 Tiratu Rd 7 33 612562 Maunga Rd 8 27 10 702646 Matamau-Ormondville Rd 9 48 11 717638 Ormondville-Whetukura Rd

(new portion) 10 21 ? 14 637756 Makaretu Rd (steep slopes) 11 24 742722 Otawhao Factory Rd 12 18 ? 13 753718 About 2 miles N of Whenu-

ahou 13 24 657518 Cowper Rd 14 42 586550 Tipapakuku Rd 15 34 12 533612 Beatty Rd 16 40 7+ 512619 Tamaki East Rd

tion) (new por-

17 32 9 ]'\150/510494 Wellington - Napier High-way

18 24 468442 Wellington - Napier High-way

19 45 6+ 656464 Weber Rd 20 28 8 N145/576505 Tiratu Rd 21 48 8 571501 Paddock S of Dannevirke 22 24 7 549526 Palmers ton - Napier High-

way

*Grid references are based on the sheet districts of the 1 : 63,360 topographical map series (N.Z.M.S. 1) and the national thousand-yard grid shown on this series.

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RHEA-AoKAUTERE ASH, DANNEVIRKE DISTRICT

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Dissected

Terrain

'" .. ~OOON ~ N.;;', J/ GIO/, Geophys, 11 (3), 1968

FIG, I-Map of river terraces in Dannevirke district. Young minor terraces and flood plains are finely stippled (A), Terrace B, the oldest post-ash terrace is coarsely stippled, Terraces c, D, and E are pre-ash terraces, Traces of active faults are shown by bold dashes, Numbers in circles mark localities mentioned in text.

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No.3 RHEA - AOKAUTERE ASH, DANNEVIRKE DISTRICT 687

TABLE 2-Generalised Aokautere Ash Stratigraphy in Dannevirke District

Member Thickness

3 in. Variable

2 in.

2 8 in.

3 3 . sIn.

4 It in.

5 3 . gin.

Variable

Description

Vegetation and topsoil Loess, generally grey to brownish grey, and com­

monly mottled reddish brown, occasionally con­taining dark brown concretions up to i in. across. Lower boundary indefinite.

White, very fine sand- to silt-sized ash; rarely pre­served and may be leached upper portion of member 4. Lower boundary indefinite.

Dark brownish grey to grey ash grading downward from very fine sand or silt to very coarse sand which contains elongate white pumice fragments up to i in. across. Commonly colour banded by iron staining. Lower boundary indefinite. (Lower­most 2 in. may be separate unit.)

Graded ash, commonly brown or brownish grey, grading downward from fine to coarse sand. Poorly preserved as a separate member. Lower boundary sharp.

White to light grey ash grading downward from very fine sand to very coarse sand. Lower i in. commonly pinkish buff.

White to light grey fine sand-sized ash. Thin pinkish buff colour band at base. Lower boundary sharp.

Loess similar to that overlying the ash but commonly darker and containing more concretions in upper part.

dark bands that contrast with the lighter coloured, finer material. The general effect of soil processes has been to enhance the boundaries between coarse and fine ash by more intense reddish brown staining of the coarse ash. In many places members 1 and 3 and the lower part of member 4 are obscured and the ash appears to consist of only two units, a thick coarse­grained, dark-coloured layer overlying a thinner fine-grained, white, or buff­coloured layer (Fig. 3).

The Taupo-Rotorua volcanic district 100 miles to the north, is the most probable source of the ash but the erratic thicknesses and the indefinite upper boundary of the ash preclude a determination of direction of origin from the direction of thickening.

The five members have essentially the same composition, mainly pumice, glass, and feldspar. Quartz is present in minor amounts, estimated as less than 3%. Hornblende, magnetite, and hypersthene are usually less than 1 % but range up to 2% in the coarser fraction. About 1 % of the ash consists of small, speckled rock fragments.

