Depositional Inodels for II1.oraine fortnation in East Antarctic coastal

J ournal qfGlaciology, flol. 43, No. 144, 1997
Depositional Inodels for II1.oraine fortnation in East
Antarctic coastal oases
SEANJ.
Department qfGeography,
Un ivmi~JI
FITZSIMO:\TS
qf Otago, PO. Box 56, Dunedin, New Zealand
ABSTRACT. Thi s pap er ex a mines th e o rig in of mora ine ridges in Eas t Anta rctic
coastal oases a nd deri\'es d epo sitiona l mod e ls a ppropr iate [o r the reco nstruc tion of
Quatern a ry histo ry. On the bas is of morphol ogy, structure a nd sedimentology, fo ur principal types of ridge may be identifi ed: (1) type A m ora ines form wh en the basal d ebri s zone
crops out near a n ice margin; (2) type B morain es form when la rge rec umbent fold s d evelop in the basa l d ebris zone; (3) ty pe C moraines a r e ice-contact sc r ees a nd fans whic h form
when debri s a cc umul ates a t steep or e1ifTed ice m a rgins; and (4-) type D morain es arc
thrust-block m o ra ines th at form when unconsolid a ted sedim ent is entrained by freez ing,
shea ring a nd thrusting of sediment blocks a t th e base of th e glacie r. Simple ca lcula ti o ns of
th e rate of d ebri s acc umula ti o n a t ice margins suggest thatt ype A , B and C mora in es take
thousand s of years to form a nd record stabl e ice m a rgins. Type D m ora ines a re struc tural
features th at m ay form rela ti vely quickl y wh en ice margins ove rride unconso lid a ted sediment. Construc ting models to ex plain the o ri g in o f the moraines is a n importa nt p a rt of
reconstructing th e Qu atern a r y hi stor y o[ Anta r cti c coas tal oases, because the m odel s
prO\'ide a bas is fo r reconstru c ting th e positi o n a nd behavi o ur of th e ice shee t during
ad \'a nce a nd r et reat.
INTRODUCTION
From recent investi gations o[ th e Quate rn a ry history on
Eas t Anta rcti c coastal oases it has bee n suggested that the
ice margin during the Las t Gl acia l Max imum was thinner
and less extensive th an previously thought (Colh oun and
others, 1992) a nd that deglacia tio n was almos t c omplete by
10000 RP (Fitzsimons and D om ac k, 1993). Th ese conclusions
arc cl earl y co ntroversial as they contradict d a ta from the
Ross Embay m ent (Demon a nd others, 1989) a nd marin e
seismic a nd core data in E as t Antarctica (Domac k and
oth ers, 1991). The controve rsy underlines the difficulties in
elaborati ng d eta il s of glacia l- i n te rglacial hi stor y (Andrews,
1992), pa rticula rl y in interpre ting frag menta r y te rrestri al
data and resolving apparent c onflicts betwee n m ar ine and
lerrestria l d a ta sources. Since th e mode and p a ttern of ice
ad\'ance a nd retreat ha\'e impl iealions for the inte rpretati on
of palaeoclim ate and ice d yna mics, it is vita l to have appropri ate depositi o nal models fo r la ndforms and sedim ents. As
ice-contact landforms, mora in es provide info rm a ti on on th e
location a nd geometr y o[ former ice margins (' Va rren and
Ash Icy, 1994), the dynami cs of ice margins (Sha rp, 1985;
Boulton, 1986) and depos iti o nal p rocesses a nd clim ate
during form ati on (Shaw, 1977a, b; Eyles a nd oth ers, 1983).
l ogether wilh appropri ate d a ting, moraines ca n be used to
reconstruct Qu atern ary eve nts and determ ine the behavi our of form er ice margins. Th e a im of thi s p ap er is to reconstruct g lac iological co nditi ons from ice-m argina l
sedim ents a nd landform s th a t have formed a t terrestrial ice
ma rgins in E as t Anta rctic co as ta l oases. Th e o bj ectives of
the stud y a rc to: (1 ) determine th e types of depositi onal environm ents th a t give ri se to th e form ation o f moraines in
coasta l Antarctica; (2) examin e relationships b etween gla256
ciological co nditi o ns a nd the sedim entology a nd structure
of m oraines; a nd (3) establish ficld c riteri a for thc r ecognition of difTere11l m oraine types a nd de\'elop depositi onal
m odel s to assist rec onstructi on o[ ice-m argin dyna m ics and
glac ial history.
Th e data used in thi s stud y consist of field obser vati ons
m ade at Vestfold Hi li s and Larsem a nn Hill s and near Casey
Base, toge ther with pre\'ious d esc ripti ons of ice-m a rgin al
features at Bunge r Hill s (Fig. I).
SEDIMENTOLOGY AND STRUCTURE OF THE
RIDGES
In order to de te rmine the struc ture, sedimentology and
origin of the ridges, small pits wer e exca\'ated to d epths of
1-4· m in the ridge crests. Th e sedimellls were reco rded
using a li thofac ies scheme based o n the wo rk o f E yles and
oth ers (1983) Clabl e I). Pebble fa bric in diamietons was
o bta ined from m eas urements of the ori entati on a nd long
a xis plunge of prol ate-shaped clasls > 2 cm in length
(n = 25). f\{eas u rem ents were plo tted on lowe r-hemi sph ere
Schmidt equ al-a r ea proj ecti ons a nd contoured according to
the method o[ K a mb (1959). Th e d a ta we re ana lyzed using
the eigenveetor m ethod of M a rk (1973). In thi s m ethod ,
eige nyector VI g i\'es the directio n o f max imum clustering,
a nd V3 indicates the directi on of minimum clustering and
is perpendicul a r to both VI a nd 112. Norm alised eigenvalues
or significa nce va lues, SI, S2, S3; indicate th e d eg ree of
clustering of th e three eigenvec to rs a nd arc calc ul a ted by
di viding each eigenvec tor by the to ta l number of sample
m easurements, N. Grain-size distributi ons of representative
samples were examined using sieves a nd a hydrom e te r.
