lakes onoke and wairarapa as modern analogues for the hautotara and te muna formations...

Sedimentary Geology, 84 (1993) 123-137 123 Elsevier Science Publishers B.V., Amsterdam

Lakes Onoke and Wairarapa as modern analogues for the Hautotara and Te Muna Formations (Mid-Pleistocene),

southern Wairarapa, New Zealand

Ken J. W o o l f e *

Research School of Earth Sciences, Victoria UniL~ersi~' of Wellington, P.O. Box 600, Wellington, New Zealand

(Received October 22, 1992; revised version accepted October 27, 1992)

ABSTRACT

Twenty-two piston cores and numerous grab samples were collected from the floors of Lake Wairarapa and Lake Onoke in the southern Wairarapa Valley, North Island, New Zealand. These samples were used along with direct field observation of lateral facies changes to produce facies models and idealised facies sequences for the sediments currently being deposited in these extensive shallow lakes.

Comparison between facies models determined for the present-day south Wairarapa Lakes and the nearby Mid-Pleisto- cene Hautotara and Te Muna Formations shows that the sedimentary environments represented by the latter are similar to those that occur today in the southern Wairarapa. The coastal-lake and lagoon complex at Lake Onoke appears to be directly analogous to the coastal-lake and lagoon complex that deposited that Hautotara Formation. Similarly, Lake Wairarapa and its surrounding Late Pleistocene to Recent aggradation surface (Waiohine Surface) provides a modern-day analogue for the Te Muna Formation.

Introduct ion

The Hautotara and Te Muna Formations (Col- len and Vella, 1984) together comprise over 400 m of Pleistocene strata in southern Wairarapa, North Island, New Zealand (Figs. 1 and 2).

Although these rocks have been previously studied (Vella, 1963; Collen and Vella, 1984; Lamb and Vella, 1987), much of the existing work is unpublished (Rodley, 1961; Thomson, 1980; Rataul, 1988) and the environment of deposition has not been fully constrained in terms of modern analogues.

The Hautotara Formation consists mainly of well sorted medium to fine loose sandstone containing locally abundant fossils (dominantly Zethalia) which indicate a shallow shoreface environment (Powell, 1979). The formation also con-

* Current address: Geology Department, James Cook Uni- versity, Townsville, Qld 4811, Australia.

tains blue-grey mudstone units which contain Austro~'enus (Chione) and Barytellina, suggesting deposition in an enclosed estuary or bay (Powell, 1979). Thin conglomerate and unfossiliferous mudstone beds are widespread throughout the formation and were described by Rataul (1988) as being of "uncertain origin". These probably rep- resent beach face and estuarine/lacustrine deposits, respectively (see below). Rataul described the paleocoastline as straight with small, enclosed embayments (p. 175) and suggested that Holocene deposits along the west coast of the North Island north of Paekakariki were somewhat analogous. Facies relationships discussed here suggest that a better analogue occurs on the southern Wairarapa coast adjacent to Lake Onoke.

The Te Muna Formation consists of seven alternations of blue-grey mudstone and iron- stained conglomerate, along with subordinate lignite, loess and tephra. The formation has been previously interpreted as being of mixed fluvial and lacustrine origin (e.g. Collen and Vella, 1984;

0037-0738/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

124 ~,.J. W O O l ,F[:

Rataul, 1988). Collen and Vella (1984) suggested that the alternations of mudstone and conglomerate in the Te Muna Formation resulted from sea-level changes (Milankovich cycles), with the mudstone members representing sea-level high- stands and the conglomerate beds low-stands. This interpretation was largely supported by Rataul (1988). Collen and Vella (t984) further concluded that the mudstone units were deposited in a wide deep lake which occupied much of the paleo-Wairarapa Valley (p. 309) and that the conglomerate members were produced by widespread alluvial aggradation.

A few kilometres west of the outcrop belt of these two formations, the present-day Wairarapa Valley is largely occupied by an extensive coastal lake and lagoon complex (see Fig. 1). Collen and Vella (1984) recognised that Lake Wairarapa was depositing mud directly on top of an extensive aggradation surface and suggested that the present-day regime was analogous to that which deposited the Late Pleistocene Ahiaruhe Formation (Collen and Vella, 1984) which overlies the Te Muna Formation to the north.

This paper examines the sedimentology of the present-day southern Wairarapa Valley and compares it with the Hautotara and Te Muna Forma- tions. The validity of the present-day regime as an analogue for Mid-Pleistocene sedimentation in the area is assessed and some regional implications of the analogue are addressed.

Modern sedimentology

Today, the southern Wairarapa Valley is dominated by an extensive coastal lagoon-lake complex which has developed on top of a widespread Late Pleistocene-Early Holocene aggradation surface (Waiohine Surface). It is likely that the lakes have formed largely in response to post- glacial sea-level rise and they are currently con- fined to the western side of the valley floor due to regional tectonic tilting.

