A mineralogical and textural study of the central North Island tephra, Okareka ash and its overlying tephric loess deposits : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Soil Science at Massey University

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A MINERALOGIC~L AND TEXTURAL STUDY OF

THE CENTRAL NORTH ISLAND TEPHRA, OKAREKA ASH AND

ITS OVERLYING TEPHRIC LOESS DEPOSITS

A T~e=~3 Preser.:~d ~s Partial Fulfil~me~t for

by

Lynette Anne Benny

Massey University, New Zealand

1982

d;

t:

^{! . ~! ••}

ACKNOWLEDGEMENTS

I wish to thank my supervisors Dr. J .H. Kirkman, and Mr. R.8. Stewart for their guidance, encouragement and advice during my thesis work.

I wish also to extend my thanks to Dr. W.A. Pullar and Mr N.M. Kennedy, Soil Bureau, Rotorua, for their

invaluable assistance in collecting soil samples for this study, and worthwhile discussion in the field .

I am grateful to Mr. D. Hopcroft and Mr. R.J . Bennett of the Electron Microscope Lab. D.S. I .R. , for their technical assistance in the electron optical work, and printing of the electron micrographs.

i i .

To Miss . J .S. Rowaxth and Mr. R.C. Wallace, my thanKs for your critical reading of, and helpful comments about the manuscript.

I would also like to thank my mother, Mrs. A.F. Benny, who typed this thesis, and finally , both my parents for

their continuing support.

J

i i i.

ABSTRACT

In Central North Island, New Zealand, Post- Okareka tephric loess rests upon Okareka Ash (c. 17,000 years B.P. ). Tephric loess accumulation occurred under semi- arid conditions which coincided with glacial advances in southern areas of New Zealand.

Morphological and grain-s ize evidence indicates the tephric loess has been derived from a localised source, most probably that of Okareka Ash material , reworked and redeposited by aeol ian processes. Optical and electron optical evidence reveals that Okareka Ash particles are angular and relatively unweathered, whereas tephric l oess grains are subangular and more weathered.

The sand and clay mineralogy of the tgphra and tephric loess are similar. Sand fractions contain mainly

rhyolitic volcanic glass , quartz, plagioclase feldspar, biotite, hypersthene, hornblende, titanomagnetite and traces of cristobalite, tridy~ite and augite, whereas clay fractions contain hal loysite, allophane, imogolite and gi bbsite in varying amounts .

Grain-s ize analysis reveals Dkareka Ash deposits show decreasing mean grain- size wi th increasing distance from s ource, are poorly-sorted, fine- skewed, ~nd

lepto/platykurti c . In contrast to t ephra, tephric loess samples exhibit a narrow mean grain- size range, and are better sorted, but show similar skewness and kurtosis values to ash. Grain-size results also

indicate that due to minimal weathering of Okareka Ash and Post- Dkareka loess, the distinction between the two deposi ts i s less wel l-defined than data from similar depos its reported by Fisher (1966). Furthermore, where ash depos its are thin, in distal areas from source, and under certain environmental conditions,

textural and mo~hological characteristics of the tephra are similar to those of the tephric loess. Nevertheless, grain-size parameters may be used to differentiate

airfall tephra and tephric loess deposits, although this

differentiation is enhanced by post- depositional weathering.

The contrasting clay mineralogies of tephra and tephric loess samples from sections of similar topography, altitude, drainage and rainfall, illustrates the problems of field sampling in weathering studies.

lV.