At most of th e locations exa mi n ed , well-prese rved ridges
Fit;:.sil71olls: JloraineJonnation ill East Antarctic coastal oases
Type A ridges
West Ice shelf
-,,
/
//'
90' E
Typc A ridges fo rm at the m a rg In of the ice sheet wh cre
basal debri s c ro ps o ut a nd acc umul a tes on the ice surface.
At the present ice margin s, ice-co red ridgcs of th is t yp c consist of acc umul a ti ons of debris up to 1.5 m thi ck a nd 300 m
w idc.
Exposures a nd pits excava ted in type A ridges re\'eal
m assive, m a tri x-supported di a mic tons with ra re laye rs of
poorl y so rted sand y grave l (Fi gs 2a a nd 3). Pa rticle-size
a na lysis of the fra cti on less tha n 4cjJ underta ke n on 12
sa mpl es shows tha t the sedim e nts a rc coa rse a nd p oorl y
so rted (Ta bl e 2). Pebblc-fabric streng ths of th e diamictons
ra nge from 0.51 to 0.81 a nd tend to be wea ker c loser to the
surface of th e ridges (Fig. 3). Direc tio ns of m aximum cluste ri ng ra nge a rc pe rpcndic ul a r to th e trend s of th e r idges
a nd in a few case oblique to th e tre nds of the ridges ('L,bl e
3; Fig. 3).
Table 2. Particfe-size characteristics qfsedimentfraction less
than -IcjJ,Jrom theJour i)'jJes qf ridges
Ridge [rl)e
Sorting
. \ illllbrr0/
salllplel
Poorl y sortl'cl (2.8 CP)
1'001'1\- ,,,rtl"d (3.31»
:-'I odera t l'i~ so rt ed 0.77 0 1
\ 'e ry poo rl y sorted :.J..20)
12
10
10
8
Jleon
gram Jl;:.r
r\
B
F'~g.
1. Location lllaj) qfp/aces lI1entioll ed ill the tn/.
occ ur as endm oraines cl ose to the ma rgin o f th e ice shee t, o r
as latera lm o ra in es adj ace nt to m a rgin s of o utl et glacicrs. No
apprec ia blc supraglacial d e bri s occ urs on th e icc shcct, a nd
the so urce o f most debris is thro ug h abl a ti o n of basa l dcbri s.
The longes t ridges a re 10 km long segments o f sinuous icccorcd m o r a ines lI'hich occ ur whcre basal d e bri s crops o ut
o n th c ice surfacc at ice ed ges (sce Fitzsimo ns a nd Colhoun,
1995, fi g. 3). Beyond th e prese nt ice edge m os t ridges co nsist
of segme nts up to 2.5 km lo ng tha t arc broken by roc k ridges.
Four d istinc t types of mora i ne ridgcs lVe re identi fi ed:
Ti,ble 1. Descrij)tion qffacies £JjJeS and codil7g lIsed ill this
Stll~)'
0.95
0.78
C
1.6
D
0.78
• Cr" phir m ean a ncl sorting \'a IUl's of part icle size in p hi (CP ) units
descr ibed by Fo l k a " d Wa rd 11957 1.
The di a mic to ns a rc acc umul a tio ns of" basal d e bri s that
have bccn re m o bilised by sedime nt fl ows. Rem o bili sation
has res ulted in rela ti\"Cly poo rl y d efin ed direc ti o ns of m ax illlum clustering, a nd slight tex tura l va riatio n is proba bly
rela ted to sorting of sediments in less yi sco us fl ows. Stronger
pebble fabrics b elow Im depth in th e cxcavati o ns ca n be
interpreted as melt-o ut till in w hi ch the fahric of the basal
debris zone has bee n prcse n "Cd . Th c fo rm ati on o f m elt-o ut
till s a nd th e preser va tion of basa l d e bri s fabrics th a t record
icc-floll' directi o n a re more likel y a ft er the scdime nt cover
cxceeds 0.5 m , a f"t er which melting slows a nd th e d ebris is
less likely to beco m e saturated a nd (low. This inte rpre tatio n
is consistent with obse n 'ati o ns o f sedim el1l flows o n ridges at
Farif.1 (otir"
Tc,ble 3. -'leall eigenvaluesJor dia micts associated with earh
1\Iud
La minated silt a n d mud
FI
D ia lll in
:-'I ass i\"l'. Illa tri x - s u p p o rt~d wit h
bo ulder to s ilt-pa ni cle sizcs
1\ la tri x-sup portcd , st ra t iri ed wit h
cobhl.. to si lt-pa rticle sizes
I) 111 m
Gran'l
Sa nd
1\ lass iw or cr udel y be dded grmT I,
poor to Ill ode r a te so rti ng
C haotic bou lde r gravel
H orizonta ll y lam in ated , mockra tel y
well-sortcd sa nd
1\l assil"c, poo rl y to m od eratel y
so rted sa nd
• 1~'o 11l Eyles a nd others 1983 1.
Dllls
Gm
Gr
Sh
Sm
moraine £)ljJe
S:~ meau
.\ toraine 'rpe
' I) 'pe A mora ines
"'I ~ ' pc
B l11 ora in f's
l ) ' pe C morai nes
l ) ' pc D ll10ra ines
'I) 'pr D m aSS i\T d ia nlicl
1) '1)(' D atte nua ted di am ict
0.7 173
0.(i629
0.7072
O.li076
0.5 [+0
0.7580
0.0898
0. 1J.+5
0. 1322
0.5·H
0.0770
0.0380
0.0902
0.1237
0.090 ~
0.1321
0. 1620
0.0590
S:l.ut.