Lake Wairarapa is a large shallow lake, 18 km long, up to 5 km wide and with a maximum depth of between 2.5 and 3 m. The lake is subject to seasonal fluctuations in water level of about a metre, but in recent years these have been greatly

reduced by drainage and flood protection works. The shallowness and relatively large fetch of Lake Wairarapa means that wave action keeps considerable quantities of fine sediment in suspension, and as a result the lake is always turbid. Prior to recent engineering works, water entered the lake from the Tauherenikau River in the north and the Ruamahanga River delta on the eastern shore, and flowed out of the lake at the southern end. However, flood protection works established the Ruamahanga Diversion and closed the southern outflow channel (see Fig. 1).

Lake Onoke is considerably smaller than Lake Wairarapa and is subject to strong tidal influence with up to 50% of the lake floor being periodi- cally exposed. The lake is formed by build up ot water behind the Onoke Bar. When the bar is breached, the lake is subject to strong tidal flows with water both entering and exiting the lake through the breach. During periods when the bar is closed, water from the Ruamahanga Diversion

i _/

\ ;~ . .~

RSTON ~,

M A R T I N B O R O U G H , ~ f

.~ /

~T

t

Road I I ] Fig. 1. Lakes Onoke and Wairarapa occupy the lower portion

an extensive Late Pleistocene to Recent aggradat~on surface

in the southern Wairarapa, North Island. New Zealand. The

outcrop belt of the Hautotara and Te Muna Formation.~

(Mid-Pleistocene) is stippled.

LAKES ONOKE AND WAIRARAPA 125

(formerly the outflow from Lake Wairarapa)

builds up behind the bar and floods extensive areas of low-lying land. However, the amount of

flooding has been greatly reduced in recent years by a combination of flood protection and drainage works.

Surface sediment and core samples were collected using a small inflatable boat and hand-op- erated piston corer. Twenty-two cores were collected from the lakes. Where possible lateral facies associations were determined by direct field observation and additional detail was added by interpretation of the core samples. The 45 mm diameter cores up to 1 m long, were split and one

half continuously sampled over 4-cm intervals for

textural analysis (Woolfe and Arnot, in prep.) and carbon/su lphur geochemistry (Arnot, 1991). Core splits and surface samples from Lake Onoke were found to contain a lagoonal-estuarine foraminifer assemblage which has not been previously documented in New Zealand (Bensley and Woolfe, in prep.).

Facies of the modern sedimentary environment

Ten distinct lithofacies are described from the modern sedimentary setting and are summarised in Table 1. These facies have been chosen as

Member

13 013 013 O~ 0 ~ O0 13 ~ o 0

13 o13 O13 O d 11 O0 O0 O0

10 ~ ~ g113 o13 o13 o 4

i ' - - § ~ _ _ _ Forrncrtlon

0 O 0 OI3 0 0 O 0

I _

° 7 : : , :::-

J

c

~ 2 -

i

n'~dst'ono

t, onci~or~ conglomerate

Fig. 2. Generalised stratigraphic column for the western limb of the Windy Peak Anticline area, showing 400 m thick sequence of

Mid-Pleistocene strata (after Collen and Vella, 1984).

representative of the sedimentology of lakes Onoke and Wairarapa and the coastal strip at Lake Ferry.

Facies Mi: Thick, well to poorly imbricate grat~el

Thick, well to poorly imbricate gravel is exposed around the margin of Lake Wairarapa where it directly underlies the Waiohine Surface. Similar gravel is currently being deposited by the Tauherenikau River immediately north of the

present-day lake margin. This facies is characterised by metre- to de-

cametre-thick sheet-like beds of moderately rounded greywacke pebbles. Metre-scale tabular crossbedding is common and was formed by the migration of midchannel and laterally attached bars. Subordinate associated lithologies include trough crossbedded sand (from dune migration in sandy parts of the channel, especially near the present-day delta) and thin discontinuous muddy lenses (which formed as small over-bank and channel-margin ponds and pools).

Facies Mii: Medium to coarse, well sorted, planar

bedded or tsery low-angle crossbedded sand with isolated pebbles and pebble patches

Medium to coarse well sorted sand with isolated pebbles and pebble patches occurs on

beaches both seaward and landward oi Onoke Bar. The facies is characterised by planar bedding and very low-angle crossbedding. Pebbles are typically small (up to 15 cm in largest diame.- ter), well rounded (0.7-0.9 on the Krumbein scale), and slightly discoid (Zingg classification). Shell concentrations are rare, but scattered shell tragments occur throughout. The sand is dark grey and is composed mainly of lithic (greywacke) fragments which are probably derived directty from uplifted basement exposed along the western margin of Palliser Bay adjacem ~,~ the Wairarapa Fault.

Facies Miii: Medium to l,ery coarse ¢loca/ly pebb/v~

planar and crossbedded sand with abundam a,ood

fragments

This facies differs from facies Mii in that ~ contains abundant decimetrc- and metre-scale trough and planar crossbedding and locally ver~, abundant twigs, branches and tree trunks. Shells and shell fragments along with small pebbles occur scattered throughout and as concentrations. Facies Miii is currently restricted to Onoke Bar where it is forming at the present d~y t~rom ::: combination of storm debris (wood) and wind- blown sand. Metre-scale aeolian crossbedding ~s common. More stable parts of the deplet:ed durlc

FABLE 1

Summary of lithofacies used in this study

Facies Description

i

ii

iii

ix

x

vi

vii

viii

ix

x

Massive, imbricate and poorly imbricate gravel, some large-scale crossbedding, scattered logs and muddy lens~-:,.

