This article was downloaded by: [UQ Library]On: 20 November 2014, At: 11:46Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

Journal of the Royal Society of NewZealandPublication details, including instructions for authors andsubscription information:http://www.tandfonline.com/loi/tnzr20

Stratigraphy of Stephens Subgroup(Maitai Group) in the CountessRange — Mararoa River area,northwestern Southland, NewZealandJ. C. Aitchison a c , C. A. Landis a & I. M. Turnbull ba Geology Department, University of Otago , P.O. Box 56,Dunedin , New Zealandb Geological Survey, Department of Scientific and IndustrialResearch , Private Bag, Dunedinc Department of Geology and Geophysics, University of NewEngland , Armidale , NSW 2351 , AustraliaPublished online: 24 Jan 2012.

To cite this article: J. C. Aitchison , C. A. Landis & I. M. Turnbull (1988) Stratigraphy ofStephens Subgroup (Maitai Group) in the Countess Range — Mararoa River area, northwesternSouthland, New Zealand, Journal of the Royal Society of New Zealand, 18:3, 271-284, DOI:10.1080/03036758.1988.10426470

To link to this article: http://dx.doi.org/10.1080/03036758.1988.10426470

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information(the “Content”) contained in the publications on our platform. However, Taylor& Francis, our agents, and our licensors make no representations or warrantieswhatsoever as to the accuracy, completeness, or suitability for any purpose of theContent. Any opinions and views expressed in this publication are the opinions andviews of the authors, and are not the views of or endorsed by Taylor & Francis. Theaccuracy of the Content should not be relied upon and should be independentlyverified with primary sources of information. Taylor and Francis shall not be liablefor any losses, actions, claims, proceedings, demands, costs, expenses, damages,

and other liabilities whatsoever or howsoever caused arising directly or indirectly inconnection with, in relation to or arising out of the use of the Content.

This article may be used for research, teaching, and private study purposes. Anysubstantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,systematic supply, or distribution in any form to anyone is expressly forbidden.Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions

Dow

nloa

ded

by [

UQ

Lib

rary

] at

11:

46 2

0 N

ovem

ber

2014

© Journal oj the Royal Society of New ZealandVolume 18, Number 3, 1988, pp. 271-284

Stratigraphy of Stephens Subgroup (Maitai Group) in theCountess Range - Mararoa River area, northwesternSouthland, New Zealand

J. C. Aitchison, *t C. A.Landis* and I. M. Turnbull **

Stephens Subgroup (upper Maitai Group) is well exposed (except for its top) forover 30 km in the Countess Range, Snowdon Peak and Bare Peak-Mararoa Riverareas. The sequence consists of 2750 m of sedimentary rocks which includevolcaniclastic sandstone, conglomerate, and siltstones, conspicuous tuffs andoccasional limestone lenses. The subgroup is subdivided into five formations; fourof them are new. Kiwi Burn Tuffs (new) is characterised by tuff. Acheron LakesSandstone (new) is a thick sequence of volcaniclastic sandstones. CerberusFormation (new) is dominated by red coloured siltstone and sandstone. EldonFormation (new) is another sequence of volcaniclastic sandstones, and SnowdonFormation is of variable lithology including volcaniclastic sandstone, conglomerate,siltstone and tuff.

Fossils are rare in the Stephens Subgroup. However, Permophorus obovatus isfound in the Retford Conglomerate Member, which has now been traced to theMararoa River. This, along with Mellarium and radiometric ages from middleSnowdon tuff, clearly indicate an Early-Middle Triassic age for the SnowdonFormation. Basal Stephens Subgroup formations may also be of Early Triassicor even of Permian-Triassic boundary age .

.Keywords: Stephens Subgroup, Maitai Group, Kiwi Burn Tuifs (new), Acheron Lakes Sandstone(new), Cerberus Formation(new), Eldon Sandstone (new), Snowdon Formation, Permian, Triassic, stratigraphy, northwest Southland.

INTRODUCTIONThe upper part of the Maitai Group (Permian-Triassic) is well exposed in the mountains

of western Southland, particularly in the Countess Range (Figs 1, 2). Here, a sequenceoriginally mapped as Countess Formation (Grindley, 1958) and comprising more than2750 m of volcanogenic sandstones and siltstones, with lesser tuffs and conglomerates,rests in depositional contact on the distinctive lower formations of the Maitai Group(Grindley, 1958; Waterhouse, 1964; Landis, 1980).

New stratigraphic information (Aitchison, 1984), here combined with a previous study(Landis, 1974) and regional mapping (Turnbull, 1986), permits subdivision of the originalCountess Formation throughout a 30 km belt extending from Countess Peak south toMararoa River. Correlation with Stephens Formation (Maitai Group) rocks on thenorthwest side of the Alpine Fault in Nelson (Fig. 1) is compelling, and we propose toelevate Stephens to subgroup status, including within it all rocks of the original Countessand Stephens formations. This proposal is broadly compatible with earlier suggestionsby Waterhouse (1964), Landis (1980), and Hyden et at. (1982). Knowledge of Stephensrocks is crucial to understanding regional structure and stratigraphy of western Southlandand northwestern Otago, relations across the Alpine Fault, and Permian-Traissicpaleogeography. In addition, Waterhouse (1964, 1979a) argues that the Permian-Triassicboundary lies within this unit.

• Geology Department, University of Otago, P.O. Box 56, Dunedin, New Zealand .•• Geological Survey, Department of Scientific and Industrial Research, Private Bag, Dunedin.t Department of Geology and Geophysics, University of New England, Armidale, NSW 2351,

Australia.

