2006 australasian-intimate meeting - gns science · charwell basin field trip 2006 – 3 geomorphic...

Charwell Basin Field Trip 2006 – www.paleoclimate.org.nz

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2006 AUSTRALASIAN-INTIMATE Meeting

Charwell Basin Field Trip

Geomorphic Responses to Climate Change in the Charwell Basin

1 December 2006

Peter Almond, Soil and Physical Sciences Group, Agriculture and Life Sciences Division, Lincoln University

Matthew Hughes, Soil and Physical Sciences Group, Agriculture and Life Sciences Division, Lincoln University

Philip Tonkin Geological Sciences Department, University of Canterbury

Right and left branches of the Charwell River just downstream of the Hope Fault. Extensive

surfaces are the Stone Jug terrace.

Australasian Quaternary Association Inc.

AQUA


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Itinerary

1. Lynton Downs – Loess sheet L1 and associated soil (2548890E, 5869710N).

2. Correlatives of the Quail Downs gravels above Green Burn Stream. View the trace of

the Hope Fault (2542320E, 5867500N).

3. Kawakawa Tephra site and view of Stone Jug aggradation gravels and the strath

beneath the gravels (2540010E, 5866280N).

4. Fill-cut and strath terraces beneath the Stone Jug fill terrace, Charwell River,

(2540230E, 5862180N).

5. “Stein Creek Gully” study site on Dillondale gravels – loess stratigraphy, slope

processes, paleoenvironmental reconstruction (2539120E, 5862000N).

6. Landsurface morphology on the Quail Downs gravels (2536510E, 5863400N).

Bibliographic reference: Almond, P.C., Hughes, M., Tonkin, P.J. 2006: Geomorphic responses to climate change in

the Charwell basin. Field trip guide for Australasian-INTIMATE meeting, Kaikoura, New Zealand. 29 November - 1

December 2006. www.paleoclimate.org.nz.

Material from this field trip guide may not be used in any publication or presentation

without prior approval of the authors.


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Geomorphic Responses to Climate Change in the Charwell Basin

Introduction

The field trip and its title owe much to the seminal work Geomorphic Response to Climate

Change by Bill Bull (1991), which features a chapter on the Charwell basin. The trip

examines the geomorphic responses of the piedmont reach of the Charwell River and its

associated terraces and hillslopes to climatic and tectonic forcing, and considers some new

paleoenvironmental data extracted from loess deposits.

The Study Area

The piedmont reach of the Charwell River occupies a structural basin bounded to the

northwest by the steep range front of the Seaward Kaikoura Range, which here rises to around

1700 m ASL (Fig. 1). The Seaward Kaikoura Range is underlain by faulted and folded,

massive to medium bedded greywacke of the Pahau Terrane. In the basin, relief is in the order

of 200 m between the channel floor of the Charwell River and the upper slopes of the fans

adjacent to the Hope Fault. The range-bounding Hope Fault moves in a right lateral sense at a

rate of about ca 33 mm/year, which, importantly, has had the effect of preserving older valley

fills and ancestral channels of the Charwell River by dislocating them in a southeast direction

from the drainage basin and active river channel. From a dated fluvial strath, Bull (1991)

estimated the uplift rate in the basin to be 1.3 ± 0.1 m/kyr; the mountains were estimated to be

uplifting 2-3 times faster. The present precipitation in the watershed ranges between 1200 and

2000 mm with mean monthly temperatures between 1°C and 14°C. In the basin comparable

climatic characteristics are 1000 to 1400 mm and 3.5-15.5 °C.

Fluvial Response to Climate and Tectonic Forcing

Five valley fills are recognised in the Charwell basin. They are, from young to old, the Stone

Jug, Flax Hills 1, Flax Hills 2, Dillondale, and Quail Downs gravels (Fig. 2). They form fill

(aggradation) terraces that are progressively higher, more dissected and more modified by

slope processes with increasing age. The bedrock straths beneath the fill terraces, representing

the valley floors prior to aggradation, form a staircase reflecting the long term incision of the


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Charwell River in response to basin uplift (Fig. 3). The timings of the phases of aggradation

were estimated by the tectonic displacement of valley fill packages, assuming a constant slip

rate of 33 mm/yr on the Hope Fault (Bull 1991). Concentrations of glass shards identified as