Medium to coarse well sorted sand. Isolated pebbles and pebble patches, planar or very low-angle erossbeddc~.

Medium to very. coarse well sorted sand. Abundant wood fragments and pebble patches. Large taeolian crossbcddmg).

Medium to coarse, locally pebbly, sand. l)ecimetre- to metre-scale trough crossbedding, abundant scour~

Medium to fine well sorted sand. Rarely preserved ripples and ripple drift lamination.

lnterbedded medium to fine sand (facies v) and mud. lnterbedded on a millimetre- to centimetre-scale.

Very, fine well sorted sand and coarse silt. Unbedded or weakly laminated scattered wood IYagments.

Weakly laminated and unbedded poorly sorted mud. Isolated plant debris including large logs.

Peat (lignite) and strongly carbonaceous mud.

Mottled sand and mud. Abundant organic fragments (roots and rootlets), some veining. Includes soils and palc0sols.


complex are being slowly vegetated (mainly by tussock).

Facies Miu: Trough crossbedded well sorted medium to coarse (pebbly) sand

When the Onoke Bar is breached, medium to coarse sand and pebbles are transported both seaward and landward by tidal flows. The ebb tide delta is poorly exposed and even at low tide is difficult to observe. Probably, much of the sediment which is deposited in the ebb tide delta

is rapidly reworked by wave action and rede- posited on the beach face or in beach-parallel bars. The flood tide delta at Lake Ferry, although not as big, is easily observed at low tide, and at times during flood tide active subaqueous dune fields with dunes up to 30 cm high can be directly observed on the delta top.

It is inferred from direct observation of the flood tide delta that both the ebb and flood tide deltas are composed of medium to coarse, well sorted sand. Pebbles (up to 15 cm in largest diameter) occur both individually and as lag sheets

50

SWCP-gb

45

40 1 35

30

25

20

15 ~ B

lO :1 --m-__ m m n m m m mmmmmmmmmmm

" m - ! • m - m - i • , - , • m • u . ! . i • i " l • i • i • i • , • m - i • | • | • , • i

- 1 - 0 . 5 0 0 .5 1 1 .5 2 2 .5 3 3 .5 4 4 .5 5 5 .5 6 6.5 7 7.5 8 8 .5 9 9 .5 1 p h i

l~cst =3,7896

50.

4 5 .

40'

3 5 -

30"

25. ~ ,

20

1 5 -

10 .

5-

O-

S W ' C P - ~ [

Rest = 15.99%

m m

i m n m m m ~ n n m m m m m m

_ m n n m m m m m m - - m m m m m u n m m m m m m m m m ~ ,

- 1 - o . 5 o o.5 1 1.5 2 2 .5 3 3 .5 4 4 .5 5 5 .5 6 6.5 7 7.5 8 8 .5 9 9 . 5 1 o phi

Fig. 3. Facies Mvii and Mviii can be distinguished on the basis their grain size distribution. Facies Mvii is characterised by tight

grain size distribution with a mode between 3 and 3.5 cb, whereas facies Mviii contains a broad grain size distribution with a mode

between 6 and 7 ~b units.

128 k i. w { ) { ) l J : l

and small-scale channel fills. Scour channels up to 4 m deep and reactivation surfaces are common.

Facies Mc: Medium to fine well sorted sand

Medium to fine well sorted sand occurs in mobile ripple fields throughout Lake Onoke, the shallow margins of Lake Wairarapa (including the neck between the lake and Allsops Bay) and in old aeolian dune fields exposed along the eastern margin of Lake Wairarapa. Coring of the subaqueous ripple fields indicates that ripple lamination is generally not preserved in the subsur- face sediment, whereas large-scale crossbedding is evident in the Holocene aeolian dunes.

In Lake Wairarapa no living or dead macro- organisms were observed. In Lake Onoke numerous Austroz.,enus (Chione) shells were observed within the facies, and the ripple fields are exten- sively grazed by estuarine gastropods (dominantly Amphibola crenata (Gmelin)).

Facies Mtq: lnterbedded medium to fine sand and mud

Interbedded sand and mud occurs in the deeper parts of Lake Onoke adjacent to and beneath the paths of major tidal gyres. Medium to fine sand is interbedded with near equal quantities of mud on a millimetre to centimetre scale. Locally, ripple fields develop on fine sandy sub- strates. Austrovenus (Chione) and various estuarine worms are locally abundant.

Facies Mull: Weakly laminated, well sorted, t~ery

fine sand

Well sorted very fine sand occurs on the crests of submerged sandbars in Lake Wairarapa. In core samples, lamination is generally only weakly visible or absent. X-ray radiography has revealed that much of this sediment is laminated on a millimetre-scale (Pickrill and Irwin, 1978).