Dow

nloa

ded

by [

UQ

Lib

rary

] at

11:

46 2

0 N

ovem

ber

2014

272 Journal of the Royal Society of New Zealand, Volume 18, 1988

/-

Countess RangeBare PI<

Fig. 1-Map of the South Island, New Zealand, showing Dun Mountain-Maitai terrane andgeographical localities of Stephens Subgroup and correlatives. (Ultramafic and associated ophioliticrocks are shown in black)

REGIONAL GEOLOGICAL FRAMEWORKSummary accounts by Coombs et al. (1976) and Suggate et al. (1978) give a regional

synthesis. Stephens rocks are included within the Dun Mountain-Maitai terrane of Bishopet al. (1985). Possible relations with rocks of the Murihiku terrane and other late Paleozoic­Mesozoic terranes will be discussed in detail later in another paper by Aitchison andLandis (in prep). Maitai rocks in northwest Southland lie within a north-striking regionalzone where they occupy the overturned limbs and axial portion of the Key SummitSyncline (e.g. Wood, 1962, 1966). Despite near-isoclinal folding and extensive lower grademetamorphism (Landis, 1974), rocks show little evidence of internal deformation; thebedding and intra-bed sedimentary structures are reasonably well preserved.

Rocks of the Key Summit Syncline are bounded on the west (in this area) by theCountess Fault (Grindley, 1958), a component of the Hollyford Fault System (Landis,1980 and Fig. 2). Within the study area, the Maitai Group rocks are mainly west-facing,and are cut longitudinally by the Upukerora Fault (Grindley, 1958) and other faults(Turnbull, 1986) which divide the group into two main slabs (Fig. 2). Maitai strata ofthe eastern slab rest in depositional contact on volcanics of the Dun Mountain OphioliteBelt at Annear Creek (Fig. 2 and Waterhouse, 1964; Pillai, 1981); elsewhere, east of

Dow

nloa

ded

by [

UQ

Lib

rary

] at

11:

46 2

0 N

ovem

ber

2014

map

1LE

GE

ND

ma

p2

osm

ap

3

ST

EP

HE

NS

SUB

GR

OU

P

...~:

I'

Eld

on

San

dsto

ne

-~

..

60

SN

Ow

oo

N,

KiW

IB

urn

Tu

tls

Wa

iua

Form

alto

n,.

SY

MB

OL

S~

3S;::

.. "C

on

tact

sq.

,;:,

..1:;

.kro

wn

app

rO

Jllf'

n..lt

e'"

Fau

lts

---

._.

..._-

-i)

;:l

Bed

don

gn

Olm

al

\~

I~

..."

......

.....:-

y~

e:..o

vert

urn

ed

~~SS

I

!IF

old

aIl

Ss

yncl

ine

-~

Fo

ssil

k>ea

hty

.11

501~

GI

~

LIT

TL

EH

IL~

rlg.

....T

OP

OG

RA

PH

ICR

EF

ER

EN

CE

NZ

.S

OU

TH

ISlA

ND

~..

~10

00

~S

trea

m~

NM

J~

T~'9

A0

00

$::?

Pe..

..6

36

77(

',om

NZ

MS

I)S

I41

1973

r~

°c;:,

..S

131

197

8"";:l

10

00

'"-

ma

p1

Mf?

ma

p2

-m

ap3

1000

~

D<:1 .g.

A'

B

Fig

.2

-Geo

logi

cal

map

so

fth

eC

ou

nte

ssR

ange

,S

now

don

Pea

kan

dM

araro

aR

iver

area

s,

show

ing

dis

trib

utio

nan

dst

ruc

ture

ofSt

eph

ens

Su

bgr

ou

pro

cks.

Mid

dle

and

up

per

Ste

phe

ns

Su

bg

rou

pro

cks

are

undi

ffer

enti

ated

onS

now

don

Pea

k.M

apgr

ids

from

NZ

MS

1m

aps

S131

and

S14

1.

Dow

nloa

ded

by [

UQ

Lib

rary

] at

11:

46 2

0 N

ovem

ber

2014

274 Journal of the Royal Society of New Zealand, Volume 18, 1988

the U pukerora Valley, they are in tectonic contact with ophiolitic melanges associatedwith the Livingstone Fault System (Fig. 1). Transverse faults with strike slip displacementare also recognised; these include the Winton Fault (Fig. 2), branches of the UpukeroraFault (Turnbull, 1986), and the Mararoa Fault (Landis, 1974), which forms the southerntruncation of the rocks described here;

PREVIOUS WORKThe first detailed mapping and stratigraphic study of the Countess Range and adjoining

areas was done by Grindley (1948, 1958). He defined the Countess Formation, regardingit as the uppermost unit of the Maitai Group, and dividing it into informal upper andlower members. Grindley also proposed a Late Permian age on the basis of a gastropodfossil collected from the upper member. Waterhouse (1960) revised an earlier identificationof the upper Countess gastropod from Shansiella (Waterhouse, in Grindley, 1958) placingit in a new genus, Mellarium, of Middle Triassic age. Later, in a major study of NewZealand Permian stratigraphy and faunas, Waterhouse (1964) re-defined the CountessFormation as "restricted to the ?Permian part of Grindley's Countess Formation", andincorporated it in his Bryneira Group. Waterhouse also reported permophorid bivalvesfrom "Triassic beds" overlying the newly re-defined Countess Formation and speculatedon the possible position of the Permian-Triassic boundary.

Landis (1974), in an account of mainly Maitai rocks in the Snowdon Peak-MararoaRiver region, generally accepted Waterhouse's (1964) restriction of the Countess Formationand presented further lithologic and stratigraphic descriptions of these rocks. He describedand named the Snowdon Formation, equivalent to Grindley's upper member of theCountess Formation (the "Triassic" of Waterhouse 1964), and showed that it extendedto the Mararoa River. It was treated as a component of the Murihiku Supergroupfollowing Campbell and Coombs (1966). In a subsequent study, Landis (1980) placedStephens (Countess) rocks, including Snowdon Formation, in the Maitai Group.