Kawakawa Tephra (26,500 cal yr B.P.)within silty lenses in the lower third of the Stone Jug

aggradation deposits provided an additional constraint. The chronology of valley filling was

used as evidence for climate control on river behaviour, and the causes couched in terms of

changes in the ratio of stream power to resisting power and exceedence of the Critical

Threshold of Power. The ages of the straths were estimated from radiocarbon dates from

organics above the straths under the constraints established by the inferred intervals of

aggradation. These estimates were then refined by correlation to high sea level stands

identified by Chappell and Shackleton (1986). Bull (1991) assumed the climatic amelioration

responsible for eustatic sea level rise also affected the balance of stream and resisting powers

and thereby stream behaviour, rather than some causative base level effect.

River downcutting with the formation of flights of fill-cut terraces and strath terraces occurred

during periods of excess stream power as the Charwell River attempted to attain base level of

erosion. The fill-cut terraces represent temporary still stands or even short-lived aggradation

resulting from complex feedbacks among different reaches of the river (complex response)

during downcutting into the valley fill. Strath terraces form as the river accommodates

episodically the bedrock uplift accumulated during and after aggradational phases. The

variations in tempo of downcutting are exemplified by the fill-cut and strath terraces formed

after 14 ka when the Charwell incised into the Stone Jug fill (Fig. 4).


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0 100 200 km

Charwell Catchment

Charwell River

QuaternarySurfaces Faults

0 2 4 km

Hope Fault

N

(a) (b)

(c)

Figure 1 (a) Location of the Charwell area, South Island. (b) Shaded digital relief model of

Charwell basin and catchment. Late Quaternary surfaces and faults are shown. (c) Orthophoto

of Charwell basin and catchment shown in 3D perspective looking northwest. Three times

vertical exaggeration.


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Figure 2

Late Quaternary terrace surfaces in Charwell basin with loess cover indicated,

underlain by a shaded digital relief model. After Bull (1991) and Tonkin and

Almond (1998).


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Figure 3. Conceptual framework for climatically controlled aggradation and degradation of

Charwell River

Figure 4. Downcutting curve of the Charwell River from 16 k cal yr B.P. to

present.

Bull (1991) argues for different climatically influenced hillslope and stream channel

processes to account for the Flax Hills aggradation, the Stone Jug aggradation and the Post

Stone Jug degradation (Table 1). These responses were inferred from paleoclimate

reconstructions of McGlone (1998/ and written comm.) for the LGM to present period, and

from pollen analysis by D. Mildenhall on samples taken from sediments above the Flax Hills


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strath. Bull assumed a treeline lowering, from the present 1200 m ASL, of 400-600 m during

the Flax Hills 2 aggradation and 800 m during the Stone Jug aggradation.

Table 1 Hillslope and channel responses during the last two major aggradation phases and the

most recent degradation phase of the Charwell River (from Bull 1991).

Geomorphic

Response

Time

interval (k

cal yr)

Climatic

variables

Hillslope processes in

drainage basin

Piedmont reach

channel

characteristics

Flax Hills 2

aggradation

38-31 Cool but

fairly wet

Frozen substrata, periglacial

procceses, gelifluction,

landslides, debris flows,

very large sediment yields

under lowered treeline

High stream power

but exceeded by

resisting power of

very large sediment

supply

Stone Jug

aggradation

29-16 Cold, dry Frozen substratum and

periglacial proceses, but less

ppt available for sediment

mobilisation, tectonically

driven landsliding

Reduced stream

power and large

sediment supply

cause threshold of

critical power to be

exceeded

Stone Jug

degradation

14-0 Warm,

increasing

rainfall to

mid

Holocene

Reduction of snow pack,

increase veg density on

slopes, rise in treeline,

increased discharge,

Reduced sediment

supply, increased

stream power

particularly mid

Holocene – rapid

downcutting

Loess Stratigraphy and Chronology

The terraces in the Charwell basin are mantled with increasing thicknesses of loess with

increasingly complex stratigraphy from the Stone Jug to the Dillondale terrace (Fig. 5). The

Dillondale terrace has been significantly modified from its original constructional form by

dissection and drainage network elaboration. Addition of loess to this evolving landform has

resulted in redistribution of primary loess by soil transport processes. The thickness and

stratigraphy of the loess mantle on flat interfluves and broad plateaux, however, are thought to

represent a complete record. The surface on the Quail Downs valley fill retains none of its

original constructional form, having evolved a ridge and valley morphology, and loess is

absent.