The grain size distribution within this facies is very distinctive and results from winnowing of the submerged bar tops by wave action within the lake. The resultant grain size distribution can be

used in the absence of all other features to sepa- rate this facies from other massive fine-grained facies found in the modern lake (Fig~ 3i

Facies Mvii was not observed in Lake Onoke.

Facies Mt'iii: Weakly' laminated and unbedded, poorly sorted mud

Much of the central parts of lakes Wairarapa and Onoke are floored by weakly laminated or unbedded mud. The mud is nearly devoid of sedimentary structures. Surface and core samples taken from numerous localities show the facies to be composed of sediment with a consistent and characteristic grain size distribution which is co~. siderably less well sorted and finer than that contained in facies Mvii (Fig. 3).

Although live freshwater mussels are locally abundant on the bottom of Lake Wairarapa, no macrofossils were observed in any of thc Lake Wairarapa cores except for scattered plant debris. In Lake Onoke, Austrot,enus (Chione) is locally abundant within this facies and was identified from a number of cores and near-surface grab samples.

Facies Mix: Peat and carbonaceous mud

Around the present-day margin ot i~oth lakes and in and adjacent to numerous small lagoons, plant debris is accumulating to form peaty deposits. At present most of the non-woody matc- rial accumulating in these areas is derived from the stems and leaves of marsh plants, in poorly oxygenated sites (such as the lagoon southeast ol Lake Ferry) black, organic-rich mud is accumulab ing. Anoxic, unbedded mud occurs in many small lagoons adjacent to Lake Onoke and adjacent to reed patches near the northern end of the lake. Twigs, stems and leaves are commonly present and are locally abundant in this facies.

Facies Mx: Mottled sand and mud

Mottled and veined lithologies arc currently developing around the margins of both lakes as a result of normal soil forming processes. 7Pypically sedimentary structures become less conspicuous


and eventually disappear as soil profiles become more fully developed. Rootlets and carbonaceous fragments are ubiquitous.

Facies models

From the present-day distribution of the ten facies defined above and their lateral transitions (see Table 2) it is possible to predict idealised facies sequences for both Lake Wairarapa and Lake Onoke. Eight sampling transects were run across the lakes. In the shallower reaches, lateral transitions were recorded by direct observation. Where the water was too deep for direct observation data were obtained by grab sampling, prob- ing the surface sediment and by considering the uppermost facies recovered in the piston cores.

Idealised Lake Wairarapa sequence (Fig. 4A)

Aggradation gravels of the Waiohine Surface (Facies Mi) are overlain by (a thin) medium to fine, well-sorted sand (facies My). The bulk of the lake sequence is represented by poorly sorted silt and clay (facies Mviii) and/or well sorted very fine sand (facies Mvii). At the lake margin carbonaceous mud and peat accumulates and locally on the drier margins soils develop. All facies have sheet-like geometries.

Lake Onoke (Fig. 4B):

Lake Onoke is made more complex by the presence of ribbon-geometries in many of the facies and rapid lateral facies changes, The main

TABLE 2

Transit ion matrix for lateral facies gradations observed along five transects from Lake Onoke and the coastal strip (o), and five

transects from Lake Wairarapa (w)

i

ii

iii

iv

V

vi

vii

vii i

ix

X

i ii iii iv v vi vii vii ix x i

WO 0 W WWW WWW WWW

WWO WO WO

000 WOO 0

O0

WOO

0

0 0 O 0 0 0 0

0 O0 WWO WWW WW

0 0

0 O0

WW

WWO

W WW WO

0 0 WWW O0 ~ WWW WO

O0 WO

0 WO 0 WWO

0

130 ~ L w ( ) ( ) l ~ :

A B

D m n "

o o 0 o o o o c~

1o o a o a o o q

o

s e a w a r d

L .....

l a n d w a r d

f a c l e l

I II III Iv v vl vl l v i i i Ix x

Fig. 4. Idealised lateral and vertical facies sequences (by transition matrix analysis (table 2)) for Lake Wairarapa I A) and Lake

Onoke (B).

body of the lake sequence is composed of three laterally adjacent facies: poorly sorted silt and clay (facies Mviii), interbedded mud and sand (facies Mvi) and well sorted medium to fine sand (facies Mv). All of these lithofacies are locally fossiliferous.

On the landward side of the lake, the sequence is capped by swamp deposits (facies Mvi) or soils (facies Mx). However, near the seaward margin of Lake Onoke the sequence passes laterally into the Onoke Bar complex, consisting of shoreface sand (facies Mii) overlain by aeolian sand and storm debris (facies Miii). Laterally the sequence is truncated by trough crossbedded channel and delta top sand associated with breaching of the Onoke Bar and strong ebb and flood tidal flows.

Hautotara and Te Muna Formations

Collen and Vella (1984) interpreted the Hau- totara Formation as being in part open marine beach face and in part enclosed estuary based largely on the macrofossil assemblage and overall lithology. In contrast, they suggested that the Te Muna Formation represented fluctuations be-

tween lake deposition and alluvial (braid plain) aggradation. As described above, all four of these environments can be observed today ira southern Wairarapa and their value as modern analogues is here tested by redescribing the Hautotara-Te Muna sequence using (where possible) the same ten facies which were identified from the m ~ e r n regime.