In a description of New Zealand Early Triassic permophorid bivalves, Waterhouse(1979a) documented Countess Range specimens of Permophorus obouatus from the SnowdonFormation. In the same paper, Waterhouse also proposed recognition of the RetfordConglomerate-Breccia Member as the fossiliferous basal component of the SnowdonFormation.

Discovery of a rich upper Permian fauna in Stephens rocks at Mossburn (Fig. 1) wasreported by Hyden et at. (1982).

Regional mapping of the Countess Range and Snowdon Peak areas at 1:50,000 scalewas recently completed by Turnbull (1986), who used Stephens Formation for all thoserocks originally described as Countess Formation by Grindley (1958), and also mappeddistinctive lithologies corresponding to some of the formations introduced here.

LITHOSTRATIGRAPHYStephens Subgroup is defined here as comprising those strata previously included within

the Stephens, Countess and Snowdon formations in Nelson and Southland. Four newformations are proposed; the Snowdon Formation is retained, and the use of the termCountess is abandoned. Mappability of the five constituent formations over a 40 km beltin western Southland is demonstrated (Fig. 2). Similar lithologic units are seen northwestof the Alpine Fault (johnston, 1981, 1982a, 1982b, 1983) although further work isnecessary before their mapping is extended. More than 2500 m of Stephens rocks areexposed in the Countess Range; Stephens stratigraphy is summarised in Fig. 3.

Stephens (spelled Stevens) was introduced into Maitai terminology by Wellman (inRigg et al., 1957), but Waterhouse (1964) was the first author to define the unit formally,designating a type section at Cape Stephens, D'Urville Island (Waterhouse 1964:24).In view of the restricted outcrop at Cape Stephens ( :=:: 1000 m of section is present), andits great distance from western Southland exposures, a southern reference section isdesirable. The main ridge of the Countess Range which, although oblique to strike, exposes2750 m of strata, is here proposed as reference section for the Stephens Subgroup.

Dow

nloa

ded

by [

UQ

Lib

rary

] at

11:

46 2

0 N

ovem

ber

2014

Aitchison et al. -Stratigraphy of Stephens Subgroup 275

SNOWDON FMN.

SILTS TONE

SANDSTONE

CONGLOMERATE IAltl lord COO9I~,a'. AkmbN )

220 - 237 M.... OU491SJ~lIa,un e_,~ 0 42tr150 1

CI:1doph~ ob!onq a

Dtc:;fO!cbym 54l. 0421 11$39

Dewn.ophxIU'n ef.~

~%US obow.at~ 043/I015.042/"06J ~rr;ync; Iltd .....~ TUFF

'~

SMITHIANEARLy T~~C

ANI SIA NM" TRIASSIC

P'''' ,'~. -- -4 MASSIVE SANDSTONE ELDON SST.- .--j .:.......:J

1000-- ·......-.-, -B REO SUSTONECERBERUS f MN.=-- & SANDSTONE-

LATE PERMIAN'?Mcmbum f...... £44/'001

=-.~-:-:e:.~$ 042/1077 000:--MASSIVE SAhOSTONE ACHERON LAKES:-...::J SST. --_.-

r0_ TUFF & MUDSTONE KfW'1 BURN TUFf S

_--..-..:1 WAUA FMN.

AGE FAUNA LITHOLOGY FORMATION

Fig. 3 -Summary stratigraphic column of Stephens Subgroup rocks as seen in the Countess Range.Dominant lithologies are shown, and the approximate positions of faunal, floral, andgeochronometric samples in adjacent areas of Stephens Subgroup outcrop (Countess Range,Snowdon Peak, Bare Peak-Mararoa River). Possible lithostratigraphic correlation of Mossburnfaunal locality (Hyden et al., 1982) is also shown.

Stephens rocks have long been included within the Maitai Group (or Maitai Series;Grindley, 1958), and we see no reason to modify this practice. The five constituentformations bear an overall lithological similarity to, and stratigraphic continuity with,the underlying Maitai formations. The presence of numerous vitric tuff beds within theStephens distinguishes it from the rest of the Maitai Group, which is only locally tuffaceous.

KIWI BURN TUFFS (new name)

Introduction and Definition

Basal Stephens strata are characterised by conspicuous light coloured tuff beds (Fig. 4)which rest on the underlying Waiua Formation. These constitute the Kiwi Burn Tuffs.The Kiwi Burn, a tributary of the Mararoa River, drains areas underlain by lowerStephens rocks (S141 GR035250)*.The unit is equivalent to the lower part of Unit 1 of the Countess Formation as describedby Landis (1974), and to the "black mudstone with discontinuous tuff beds" mapped byTurnbull (1986). The base of the Kiwi Burn Tuffs is defined as the first tuff overlyinglaminated hematitic siltstones and sandstones of the Waiua Formation (S141 GR050383in type section). The Kiwi Burn rocks rest confomably upon the Waiua Formation withinthe study area. In the Tapara Range, 12 km north of Countess Peak, Grindley (1958)

* Grid references are taken from the 1:63360 scale NZMS 1 series topographical maps S131and S141. These are used in preference to metric grid references, as 1:50,000 NZMS 260series maps will not be available for some time.