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Loess sheets show systematic spatial relationships with aggradation terraces and they are

presumed to have genetic relationships to valley fills. Loess is produced in aggradational

phases when rivers are not constrained to narrow, incised valleys. The broad surfaces of the

alluvial fans forming during aggradation provide a large loess source area, and climatically

mediated erosion processes in the hillslopes such as frost shattering promote formation of silt-

sized material suitable for wind transport. Loess produced during a given aggradation event is

found on all higher terraces so long as erosion on those surfaces is minimal (Fig. 5).

Boundaries between loess sheets are marked by strongly expressed buried soils presumed to

have formed during phases of river downcutting when loess supply was mimimal. Soil

modification of the loess can be divided into an upbuilding phase with relatively weak soil

development when soil and loess accumulation occur together, and a topdown phase when

loess accumulation is minimal and strong soil modification takes place (Almond and Tonkin,

1999). Buried soils tend to have similar morphologies to surface soils except for the absence

of organic rich A horizons, i.e. a mottled, clay rich horizon with Mn-Fe nodules (Btgc) over a

dense subsurface horizon with or without significant accumulation of clay (Bxg or Btg) (Fig.

6). This morphology corresponds to Perch-gley Pallic soils of the NZ Soil Classification

(Hewitt, 1998) and Epiaqualfs of the USDA Soil Taxonomy system (United States. Natural

Resources Conservation Service., 1999). The degree of clay accumulation particularly is

presumed to be indicative of the length of time of soil formation.


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Figure 5. Loess thickness and stratigraphy for terraces of the Charwell River.

Accepting the above conceptual framework a robust chronology of loess accumulation would

be a test of the (climatically controlled) aggradation/degradation chronology of the Charwell

River.


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Figure 6. Loess stratigraphy across Charwell River terraces (Milne et al., 1995)

Morphological Evolution of the Charwell Terraces

The morphological evolution of the Charwell terraces is driven by long term base level fall as

the Charwell River responds to tectonic uplift of the basin. The base level fall signal

propagates to the rest of the landscape through tributary streams. The changes in drainage

networks, loess stratigraphy, and topography across the fill terraces suggest the following

evolutionary sequence.


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• Immediately after abandonment of a valley fill by river incision, steeply sloping and

steep-sided gullies form at terrace edges by spring sapping initiated at the gravel/strath

interface, and fluvial erosion (Fig. 7)

• A positive feedback results as an extending gully intercepts a larger area of the

groundwater and its drainage area increases. (Stone Jug)

• Loess accumulates episodically on terrace tread remnants producing a loess-mantled

terrace. (Flax Hills 1 and 2)

• Eventually drainage density is sufficient that competition between neighbouring gullies

for groundwater, throughflow and overland flow is severe enough that some become

inactive.

• Diffusion-like processes transport loess into gullies and round out gully walls to form

convex hillslopes. Some gullies infill.

• Ongoing extension and elaboration of the drainage network increases the area of

hillslopes and decreases the area of terrace tread. A loess-mantled downland results.

(Dillondale)

• Erosion rate increases as the proportion and convexity of hillslopes (driven by ongoing

incision) increase.

• Eventually hillslopes are ubiquitous and the erosion rate exceeds long term average

loess accumulation rate. Erosional ridge and valley terrain results. (Quail Downs)

• A (flux) steady state landscape results when erosion rate equals base level lowering rate

(ca 1.3 mm/yr). (Quail Downs??)

Figure 7. Evolving gullies in the Stone Jug terrace (Photograph P. Almond).


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The above qualitative description can be quantified using terrain attributes derived from a

DEM. The slope angle and curvature data shown in Fig. 8, although compromised by a low

resolution DEM, show a trend toward increasing average slope angle, greater range of slope

angles, and an increasing variation in curvatures as terraces become older.