Facies of the Mid-Pleistocene sedimentary environment

The following ten facies are based on the facies predicted by using lakes Onoke and Wairarapa as a model for Mid-Pleistocene sedimentation (Hautotara and Te Muna Formations) along the western flanks of the present-day Windy Peak Anticline (Collen and Vella, 1984)i Differ- ences between the observed Pleistocence facies (described below) and the observed modern-day facies (described above) are largely explained by preservation bias, changes in sediment source, cementation, and weathering. Facies annotated with the same numbers are believed to be direct correlatives (e.g. Pii is the inferred correlative of Mii).

LAKES O N O K E A N D W A I R A R A P A 131

Facies Pi: Thick, imbricate and poorly imbricate conglomerate

Thick conglomerate units occur throughout the Te Muna Formation. They typically directly over- lie massive blue-grey mudstone beds (facies Pvii and Pviii); the lower contact of the conglomerate units can be either planar or strongly channelled. Bedding is usually developed on a scale of 1 to 4 m and is defined by slight changes in clast size, imbrication and the orientation of large cross- beds.

Clasts are typically moderately well rounded (0.5-0.7 Krumbein scale) and are composed almost entirely of greywacke. Paleotransport directions are from the northwest suggesting deriva- tion from the axial ranges of the North Island, where likely parent material is still exposed today. Many of the conglomerate units contain zones of intense iron staining and cementation. It is unclear how and when this staining occurred. Isolated carbonised logs, log impressions and thin mudstone and trough crossbedded sandstone lenses occur in some of the conglomerate beds.

The facies resembles facies Gm of Miall (1977) and is here believed to be directly analogous to facies Mi described above. The conglomerate units of the Te Muna Formation are interpreted as having been deposited on an aggrading braid plain as suggested by Collen and Vella (1984).

Facies Pii: Medium to coarse, well sorted, planar bedded or very low-angle crossbedded sandstone, pebbly sandstone thin conglomerate beds

Medium to coarse, well sorted, pale brown sandstone containing locally abundant shells and shell fragments ( Austrovenus (Chione), Barytel- lina, Zethalia) is restricted to the Hautotara For- mation. The sand is commonly loose and un- weathered with many shells (Zethalia) retaining their original colour. Lamination is developed on a centimetre-scale and in larger outcrops very low-angle crossbed sets may be evident, although these features are commonly masked by weathering. Some bedding surfaces are littered with small very well rounded pebbles.

Thin pebble conglomerate beds occur in a

number of localities. These conglomeratic beds differ from those of facies Pi, in that they are

finer, better sorted and the clasts are more rounded (0.7-0.8 as compared with 0.5-0.6, on the Krumbein scale). Locally such beds contain broken bivalve shells. The facies is directly analogous to facies Mii which occurs today in Lake Onoke.

Facies Pill: Medium to very coarse (locally pebbly) planar and crossbedded sandstone with abundant wood fragments

This facies has not been recognised in either the Hautotara or Te Muna Formations. The absence of this facies is not unexpected. The present-day equivalent is restricted to a belt about 50-100 m wide along the Onoke Bar. Migration of the present-day bar either landward or seaward would result in erosion and reworking of this facies along either the seaward or landward margins of the bar, respectively. As such this facies has a very low preservation potential. Al- though old dune fields are currently preserved along the eastern margin of Lake Wairarapa, suggesting that this facies may have been more widespread in the past, these too have a very low long-term preservation potential and they are likely to be reworked rather than mantled by lake or river sediments. Such reworking is currently depositing facies Mv in the neck between Lake Wairarapa and Allsops Bay.

Facies Ply: Trough crossbedded well sorted medium to coarse (pebbly) sandstone

Trough crossbedded sandstone units are common in the Hautotara Formation and occur as a minor lithology associated with thick conglomerate units in the Te Muna Formation. Within the Hautotara Formation this facies consists mainly of medium to coarse sandstone and loose sand with locally abundant abraded (reworked a n d / o r transported) shells and shell fragments along with isolated pebbles and pebble lags. Facies Piv is considered to be directly analogous to facies Miv which can be observed in the flood and ebb tidal deltas associated with breaches in the present-day Onoke Bar.

132 r,~. WOOl ~..t

Facies Pv: Medium to fine well sorted sandstone

Medium to fine, well sorted and very well sorted sand occurs as thin beds in both the Te Muna and Hautotara Formations. Within the Hautotara Formation this lithology may contain abundant broken and complete bivalves (mostly Austrovenus).

This facies is interpreted as being deposited by migrating subaqueous ripple fields within lagoon and lake complexes and is considered directly analogous to facies Mv. The absence of preserved ripple lamination is taken to indicate that the rate of reworking exceeded the rate of nett deposition.

Facies Pvi: lnterbedded medium to fine sandstone and mudstone

Interbedded medium to fine sandstone and mudstone occurs in the Hautotara Formation and near the base of the Te Muna Formation in the type section proposed by Collen and Vella (1984). The two lithologies are interbedded on a centimetre-scale and are present in nearly equal abundance. Known occurrences are unfossiliferous but biogenic borings closely resembling Skolithos are common in the basal Te Muna Formation and/or uppermost Hautotara Forma- tion.

