The Illarine record of the Russell Fiord outburst flood, Alaska, V.S.A.

of Glaciology 22 1996
Intern a ti onal Glac iologica l Society
Annals
©
The Illarine record of the Russell Fiord outburst flood,
Alaska, V.S.A.
ELLEN
Departmellt
A.
COWAN ,
of Geology, AfJpalachian State University, Boone, NC 28608, U.S.A.
PA UL
R.
CARLSON ,
U.S. Geological Survey, N/ mlo Park, CA 94025, U.S.A .
Ross D. POWELL
Department oJ GeoLogy, Northem ILlinois University, DeKaLb, IL 60115, U.S.A .
ABSTRACT. Th e ad va nce of Hubbard Glaci er, nea r Yaku ta t, Alaska, .S.A. , in
sprin g 1986 bl ocked th e entra nce to Russell Fiord with an ice-and -sed iment dam ,
behind which a lake form ed. Th e water level in Ru ssell Lak e rose to 25.5 m a.s. l. Th e
d a m ca tastrophicall y failed in O ctob er 1986, releas ing 5.4 km 3 of water into
Dise ncha ntm ent Bay. Hi gh -resoluti on se ismi c-refl ec ti on profi les show a 7.5 km long
ch a nnel sys tem cut into a nd buried by g lacim a rin e sed iment, re presented by
co ntinu ous, pa rallel refl ec tions. The chao ti c seismi c facies fi lling th e channel is
interpreted to be d ebris flow d eposits. A gravity core from ch a nn el-overb a nk d epos its
co nta ined sa nd y diami cton with mud clasts. Above th e cha nn el a 1- 2 m thick sedim ent
dra pe ex tend s ac ross the bay . L a min a ted mud , fining -upwa rd sand bed s a nd
di a mi cton were recovered from thi s unit. Th e sedim ent-drap e deposits were produced
by suspen sio n se ttlin g fro m turbid p lum es a nd non-cha nn elized turbidity currents
genera ted b y th e outburst fl ood.
INTRODUCTION
Hubb ard Gl acier in so uth-centra l Al as ka , U.S.A ., has a
hi story of adva nces ac ross the na rrow mouth of Russell
Fi o rd , cl os in g off th a t inl et to form a glac ier-damm ed lake
(Ca rlson a nd oth ers, 1992; Fi g . 1). Th e most recent
closure occ urred in Ma y 1986 . For a 5 mo nth p eri od, a n
ice-a nd -sediment dam trapped fresh water in Ru ssell
Fiord , formin g Ru sse ll L a ke, th e level of which rose to
25.5 m a.s. 1. (Mayo, 1988 , 1989 ) . On 7 O cto ber 1986, la ke
wa ter began to seep through crevasses in th e 150 m wid e
dam , a nd on that d ay th e d a m ca tas trop hi cally failed ,
3
releasing a n es tim a ted 5 .4 km o f wate r during a 30 h
peri od into Di sen cha ntment Bay (Fi g. 1; M ayo, 1988 ) .
M ayo (1988 ) reported that th e outburst flood erod ed a
300 m wid e swa th of a lluv ial fan a nd j ointed bedrock from
th e shore nea r the glacier dam. A fl ood-wa ter velocity of
II m s I was measured in th e outburst cha nnel, 4 h after
p ea k discha rge (M ayo, 1989 ) . Th e momentum of thi s
disc harge ca rried turbulent wate r 6 km ac ross Di sencha ntment Bay wh ere it washed aga inst the terminu s of
Tu m er G lac ier (Fi g . I ) . Th e turbid pi um e from th e
Russell Fiord outburst reac hed th e mou th of Ya kutat
Bay, a distance of 50 km (Seitz a nd oth ers, 1986 ).
This stud y d oc um ents th e m a rin e record of this
ca tastrophi c fl ood in Dise ncha ntment Bay . U sing hi ghresolution, Huntec d eep-tow seismic-refl ec tion profi ling,
194
Yak utat
Bay
Fig. 1. Location map oJ Hub bard GLacier, RusseLL Fiord
and Disenchantment Bay.