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Perc

ent F

req

uen

cy

0

1

2

3

4

5

0

5

10

15

20

25P

erc

ent

Fre

que

ncy

0

2

4

6

8

10

0

5

10

15

20

25

30

Perc

ent

Fre

que

ncy

0

2

4

6

8

10

0

5

10

15

20

25

30

Perc

en

t F

requ

en

cy

0

1

2

0

5

10

15

20

Perc

en

t F

requ

ency

0

2

4

6

8

10

0

5

10

15

20

25

0 10 20 30 40

Perc

ent F

req

uen

cy

0

1

2

-8 -6 -4 -2 0 2 4 6 80

5

10

15

20

Stone Jug

N=1436

N=15309

late Flax HillsN=3676

early Flax HillsN=871

DillondaleN=4819

Quail DownsN=5746

Slope (°) CurvatureConvex Concave

post Stone JugN=1436

post Stone Jug

Stone JugN=15309

late Flax HillsN=3676

early Flax HillsN=871

DillondaleN=4819

Quail DownsN=5746

(a) (a)

(c) (d)

(e) (f)

(g) (h)

(i) (j)

(k) (l)

Figure 8. Frequency distributions of slope (°) and curvature for the late Quaternary terrace surfaces in

Charwell basin, derived from a 25 m digital elevation model. Data describe terrain characteristics for

post Stone Jug (a, b); Stone Jug (c, d); late Flax Hills (e, f), early Flax Hills (g, h); Dillondale (i, j);

Quail Downs (k, l).value after 10 k cal yr B.P..


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Transport processes and slope evolution

A small gully (Stein Creek gully) on the Dillondale terrace has become the focus of detailed

studies of soil transport processes, slope evolution, and paleoenvironment. Roering et al

(2002; 2004) used the distribution of Kawakawa Tephra in soils along a hillslope transect

from interfluve to the floor of the gully to investigate the kinds of soil transport processes

operating, and to parameterise a soil transport model with the aim of quantifying erosion.

Interfluve sampling site

Hollow sampling site

N

Figure 9. Location of Stein Creek gully and sample sites.

At the interfluve Kawakawa Tephra occurs as dispersed glass grains showing a clear peak

concentration, assumed to be the primary emplacement horizon, at about 0.8 m depth.

Downslope, as curvature increases, the tephra peak is progressively exhumed until, at about

one third the way down the slope it becomes completely dispersed in the top 40 cm (Fig. 10).

This pattern is consistent with a slope dependent transport process (soil creep) driven by

bioturbation involving the upper 40-50 cm of soil. Roering et al (2002) concluded the

bioturbation mechanism was most likely tree-throw.

Slope dependent transport is modelled by an equation of the form:

x

zKqs

∂

∂=


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where qs (L2T

-1) is the soil flux, K (L

2T

-1) is a transport coefficient, and

x

z

∂

∂ (LL

-1) is the slope

gradient.

Roering et al. (2004) showed that the concentration and distribution of glass downslope was

best explained by a history of soil transport characterised by two different K values: a low K

value for between 26.5 k cal yr B.P. and about 10 k cal yr B.P. and a much higher K from 10

k cal yr B.P. to present. The pattern of glass distribution they took as evidence for a reduced

intensity of bioturbation and soil transport under LGM-late glacial grassland and an increased

intensity of bioturbation and soil transport after the Holocene reforestation. This finding

contrasts with the scenarios advanced by Bull (1991) for temporal variation of sediment

production in the mountainous drainage basin of the Charwell River. In this part of the

landscape grasslands during glacial climates were inferred to have increased sediment supply,

whereas recolonisation of slopes by trees in interglacials reduced it. It appears that different

parts of the landscape respond differently to climatically induced vegetation changes.

Figure 10. Slope morphology and patterns of tephra distribution in soils along a slope transect into

“Stein Creek Gully” on the Dillondale terrace.

Paleoenvironmental History of Stein Creek gully

The PhD study of Matthew Hughes has focussed on reconstructing the paleoenvironmental

history of Stein Creek gully. The study has involved examination of the soil chemistry and

biogenic silica (phytoliths and diatoms) from the loess on the interfluve and the reworked


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loess in the gully (Fig. 9). Age control, of varying veracity, has been provided by Kawakawa

Tephra, luminescence dating, and correlation to an established biostratigraphic datum.