0.0132
0.039
0.0+88
0.cJ7:l8
0.0620
0.007
;'\IOle: St gi\"Cs th e st re ng th ofdustcring a b o ut t he principa l eige ll\TClor, 5 3
th e strengt h or clu ster ing abo ut the e i g~ Il\'cc t o r \ :1.
257
Joumal qfGlaciology
b
Fig. 2. (a) Nlassive, matrix-sujJported diall1ictexjJosed inthemst oJa moraineJorll1s as debrisji'ol1l the basal debris zone lI1elts
and accull1ulales. ( b) ( jJ-lI'a1 ped basal debris zone qfllte ice sheet in contact with and deforming Ihe /I1mgillal snou' wedge. The
cliffis about 30 111 high. (c) Lmge re(umbentfolds exjJosed in an ice-cored moraine. The cliffis about8m high.
the present ice marg in th at exhibit \'ariable Oow rates
related lo water content and viscos ity (Fitzs imons, 1990).
Type Bridges
The ice cores of type B ridges show evidence of intense compressi\'e deformation th at has gene rated large rccumbent
folds within basa l debris zones. In many o th er respects type
B ridges are simil ar to typ e A ridges.
Sed iments at the cres t of ty pe 13 ridges co nsist of massi\'e,
matrix-supported diamictons, stratifi ed diamictons a nd
crudely stratified gravel. Poorl y defined stra tifica ti on with in
stratified diamictons, and co ntac ts between diamictons a nd
grave l, show th at the sediments dip down the di stal slope of
b~~
c~c!c5J
0
0
,
RN
0
o~ ·
o·
. ' D
' 0.0 '
0'1) "
.~
0',
1
. ci 0
o
E
.c
Cl.
Q)
o
o ..
.0
'0, .
Dmm
v, = 139.7P, = 54.5S, = 0.594
'.0
•0
~.o ~
..
0'· •
•
0
.
0,' ,
" ?~
,0 .
o.
o .
••
0
~
Gms R@
~
N
~
V, = 135.9P, = 58.2S, = 0.7492
Fig. 3. Sediment({1~y logs qfsedimentsJrom the msts rif type A ( lift ) and 1J'f;e B (right ) momilles. The contouT interval q! the
Sdzll7idtnets is two standard deviations. VI and PI give the azimuth alld plunge qfthe jJrincipa! eigel11'ectol; SI gives the strength
qfcluslering abolltthe principal eigelluect01; and R shows the trend qfthe momine ridge.
258
Fit.~s ill7olls:
th e m oraines a t a ng les of 10- 25 (Fig. 3). Pa rticl e-size
a na lys is often samples shows th at th e sediments a rc slight ly
fin e r a nd ha\'e a simil a r so rting to sedime nts found in ty p e
A ridges (Table 2). Pe bble fabrics of di a m ictons ha\"C unimod a l a nd occasio na ll y bim odal pa ttern s (Fig. 3). SI ya llles
for th e eige l1\Tc tors r a nge fr om 0.52 to 0.85 (Fi g. 4) a nd ge ne ra ll y increase in stre ng th with increased depth below m o rain e surfaces. Alth o ug h ge nerally pe r pend icula r to th e
t re nd s ofriclges. som e fa brics a re nea rl y p a ra llel to th e tre nd
o f th e ridges (Fig. 3; Ta ble 3). Expos ures of" ice-cored ty p e B
ridges in the \'es tfo ld Hills sholl" th a t b asa l debr is has bee n
defo rm ed into a seri es o f la rge rec Llmbent folds (Fig. 2c ).
Jloraillejorma/ioll ill East AIl/arclic coastal oases
Type C ridges
Type C ridges form sha rp-c rested cuspa te r id ge segments up
to 20 m hi gh a nd 500 III lo ng. lee-cored t yp e C ridges a re
assoc ia ted with ice cli f1s wh e re abla ti o n o f basa l debri s
res ults in debris ia lling fr o m a nd acc umul a ting a t th e foot
of th e cliffs (Fig. Sa ). M ost t yp e C ridges ha ve asymmetrical
profiles (Fig. 5a ) charac te ri sed by p rox im a l slo pes 0(, 25 15°
a nd di sta l slopes of 15- 25 .
a
0.3
A Type A ridges
6 Type Bridges
0.2
6
M
<J)
6
6
6
6
A
0.1
A6 Af:;
A
6
a
0.0
0.40
0.70
0.60
0.50
0. 80
0. 90
1. 00
0.90
1.00
S,
0.3
••
•
0.2
•
.
•
6
~
M
<J)
•
0.1
Type C ridges
6 Type D ridges
6
•
b
•
.6
•
A
6
/IlO.
00
0.50
0 . 40
0.70
0 .60
0.80
S,
0.3
Waterlain
glacigenic sediment
(n=')
Range
S.D.
.
-' k
j
Mean
0 .2
_+~tniC
sedim:~t_flows
M
__ .
<J)
(n=20)
0 .1
Lodgement till
(n=9 )
C
Melt-out till
0 .0
0.40
0. 50
0.70
0. 60
t
Debris -rich
~ basal ice___
(n=38)
(n=8)
0.80
0.90
1.00
S,
Fig. -I. Fabric dataji"Oll7 sedill/ents of ~) 'jJe .r1 and IJ'jJe B moraill es ( a), £l'jle Ca ll d ~vjJe D 1II0railles ( b ) alld da tafrollljive
diJjaelltlllodem glacial I' III 'iroll III ell Isjro 111 DOll'desl1'ell and
olh ers (1985) jor cO /lljJarison ( ("). SI all d S l are erjllailled ill
Ih e 1n l.
Sediments in typ e B r idges arc \"C r y simi lar to th ose in
typ e A rid ges, a nd a rc a lso int erpreted as the product o f resedim e nt ati on of basa l de bris by sedim e nt nowage o n th e
dista l side or th e r idges. A lth ough th e re a rc minor dilfe renees in the structure, particle size a nd sedim enta ry cha l-a cteri sti cs of th e sedim e nts o f type A a nd B ridges, th ey a rc
alm ost indi sting uisha ble.