Cowall olld ol/iers: R lIssell Fiord olllbllrsl flood
12 kH z ec ho -so und p ro filin g, a nd g ra \'ity co rin g \I'e
map ped th e fjo rd 0 00 1' durin g t\l'O cru ises o n th e RI\'
:lljJ/ia H e/il durin g 1993 a nd 199+ ((:o \l'a n a nd o th ers,
199+ ) .. \I thoug h th e fl ood d ep os it s han' bee n b uri ed sin ce
1986 b y g lac im a rin e sed im c nt , they are id en tifi ed in th e
su b-sur face by se ismic p ro fil ing and arc samp led by coring.
BATHYMETRY OF DISENCHANTMENT BAY
BEFORE AND AFTER THE OUTBURST FLOOD
A to ta l of 155 km o r ec ho-so und pro fil cs sp aced in a
0 .5 km g rid w as co llccred durin g 1993 a nd 1994 a nd uscd
to m a p th c ba th ymet r y o f Di se nc h a ntm cnt Bay (Fig . 2 ).
:--\ a\'iga ti o n was b y G PS upd ated a t 30 s inte n ·a ls. An
iso p ac h m a p o f sed im el1l acc umul at io n (Fig. 3 ) was
d raw n by co mpa rin g this 1993 9+ m a p lI'ith b a th y m e tr~ '
ch a rt ed b ~' th e U.S . N a ti o na l O cea ni c a nd A tm osp h eri c
A dmini strat ion du r ing 1978 . E rro rs may b e il1lrodu ced
fi'om ho ri zo nta l pos iti o nin g of trac k-lin es, tid a l \'ar ia ti o ns
a nd th e ec ho-so un de r in strum e nt. \\'e es tim a te th a t th e
c umul a ti \"(' erro r fo r eac h iso p ac h is ± I m .
On a ba th y m etri c m a p co mpil ed fr o m th e 1978
so undin gs, th e co n to urs d ee pe r th a n 230 m show a cha nn el
t ha t o ri g in a tes 0 11 the fj o rd 0 00 1' bel0 11' th e entra nce to
Russe ll Fi ord (sce m a p in Ca rbo n and o th ers ( 1992 )) . On
Fig . 3. iJ o/)ach mal) cif D isenc/wlllmenl Brl)' showing llie
difference ill de/lllis bfllealli Ihe waler slllface and lite bed
betu'eell the 1978 . \ D AA bat/z)'metric SUr/'f] ([nd this
stIl1)'·
139"32'
Fig . 2. B al/o'mell)' cif Disellr/wnlmenl BrO' III me/el's
durillg 1993 alld 1994 u'ilh l/ie ollllille oI Ihe bilriedcholllle/ S)'.ltell1 slIlNrillljJosed. Locatioll ~f Ihe gr([l'iO' core
alld seismic-reflectioll jJroji"fes. 1 alJo Shall'll. H ubbard
Glarier termilllls a ,l oI6 . lllgllst 199-1 iJ .\//O[l'II . S/)OI dejJlhs
( ill //let er,, ) l('ere llleasllred Oil l/ie .1(///11' dr~l' .
c:
the 1978 m a p. th e cha nn el is no t o bse rvcd a t \l'a ter de pth s
g rea ter th a n 250 m , towa rd th e so uth ern end o r Dise nchan tm ent Bay . Co n to urs on th e 1993- 94 m ap (Fig. 2 )
sho\l' no c\'id ence of a cha nnel in th a t loca ti on .
.-'\ m ax im um sh oalin g o f 35 m b etll'ee n 1978 a nd 1994
lI'as m eas ured nea r th e Hubba rd G lac ier terminus (fi g .
3 ) lI'he re sedim ent acc umul a ti o n is expected to b e hig h
beca use o rits prox im it y to th e g lac ier. During thi s p eri od ,
th e te r m inu s o[ H ubb a rd G lacier ex p eri e n ced bo th lo ng te rm a nd season a l a d I'Gl n ces (Kril11mel a nd Traba nt ,
1992 ). Fa ilure of th e d isia l sid e o f th e m o ra in a l b a nk
could res u lt [rom o\'e rstee p e nin g du e to ra pid sedim e ntati o n o r g lac ia l ach"a nce. S hoa lin g a lo ng th e termi n us
wo uld occ ur as sedim e nt was d e posited b y m ass m o \'cm e nt a nd sedim ent-g ra \'it y fl Ol\·s.