This facies closely resembles facies Mvi recovered in core samples from Lake Onoke and is believed to result from migrating ripple fields (medium to fine sand) and suspension settling (mud).

Facies Pvii: Weakly laminated, well sorted very fine sandstone and mudstone

Most of the fine-grained lithologies of the Te Muna Formation fall into this facies. The facies is typically blue-grey, homogeneous and structure- less. The sediment is well sorted with a mode of about 4-4.5 4) and a slight positive skew, suggesting it is deposited from weak traction currents, probably ripple-induced, rather than by direct suspension settling. In nearly all respects the facies is indistinguishable from the present-day facies Mvii.

Facies Pviii: Weakly laminated and unbedded, poorly sorted mudstone

Weakly bedded and unbedded, poorly sorted mudstone occurs in both the Hautotara and "I~e Muna Formations. In the field this lithology is difficult to differentiate from the slightly coarser very fine sandstone and mudstone of facies Pvii. However, the grain sizes of these two facies are very distinctive and suggest that they were deposited by different processes.

Within the Hautotara Formation this facies is commonly fossiliferous, the most common genera being Austrovenus ( Chione ) and Barytellina. Both of these genera have been used previously to infer deposition in sheltered estuarine conditions (Powell, 1979). Similar facies in the Tc Muna Formation are for the most part unfossiliferous, with the exception of isolated plant fragments (including tree trunks) and rare, decalcified freshwater mussels (Hyridella) which were reported by Collen and Vella (1984) and Rataul (19881).

This facies was most likely produced by the slow settling of suspended load in highly turbid, sheltered water bodies. The equivalent present- day facies (Mviii) currently dominates much of the central portion of Lake Wairarapa and parts of Lake Onoke.

Facies Pix: Lignite and carbonaceous mudstone

Lignitic layers and mudstone beds containing plant fragments are common in the Te Muna Formation and have also been reported from the Hautotara Formation (Rataul, 1988). Within the Te Muna Formation these beds generally occur near or at the top or base of thick mudstone units (facies Pvii and Pviii) and are typically directly and sharply overlain by massive imbricate conglomerate (facies Pi).

Plant fragments, where preserved, are typically stems and leaves of marsh-dwelling genera similar to the present-day wetland flora found around the margin of Lake Wairarapa.

Facies Px: Mottled lithologies

Mottled and veined lithologies occur widely in the Te Muna Formation and have also been


A 13

s e a w a r d l a n d w a r c l

i___ _ =~___ ~ ......

::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

~ q

. : : . : . i:: ~ _o~ _o~ _o~ ]

f o c l e l

I II IU Iv v vl vil vlil Ix x

Fig. 5. The idealised facies sequence for the Hautotara Formation derived by transition matrix (B) compares closely with the facies model derived from Lake Onoke (A). Scale as for Fig. 4.

reported from the Hautotara Formation (Collen and Vella, 1984; Rataul, 1988). These horizons are interpreted as paleosols and at least in one locality they are associated with tree stumps which are probably in situ (Collen and Vella, 1984).

Unassigned lithologies

Although no lithologies from the Hautotara or Te Muna Formation remain unassigned to facies in the above classification, it is worth noting that

A

0 0 0 0 0 0 C] O O 0 O 0 0 0 i

O 0 O 0 O 0 c I

ii!~iiiiii!!i~i~i~i~iiii!~ii!!i!i~i~i~i~iiiiii!!i!iliiiiiiii t O ° O OO O 0 q

o o o o o o o q

B

C~ C~ C' C~ J

f a c i e s

iw 0 O 0 O 0 go oo oo o?2

I U Ill Iv v v l vii viii Ix x

Fig. 6. Generalised facies sequences in the Te Muna Formation (B) closely resemble facies sequences predicted by using Lake Wairarapa as a model (A), the notable difference being a change in the relative abundance of facies vii and viii. Scale as for Fig. 4.

two lithologies placed within the facies frame- work have no direct modern analogues in Lake Onoke and Lake Wairarapa. These are the Up- per and Lower Te Muna Tephras and a number of horizons interpreted as loess (Collen and Vella. 1984; P.A.R. Shane, pers. commun., 1991).

Facies sequences

Idealised facies sequences for the Hautotara and Te Muna Formations were established by redescribing established measured sections, using the facies described above. Additional data points were derived from vertical facies transitions exposed in stream cuttings. The proposed new standard (type) section for the Hautotara Formation (Rataul, 1988) was also redescribed. Transition matrix analysis (similar to that presented in Table 2) was then used to produce an idealised facies sequence for the Hautotara Formation. This is compared with the Lake Onoke sequence in Fig. 5. It can be seen that the generalised facies relationships present in the standard section are similar to the Lake Onoke analogue, with the notable absence of facies iii (aeolian dunes, which are currently restricted to the supratidal part of the Onoke Bar).

Similarly, facies relationships in the Te Muna Formation yield (using transition matrix) an idealised sequence which is indistinguishable to that predicted by using Lake Wairarapa as a modern analogue (Fig. 6).