The pumice occurs as clear or milky white, elongated fibrous particles. The glass, commonly occurring as shards, is clear and vesicular with a refractive index between 1'500 and 1'505. Petrographic and X-ray exam­inations indicate that the glass and pumice are partly cryptocrystalline. The feldspar is sodic plagioclase in the form of prismatic to blocky, clear crystal fragments.

Geology-12

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688 N.z. JOURNAL OF GEOLOGY AND GEOPHYSICS VOL.li

FIG. 2-Photograph showing Aokautere Ash (between white lines) forming a projecting ledge in a cutting at Weber Road (N150/656464).

FIG. 3-Photograph of Aokautere Ash near Ormondville (N145/717638). Light band about 2 in. thick just below head of hammer includes ash members 4 and 5. Top of ash is poorly defined and lies about half way between light band and top of exposed face. Scraper marks poorly defined "iron-pan" in finely laminated lake deposits 8 in. below base of light band.

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No.3 RHEA - AOKAUTERE ASH, DANNEVIRKE DISTRICT 689

Quartz occurs as clear and more or less equidimensional microscopic frag­ments. No well defined quartz peaks were observed in X-ray analysis of the ash. The hornblende occurs as black prismatic crystal fragments and the magnetite as black equidimensional euhedral crystals. The hypersthene is translucent green, euhedral to subhedral crystals, many of which contain magnetite inclusions. The rock fragments are opaque, commonly milky red, green, blue, or grey and contain many dark unidentified inclusions.

All particles are angular and range in size from 0'001 mm or less to about 10 mm.

LOESS

Medium sand- to clay-sized sediment of uniformly massive texture over­lies terraces and other surfaces older than Holocene, and is considered to be loess. Similar material that underlies the ash is probably loess also. Loess with a similar relation to the terraces and ash has been described from the Palmerston North district by correlation. The loess is grey to brownish grey, often with mottlings and small dark brown nodules. Grain size ranges from clay to medium sand, silt-sized particles being dorqinant. Quartz and feldspar (sodic plagioclase and possibly some orthoclase) are the principal minerals and hornblende, magnetite, and muscovite the minor minerals of the non-clay fraction. The clay-sized fraction was not examined. The feld­spars are clear, or white to flesh coloured, and Me sub-rounded to rounded. The quartz is clear and sub-rounded to angular. The dark brown nodules are usually smaller than 2 mm and consist of quartz and feldspar grains loosely cemented by a dark brown matrix. The nodules occur throughout the loess but are generally more abundant immediately below the ash.

The loess has protected the ash from erosion and soil mixing processes and, in general, the ash is best preserved where the loess covering is thickest. Surfaces older than the ash are commonly covered by up to 8 ft of loess. Of terraces younger than the ash, the eldest is generally covered by less than 2 ft of loess and the youngest, probably Holocene in age, are without significant loess.

The loess of the Palmerston North district is considered by Cowie (1946b) to have been blown from nearby aggrading river flats by north-westerly winds, and a similar origin is probable for the loess in the Dannevirke district.

TERRACES

Nearly all of the Dannevirke district is terraced, but many terraces are only a mile or so long. Of the four main terraces recognised (B, C, D, and E in Fig. 4), one (B) is younger than the Aokautere Ash and three are older. Minor terraces younger than Terrace B together with the flood plains of the rivers are grouped under A.

The three pre-ash terraces recognised at most places are moderately dissected and gently undulating, and are covered by thick loess. The vertical interval between successive terrace surfaces is usually several tens of feet.

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No.3 RHEA - AOKAUTERE ASH, DANNEVIRKE DISTRICT 691

The youngest pre.ash surface, Terrace c, is commonly covered by at least 2 ft of loess. Terraces C and B are separated by a vertical interval of about 8 ft poorly marked at most plact!s by a riser that is well rounded, gently sloping, and everywhere indistinct. Distinction between Terraces Band care particularly difficult in the extreme south·west corner of the mapped area.

An anomalous terrace sequence I! miles south·east of Dannevirke appears to contain four and not the normal three pre.ash terraces. The anomaly is probably due to the western part of Terrace c having been upthrown by a short north·east striking fault.