\\' ith di sta nce fro m Hubbard Gl ac ier terminus, g lac im a rin e sedim enta ti on no rm a ll y d ec rease ' ex ponenti a ll )"
a nd co nsistentl y (cL Cowa n a nd Powcl!. 199 1). Th erc/o re,
la rge, irreg ul a r a reas or hig h sedim ent acc umula ti o n a re
interpreted as a produ c t orth e outburst fl ood . F o r exa mpl e,
be tll"Cc n th e entra n ce to Russell Fi ord a nd H ae nke a nd
Turn er G lac ier 12- 18 m of sedim ent was d epos ited (Fig .
3). During th e o utburst Oood , surface disc ha rge Oowed as a
j et across th c Gord to \·;'a rd Turn er a nd H ae nk e Gl ac iers.
Ph otOgra phs o f th e outburst fl ood (as in Seitz a nd o th e rs,
1986 ) indi ca te th a t ed ch 'in g occ urred a t th e m a rgin s o f th e
j et as th e sed im ent-l a d en fl ood wa ter mi xed with th e lI'a ter
in Di se ncha ntm ent Bay. Th e thi ckes t sedim ent acc umul a tio n occ u rs und er th e ap pa rent ax is of th e je t, a nd d ec reases
late ra ll y to less th a n 8 m thi ck. Th e la rgc, irreg ul a r a reas o f
hi g h sedim ent acc umul at io n d OlI'nG o rd a rc a lso th o ug ht to
195
C:owall and others : RlIHell Fiord olltbllrst j700d
h a\"(' res ulted fro m th e ou tburst noocl. Th e thi c kest
sed im e nt acc umul a ti o n occ u rs in th e ce nte r o r th e fj o rd .
II'ith less th a n 8 m d eposited o n th e m a rg ins. E lo nga te a reas
with sedim e nt acc umu la ti o n 12- 14 m th ick (Fi g . 3 )
coin cid e w ith th e c h a nn e l o n th e 1978 ba th ym e tri c m a p .
Sedim e nt acc umul a ti o n vl'as t'l" id e l1ll y e no ug h to fill th e
ch a nn e l co mpl e rel\'. ex pl a ining \I'hy it d oes no t appea r o n
th e 1993- 9+ ba th ym e tri c m ap (Fig . 2) .
G lac im a rin e scdim e nt fill co nsists o r \"a ri a bl e-a m p litud e.
co ntinu o us pa ra ll e l re nec ti o ns o n th e hi g h-reso luti o n
seis m ic-re n ec ti o n pro fi les f"ro m Di sen c h a n t m e n t Bay .
Th e c h a nn e l sl'ste m c ut into a nd buried be n ea th th ese
glac im a rin e sedim e nts (Fi g . 4) is 0 .25- 0 . 75 km in width
a n d 3- 7 m in d ep th . Th e c h an n el is fill e d II'ith a
disco ntinu o us c h ao ti c seismi c rac ies th a t is inte rpre ted as
hi g h-I'iscosit y sed im e nt -g ral'it y fl ow d e pos its. T wo sm a ll
tribut a ry buri ed c h a nn e ls begin mid-fjo rd a nd jo in to
At th e h ead or th e buri ed- c h a nnel sys te m , th e eas te rn
tribut a rv c hann e l is d ee per a nd b e tt e r d efin ed th a n th e
II"('s te rn c ha nn e l (Fig. +) . A brupt trun ca ti o n o f" p a ra llel
re fl ec ti o ns b y a di sco ntinu o us c h ao ti c fill in th e eas te rn
c h a nnel indica tes er os io n o f g lac im il rin e scdim e nt b y
se dim e nt- g r a \'it y fl o ws. Th e c h a nn e l was pro b a bl y
e nl a rged b y th e 1986 o utburst fl ood prio r ro fi ll in g.