Discussion

Facies associations and paleontology show that lakes Onoke and Wairarapa can be considered modern analogues for the Hautotara and Te Muna Formations, respectively. However, there remains a number of questions worthy of discussion: (i) how strong are the analogies, (ii) what implications do the analogues have for the relationship between the two formations, (iii) can these formations provide additional information on the timing of regional uplift and deformation, and (iv) with the benefit of these modern analogues is it possible to constrain the mechanism that produced the apparent cyclicity in the Te Muna Formation.

Strength of analogue

From the facies associations and overall facies geometries, especially if the paleontological con- straints are considered, there is a s tro,g case tot concluding that the Hautotara F:ormation was deposited in and adjacent to an ~)pcN coa,~t- lagoon complex similar to the present-day Lake Onoke system. It is not easy to proceed quantit~- tively by looking at the relative proportions ot the facies preserved in the Hautotara Formation and comparing it with that obse~,ced in the present-day setting because the preservation potential of many facies is uncertain. However, it may be possible t:~; say something about energy levels a~d prow- nance based on grain size and petrology.

The grain size of facies ii present ~::)TI both the modern beach face (Mii) and in the l:tautotar~ Formation (Pii) is almost identical (very stro~tg mode at 3 &), and this also corresponds closely to the grain size found at the base of co~es SW(:t' 11-13 taken from a lagoon east of Lake Onokc. This match in grain size suggests that the energy level present on the Hautotara beach face (and presumably the beach profile) was similar to that of the present-day beach. However, a striking difference is the colour (and petrology) ot: the sand.

Hautotara sandstone beds are domin~antty pale brown and are composed mostly ,.~I quar[z~ feldspar and pale lithic fragments. The present.- day beach face is composed of dark-grey titbit sand derived from direct erosion of uplifted l¢~v- lesse greywacke exposed in both the Rimutaka and Aorangi Ranges (see Fig. 1). Thi~ suggests either that the sand was derived from a different parent material or that it underwent more pr~- longed transportation prior to deposition, or tha~ it was deeply weathered in situ prior t~ erosion and transportation. Either way this observation suggests that greywacke basement was not exposed to direct coastal erosion close to the Haw totara beach face-lagoon complex. This constrains the timing of major uplift along Ihe range fronts (Rimutaka and Aorangi) to at most la~e Hautotara times.

The similarity between sediments ot the Te Muna Formation and those of Lake Wairarapa is


very strong. However, there is one noticeable

difference in the grain size distributions. The present-day lake is dominated by poorly sorted mud, facies viii (mode about 6 ~b), whereas the fine-grained beds in the Te Muna Formation are dominated by well sorted very fine sand and mud (facies vii).

Despite their grain size differences, both facies are almost devoid of obvious lamination, suggesting that sediment mixing, probably due to wave action, was a major factor in their formation. As both facies (vii and viii) are observed in the present-day lake, it is likely that the change in their relative abundance observed in the Te Muna Formation reflects only a minor difference between the Te Muna and present-day lakes.

The well sorted nature of facies vii suggests that it is the product of either deposition from slow traction currents or winnowing of pre-existing poorly sorted sediments (?facies viii). These two possibilities are not easily resolved and have little bearing on the final interpretation. Either the Te Muna Lake was for the most part slightly shallower or had greater fetch than the present- day lake allowing more intense sorting and winnowing by wave action, or the system was more open allowing much of the fine-grained suspended load to exit the system (presumably to the sea). In either case it is likely that Lake Te Muna was not significantly deeper than the present-day lake. If it were, then its sediments would be expected to be more strongly laminated--ref lect- ing individual major storm (flood) events. It has been suggested that such flood events may be preserved in the more strongly laminated Mem- ber 2 of the Te Muna Formation (Rataul, 1988).

If, as in the modern setting, the paleolake(s) did not extend over the entire alluvial aggradation surface, then major time breaks (equivalent to the time represented by a single mudstone unit) must be locally missing from within the conglomerate beds of the Te Muna Formation. The nature of the lithology is likely to make any such break difficult to recognise and any paleosols developed on this lithology would likely have low preservation potential, but a careful study of iron staining or of weathering rinds may further constrain the model. Paleosols developed

in mudstone units within the Te Muna Formation show that lake deposits were subjected to pro- longed subaerial exposure as the paleolakes shrank or migrated across the basin.

Relationship between the two formations

As the proposed modern analogues of both formations occur laterally adjacent to each other, it is necessary to consider the possibility that the two formations may be in part laterally time- equivalent. This becomes important when considering the timing of major deformational events, because at (at least) one locality (Huangarua Syn- cline, NZMS 260 $27 196920; see Lamb and Vella, 1987, for discussion) an angular unconformity of up to 70 ° separates the two formations. Elsewhere the two formations appear conformable.

