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

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  • 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have been previously studied (Vella, 1963; Collen and Vella, 1984; Lamb and Vella, 1987), much of the existing work is unpublished (Rodley, 1961; Thomson, 1980; Rataul, 1988) and the environment of deposition has not been fully constrained in terms of modern analogues.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 con- taining locally abundant fossils (dominantly Zethalia) which indicate a shallow shoreface envi- ronment (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 de- posits, 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 lig- nite, 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 conglomer- ate 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 de- posited 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 pre- sent-day regime was analogous to that which de- posited 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 com- pares 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 implica- tions of the analogue are addressed.

    Modern sedimentology

    Today, the southern Wairarapa Valley is domi- nated by an extensive coastal lagoon-lake com- plex 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 consid- erable 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

    .~ /

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    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 col- lected using a small inflatable boat and hand-op- erated piston corer. Twenty-two cores were col- lected 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 previ- ously 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 ex- posed 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 iso- lated pebbles and pebble patches occurs on

    beaches both seaward and landward oi Onoke Bar. The facies is characterised by planar bed- ding 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 west- ern 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 oc- cur 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.

  • LAKES ONOKE AND WAIRARAPA 127

    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-

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    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 com- mon.

    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 lam- ination 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 numer- ous 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 quan- tities of mud on a millimetr...

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