Th e buri ed c ha nnel ro ll o ll's a sinu o us co urse a nd
becom es d ee p e r a nd w id e r fa rth e r d Oll'nfjord. Th e b uri ed
6
3
c h a nn e l sys te m co nt a in s a n es tim a ted 9 .6 x 10 m o f
deb ris-fl o w d e p os its. C h a nn e l-fill a nd o \'e rb a nk d e posits
a re re prese nt ed o n h ig h-reso l u ti o n seism ic-re fl ec t io n
pro lil es as a c hao ti c seismi c fac ies simi la r ro th e se ismi c
sig na ture of" s lid es a nd nOIl's o bse J"\'ed within o th e r
g lac im a rin e se llin gs (Ca rl so n , 1989 ) . I n pla ces, th e
c hao ti c se ismi c fac ies fillin g th l' c h a nnel ex te nd s across
th e fj o rd as a we d ge . Th e loca ti o n o f th is fac ies b eyo nd th e
sid e of" th e ch a nnel in d ica tes th a r th e sedim e n t-g ra l'ir y
11 0ll's o l'e rt o pped th e c h a nnel a nd sprea d ac ross th e Qo rd
11 00 1' (Fi g. 5 ) . Th e 1- 2 m thi c k sedim e nt dra pe a bovc th e
c h a nn el fill is re prese nted o n seismi c pro fil es b y a n o p aq ue
fo rm o n e c ha nn el 1. 5 km d ow n fj o rd ( Fi g . 2). Th e c hann e l
e nd s so uth o f H ae nk e I sla nd, a ft e r a tt a inin g a n QI'e ra ll
len g th o f" 7.5 km.
Th e c ha nn e l sys te m vl'as e rod ed pri o r to 1978 , pro ba bl y
by turbidit y c urre nts o ri g in a tin g a lo ng th e 11l 0 ra in a l ba nk
in fr o nt of Hubba rd Gl ac ie r. Th e so urce o r th e turbidit y
II'l'akl y s tr a tifi e d se ismi c fac ies e xte ndin g across
Di se nc h a ntm e nt Bay. A seismi c p ro fil e a lo ng th e c h a nn e l
ax is sho ll's th a t th e c h a nn el e nd s a bruptly as th e c hao ti c
rac ies thin into p a ra lle l co ntinu o us r e ll ec ti o ns to th e sO Llth
(Fi g. 6 ) . Th e 5 m [hi c k c h a nn e l fill g r adu a ll >' thin s ro a
lm thi c k di sco ntinu o us d e p os it th a t was id e ntifi ed o n a n
c urre nts m ay h a \'e bee n pe ri odi c slumping a nd fa ilure o r
the Hubba rd G lac ie r m o ra in a l ba nk as it was built up a nd
ol'e rstee pe ned. Thi s 11l 0 ra in a l ba nk was el'C ntu a ll y pu shed
a nd sq ueezed bl' rh e ach 'a nc in g g lac ie r in 1986 so th a r its
top was a bol"C sea lel'C l aga inst G il be rt P o int w he re it
fo rm ed p a rt of th e Ru ssc ll L a ke d a m (\l a)"o, 1989;
a dditi o n a l seismi c p ro fi le co ll ec ted p e rp e ndi c ular to th e
Qo rd ax is 0 .5 km beyo nd th e e nd o f th e c h a nn e l.
Pos t-l1 ood g lac imarin e d e pos its ab o l "(' th e c ha nnel tilin
d OIl' nO o rd away ri'o m th e g lac ie r so urces, as ex p ected . I n
pro fil e A- A', 7 m of sedim e t1t h as acc umul a ted a b ol'C th e
c h a nn e l-fill d e p os its in th e n o rthw es t n ea r Turn e r
Tra b a nt a nd o th e rs, 199 1) . During th e o utburst Oood th e
c ha nn e l sys te m was co mpl e te lv fill ed a nd buri ed b y d ebri s.
S in ce 1986, the c ha nn els hal'e no t beco m e re-es ta bli shed .
G lac ie r , a nd 5 m has acc umul a ted in th e so uth eas t (Fi g .
4) . I n pro fil es B- B' a nd C - C ', '~m o f" p os t-Il oocl
acc ul1lul a ti o n cOI'e rs th e c h a nn e l fill . }\ ce umul a ti o n ra tes
THE BURIED-CHANNEL SYSTEM
to
A'
A
SOUTHEAST
NORTHWEST
225
~-:::::~~~~_________P~O~S:t~-1~9~8~6~g~l~ac:i:ma~r:in:e~s:e:d:im::e:n~t____~~~~~~~~__
-1
240
0.30
0.32
~
!l!Q)
E
0.34 F
~
0.. 255
channel fill
Q)
o
2 225
0.30
240
0.32
~
fU
~
6
~
0.34
o
0.5
1.0
1.5
2.0
km
Fig. 4. High-resolutioll seismic-rejlectioll Profile A 0"1 ' ShOll'illg cross-sectioll
Figure 2 Io r location.