Dow

nloa

ded

by [

UQ

Lib

rary

] at

11:

46 2

0 N

ovem

ber

2014

276 Journal of the Royal Society of New Zealand, Volume 18, 1988

Fig. 4 - Steeply dipping tuff beds of Kiwi Burn Tuffs exposed at a reference section on the ridgeleading east from Countess Peak. Photograph shows view southwards from Countess Peak.

describes basal Stephens strata as resting in erosional contact upon Greville Formationwith Waiua rocks missing. At the Stephens' type locality on D'Urville Island, Waterhouse(1964) describes basal Stephens conglomerate scoured into the Waiua. Re-examinationof these localities by one of us (C .A. L.) revealed widespread evidence for faulting at theStephens-Greville contact in the Tapara Range, whereas no unconformity could berecognised; the erosional nature of the base on D'Urville Island was confirmed. The typesection is designated where these rocks are best exposed, which is on the ridge leadingsoutheast from Snowdon Peak (Fig. 5). A reference section is proposed on the ridge leadingeast from Countess Peak to the saddle between Annear Creek and Upukerora River.

LithologyKiwi Burn rocks are dominated by fine- to medium-grained, light coloured, vitric tuffs

in beds up to 1 m thick. Associated beds include fine-grained, dark coloured (but lightweathering) tuff, volcaniclastic sandstone and siltstone and minor conglomerate. Small(10 cm maximum dimension) concretionary blocks of calcareous sandstone and siltstonemay be seen near Snowdon Peak (S131 GR057378).

Outcrops typically weather to poorly vegetated, fine grained angular screes of distinctiveyellowish grey to brown colour. The tuffs characteristically appear cherty on freshly brokensurfaces, and many also show distinctive metasomatic mottling (e.g. Landis, 1974, fig. 5A).Metamorphism has produced lawsonite-pumpellyite-quartz rich assemblages.

Distribution and ThicknessKiwi Burn Tuffs are exposed from the ridge north of Countess Peak south along the

east side of the Countess Range to the Upukerora River. Good exposures also occuron the eastern side of Snowdon Peak (type section). Outcrop is largely obscured by bushand screes in the Bare Peak-Mararoa River area. Thickness is 175 m at the type section,155 m at Countess Peak and approximately 150 m at Mararoa River.

Dow

nloa

ded

by [

UQ

Lib

rary

] at

11:

46 2

0 N

ovem

ber

2014

A itchison et al. -Stratigraphy of Stephens Subgroup 277

Fig. 5 - Ki wi Burn T uffs exposed on th e east face of Snowdon Peak . T he type sect ion is on theridge sloping from the top left to bottom right of the photograph.

ACHERON LAKES SANDSTONE (new name)Introduction and Definition

The name Acheron Lakes Sa nds tone is introduced for th ick bedd ed volca niclas ticsands tones ove rlyi ng the K iwi Burn Tuffs. The base is drawn a t the first conspicuo usvolca niclastic sandstone (S13 1 G R0236 06 in type section) overlying the highest vitric tuffor mudstone of the Kiwi Burn Tuffs, and is conformable . The form ation is equivalentto the basal part of the Countess Form ation as described by G rindley (1958) andW aterhou se (1964), and to the upper part of the Countess Form ati on Unit 1 of Landis(1974). T he sandstones are name d after Acheron Lakes in the Livin gston e M ountains,east of the sout he rn mos t Acheron Lakes Sandstone outcrop s in the Countess Ran ge.

The type sect ion is on the north side of Countess Peak (D42 G R0236 06) along theCountess R an ge to the upper contac t (with Cerberus Format ion ) at the col betweenCountess Peak and Peak 5700 (N ZM S 1 Sheet S131 ) .

LithologyAcheron Lakes Sa ndstone is dominated by thi ckly bedded (1-2 m) gree nish-grey

coloured, massive, medium to coarse gra ined volcaniclas tic sands tone . Assoc iatedlith ologies inclu de thinly bedded siltstone and fine sandstone , red and green mu dston eand occasional tu ff to the south of Countess Peak (S131 G R022600) and a resistantcong lome ra te on Snowdon Peak (S141 GR046368). San dston es are characteristicallymoderat ely sor ted , ranging in com position from volcarenite to felds pa thic volcarenite .Bedding is com mo nly amalga mated . The dark greenish grey colour reflects a basalticandesite derivati on and an ab undance of authigenic iron-rich pumpellyite and chlor ite.Detrita l quartz gra ins are rare to absent.

Distribution and ThicknessAchero n Lakes Sa nds tone is expose d throughout the mapped area (Figs 2, 6) forming

much of the crest of the northern C ountess Range, and the upper slopes of Snowdo nPeak . It is approximately 550 m thick at Countess Peak , and 630 m at M ara roa Ri ver.Preliminary work at Snowdon Peak indicat es a thi ckn ess of approxi mately 680 m .

Dow

nloa

ded

by [

UQ

Lib

rary

] at

11:

46 2

0 N

ovem

ber

2014

278 Journal of the Royal Society of New Zealand, Volume 18, 1988

Fig. 6- Lower Stephens Subgroup rocks exposed on an unnamed peak (composed of AcheronLakes Sandstone), SE of Countess Peak. Beds are steeply east-dipping and overturned. Viewlooking south down the east side of the Countess Range. East of Upukerora River are CaplesTerrane (CT), and Dun Mountain Ophiolite Belt (DMOB) melange rocks. Other rocks are WaiuaFormation (W), Kiwi Burn Tuffs (K), Acheron Lakes Sandstone (A), Cerberus Formation (C),Eldon Sandstone (E), and Snowdon Formation (S).

CERBERUS FORMATION (new name)

Introduction and Definition

This unit, characterised by reddish siltstone, also contains other fine-grained lithologiesand intervenes between massive sandstones of the Acheron Lakes Sandstone and EldonFormation. The base, drawn where reddish siltstone dominates over thicker beddedgreenish sandstone, is conformable and transitional. The unit is equivalent to the lowerpart of the Countess Formation Unit 2 of Landis (1974) and the red mudstone mappedin the Countess Range by Turnbull (1986). The name is taken from Mt. Cerberus, apeak near South Mavora Lake, 6 km east of Snowdon Peak. The type section extendsacross a col between the Winton Peak-Countess Peak ridge and an un-named peak tothe east (S131 GR026587).