Vella and Collen (1984) suggested that there was a significant break between the Hautotara Formation and Te Muna Formations. They corre- lated the Hautotara Formation with the Mara- hauan Substage (New Zealand substage) and suggested that the formation is no younger that 1.05 Ma. However, there is some question as to the reliability of this correlation as there are no strongly age diagnostic fossils. The formation was assigned to the Nukamaruan Stage by Rodley (1961) and Vella (1963) based on the presence of Tawera subsulcata, Barytellina anomalodonta and Austrovenus ( Chione ) stutchburyi crassitesta.

A reversed palaeomagnetic polarity has been determined for the Barytellina Siltstone Member (Kennett et al., 1971) and this has been considered consistent with an age of just over 1 Ma (Vella and Collen, 1984). However, reversed po- larities also occur in the lower half of the Te Muna Formation (Vella and Collen, 1984; Shane and Froggatt, 1991). Shane and Froggatt (1991) correlate the Upper and Lower Te Muna Tephras with Rewa Pumice and tuff b (in deep-sea cores RC12-215 and RC9-113) respectively, on the basis of glass chemistry and paleomagnetism, and suggested fission track ages of ca. 0.74 to 0.75 Ma.

On the basis of the available age control it is not possible to determine exactly how much (if

any) time is missing between the Hautotara and Te Muna Formations. Near the core of the Huangarua Syncline a striking angular unconformity separates the two formations (Lamb and Vella, 1987). However, at other localities the two formations appear conformable.

Uplift and deformation

A 70 ° angular unconformity and considerable erosional relief is developed between strata of the Hautotara Formation and overlying conglomerate (believed to be Te Muna Formation) exposed in the northwestern limb of the Huangarua Syncline (NZMS 260 $27 196920). This shows that significant deformation occurred in the area subse- quent to Hautotara deposition and probably during Te Muna deposition. Collen and Vella (1984) suggest that the top of the Te Muna Formation is about 0.4 Ma, thus constraining the timing deformation within the Huangarua Syncline to between 1 Ma and 0.4 Ma if the overlying conglomerate is correctly assigned to the Te Muna For- mation.

Pale-brown, quartzo-feldspathic sandstone in the Hautotara Formation contrasts strongly with the dark lithic sand which is being deposited on the present-day beach face. This petrological change in the composition of beach sand is most easily explained by a change of the dominant source rock, suggesting that Torlesse greywacke was not exposed along the coastline during Hau- totara times. This constrains the main phase of uplift along the western side of the Wairarapa Valley (Rimutaka Range), indicating that uplift had not exposed significant greywacke basement at the coast 1 m.y. ago. However, well rounded greywacke clasts in the Hautotara Formation (and in the upper part of the underlying Pukenui Limestone) suggest that greywacke basement was exposed to the northwest presumably in the Tararua Range, and possibly in the Aorangi Range to the southeast (Vella and Briggs, 1971).

Paleocurrent determinations in the Te Muna Formation (mostly from pebble imbrication) show that sediment transport was dominantly from the north or northwest, whereas the present-day sediment transport direction in the Huangarua River

Catchment is dominantly from the southeast. Southeasterly paleocurrent directions in the Te Muna Formation, together with the presence of thick lacustrine sequences show that uplift along the Windy Peak Anticline did not effect the regional paleoslope until after the end of Te Muna Formation deposition at 0.4 Ma.

Cyclicity in the Te Muna Formation

Alternations of mudstone and conglomerate in the Te Muna Formation have been attributed to climatic and sea-level changes (Collen and Vella, 1984). They identified seven and a half alternations and suggested that these represented Mi.. lankovich cycles. It is inferred that the presence of extensive lacustrine deposits on top of braid plain gravels, as suggested by Collen and Vella (1984), records sea-level high-stands. However, lakes could only form if bed load and flashiness of the fluvial system were reduced. This suggests that vegetation changes in the catchment and ultimately climate charges are responsible for the observed mudstone-conglomerate alternations. Although climate and sea-level change are closely linked, no direct evidence of sea-level change is recognised in the Te Muna Formation.

Conclusions

Sediments are currently being deposited i~ Lake Wairarapa, Lake Onoke and along the southern Wairarapa Coast near Lake Ferry that closely resemble those preserved in the Te Muna and Hautotara Formations. Facies models based on the present-day configuration of the south Wairarapa coastal, lake and lagoon complex give rise to sequences which are like those preserved in the Mid-Pleistocene rock record.

Comparison of lithofacies associations, bed geometries and grain size analysis leads to the con- clusion that Lake Wairarapa and the Waiohine Surface, which occurs both laterally adjacent to and directly beneath modern lake sediments rep- resents a modern analogue for the Tc Muna Formation. Similarly, Lake Onoke and the adjacent beach face-bar complex provides a modern analogue for the Hautotara Formation.


Acknowledgements

This paper stems from work undertaken as part of the South Wairarapa Coring Project which was funded by the Internal Grants Committee, Victoria University of Wellington. I am indebted to Malcolm Arnot who provided valuable assis- tance in the field and much useful discussion, and to Andrew Sutton who did most of the grain size analysis.

Phil Shane and May Bensley read early drafts of the manuscript and Peter Barrett, Paul Vella and John Collen critically reviewed the manuscript prior to submission.

References

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lakes onoke and wairarapa as modern analogues for the hautotara and te muna formations...

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