196
1
F
DJ maill
channel alld
Iriblll(/I~V
c/zallllel. See
C01call alld otliers : Rl/,l.Ifll Fiord olltbunt J700d
8'
8
WEST
225
0.30
EAST
post-1986 glacimarine sediment
~
flood-sediment drape
I
I
1
-'\
240
7
\...l
i
overbank
chan~el fill
overbank deposits
-0.32
~
deposits
g
<ll
E
..c
0.34 ~
li255·
<ll
<ll
o
>
(Il
'-
0.30
2225
t=
~
;::
~
6
~
240
0.32
255
0.34
o
0.5
1.5
1.0
km
Fig . 5. Profile B- B' SIIOlI ,illg crO.I.I-.\{'rtiollal l,iell' oj {Ita lllle/ ({lid O1'crb({lIk de/iO,li/.l. T Ite rore described ill Ihi,l ,Ill/Ill' ll'aS
col/eelI'd 1I{'({r lhi.\ /JI'ofi/e ( see ((/Tow ) . Se!' Figure 2jor lowlioll .
sufli cicnth- lo ng to pene tra te in to rh e chao ti c se ism ic
fac ies represe ntin g th e fl ood depos its IF ig. 2). Til e +.6+ m
long co rc \\'as coll ectcd near the eastern ed ge of th e
c ha nn cl thro ugh Il oocl -clra pc a nd ch a n nc l o\T rba nk
d e pos it s Fi g. 5 . Th e 10\\'er 1.7 111 o f' thi s co r e is
int erpreted a s d epos its ri-o m til l' 1986 o utburst fl ood .
Th e lo\\'('s t -~+ cm of th e co re is co mposed of di a mi cto n
F ig. 7) \\ hi ch grad es fi'o m a mass iq' di a m icto n [+32-
of 0.9 0.5 m yea r I a re \\'ithin th e ra nge ex pec ted [o r
glac im a rin e sedim entati on sin ce th e 1986 outburst fl ood.
LITHOLOGY OF FLOOD DEPOSITS
-'\ lIm ero ll s at tempt s a t gn\ \'ity co ring in Dise ncha ntm ent
Ba\' res ulted in th e coll ec tio n of onl y onc co re th a t \\'as
C'
post-1986 glacimarine sediment
C
SOUTH
225
flood -sediment drape
...,I
1
NORTH
0.30
~
240
chan~el fill
.1
run!out
deposits
end of
channel
0.32
~
g
<ll
E
..c
o.34F
li255
Qi
>
ro
Q)
o
0.30
2225
t=
~
;::
~
6
~
i~~~;j:ti:~
240
0.32
0.34
255
o
0.5
1.0
1.5
km
Fig . 6. /) i.l'la/ fIId of cll(lIlIlel S!tOll'lI ill Pro/ill'
g/acilllarilll' sedilllelll .
c:
C' ll'lt ere Ihe chaotic r!tallllel/ill illlelfillger.1 with sei!Jllliw/f)' .l/rattjied
197
Cowan and oth ers: RllsselL Fiord olltburst flood
464cm ) to stra tifi ed sa nd y di a mi cto n (420- 432 cm ). Th e
massive di a micron co nta ins mud c1 as ts up to 10 cm in
di a meter, pebbles, a nd coarse sa nd lenses . Stra tifi ca tion is
p rodu ced b y con centra ted laye rs of pebbl es where lo ng
axes a re a li gned p a ra ll el with th e bed , a ltern a tin g with
laye rs of sa nd y mud. Th e di a mi cton coin cid es with th e
chao ti c se ismic facies loca ted a bove th e channel fill. W e
beli eve th a t th e di a mi cto n was ra p idl y d eposited by
d e bri s (lows o ri gin a tin g fr om th e o utburst m ov in g
throug h th e cha nnel. Th e pe bhl es a nd mud clas ts we re
supported within the fl ow by th e fluid ma trix form ed with
clay (Fi g . 7) . Th e a ltern a tin g pe b ble-ri ch beds a nd sand y
mud in upper stra tifi ed di a mi cto n may hcl\'e formed by
shea rin g within the d ebris (l ow (H a mpton, 1975 ) o r from
sm a ll epi sodi c now eve nts as th e fa ilures d ecreased in size
a nd n um beL
Th e 1. 34m of fl ood d e posit a bove th e di a micton s is a
fin ing-upwa rd sequ ence of la min a ted mud interstra tifi ed
with sa nd a nd di a mi c to n bed s. It is represe nted on
se ismi c- re(l ec tio n pro fil es as a n o p a qu e to wea kl y
stra tifi ed seismic facies fo rmin g a dra pe a bove ch a nn elfi ll a nd ove rba nk d ep os its. Lmm edi a tcl y a bov e th e
di a mi clO ns is a 4cm thi ck la min a ted bed with sh a rp
co ntac ts oTadin g fr o m m edium sand to \'e ry fin e sa nd. As
th e sa nd fin es upwa rd , sil t co ntent in creases (Fi g . 7) .