The loess on the interfluve shows three loess sheets typical of the Dillondale terrace. The

surface soil, formed in L1 (1.8 m thick), is a Pallic soil with a well developed fragipan and

evidence of perching of water above that horizon in the form of a Bg horizon. The Kawakawa

Tephra primary depositional layer is at 70 cm. The first buried soil, formed into the top of L2,

has a similar morphology, but L2 is only 0.8 m thick. L3 is about 2 m thick and the soil

formed in it has very high clay content, many strongly cemented Mn nodules, and well

developed clay skins in the upper 1 m. This soil stands out as being more strongly developed

than the surface soil or the soil in L2 (Fig. 11).

Phytolith data, summarised in terms of shrub/tree types and grass types, show:

• Grasses dominated during accumulation of L1, until sometime after 26.5 k cal yr B.P.

when trees/shrubs returned to the landscape.

• L2 accumulated under a more shrub/tree rich flora than L1

• L3 started accumulating under a grass rich flora which was replaced by probably tall

forest. The upper part of L3 marks the transition to the more grass rich flora of L2. .

The phytolith data in combination with the soil phosphorus data, which show strong depletion

of P in L3, suggest the soil in L3 formed for a long period under a climate regime that

supported tall forest. Tentatively, we assign part of the soil development in L3 to MIS 5 and

at least the lower part of the loess sheet to MIS 6. Hence, L2 and L1 both formed in the last

glaciation (MIS 4-2).

We have no reliable age constraints on this loess sequence other than Kawakawa Tephra. An

OSL age from below the tephra replicated the age under-estimation problem luminescence

dating seems prone to in the South Island (Almond et al., 2007). Total element analyses from

glass grains extracted from deeper in the section showed no clear affiliations with older

tephras than Kawakawa, although the results are tantalising (Figs 12 and 13).


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Dep

th (

m)

0

1

2

3

4

4.6

Soil Stratigraphy& Loess Sheets

Ah

Bg

Btg(x)

bBw(g)

bB(x)(g)

b2Btg1

b2Btg2

b2Btgc1

b2Btgc2

b2Btg3

L1

L2

L3

b2Bt

600

Total Phosphorus µg g-1

0 10020 40 60 80

P Fractions % of Total

PCa PFe/Al POcc POrg

0 200 400

PT

Trees/Shrubs & Grasses (%)

Grasses

Trees & Shrubs

OSL18.1±1.3

Fig. 11. Summary diagram of interfluve soil and loess stratigraphy (left), P fractions (centre) and

phytoliths (right). Stratigraphic positions of Kawakawa Tephra (Kk) and OSL date are indicated.

Fig. 12. Plot of CaO vs. FeO contents of glass shards from the interfluve and hollow loess deposits

(open circles) and CaO vs. FeO contents of known North Island late Quaternary rhyolitic tephras.


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Fig. 13. Plot of CaO vs. FeO contents of glass shards extracted from specific depth increments in the

interfluve loess deposit, and CaO vs. FeO contents of known Kawakawa Tephra glass shards.

Depth variation of phytolith abundances from a core taken from the gully floor (hollow) show

a similar but expanded pattern to that of L1 on the interfluve. This suggests the gully fill

represents loess transported from the slopes since the beginning of accumulation of L1 to the

present day. The expansion of the record from 1.6 m depth to 7 m gives much higher

resolution by reducing the effects of mixing of phytolith zones and blurring of boundaries by

bioturbation. Age control for the section is again problematic. Kawakawa Tephra appears as a

glass peak at 2.6 m depth. An OSL age immediately above underestimates the tephra age by

as much as 13 ka and an age 2 m below under-estimates the tephra age by ca 6 ka. We

consider an OSL age of 28 ka at 5.6 m also to be an under-estimate. The OSL age of 8.75 ka


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at 1.1 m appears to be reasonable. This depth marks the transition from grass dominated flora to

a flora with an increasing tree/shrub component. Pollen analysis and radiocarbon dating from local

sites place this transition at 10,900 ± 300 cal yr B.P. (McGlone et al., 2004). This datum and that

provided by Kawakawa Tephra can be used to test the hypothesis of Roering et al. (2004) that soil

transport under grassland was slower than under Holocene forest. Rates of soil transport can be

estimated from the rate of gully infilling. Gully fill volumes between the surface and the Holocene

forest transition datum (HFTD), and between the HFTD and Kawakawa Tephra indicated filling rates

were 0.11 ± .04 mm/yr and 0.05 ± 0.03 mm/yr, respectively.