LithologyCerberus Formation consists predominantly of siltstone and is finer-grained and thinner­

bedded than overlying and underlying units. The abundance of interbedded greyish-red,grey, and (less commonly) greenish grey siltstones and fine sandstones is distinctive. Thered beds are hematitic, occasionally calcareous, and slightly more quartzose than otherunits. Grey, thin bedded, graded, fine grained volcaniclastic sandstones are also common.It frequently contains mudstone rip-up clasts. Light brown and olive grey mudstone isalso widespread.

Subunits of red, brown or greenish beds are recognisable, and may prove to bemappable members. Lateral variation can also be recognised, for example red sandstoneexposed on the ridge dividing Walker and Mackay creeks (Fig. 2) is represented by redsiltstone on Peak 3925 near the Mararoa River.

Dow

nloa

ded

by [

UQ

Lib

rary

] at

11:

46 2

0 N

ovem

ber

2014

Aitchison et al. -Stratigraphy of Stephens Subgroup 279

Distribution and ThicknessCerberus Formati on is found th roughout the Countess R an ge, where it is com mo nly

expose d in cols and gullies, reflectin g its finer grain size and thinner bedding. Thicknesshere ra nges between 115-230 m . Sim ilar lithologies on the south end of the lon g spurleading sou th from Snowdon Peak are exposed in a gently sout hward plunging syncl ine .H owever , furthe r mapping is needed in th e Sn owdon Peak area before thi s correla tioncan be confirmed. A complete seque nce is recognised to the south at Bare Peak and onthe ea st side of Peak 3925 (Fig . 2) where a 300 m section is presen t.

ELDON SANDSTONE (new name)Introduction and Definition

Thick bedd ed to massive , medium to coarse grained volcanicl astic sandstones overlyingthe Cerb eru s Formati on are here named Eldon Sandstone. The u nit is strongly induratedand resistant , supporting high ground , including th e summit of W int on Peak (F ig. 7).The base of the Eldon Sandstone is abrupt and conformable and is defined by the incomingof thick-bedded, medium to coarse grained sandstones above the thinner-bedded, finergra ined strata of the Cerberus Formation. Eldon Sandston e is equivalent to the upperpa rt of Grindley's (1958) lower member (YC 1) of th e Countess Formati on and to Unit 3of the Countess Formation as desc ribed by Landis ( 1974) . The formati on is nam ed afterMt. Eld on , wh ich is immediat ely north of Mt. Cerberus in the Livin gston e M ountains.

Fig. 7 - Sou th face of W inton Peak , beds overturned. Top of Winton Peak is com posed of Eldo nSandston e. Below Wi nton Peak the con tac t with Snowdon Form ati on is clea rly exposed . Notewhit e colou red tuff hori zon s and fossiliferous Retford Conglomer ate M em ber . Also note cyclicsedimen ta tion pattern s.

LithologyEldon Sandstone consists almost entirely of thi ckly bedded , mod erat ely to well sorted,

medium sand to granule grade volcani c sandstones and gra nule conglomerates. Individu albeds are characteris tically graded and amalgamated. Som e contain dish struct ures . Rocks

Dow

nloa

ded

by [

UQ

Lib

rary

] at

11:

46 2

0 N

ovem

ber

2014

280 Journal of the Royal Society of New Zealand, Volume 18, 1988

are green or greenish-grey in colour, reflecting basic volcanic composition and theabundance of iron-rich pumpellyite and chlorite. Pyrite and plant scraps are also present.Detrital pyroxene is conspicuous in some hand specimens.

Distribution and ThicknessEldon Sandstone forms a distinctive band extending obliquely across the Countess

Range from the divide between Walker and Mackay creeks south to the U pukerora River.It is also exposed in the Bare Peak-Mararoa River area, where it supports the summitof Peak 3925. A maximum thickness of 550 m is recorded at Peak 5600 in the CountessRange; to the north thickness reduces to 300 m while to the south at Mararoa only 150 mis exposed.

SNOWDON FORMATIONIntroduction and Definition

The Snowdon Formation was introduced by Landis (1974) in large part to formaliseand upgrade Grindley's (1958) upper (YC2) member of the Countess Formation. It ischaracterised by a varied lithologic assemblage consisting mainly of interbedded sandstoneand siltstone units in varying proportions and colours, but also includes conspicuousconglomerate and tuff. Molluscan and plant fossils are also present. Thus, the SnowdonFormation lacks a distinctive simplicity. The base was originally taken as "the lowestconspicuous tuff, conglomerate or unconformity" (Landis, 1974). We propose a revision,lowering the base slightly to a lithologic break mappable throughout the area investigated:the base of the Snowdon Formation is here defined as the first appearance of fine grained,thinly bedded sandstone and siltstone (S131 GR021570 in type section) above the EldonSandstone. The type section is modified to commence just beneath the summit of WintonPeak, on the south ridge, continuing in a southerly direction along the crest of the CountessRange to where the formation is truncated by the Countess Fault (Fig. 2). The top ofthe unit is everywhere removed by faulting.

Lithology and SubdivisionThickly bedded volcaniclastic sandstones and mudstones form the bulk of the Snowdon

Formation. These characteristically show sedimentary structures indicating turbiditycurrent deposition. Maroon (greyish purple) siltstone and fine grained sandstone thatare conspicuous near the base are well exposed on the north ridge of Winton Peak. Whiteweathering (very light grey to yellowish grey) vitric tuffs rich in laumontite are alsoconspicuous, especially near the base of the formation. These are found as isolated thinbeds and as composite beds up to 2 m thick.