Fig. 5. (a) All ice-CUll/act scree jorill illg al the ice 1II(lIgin
( left) alld tll'Oire-cored ire-coli /act saee,\ adjacelll la the ice
ilia/gill. ( b ) Poor[p sorled g ra NI ol'erlaill ~) ' {amillaler! sand
alldgral1e{, alld a dast-supported dia micl er/losed ill /he crest
of the ice-con/act scree.
Sedime nts ex posed a t th e c res ts of"t ypc C ridges ( Fi g. 5)
show a r a nge of" srdilll ent a r y fac ies. including l11 ass i\T a nd
stra tified g ra\TIs. hori zo nta ll y la m inated a nd c ross-bedded
sa nds, b o ulderv gra\-c ls with lcIl SC'S or fin e-g ra in ed sediIllellts, m ass i\T matri x-suppo rted dia mic to ns, stratified diamictons a nd l11uds (Figs 5 a nd 6). Pa rt icle-size a na lys is of
these sedim ents sho\\'s th a t th ey ra nge fr o m modera tel y
sorted to ye r v poorl y sorted , but o n anTage a re modera tel y
so rted (Tab le 2). Pa rticles up to 0.8 min di a m e ter a rc CO I11mon and occ ur with chao ti c mi xtures o rdi a mi ctons, gravel
a nd well -sorted a nd stra tifi ed sand. :-Iost ex posures show
that th e sed iments co nt a in \\'e ll- presrrn'd stra tifica ti on th a t
d ips down the d istal slop e o f th e moraincs a t a ngles of 5 20 .
259
Journal qfClaciology
VI = 203.8PI = 2.6-
5, :; 0.52
v, = 357.9P, = 14.5 5, :; 0.5715
E
.c:
a.
Q)
o
v, :; 34.3P, = 33.45, = 0.507
V, :; 145 .7P, = 2.5·
5, = 0.526
V,:; 17.4-
V, = 274.7'
P, = 8.8 0
S, = 0.4809
P, = 16 .1'
S,=0.5517
VI:; 3.2P, :; 27.5-
5,:;
O.~811
Fig. 6. Sedimentmy logs cifsedimellts.from the crests of ice-contact saees. The contour interval ofthe Scftmidt nets is two standard
deviations. VI and PI give the azimuth and plunge of the jJrincipal eigenvect07; 51 gives the strength cif clustering about the
principal eigenvector, and R shows the trend cif the moraine ridge.
The pebble fabric of diam ictons a nd mass ive grave ls is
tranS\'el-se or oblique to th e (rend of the ridges (Fig. 6), a nd
the clustering abo ut th e mean axis ranges from moderate to
strong (51 = 0.54-0.86; Fig. 7; Table 3).
Gravel a nd sand facies were deposited by meltwater
stream s on the distal slopes o f the ridges. Th e p ebbl e fabric
of di a m ictons is typical of sediment Oow d eposits (Lawson ,
1979, 1981) which are widespread on recently formed ridges.
Th e association of di amicton, grm'el, sa nd a nd the bouldery facies suggests that bot h allu\'ia l and collu\'ial processes a re important during the form ati o n of th e ridges.
The chaotic bouldery lithofacies is interpreted as the product of simulta neous acc umul ation of a llu vial a nd massmovement deposits, i.e. la rge pa rticles fal l or roll into accumulating alluvia l deposits and sediment fl ows.
Typ e D ridges
Type D ridges form a long th e latera l m a rg ins of outlet
glacie rs, particularly where ice OOlVS across marine inlets
or lakes. The ridges a re up to 20 m high with proxima l slopes
of arou nd 30 a nd dista l slopes of around 25 °. As the ice core
melts, large tension cracks develop a long the ridge crests.
Sediments from type D ridges (Fig. 7a ) co nsist of stratified di amictons (Fig. 7b), m assive diam ictons and rare laye rs
of hori zonta lly laminated sands (Fig. 8). Pa rticle-size a nalyses of eight samples show th at the sed im ents are fin er than
sedi m ent from other ridges and cons iderably less so rted
(Ta bl e 1). M any expos ures d isplay low-angle thrust-faults
a nd sheared zones that consistently dip in a n up-glacier
direction at a ngles of 10- 25 (Fig. 8). The pebble fabric of
the diamictons can be divided into a group characterised
by weak fabrics associated with stratifi ed di am icto ns
(51 = 0.45- 0.57), and a group of stronger fabrics adj ace nt
0
260
to low-ang le fau lts (51 = 0.67- 0.85; Figs 7 and 8; Tabl e 3).
M assi\"C diamictons freque ntl y co ntain ab undant shell fragments, and stra tifi ed di ami ctons occas iona lly conta in beds
of shells, so m e of whieh are in growth position . Radi ocarbon dates from Latllmllla shells ga\'e age of 9920 ± 100 BP
(SUA 2924) from a ridge a bo ut 20 m from th e ice edge,
5070 ± 80 BP (SUA 2923) from a ridge about 40 m from
the ice edge a nd 2010 ± 110 BP (SUA 2922) from a ridge
abo ut 500 m from th e ice edge (Fitzsim ons a nd Dom ack,
1993).
The m assive and laminated diamictons h ave a weak
pebble fabric similar to previously studi ed ice-rafted di amictons (Domack and Lawso n, 1985; Dowd eswel l and
oth ers, 1985; D owdeswell a nd Sharp, 1986). Th e distinctive
fabric (Table 3), toget her with la minat ion, a nd m arine shell
fragments a nd layers, sugges ts that the di am icto ns a re glacima rine sediments. Pebbl e fabrics of attenuated dia mictons
(faulted a nd sheared ) have si m i lar strengths to deformed
lodgement till s, as described by Dowdeswel l a nd Sharp
(1 986). The incr eased fabric strength (Table 3) is interpreted
as a co nseq uence of atten ua ti o n by sheari ng as th e bl ocks
were either detached or deposited. Preservation of beds of
shell s and la min ations within the diamictons suggests that
at least som e of the sediment may have been frozen during
entrainment a nd transportation.