Except where disturbed by faulting as at localities 1 and 2 (Fig. 1), post ash terraces are flat with few undulations, and are commonly covered by gravelly soil and marked by shallow branching channels that show on air photographs. They have well defined and steep risers. Terrace B is under· lain by as much as 10 ft of fresh gravels. At locality 3 (Fig. 1) the fresh gravels disconformably overlie strongly iron·stained non·marine gravels that contain abundant shell fragments derived from the early Pleistocene marine sediments. The iron stained gravels are inferred to be aggradational and to belong to one of the pre·ash terraces.

The exact time plane represented by the Aokautere Ash is invaluable for terrace correlation, and those terraces that are closest in age to the ash can be correlated with reasonable certainty. In the Palmerston North district the Milson is the youngest terrace that is older than the ash, and the Ohakea is the oldest terrace that is younger than the ash, and both are well defined and fairly extensive (Cowie and Wellman, 1962; Cowie, 1964a, b).

In the Dannevirke district Terrace c, the terrace immediately older than the ash, is well defined and fairly extensive and is correlated with the Milton Terrace. Terrace B, the terrace immediately younger than the ash, is the most extensive terrace in the district, is well defined, and is correlated with the Ohakea Terrace.

By comparison with terraces in the Manawatu district, Terrace c is more rounded and more subdued than the Milson, and not as far above Terrace B as Milson Terrace is above the Ohakea Terrace. The correlation of Terrace B

with Ohakea Terrace is more certain than is the correlation of Terrace c with Milson Terrace.

Terrace E, the highest and oldest extensive terrace and Terrace B, the most extensive surface, almost certainly represent significant periods of aggradation. That is probably true also for Terraces c and D. An undeter· mined thickness of gravel is shown underlying each of the four older terraces in Fig. 4.

By contrast the recent terraces that are grouped together as "A" are not individually extensive and appear to mark pauses in the period of river downcutting that is probably still continuing.

The three main terrace divisions, pre·Aokautere, oldest post·Aokautere, and younger terraces generally correspond with the soil sets 77, 77b, 78b, soil sets 75, 75a, 78, and soil set 1 respectively. This is a clear indication that the soils are related to terrace age, but only by use of Aokautere Ash is it possible to make direct correlation.

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692 N.Z. JOURNAl, OF GEOLOGY AND GEOPHYSICS VOL. 11

Comparison of soil development on the Ohakea Terrace with that on a carbon dated 3,OOO-year-old terrace at Rangitawa Stream led Cowie and Wellman (1962) to conclude that the Ohakea Terrace is at least 10,000 yr old.

The present study shows that Terrace B of the Dannevirke district, which correlates with the Ohakea Terrace of the Manawatu district, originally extended over about a quarter of the mapped area. Its size and aggradational character indicate that it was probably formed in a climatic environment different from that of the present day; its age suggests that it was formed during the last main cold period of the Last Glaciation.

ACKNOWLEDGMENTS

The writer is indebted to Mr J. D Cowie of the New Zealand Soil Bureau for information about the Aokautere Ash in the Palmerston North district, and to Professors H. Wellman and ]. Bradley of Victoria University of Wellington, for field supervision and editing, respectively. The study was sponsored by Victoria University and supported by the United States Educational Foundation in New Zealand.

REFERENCES

COWIE, J. D. 1964a: Aokautere Ash in the Manawatu District, New Zealand. N.z. II Geol. Geophys. 7 (1): 67-77.

COWIE, J. D. 1964b: Loess in the Manawatu District, New Zealand. N.z. JI Geol. Geophys. 7 (2) : 389-96.

COWIE, J. D.; WELLMAN, H. W. 1962: Age of Ohakea Terrace, Rangitikei River. N.Z. JI Geol. Geophys. 5 (4): 617-9.

N.Z. SOIL BUREAU, 1954: General survey of the soils of North Island, New Zealand. N.Z. Soil Bur. Bull. 5.

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