Conglomerate horizons form a conspicuous though minor component. They exhibitlateral as well as vertical grading; thus a conspicuous cobble conglomerate exposed onthe west side of Peak 5600 (Fig. 2, map 1) can be traced north approximately 1 km tonear Winton Peak, where it is a granule conglomerate. Another conglomerate comprises2 m of vertically graded granule to coarse sand grade sediment rich in shells of the bivalvePermophorus obovatus, which Waterhouse (1979a) named Retford Conglomerate-BrecciaMember; we have traced this unit over 2 km from the head of Walker Creek(S131 GR016586) to a small promontory south of Winton Peak (S131 GR025564, Fig. 7).Boulders of the same lithology were collected on the slopes of Peak 5600 (D42 GR032556)and similar rocks are also exposed in a comparable stratigraphic position near MararoaRiver (S141 GR062250). Further work may show the Retford Member to be a regionallyextensive unit within the basal Snowdon Formation. We note, however, that the unitis rarely a breccia and suggest a name revision to Retford Conglomerate Member. Furtherformal subdivision of Snowdon rocks is not attempted here, although conspicuous palebuff laumontite tuffs (Landis, 1974:241), conglomerates, and maroon argillaceous horizonsall provide useful marker horizons within the dominantly sandstone-siltstone sequence.

Distribution and ThicknessSnowdon Formation is exposed in a SSE-trending band along the western flank of

the Countess Range and into the Upukerora Valley. The formation is again seen at Bare

Dow

nloa

ded

by [

UQ

Lib

rary

] at

11:

46 2

0 N

ovem

ber

2014

Aitchison et al. - Stratigraphy of Stephens Subgroup 281

Peak and continues to Little Hill (S141 GR053212). The top of the Snowdon is notexposed; the uppermost beds are in fault contact with Cenozoic sediments. More than1200 m thickness is exposed in the Countess Range, and at least 1300 m along theMararoa River. Snowdon rocks have not been mapped north of the Countess Range.A thin, fault-bounded sequence of distinctive laumontitic, vitric tuffs and purple siltstonesis exposed near Whangamoa Head 30 km north of Nelson City (Landis, 1969). The rocksare surrounded by older Stephens rocks, but are quite distinctive in both lithology andlower degree of metamorphism, and may be Snowdon Formation correlatives.

PALEONTOLOGYStephens Subgroup strata within the study area contain few macrofossils. Only the

Snowdon Formation has yielded substantial material, including molluscan and plantremains in some abundance as well as rarer brachiopods and echinoderm fragments.In addition, Acheron Lakes and Kiwi Burn formations have yielded some poorly preservedmaterial. Stephens rocks near Mossburn, south of the present study area, and in theNelson region, also have significant faunas.

Early discoveries and descriptions of Mellarium and Permophorus from the SnowdonFormation are summarised in the introduction to this paper. We concur with Waterhouse(1979a) that Permophorus is abundant in the Retford Conglomerate Member. In addition,Permophorus is found in rocks of a similar lithology in the lower part of the SnowdonFormation north of the Mararoa River (D43/fD15). This locality has also yielded arhynchonellid brachiopod, which is regarded (Dr D. A. B. Macfarlan, pers. comm. 1984)as probably belonging to a new genus of Early Triassic age. A similar poorly preservedrhynchonellid is also found in Retford Member beds in the Countess Range. This locality(D42/fD65) was first reported by Hyden et at. (1982), but we note that the location citedby them (Mackay Creek) is incorrect. The specimen was collected from a small promontory(S131 GR025564) near the head of the stream draining southeast from Winton Peak intothe Upukerora Valley. Permophorus and crinoid fragments were also found upon recollectionof this locality, and along strike 200 m west of the summit of Winton Peak at D42/fl06.Well preserved crinoid plates and articulated columnals, poorly preserved molluscan"fragments and plant stems were reported by Landis (1974) from Peak 3925 near theMararoa River (D43/f751O), and we report further crinoid fragments and an indeterminateseptate fossil nearby at D43/fD14. Hyden et at. (1982) report crinoid columnals withatomodesmatinid prisms from D42/f7524, a locality in the southern Countess Range.A report of bryozoan and crinoid fragments, a rhynchonellid and a terebratulid (bothindeterminate) from Snowdon Formation rocks on Snowdon Peak requires comment:these rocks (D42/fD77), originally mapped as Snowdon Formation (Landis, 1974) probablybelong to Acheron Lakes Sandstone.

There are plant fossils in Snowdon rocks south of Winton Peak. First reported byGrindley (1958), they were recollected by Retallack (1977), who reported (fide Hydenet al., 1982) that his collection (D42/f7 539) is possibly a recollection of Grindley's originallocality (D42/f7511). The flora includes Cladophlebis oblonga (Halle), Dicroidium sp. andDesmiophyllum cf. indicum Sahni. Extremely comminuted plant fossils were also seen byAitchison (1984) in boulders of the Eldon Sandstone (S131 GR027578), by Landis (1974)in Snowdon tuffs (S141 GR054265), and Turnbull (1986) in tuffs SSW of Snowdon Peak(S141 GR043357), but these have not been collected.

Trace fossils are also found in Snowdon rocks, particularly in the Countess Range.Most conspicuous of these is Scalarituba (e.g. at D42/fl07, south of Winton Peak).