PROCESSES OF RIDGE FORMATION: DEPOSITIONAL MODELS
Type A and B ridges are accumul ations of basa l debri s that
have bee n red eposited by sed im ent Oows and meltwater.
These accul11.ulations occur a t the margins o f ice sheets
where deform ed basa l debri s crops out on th e ice surface
Fit::,simolls: 11/ominejormation in E ast Antarctic coastal oases
m e nt di scha rges a t ice edges f'ro m m easurements o f ice
\ '(' Iocit y, debri s co nce ntrati on a nd thic kn ess of debri s-bea ring ice, ?lleas urem e nts o f th e \'eloe i t y o f th e ice m a rg in a t
Bun ge r Hill s ra ng e fr om 0,5 to O,lm a I (Sim ono\', r. ?\l. ,
19 71 , c ited in \\'isni e wski, 1981), \I'hile d ebris-concentra ti o n
m eas uremcnt s r a nge fro m 0,11 % to 13,8"l(" \\'ith a m ea n oC
1.78 '1.) (\ c\'I c)'e\', 196+). Using th ese d a ta, t he la rge inn er
m o r a ine at \'estfo ld Hill s \\'ou ldt a ke 2157- 43 1+ yea rs to fo rm
(' table +), Altho ug h th ese estim ates arc based on th e dubi o us
ass umpti on o frela ti\'e1 y consta nt d e bri s di scha rge a t the ice
m a rg in , th ey sugges t th at th e ice m a rg in a t th e locati o n
sh ow n in Fig ure 5a h as bee n a t its prese nt pos iti o n fo r a t
leas t 2000 \'('a rs,
a
Table.J.. Estimates ifthe time takenfor (vjJe "1a/ld c: ridgfs 10
joml
Ri«(£:f
Ire retorill'
l'v/llme
III
,\
• Le ft ridge
II I
22'>
C'
Ct
t
,
0 11
mOl
I ~
O"j l.()
Thirkll ell Dj'
h(l.\at de/ni,
7 imp lakelllo
III
)t'api
:)
3,)
()j
1.0
5
1<J2
0..1
l.()
,"I
jiJ/'lIl
2,')26 jo::;(;
39:1 786
'Z1.'i7 I:ll+
Fig ure: 5a.
R ight ridge on Fi gure Sa.
~ Sil11 onm'r l97 11cil ecl in\\'isll ic\\', ki 19R1,
FI~~, 7 (a) A lhmst -block moraine adjatell t10 the margin a/all
olltlet glacier (right), alld a slllall jJ/l.I/i moraille 011 t/i f /Jro\ imal slojJe if the thrust -block moraine (Itji), (b) Stratified
glaciolaCllstrinf Sfr/illll'llt fI/Josed in the ere,lt q/a thrust -block
morallle,
( Fi g, 9a ), U pwa rd 0 0 11' of'b asa l d e bri s zo nes is o ft e n a llributcd to compress io n ge nera ted as ice meets m a rg in a l \I'inddrifted sn oll' (H oo ke, 1970: Fit zs im o ns a nd Co lh o un , 1995).
A ltho ugh no t w idely repo rted, large-sca le rec umbe nt foldi ng o f' th e basa l d ebri s zone o f ice ca ps has lx-e n inte rpreted
as a co nsequence ofsma ll-sca lc d e p a rtures f'rol11 stea d y-sta te
conditi o ns by HudlcsLO n (1976) \I'h o suggested th a t rec umb e nt fo lding p a ra ll el to ice m a rg in s l11 ay turn o ut to b e m ore
CO l11m on th a n is a pprec ia ted, Th esc m ora ines a lT co mmo n
a t th e m a rg in s o Cice ca ps in G ree nl a nd. Ba llin a nd Ellcsm e re Isla nd s (Arcti c Archipel ago ), a nd so uth Victori a
L a nd , A nta rc tica. II'here they fo rm perm a nent features th at
re prese nt m a rg i n a l defo rm a tion a nd abl a ti o n o f basa l
d ebri s, Th e), h a\'e bee n ca ll ed sh ea r m ora ines, Thuk Ba flin
m ora ines, inn er m ora i nes a nd ice-co red m ora i nes (Bi shop,
1957; \\'ee rtm a n , 1961; Hoo ke, 1970, 1973; So uchez, 1971), The
te rm inner m o r a ine has th e ach-a ntagc of desc ribing th e
locati o n where th ey begin ( Fi g, 9a ) a nd avo id s th e eo ntro\Trs ia l implicati o n o f' shea rin g (H oo ke, 1970),
Th e time ta ke n f() r inn er m o r a in es to form c an b e estim a ted b), ca lcul a ting rid ge \'o lum es fi-om sun'Cys a nd sedi-
Ty p e C ri dges a r c intcrpreted as ice-co ntact fa n s a nd
se r ees th a t f()I'm acUace nt to \'C rLiea l o r steep ice m a rg i n s
(Fi g, 9 h ), An a lte rn a ti \,(, intC'rpre ta ti o n is th at th e rid ges
h a\'e acc umul ated where ice-marg in a l strC'a ms h3\'e flow ed
b e t\l'Ce n th e ice m a rg in a nd proglae ia l rid ges, This a lte rn atin' is co nsidered a n unlike'" interprc ta ti o n beca use sedin1C' lll a r y stru ctures co nsistentl y dip "l\\'ay ri'olll the ice
m a rg in , Ice-co ntac t fa ns a nd sc reC's tcnd to fo rm du r in g p eriods o r zero o r nC'gat i\'C' m ass ba la ncc if th e debri s supply to
th e ice m a rg in is hi g h enoug h, a nd th e retreat rate is low
cn o ug h, for de bri s to acc umul ate, In co nditi ons 0 (' p ositive
m ass b a la nce, icc-co nt ae t fa ns a nd sc r ces a rc unlike ly to