Acknowledgements Abstract

Table of Contents List of Tables List of Figures Nomenclature Abbreviations

TABLE OF CONTENTS

Chapter 1 Introduction

SECTION A: Mineralogy and Morpholooy

V •

Page No.

l l l l l V

V l l l lX

xiii

X lV

Chapter 2: Literature Review 3

2.1 Distribution of airfall deposits rn 3 New Zealand

2. 2 Tephra and loess deposits of Central 6 North Is land

2.3 Okareka Ash and overlying loess deposits

2.4 Weathering studies of tephra and tephra-derived soils

Chapter 3: Materials and Methods 3.1 Field Techniques

3. 2 Instrumental Techniques

Chapter 4: Results

4.1 Sand and silt mineralogy

4.1 .1 Okareka Ash Formation : Trunk Rd 4. 1. 2 Okareka Ash: remaining sections

4. 1. 3 Post-Dkareka loess

4.2 Sand and silt morphology

4. 2. 1 Okareka Ash

4.2.2 Post- Dkareka l oess

4. 3 Clay mineralogy

4. 3. 1 Okareka Ash: Trunk Rd

4. 3. 2 Okareka Ash: remaining sections

1 0

1 2

1 9 1 9 23

26 26 26 26 28

29·

29 35

46 46 49

Vi.

Page No.

4.3.3 Post- Dkareka loess : Trunk Rd 59 4.3.4 Post- Okareka loess: Gavin Rd and 59

Te Ngae sections

4. 3. 5 Post- Dkareka loess: remaining 61 sections

4.3.6 Gibbsite in clay fractions of ash 71 and loess deposits

Chapter 5: Discussion 77

5. 1 Okareka Ash sand and s i l t mineralogy 77 5.2 Post- Dkareka loess sand and s i l t 78

mineralogy

5. 3 Dkareka Ash sand and s i l t morphology 80 5.4 Post- Dkareka loess sand and s i l t 81

morphology

5.5 Weathering in Dkareka Ash and Post- 82 Dkareka loess deposits

SECTION B: Grain- size Analvsis

Chapter 6: Literature Review 6.1 Textural Studies

91

Chapter 7: Materials and Methods 7. 1 Analytical Techniques

95 95

Chapter 8: Results and Discussion 8.1 Grain-size parameters

96 96 8. 1 . 1 Mean grain-size: 96

8 . 1 . 1 Mean grain-size:

8. 1 . 2 Sorting

8. 1. 3 Skewness

8. 1 • 4 Kurtosis

8. 1 . 5 Mean grain-size

8 .1. 6 Mean grain-size and kur tosis

Dkareka Ash

Post- Dkareka loess 98 1 03 1 05 1 06 versus sorting 109 versus skewness, 111

8 .1. 7 Mean grai n- size versus Mz / Mz + Q'\ I 111

8.1 .8 Mean grain-size versus KG/ KG+ Mz 115

Chapter 9: Conclusions

References

Appendices

vii.

Page No. 1 1 8

1 21

1 31

LIST OF TABLES Table No.

2

3

Correlation of late Pleistocene tephra deposits in Central North Island, with North Westland glacial advances

Frequency of mineral species (percent) in very fine sand (63 - 125,rm) and fine sand ( 125 - 2sorm) fractions from

selected beds in Okareka Ash Formation, Trunk Rd section

Summary of Mineralogical and Morphological Results

V l l l.

Paqe No. 7

27

76

lX.

LIST OF FIGURES Figure No .

Generalised map of airfall deposits in North Island , New Zealand

Page No.

s

2

3

4

s

6

7

8

9

1 D

Isopachs showing distribution of Dkareka Ash

Site locations , Rotorua district, Central North Island

Stratigraphy of transect sections Stratigraphy of Te Ngae , Trunk Rd and Gavin Rd sections

Scanning electron micrographs of :

A - rhyolitic volcanic glass particles, B - plagioclase feldspar grain exhi bi t ing albite (multiple) twinning , separated from Dkareka Ash samples

Scanning electron micrographs of :

A - quartz grains , B - bipyramidal- shaped

1 1

20

21 22

30

31

quartz grain, separated from Dkareka Ash samples Scanning electron micrographs of

hypersthene grains separated from:

A - Dkareka Ash samples, B - Post- Dkareka loess samples

Scanning electron micrographs of

hornblende grains seoarated from: A - Dkareka Ash samples , B - Post-Dkareka loess samples Scanning electron micrographs of biotite mica grains separated from : A - Dkareka Ash samples, B - Post-Dkareka loess samples

32

33

34

11 Scanning electron micrographs of titano- 36 magnetite grains separated from: A - Dkareka Ash samples , B - Post -Dkareka loess samples

12 Scanning electron micrographs of Dkareka 37 Ash particles separated from sections: A - close to source (RAL), B - intermediate (CA) , and

1 3

C - furtherest (EA) to ash source

Scanning electron micrograph of plagioclase feldspar grains separated from Post-Dkareka loess samples

38

X •

Figure No. Page No.

1 4

1 5

1 6

1 7

Scanning electron micrographs of plagio- 39 clase feldspar grains separated from Post-

Dkareka loess samples : A - feldspar prism

coated with short- range order material, and at B - higher magnification, C - feldspar

prism showing dissolution etch marks, and D - at hi gher magnification

Scanning electron micrographs of:

A - exposed cleavage planes in a biotite mica grain , and B - a pumiceous particle separated from Post- Dkareka loess samples

Scanning electron micrographs of:

A - a s i l t- sized aggregate, with, B - clay- 40

42

43

size material forming on grain fragments within the aggregate , which has been separated from Post-Dkareka loess samples

Scanning electron micrographs of Post- 45 Dkareka loess particles separated from sections:

A - close ( AL

1), B - intermediate ( CL

2), and C - furtherest (EL

1) to ash source

18 X-ray diffraction patterns (~) of the clay 47 ( < 1.Dfm) fraction from Dkareka Ash,

1 9

20

21

22

23

Ngongotaha section ( -1

Infra- red s pectra cm ) of Dkareka Ash

C l a y ( < 1 • D

rm )

^{f r a C t i On s from selected}

sections

Transmission electron micrograph of < 1 . o,rm size clay of Okareka Ash at Trunk Rd section

(RA bed)

Transmission electron micrograph of <1 . orm size clay of Dkareka Ash at Trunk Rd section

(RAL bed)

Transmission electron micrograph of < 1 • orm size clay of Dkareka Ash at Trunk

Rd

section, showi ng surface boiling affects on halloysite tubes and hexagonal-shaped material

Differential thermal curves (DC) of

Dkareka Ash clay ( < 1 . o,m) fractions from

48

so

51

52

53

X i .

Figure No. Page No.

24

25

selected sections

T ra nsmis s ion electron micrograph of < 1 . DJm size clay of Dkareka Ash at Te Ngae section Transmission electron micrograph of <1 .Dfm size clay of Okareka Ash at Pukehangi Rd section

55

56

26 Transmission electron micrographs of < 1.

or

⁵⁸

size clay of Okareka Ash at: A - Tarukenga

27

28

29

30

31

and B - Highland Hill sections

Transmission electron micrograph of < 1.

or

size clay of Post -Dkareka loess at Trunk Rd section

60

- 1 6

Infra-red spectra (cm ) of Post- Dkareka 3 loess clay ( <1 .O,m ) fractions from selected sections

Differential thermal curves (°C) of Post- 64 Dkareka l oess clay ( < 1.0fm) fractions from selected sections

Transmission electron micrograph of <1 ·o/m size clay of Post-Okareka loess at Pukehangi Rd section

Transmission electron micrograph of < 1 .~rn size clay of Post-Okareka loess at Ngongotaha section

66

68

32 Transmission electron micrograph of < 1. ~m 69 size clay of Post-Okareka loess at Tarukenga section

33

34

35

36

Transmission electron micrograph of < 1 · t:rm size clay of Post-Dkareka loess at Dalbeth Rd section

Transmission electron micrograph of < 1 .D/m size clay of Post-Dkareka loess at Highland Hill section

Differential thermal curves (DC) _{of Post-} Okareka loess (basal sample) s i l t and clay fractions at Dalbeth Rd section

70

72

73

Differential thermal curve (DC') _{of a} 74

crushed sample of well- ordered crystalline gibbsite

xii.