Th ere a re severa l additio na l g ra d ed sa nd bed s up-co re.
W e in terpret th ese as n on-cha nn eli zed turb idit y-current
d e posi ts beca use of th e prese n ce of sh arp b asal co n tac ts
a nd medium sand- pa rti cle size. Th e thinl y la min a ted
mud is interpre ted as a suspe nsi o n d eposit th a t se ttl ed
from th e turbid plum e fo rm ed as Russell L a ke dra in ed.
Th ese cycl o psa m s a nd cycl ope ls co nsist o f pl a n a r
la min ae fo rm ed o f coa rse silt a nd fin e sand a ltern a tin g
with mud. Th ey a ppea r simil a r to th ose co llec ted in
sedim ent tra ps fr om o th er hi g h-d isch a rge events in
simil a r tempera te g lacim a rin e se ttin gs (C ow a n a nd
oth ers, 1988 ) . M a ny la min ae a re d eform ed b y dropston es ra ft ed from ice bergs . Thin di a mi ctons within thi s
la min a ted sequ ence were produ ced fr om pe bbles a nd
sa nd d e pos ited a fter ra ftin g by ice bergs crea ted w hen the
ice d a m coll a psed . Di a mi cto n lenses we re pro ba bl y
p rodu ced w hen icebergs ro ll ed a nd dum ped th eil- d e bri s
to th e Qo rd floo r.
260
M o re th a n 150m o f sedim e n t has acc umul a ted in
Di se nch a ntm ent Bay sin ce d eglac ia ti o n in th e la te 19th
century. Th e record of th e 1986 outbu rs t fl ood , th e larges t
kn own thi s cen tury, is prese rved within th e upper 15 m of
thi s sedim ent fill. R a pid acc umula ti on of pos t-nood
glac im a rin e sediment has ensured prese rva ti on of th e
fl ood d eposits, a t leas t until th e nex t adva nce of Hub ba rd
G lacier a long Disencha ntm ent Bay .
A sha ll ow ch a nn el sys tem was erod ed into the fj ord
0 00 1' pri or to th e 1986 outburst Oood . Gi ve n th e seism ic
ch a rac ter o f th e fj ord fi ll o[ co n tin uous pa rall el re!1 ecrions,
th ese d eposits a re int erpreted as ice-prox im a l glacim a rin e.
Sedim ents a re un cha nn eli zed turbidites, bergsto ne mud ,
cycl opsams a nd cycl opels (Cowa n a nd oth ers, 1994 ) .
Som etim e pri or to the ba th ymet ry und erta ken in 1978 ,
th a t sedim en ta ry regim e cha nged loca ll v so th at erosion
198
50
100
I
I
·
D = diamicton
F = fine sediment
"
S = sand
I = laminated
·
·
= massive
= graded
= stratified
• = dropstone
m
g
s
Cl
gravel
c:J
sand
=
_
silt
clay
-
~
E340
~
~350
0
()
·
.~360
J::
0..
Q)370
0
·
380
·
390
-
400
410
!!!
420
430
"-"
-
440
DISCUSSION
Particle-size
Distribution (%)
Uthologic
Log
450
coarse
sand
0./ 100uest
19-1 rm cif core with flood dejJosits. Litho{og)' is based 011
visual (ore descriptioll alld I-radiographs.