fo rm. beca use th ey will he o\'(' rridd c n a nd incorp o ra ted
\\'ithin th e basa l d c bl-is zo ne as th e icc a nd debri s a pro n is
e ntrain ed (Shall', 1977 b; E\'a ns, 1989). Fa ns a re morc like ly
to fo rm in circum st a nces \\'here fr ee ly a \'a il abk meltwa te r
res ult s in sig nifica lll resedim enta ti o n a nd was hingo f'd e bri s,
whelTas ice-co ntact sc re es record dep os iti o n \I'ith out s i g ni(~
iea nt melt\l'ate r, In a IT \'ic\\' of th e o ri g in o r ice-cont ac t stratified ridges, Wa rren a nd i\ shle y (199+) arg ued th e
impo rt a nce of di stin g ui shing ridges tha t f(>rIll perpe ndi c ula r to ice m a rgins (es ker s) a nd ridges th a t fo rm pa ra llel to
ice m a rg in s (m o r a in es ), /\Ith o ug h ice-eo lll ac t stratifi ed
m o ra ines hm'C bee n \I'idel)' dcscribed in Qu atern a r y se ttin gs, m ost h3\'(' kJrmcd in subaqueo u s Cll\'ironm cnt s and
h m'(' b ee n ca ll ed delta m ora ines (Sy n g c, 1950; Fyfc, 1990;
Sh a rp a nd C O\\'<l n, 199 0), There h a \'e b ce n rcl ati n' ly few
field-b ased sedim e nlo log ica l studi es o r s ub ae ri a l ice-co ntac t
se rees a nd f~lIl s o n t h e m arg ins o l" ex isling g lac iers (Bo ulto n,
1986). Bo ulton (1986) sugges ted th a t la rge push mora in es a rc
fr equ c ntl y assoc ia ted with terrestri a l ice-cont ac t ra ns, H e
a rg u c:'d th at th e ra n s prO\'idc nuclei ft)r the d e\'C loplll e nt o f
pu sh m o ra ines by tra n smitting stresses into th e sedim e nt
a nd pl'O\'iding m a te ri a l fr om whic h push m ora ines fo rm ,
Th e abse nce of' sedim e nt a ry a nd m o rph o logica l e\ 'ide ll ee
261
J ourna/ qfGlaci%gy
V1 =343.9°
P1 = 17.4°
S1 =0. 51 53
V1 = 158.7"
P1 = 13.7"
S1 = 0.5381
V1 = 159.7"
P1 = 18. 0°
S1 = 0.61 51
V1 =152.9°
P1 = 15.8°
S1 =0.7601
V1 = 127.7"
P 1 = 0.4°
S1 = 0.8523
Fig. 8. SedimentOl) logs of sedimentsjrom the crests qf tILmst -block moraines. The colltollr illterval qf the Sclzmidtnets is two
standard deviations. VI and PI give the azill1uth and /J/zlllge qfthe /)rincipal eigenvect01; S I gives the strength of clustering about
the /)1'inet/Jal eigenverlO1; and R shows the trend of the 1I10raille ridge.
of glaciotcc to nic deform ation o f sediments in typ e C moraines suggests th at push mor a in es are not associa ted with
these ridges.
Th e time ta ken for type C ridges to form ca n be estim ated using th e ice-velocit y a ncl debris-co ncentra tion da ta
summ ar ised in 1a ble 4. The ice-co red ridge on th e left of
Fig ure 5a wo uld ta ke 393-786 yea rs to fo rm, a nd th e ridge
on the right o f Fig ure 5a wo uld ta ke 21574314 yea rs to form
(Table 4). Th ese estim ates sugges t th at the ice ma rg in at th e
location shown in Figure 5a h as been within 200 m of its
prese nt posit io n for at least 2500 yea rs.
Stable
a
S
d
...-----~~':,[
Advanc ing
Type D ridges a re interp reted as th r ust-block mo raines
tha t hm'e form ed a t the margin s of o utlet glaciers. \ Vith in
such glaciers, entra in ment and stacking of layers of unconso li da ted debris on the distal shores o f fi ords a nd la kes ta kes
p lace (Fig. 9c ). Entrai nment processes inyolved in th e fo rm a ti on of these ridges are thoug ht to involve margin a l accre tion of' ice a nd debri s. T he processes of entra inment
involved in the fo r mation of these I-idges are desc ribed in
greater deta il by Fi tzs imons (1997). Thrust-block 11l0r a ines
a re ridges that co nsist of stac ked blocks of L1llco nsolida ted
sedim en t th at we re froze n when d eformed (E" ans, 1989;
=?4
St able
Retreating .
Stab le
Retreating .
~ A
b
Advancing
~~
-.
: :
... .. :
.- (. -.-.. - .-
.
--.-
7
Fig. 9. Depositional modelsjo'!' theJollr £V/Jes ofl'idges. ( a) 1j/Je A 1I10rainesJo /'m where the basal debris zone crops oul 011 the ice
surJace. 1jj;e B mominesjorm where the basal debris ZOlle iJ diform ed by Imge-scale recumbentjolds. ( b) 7jpe C morainesJorm at
stationary or slow-moving cLijJed II1mgills as ice-contacljillls and saees. (c) 7}fJe D moraill es are thruJt -block momines that have
Jorm ed as la)'ers qfunconsolidatedglacimarine sedim ents are entrained and de/Josited on LIte dislal shores ofmarine inlets.