Figure No. Page No.

97 37

38

Mean grain- size of Okareka Ash and Post-

Okareka loess deposits in relation to distance from ash source

Grain-size cumulative curves for Okareka Ash deposits

99

39 Variation in sand , s i l t and clay percent in 100 Okareka Ash and Post- Okareka loess deposits

40

in relation to distance from ash source Grain-size cumulative curves for Post- Okareka loess deposits

1 01

41 Grain-size cumulative curves of Okareka Ash 102

42

43

44

45

46

47

48

49

and Post- Okareka loess deposits from Ngongotaha section

Sorting of Okareka Ash and Post- Okareka loess deposits in relation to distance from ash source

Skewness of Okareka Ash and Post-Okareka loess deposits in relation to distance from ash source

Kurtosis of Okareka Ash and Post- Okareka loess deposits in relation to distance from ash source

Mean grain- size versus sorting plots for Okareka Ash and Post-Okareka loess deposits Mean grain-size versus skewness plots for Okareka Ash and Post-Okareka loess deposits Mean grain- size versus kurtosis plots for Okareka Ash and Post- Okareka loess deposits Variation of mean grain-size versus the mean grain- size - sorting coefficient ratio

(Mz/Mz + Qs.I) for Okareka Ash and Post- Okareka loess deposits

Variation of mean grain-size versus the

kurtosis - mean grain-size coefficient ratio (KG/KG + Mz) for Okareka Ash and Post-

Okareka loess deposits

1 04

107

1 08

1 1 0

1 1 2

1 1 3

1 1 4

1 1 6

xiii.

NOMENCLATURE

Sections : (except for Trunk Rd , sections sampled are given a letter symbol , listed below , with increa^. sin^. g distance from Okareka Ash source)

G Gavin Rd

A Okareka Quarry I Lynmore

H Te Ngae

8 Pukehangi Rd J Ngongotaha C Tarukenga D Dalbeth Rd E Highland Hill K Kuhatahi

At Trunk Rd section, due to the thickness of Okareka Ash , samples were labelled according to -

firstly , the predominance of

R rhyolitic (pale grey)

or 8 basaltic (dark grey-black) material and secondly, the grain-size by a letter subscript -

A ash L lapilli c coarse f fine

Ash and Loess : l oess samples were labelled - L l oess

A Ash

and given a number subscript, from top to base, depending upon the number of samples taken -

e.g. Okareka Quarry - section A

channel

sampled AA

upper

mid basal

Okareka Ash

Post-Okareka loess

ABBREVIATIONS Al Aluminium

~ ^{angstrom ( 1 0- 1 m)} An

Cpx Bt cm DC

anorthite augite

biotite mica centimetre (1D-2

m) degrees celsius

differential thermal analysis

XlV .

DTA Fds

g

feldspar (includes alkali and plagioclase) gram

>

Hb H Opx

IR kHz km Kw

greater than hornblende hydrogen hypersthene

infra-red spectroscopy kilohertz

kilometre kilowatt kurtosis less than

Mz mean grain-size m metre

mm 0

%

Qz

0

(v)ps RI SEM Si SKI (s)fs sym

°'I

8

micrometre milligram millimetre oxygen percent quartz phi scale

(10- 6 m) (10-³g) (10-3m)

(very) poorly-sorted refractive index

scanning electron microscope silica

skewness

(strongly) fine-skewed symmetrical

sorting the ta

N.B.

FeD titanomagnetite

TEM transmission electron microscope VG volcanic glass (rhyolitic)

XRD X-ray diffraction

for consistency all grain-size calculations were carried out at

rm

^{sizes to equal}

⁰

(phi-scale) divisions (e .g. 63fm 4.50)

xv.