Phytolith ZoneBiogenic Silica

Grasses

Trees & Shrubs

Diatoms

AhBg

Bgc

Bwg

Bg2

Bwg2

Br

Bg3

bBwg

Soil Stratigraphy

OSL8.75±0.73 ka

OSL13.1±1.1 ka

OSL20.6±0.1 ka

OSL28.3±1.6 ka

Fig. 14. Summary diagram of hollow soil stratigraphy (left) and biogenic silica (centre). Phytolith

zones are shown on right. Stratigraphic positions of Kawakawa Tephra (Kk) and OSL dates are

indicated.


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Chronostratigraphic Summary

Interval (k cal yr) Valley fill

Bull

(1991)

Revised

MIS Loess

sheet

Interval

(k cal yr)

Loess sheet

drapes

Stone Jug 16-29 L1

Flax Hills 1 31-38 L2

Flax Hills 2 43-49 L3

Dillondale ?

Quail Downs ?


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References

Almond, P.C., Shanhun, F.L., Rieser, U. and Shulmeister, J., 2007. An OSL, radiocarbon and

tephra isochron-based chronology for Birdlings Flat loess at Ahuriri Quarry, Banks

Peninsula, Canterbury, New Zealand. Quaternary Geochronology.

Almond, P.C. and Tonkin, P.J., 1999. Pedogenesis by upbuilding in an extreme leaching and

weathering environment, and slow loess accretion, south Westland, New Zealand.

Geoderma, 92: 1-36.

Bull, W.B., 1991. Geomorphic responses to climate change. Oxford University Press, New

York, 326 pp.

Chappell, J. and Shackleton, N.J., 1986. Oxygen isotopes and sea level. Nature, 324(6093):

137-140.

Hewitt, A.E., 1998. New Zealand soil classification. Landcare Research science series,.

Manaaki Whenua Press, Lincoln, N.Z., 133 pp.

McGlone, M.S., 1998. New Zealand. In: B. Huntley and T. Webb (Editors), Vegetation

history. Dortrecht, Boston, pp. 557-599.

McGlone, M.S., Turney, C.S.M. and Wilmhurst, J.M., 2004. Late-glacial and Holocene

vegetation and climatic history of the Cass Basin, central South Island, New Zealand.

Quaternary Research, 62: 267-279.

Milne, J.D.G., Clayden, B., Singleton, P.L. and Wilson, A.D., 1995. Soil description

handbook. Manaaki Whenua Press, Lincoln, N.Z., 157 pp.

Roering, J.J., Almond, P., McKean, J. and Tonkin, P., 2002. Soil transport driven by

biological processes over millennial timescales. Geology, 30: 1115-1118.

Roering, J.J., Almond, P., Tonkin, P. and McKean, J., 2004. Constraining climatic controls on

hillslope dynamics using a coupled model for the transport of soil and tracers:

Application to loess-mantled hillslopes, South Island, New Zealand. Journal of

Geophysical Research, 109: F101010, doi: 10.1029/2003JF000034.

Tonkin, P.J. and Almond, P.C., 1998. Using the soil stratigraphy of loess to reconstruct

landscape histories of north-eastern and western lowlands of South Island, New

Zealand, 8th Biennial Conference of the Australian and New Zealand Geomorphology

Group, Goolwa, South Australia, pp. 55.

United States. Natural Resources Conservation Service., 1999. Soil taxonomy : a basic system

of soil classification for making and interpreting soil surveys. U.S. Dept. of

Agriculture Natural Resources Conservation Service : For sale by the Supt. of Docs.

U.S. G.P.O., Washington, DC, 869 pp.

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