262
Fil<:,siIllOIlS: .Iiorainejorma /ioll ill Eas/ Antarc/ic coastal oases
H a mbre), a nd othe rs. 1996). Th ey a rc a subset of push m o ra in es, which is a ge ne ra l term for ridges formed by pmg lacial defo rm a ti o n (Bo ulton, 1986). Thrust-bl oc k mora ines a re
common in :\fo rth e rn H emisphe re mid- a nd hig h-latitude
a reas wh ere th ey hm'C bee n ca ll ed pseudo-l11oraines, pLlsh
m o ra ines, thrust m o ra ines, ice-pushed rid ges, push ridges
or ice-thrust ridges (Kupsch, 1962; M o ra l1, 1971; Bo ulton,
1972; Va n dcr\Va tere n, 1985; Hambrey and Huddart, 1995).
Altho ug h th e m o ra ines ca n hal'e a simil a r appea ra nce to
th e cnd mora ines desc ribed above, they arc structura l
ra th er th a n co nstruc ti o nal la ndfo rm s. C o nsequentl y, th e
reaso ning used to es tim ate the tim e ta ke n fo r ty pe A a nd C
rid ges to fo rm is in a ppropri ate. Althoug h rcla til'cly hi g h
I·cloc ities or o utlet g laciers (9 0~ 1200 IT!. a I) mean th a t sm a ll
p e rturbatio ns in di sc harge may res ult in substa mi a l c hanges
to the ice-m a rg in a l positi on, a t prese nt th e ma rg ins a re
g ro unded a nd see m rcl atil'Cly sta bl e. Three radi oca rbo n
d a tes (i'om thrust-bloc k mora ines in the Vestfold Hills suggest th a t three rid ges close to th e ice margin p ostda te
700 BP (Fitzs imo ns a nd D omack, 1993). Although th e size of
th e thrust-bl ock m o ra ines is similar to th a t of a nl1u a l ridges
produced by snoll'-ba nk pushing, as d escribed by Bi rni e
(1977), a nd small-pu sh ridges occ ur OIl the proxim a l side of
som e o f' the thrust-block moraines (Fig. 7a ), a nnu a l rid ges
d o no t fo rm a t a ny o f th e ice ma rgins exa min ed.
ACKNOWLEDGEMENTS
Thi s wo rk was supporte d by th e Austra lian N'ati onal Anta rctic R esea rch Exp editi o ns, Amarc ti c Sc ience Adl'isol' Y
Committee grants, a nd a Rectors grant from th e Australi a n
D efe nce Fo rce Academ y, Unil'Cl'sity 0 (' N e w South \ Va les.
Logisti ca l support was prOl'ided by th e Au stra li an Anta rc tic
Dil·ision. I th a nk D. G o re, 1\1. Gas pa ron and R. Pay ne for
assista nce in the field , P. H o ll and for c ritica l comments on
the tex t. a nd B. i\100ne y fo r drawing th e di ag ra ms.
REFERENCES
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28 1.
Birn ie. R . \ '. 1977. .\ ,now-ba n k p ush mecha nism li)l' the 1()J'Il1at ion ,,['som e
"a nnu a l" moraine r idges.] (;/ario/ .. 18 78.77 85.
Bi sho p. B. c:. 1957. Shl'a r m ora ill l's in t hc Thul c arca, no rth wcst Gree nl a lld.
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Bo ullOn . C . S. 1972. The role or therma l ri'gime in g la c ia l ,,'dilllentat ioll. I II
Pr ice. R . .J. a nd D. E. Su~dl'n. tOIll/". Polar gI'IIIlIor/J/r%gle London . I n stiIU tl' or Hritish Geograp her,. I 19.
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tnrc, t r i,i\ l'!I\' iro nllH'nt s. Sl'dill/(,/llo/I~~I' 33 j l. (i77 698.
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.\l1l a rn ic ice ",lulI1(' a nd cO llt ribution to sea- I('\'(' I 1:t11 a t 20.000 \T BP
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CONCLUSIONS
1. Th e sedil11 ento logy a nd structure of th e ridges toge ther
with obsen 'ati o ns or cont empo ra r y deposition a l processes show th a t fo ur types o('m o ra in es ca n be ide ntifi ed
in Eas t Ant a rctic coasta l oases, a lth o ugh onl y three can
be di sting uished fr om structure and sedime ntology
a lo ne.I>· pe A, B a nd C ridges a re c onstructiona l fea tures
th a t ca n be used to reconstru ct th e pos ition of the ice
sheet, \\·hcreas ty p e D ridges a rc strLlc tural features th a t
reco rd fluctu a ti o ns in the marg in s o f o utlet glac iers.
2.
I nte rpreta ti on o f' th e sedimento logy a nd st ruct ure 0 (' th e
rid ges suggests th a t mos t scdim e nts han' been de p os ited
b y sedim cnt (l o ws a ndmellwa te r (l o ws. The importa nt
role or melt wa te r in th e depositi o na l e!l\·irollme nts re prese nted is a co nsequence of re la til'C' ly wa rm summer
l11 0 nths ge nera ting sig nifica nt qu a ntiti es of melt wate r.
arCl ica. Qyal, R I'I .. 31 2 '. \.)1 182.
DOI n ae k, E. \\'. a nd D. E. L awso n. 19H.'i, Pebble 1, Ii) ri c ill a ll ic('-ra lkci c1i a mi c ton . .7 (;1'01., 93 5'.577 591.
\)om ack. I':. \\ .... \ ..J.T..J u ll ami S. :\akao. 1991. ,\ ch-ance of' Ea,1 .\nt a rct ic
()utkl p;la ciers clur ing till' hypsi th erlll,li : impli ca tions lu r tilt' \'olul11 c
,[;t Ic o r the r\ lllarn ic ice sh ee t under .!!;Ioba l war ming. (;fO/~~)'. 19 11).
10.)9 J( )(i:Z.
J)O\\ c!I·s \\ I·II . .J. .\. allcl ;\I..J. Sh a r p. 1')Hli. Charac ter izat io n orpelJblc labr ics in Ill ociern terr est rial g la cigc llic scdiJ11L' l1l s. ,)'('(lilll(' lIfolo,I.t.l~ 33 (5 ).