Near Mossburn, 30 km to the southeast of Stephens outcrops along the Mararoa River(Fig. 1), a rich late Permian fauna (Hyden et al., 1982) is described from a Lm thicklens within rocks mapped by them and by Grindley (1958), as Countess (Stephens)Formation. Detailed correlation of the fossiliferous strata remains uncertain; the fossilswere collected from a poorly-exposed granule-pebble conglomerate 5 m beneath aconspicuous atomodesmatinid-bearing cobble-boulder conglomerate. The latter is wellexposed in the "Mossburn Quarry", a road metal excavation on the true left bank of theOreti River, approximately 2.5 km west of Mossburn. Hyden et at. (1982) interpret the

Dow

nloa

ded

by [

UQ

Lib

rary

] at

11:

46 2

0 N

ovem

ber

2014

282 Journal ofthe Royal Society of New Zealand, Volume 18, 1988

fossiliferous strata as lying within the highest 5 m of the Countess Formation, whereasthe coarser conglomerate is taken as the base of the Snowdon Formation. We have visitedthe Mossburn fossil locality and agree with Hyden et at. regarding correlation with theStephens. However, neither the fossiliferous outcrop nor the overlying conglomerate canbe readily correlated with specific horizons of Stephens rocks in the Mararoa River­Countess Range area. On the basis of stratigraphic position (the fossil horizon is 700 mabove the base of the Stephens) and general lithology, correlation with the Eldon orAcheron Lakes sandstones is equally possible. Some doubt must exist about the recognitionof the basal Snowdon Formation at Mossburn by Hyden et al. (1982). Although theirmapping is certainly plausible, it requires approximately 50 % reduction in thickness ofStephens Subgroup strata below the Snowdon Formation from the Mararoa River 20 kmto the north. Equally possible is that the conspicuous conglomerate may be a lens (probablychannelised) within pre-Snowdon Stephens rocks. As discussed in the previous section,other Stephens conglomerates show lateral as well as vertical size grading. StephensSubgroup is interpreted as a deep water mass-flow sequence (Aitchison, 1984; Aitchisonand Landis, in prep) and we emphasise that such variability, as well as discontinuity,is to be expected.

Furthermore, we have reservations regarding relations between the fossiliferous rocksat Mossburn and adjoining sediments. Although described as directly underlying theconspicuous cobble-boulder conglomerate, outcrop relations are poor and do not excludethe possibility that the fossiliferous granule-pebble conglomerate may be part of the moreextensive and coarser overlying conglomerate (observations of J.C.A. and C.A.L).Alternatively, the fossiliferous rock itself may be an isolated boulder or raft at the baseof the main conglomerate. As support for this latter possibilty, we cite the recent discovery(Campbell et al., 1984) of a comparably large, but different, late Permian fauna in a4 m long debris flow boulder of coarse tuffaceous sandstone in the conglomeratic QueensBeach Member, the lowest unit exposed in the Stephens Subgroup at Stephens Island,Nelson (Fig. 1). If exposure at the Stephens Island locality was imperfect, it would havebeen easy to mistake the fossiliferous boulder for part of the sequence.

Fossils can also be seen in road-cuttings in volcanic sandstone, along State Highway94 at Rocky Point 2 km northwest of Mossburn, and approximately 1200 m across strikefrom the locality reported by Hyden et al. (1982). These rocks were described by Coombs(1950:429) and mapped by him as undifferentiated strata of probable late Paleozoic age;they are mapped by Hyden et at. (1982), provisionally, as Snowdon Formation. Herewell-preserved vertebrate remains, originally discovered by Y. Olive-Crena, I. Raineand H. J. Campbell, were collected in 1984 by two of the authors a.C.A. & C.A.L.),R. E. Fordyce and R. K. Bearlin, and are awaiting description. Unidentified plant fossilsare also abundant at this locality.

AGE

The age of the Stephens Subgroup is of considerable interest since it is likely that thePermian-Triassic boundary lies somewhere within the unit (Waterhouse, 1964, 1976,1979a; Landis, 1974). Unfortunately, age constraints remain sparse and are to some degreeambiguous. Waterhouse (1964) proposed that the system boundary lies at, or near thebase of the Snowdon Formation, while Landis (1974) maintains that it is equally likelyto lie somewhere within older Stephens strata.

Age of the Stephens has been reviewed recently by Hyden et at. (1982) who expresssome reservations about the earlier opinion of Waterhouse (1964, 1979a), that Snowdonstrata are Triassic. They consider the strata which they map as basal Snowdon Formationto be Permian. This view has been tentatively supported by Campbell et at. (1984).Nevertheless, some doubt must remain regarding the Permian age of the Stephens. Furnishet al. (1976) argue that the Durvilleoceras-bearing Greville strata in Nelson area are of Triassicage, a view disputed by Waterhouse (1979b). The recent discovery of Durvilleoceras inGreville strata of the Upukerora Valley immediately east of, and downsection from,Stephens rocks of the Countess Range (Pillai et al. manuscript) revives the interpretationthat the Permian-Triassic boundary might significantly underlie Stephens Subgroup.

Dow

nloa

ded

by [

UQ

Lib

rary

] at

11:

46 2

0 N

ovem

ber

2014

Aitchison et al. - Stratigraphy of Stephens Subgroup 283

There can be no doubt regarding the Triassic age of the Snowdon Formation. Mellarium,a Middle Triassic gastropod, is found within the unit (Waterhouse, 1960 and Fig. 3), andPermophorus obovatus, an Early Triassic bivalve, near the base (Waterhouse, 1979a, andFig. 3). Furthermore, isotope dating of pyrogenic zircons from tuffs in the middle Snowdonyields Middle to Late Triassic dates (D. L. Kimbrough, in Pillai et at. manuscript).

Pre-Snowdon Stephens and Waiua strata lack demonstrably in situ diagnostic fossilsand could equally well be Late Permian or Early Triassic; this topic is further discussedby Pillai et at. (manuscript).

ACKNOWLEDGMENTSRoger McNaughton and james Barringer assistedj.C.A. with fieldwork,

Don Weston helped produce the photographs, and a University of Otago ResearchGrant helped defray the cost of fieldwork for j.C.A. and C.A.L.. D. L.Kimbrough, D. A. B. MacFarlan and D. D. L. Pillai discussed and made availabletheir unpublished work on Maitai rocks. We thank j. D. Campbell and D. S.Coombs, H. j. Campbell, and G. W. Grindley for helpful comments on themanuscript.