Fig. 7. L ithology alld /Jartide- si:::.e distributioll
in stead of d eposition occ urred o n th e Qord fl oo r, [o rmin g
th e cha nn el. Cha nn els a re comm on on th e fl oors of'
ma ri ne-o u twas h Qord s \V i th cl el tas a t th e fj ord head . hu t
a re un co mmon in glac ia l Qo rds (Ca rlso n a nd oth ers,
1989 ) . A hi g her fores lopc a ng le is p ro ba bl y impo rta nt in
genera ting surge-type turbidi ty currents with suffi cient
COWGn alld olhers : RlIssell Fiord olllbursl flood
energy to erode th e sea Ooor to produ ce chann els (Ca rlso n
an d others, 1989 ). \\l e beli e\'e th at th e erosiona l regime
may ha \'e bee n in iti a ted as the mora in al ban k was
oversteepened in front of Hu bbard G lacier during the
ad\·a nce. \ Ve canno t id ent ify a loca l so urce a rea of
turbidity-curreIH genera ti on a long th e Hubba rd G lac ier
mo rain a l bank. H oweve r, ma rin e-outwas h d eltas ha\'e
simil a r situ a tions where num ero us feed er gulli es ap pear to
become loca li zed into a sm a ll numb er of ch a nn els
(Carlson a nd others, 1989 ).
Th c 1986 flood even ( was ene rge ti c enou g h to
ex te nd the zo n e of deb ris-fl ow d e positi o n into th e a rea
o f the ch a nn els, w h ereas previo us debris fl ows likel y
transform ed to er os ive turbidity currents b y th e tim e
the fl ows reached th a t a rea. \II/e ca n id entify severa l
sou rc es of sedim ent that mi g ht h ave initi ated d eb ri s
fl ows . F o r exa mpl e, th e sub ae ri al m o ra in e that form ed
th e d a m blocki ng the en trance to Russe ll Fi o rd was
removed by th e ou tburst fl ood (Tra bant a nd others,
199 1) . An a ll uvi a l fa n that ex tended 200- 300 m from
G ilb ert P oint into Ru sse ll L a ke was co mp le tely erod ed
duri ng th e first few hours o f the fl ood (M ayo, 1989 ) .
These featu res m ay h ave been rem oved by b loc k failure
a nd slidin g wh ich tra nsform ed in to la rge d e bris fl ows in
th e c ha nnel.
Turbidites and suspensio n deposits a ppea r as a n
opaqu e to weakl y stra tifi ed se ismi c fac ies ex tendin g across
th e fl oor of Dise nchantment Bay above th e cha nnel fill.
Th e suspe nsion d eposits a bove th e cha nn el fill a re thi nl y
lam in a ted sands a nd mud s th a t res ult from flu ctu a tion s in
sed im ent supply or in velocity of th e interOows or overfl ows
tra nsportin g th e sediment. Evid ence of ice berg-raft ing in
th e form of both isola ted drops ton es a nd iceberg-rafted
di a mi cton is ab u ndant withi n the lam in a ted mud s
deposited fro m suspe nsion. Debris-l aden icebergs we re
a bund ant when Hubbard G lacier retrea ted 500 m by
calving a t th e entrance to Russell Fi ord , as th e openin g
widened and d eepened during the fl ood (M ayo, 1989 ).
ACKNOWLEDGEMENTS
Thi s resea rch was fu nd ed by U.S. Nat iona l Science
Foundation gra nts OPP-9223990 to Cowan a nd OPP-
9223992 to Powell. Add i ti ona l su pporr was provid ed by
th e U.S. Geo logica l Sun'ey Marin e Geology Bra nch. W e
wish to thank th e Capta ins a nd crew of the R /V AI/J/w
H elLI for their ded ica ti on to co ll ection of the se ism ic
profi les and cores. G. B. Standen , Geoforce Consultants
pro\'id ed expert ise in co ll ection of hi g h-qu a li ty H untec
profi les. Th e 1993 and 1994 bathymetr ic d a ta fo r
Di se ncha ntm ent Bav were a na lyzed by C. A. Na il. D.
L awson , E . R eimnitz a nd D. Trabant made helpfu l
comll1 en ts on the man uscri pt.
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Carlson. P. R .. R . D. Po\\'cll and D. r-1. R earic. 1989. Turbidit y current
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lO
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or
199