699 710.
DllIIdes\\I· II..J. .\ .. H.J. Hami> rc) a nd R. " ·u. I<)H.). ,\ co l11 parison or c1ast
l: dJri c ;llld shapl' i ll Late Prl'c illl1bri an :tnd modern g la c igc ni c scci irn e llls.
]. Sedilllt'lIl. Prlml.. 55 .) . (il)! 70 L
EI·a ns. D. .J. .\. 19B9. T he nature orglacile('lon ic strllCl ll n' and scci illl ctlls a t
sub-po lar g'lac icr 1ll(lrgill .'-l, Ilorth \\('.., t ElI l'slll cre Island , Canada. ( ,'fOg}:
. 11111 .• 7lA :1 +1. II:l In
E\' lcs. '\.. C. 11. E\ band .\ . ;\ I i,tl l. 19B1 Litholac ie, I ypc, a nd \'(' rtica l p rolil e Illockl s: a n a lt ertla ti \T 'Ipproach 10 lile clesc ript io n a nd ell\';ro nlllcntal i nuTprcuH ioll ui' g l.lc i;t\ d ia miu alld di all1in itt :,cqu l' nccs.
S,dilll""I()h~r;I' 30 :1. :193 f1 0.
Fill.~i l11 o l1 "1, S. .J. 1990. k l'- maq.!,' illal dl'pm, itiona l pro('('''Is(':-. ill <l polar nUl rit i Illl' l'Il\'i ron l1lell!. \ 'cst lc) ld Hill s, . \ nlarnica. ). ( ;/ario/ .. 36 12+, 279 :286.
Fit l.~ in lo ll "l. S. .J. 19Q7. Entra i ll llll"l ll of'glaciollla ri lll' "Icd i nll'll h and f(Jr!n al ion
3. Rela til'C ly lo\\' sediment di scha rges at th e marg in of Lh e
ice shee t mea n th a t t ype A, B a nd c: rid ges re prese nt
lo ng peri ods o rre la til'C ice-m a rg in sta bilit y beca use th ey
ta ke long peri od s to form. 1)'pe D rid ges (arm relativel y
ra pidl y beca use th ey a re structura l fo rm s that co nsist of
defil rmed glac io l1l a rin e a nd glaciolacuslrine scdim e nts.
+. L a rge type A m o raines at the Vestfolcl I-Till s, Case )' and
th e Bunge r Hi lI s support prCl'ioLl s conclusions co nce rning the rela til'e sta bilit y of prese nt te rres trialm a rg ip s of
th e Eas t Ant a rc ti c ice sheet. l\Iultipk t ype C ridges a nd
d a ted type D rid ges in th e \'estfo ld Hill s a rc a lso c on siste ntll'ith th e I·ie\\, th a t the ice m a rg in has bee n rcla til'e ly
sta ble in th e las t (CII' tho usa nd yea rs.
5. This stud y o('th e structure. sedim e ntology a ndmorph o logy of the rid ges has prOl' idedthree d e positi ona ll1lodel s
th a t can be used as a bas is (o r reco nstructing ice-marg in
d y na mics a nd g lac ia l history in Eas t Anta rcti c coasta l
oases.
or lhr ust -bloc k
Ill ora i nl's at tile margi n
(II' Sorsdal G lac il'r,
E as l
,\Ilt a rc ti c a . Earl/r SU(jiICf P mCf,IlfJ und iJ(/J{ljimlrl, 22, 17;) 187.
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1-la l11\)IT). 1\1..J. and D. !-Iudd ar t. 1'19.1. Englac ia l a nel p roglar ia l glac iote cto ni c p ro('(':-'<"l':"! al the "n ou t or (\ lh('rJlla ll ~ cOlllpl cx glacier ill S\'a lha rd . }. Qjlal. SIi., 10 I , :11:1 326.
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Thrus ti ng and (kbri:-. l' llt ra innll'll l ill <I surg ing g la cie r: Bakani nbrec n ,
S"" II)a rei .. l Ull . (;/w;u/., 22, 2 11 2~ 8.
Hookc. R . 1,I'B. 1'170. ;\I or p ho logr n i' till' in'-sh el' t Ill ilrg in nea r Thuic,
Gree nl a n d.)' (;/alio/ .. 9 :)7, 30:1 :1~ I.
Hooke. R , \'c i3. 1971 Fl o\\' nea r til e ma rg-ill "rt he Ba rn es Ire Cap. and th e
cicI'(' lo pmc lll of'icN'o/Td lll o ra il1l's, (;pn/. SOl. . Im. /3 u/l. , 84 1121. :$929 39c~ 8.
Hudleston . p..J. 197(;. Rec u lll bn ll lo ldi ng in till' base o r t he Ba rncs Ice C a p.
Ba lEn Isla nd. :\orth lll'st ·li:rrit o ri es. Ca nada. (;('o/. Sor.. 1111. 1311/1.. 87 12 .
168+ I()92.
263
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2 i.55 71.
\\'a rren, \I'. P. a nd C . i\ I. ,\ shley. 199+. Orig ins or the ice-cont act stra tifi ed
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\\ce rt man. J 196 1. l\lec ha ni sm fo r th e ro r lllat io n or in nl'r m Ol'a ines rou nd
nea r thc edge o )' co ld ice caps a nd ice s h ee ts. J (;Jariol., 3 (30 ), 965 97ft
\\'isni c\\'ski. E. 1981. I\ lora inc ron n s a nd deposits or ,\ nrarc tic ice sheet at
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Yc ,·teyl" ·, S. A. 196+. D e te rmination o f' the a m o unt or m o raini, materi a l
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Bull. 3. 7 9.
MS received 16 ]u(y 1996 alld accepted in TevisedJo nn 12 D ecember /996
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