REFERENCESAitchison, j. C., 1984. Stephens Subgroup (Upper Maitai Group) in the CountessRange-Mararoa River

area. Unpublished M.Sc. thesis, University of Otago.

Bishop, D. G., Bradshaw, J. D., and Landis, C. A., 1985. Provisional Terrane Map of SouthIsland, New Zealand. In D. G. Howell (Ed.): Tectonostratigraphic terranes of the Circum-PacificRegion, pp. 515-521. Circum Pacific Council for Energy and Mineral Resources. Earth ScienceSeries No. 1.

Campbell, H. j., Coleman, A. C., johnston, M. R., and Landis, C. A., 1984. Geology ofStephens Island and the age of Stephens Formation. NZ Journal of Geology and Geophysics27: 277-289.

Campbell, j. D., and Coombs, D. S., 1966. Murihiku Supergroup (Triassic-jurassic) ofSouthland and South Otago. NZ Journal of Geology and Geophysics 9: 393-398.

Coombs, D. S., 1950. The geology of the Northern Taringatura Hills. Transactions of theRoyal Society of NZ 82: 65-109.

--- Landis, C. A., Norris, R.j., Sinton,j. M., Borns, D.j., and Craw, D., 1976. TheDun Mountain ophiolite belt, New Zealand, its tectonic setting, constitution, and origin, withspecial reference to the southern portion. American Journal of Science 276: 561-603.

Furnish, W. M., Glenister, B. F., Kummel, B., Spinosa, C., Sweet, W., and Teichert,C., 1976. Reinterpretation of ceratitic ammonoids from the Greville Formation, NewZealand. Geological Magazine 113: 39-46:

Grindley, G. W., 1948. The Reconnaissance Geology of the Eglinton and East Branch Valleys, westernSouthland. Unpublished M.Sc. thesis, University of Otago.

1958. The geology of the Eglinton Valley, Southland. N Z GeologicalSurvey, Bulletin58.

Hyden, G., Begg, j. G., Campbell, H. j., and Campbell, j. D., 1982. Permian fossils fromthe Countess Formation, Mossburn, Southland. NZ Journal of Geology and Geophysics 25:101-108.

johnston, M. R., 1981. Sheet 027 AC-DunMountain (1st ed.). Geological map of New Zealand,1:50000. Wellington, Department of Scientific and Industrial Research.

1982a. Sheet N27-Richmond (1st ed.). Geological map of New Zealand, 1:50000.Wellington, Department of Scientific and Industrial Research.

1982b. Sheet N28 BD-Red Hills (1st ed.). Geological map of New Zealand, 1:50 000.Wellington, Department of Scientific and Industrial Research.

1983. Sheet N28 AC-Motupiko (1st ed.). Geological map of New Zealand, 1:50000.Wellington, Department of Scientific and Industrial Research.

Dow

nloa

ded

by [

UQ

Lib

rary

] at

11:

46 2

0 N

ovem

ber

2014

284 Journal of the Royal Society of New Zealand, Volume 18, 1988

Landis, C. A., 1969. Upper Permian rocks oj South Island, New Zealand-lithology, stratigraphy,structure, metamorphism, and tectonics. Unpublished Ph.D. thesis, University of Otago.

1974. Stratigraphy, lithology, structure and metamorphism of Permian, Jurassic andTertiary rocks between the Mararoa River and Mt. Snowdon, Western Southland, NewZealand. Journal of the Royal Society oj NZ 4: 229-251.

Landis, C .A., 1980. Little Ben Sandstone, Maitai Group (Permian): nature and extent inthe Hollyford-Eglinton region, South Island, New Zealand. NZ Journal oj Geology andGeophysics 23: 551-567.

Pillai, D. D. L., 1981. The Geology ojAnnear Creek. Unpublished B.Sc. (Hons) thesis, Universityof Otago.

Retallack, G. J., 1977. A biostratigraphyjor terrestial Triassic rocks oj Gondwanaland. UnpublishedPh.D. thesis, University of New England, Armidale, NSW.

Rigg, T., Chittenden, E., and Hodgson, L., 1957. A survey of soils, vegetation, and agricultureoj the eastern hills, Waimea County, Nelson. Cawthron Institute, Nelson.

Suggate, R. P., Stevens, G. R., and Te Punga, M. T. (Eds.), 1978. The Geology o]New Zealand.Government Printer, Wellington.

Turnbull, 1. M., 1986. Sheet D42 BD & part sheet D43-Snowdon. Geological Map of NewZealand 1:50 000. Wellington, New Zealand, Department of Scientific and Industrial Research.

Waterhouse, J. B., 1960. Mellarium, a new Triassic pleurotomariid gastropod from NewZealand. NZ Journal oj Geology and Geophysics 3: 271-283.

1964. Permian stratigraphy and faunas of New Zealand. NZ Geological Survey Bulletin

72.

1976 Permian-Traissic boundary in Ncw Zealand. NZ Journal oj Geology andGeophysics 19: 373-385.

1979a. A new species of Permophorus Chavan (Bivalvia) from the Early Triassic ofNew Zealand. NZ Journal oj Geology and Geophysics 22: 743-749.

1979b. The debated age of Durvilleoceras Waterhouse. Geological Magazine 116:385-392.

Wood, B. L., 1962. Sheet S22- Wakatipu. Geological Map of New Zealand. 1:250000.Wellington, Department of Scientific and Industrial Research.

1966. SheetS24- Invercargill. Geological Map of New Zealand. 1:250 000. Wellington,Department of Scientific and Industrial Research.

Received 18 December 1986; accepted 15 September 1987

Dow

nloa

ded

by [

UQ

Lib

rary

] at

11:

46 2

0 N

ovem

ber

2014