Dean A. McManus
Department of Oceanography
University of \Tashington
Seattle,\Washington
Glassificationsof SubmarinePhysiography
in lhe Gulf of Alaska'
Soundingsin the Gulf of Alaska have been made systematicallysince the middle 1920's.
On the basis of these soundings certain areas of similar topographic relief have been
defined as physiographicprovinces and placed into a physiographicclassification. As
the data have increasedand as new sounding techniques have been developed,the
province boundaries have been altered aod even the classification system has been
changed. Although these modifications io apptoach to researchon submarine physiography are apparent in studies of each ocear, an excellent illustration is provided by
the Gulf of Alaska becauseof its small size aod diversified physiography.
NonprecisionEcho Sounding
Murray (1!41) presenteda summary of rhe topographic data available for the Gulf
of Alaska through 1939, although he did not recognizephysiographicprovinces. The
sounding lines were .locatedby asronomic fixes and dead reckoning,and the soundings
wete recorded on the averageat intervals of 1 nautical mile (Murray, 1941). The
topographic chan of the dara showed a conrineotal shelf and slope and the Aleutian
french. The floor of the Gulf of Alaska, however, was depicted as a rather smooth
tegion with severallarge seamoLlnts.
The major part o{ the floor, haviog relief too small
to be measuredby the existing equipment, was not consideredso important as the
more obvious and easily measuredseamoLrnc.Consequently,topographic descriptioo
emphasizedlhe seamountsof several hundred fathoms relief.
Eardley ( 1951) used the data reportedby Muray (I94I) as tbe basisfor identifying
a Region of Great Seamountsin the Northeast Pacific, as outlined in Figure 1. By
transferring the boundariesof Eardley'sregion to the most recent geneml topographic
base chart of the Gulf of Alaska (Hudey, 1960), the inclusion in the region of most
of the seamountsand ridges known today is revealed.No attempt was made by Eardley
to determine rhe physiographic significance of the region, becausetopographic data
were scant and other types of data were practically nonexistent.
'Contribution No.396, Department of Oceanography,University of \trashington, Seattle,\trash'
i18
Northwest Science,Vol. 41, No. 3, 1967
Figlte 1. Physiographic regions in the Gulf of Alaska according to Eardley (1951).
the seamounts discovered since 1951 are in the Reeion o{ Great Seamounrs.
Most of
The {irst associationof typesof bottom relief into physiographicprovinceswas made
by Menard and Dietz ( 1951 ) in their descriptionof a Gulf of Alaska SeamounrProvince
and a Ridge and Trougl.r Province ( Figrire 2). This rvork was based on some of the
eadiest coltinuously recorded echo-soundingprofiles in rhe region. The previous,
discrctc-sor.rndings
ciata were thereby supplementedby continuous dara in rhe form of
graphic profiles. The small size of the recording paper and the nature of the echosoundiog system prevented resolution of fearureshaving relief of tens of fathoms, so
rhat emphasisremained on thc positive relief fearuresof l.rundredsof fathoms reliefthc seamountsand ridges.
The basic characterisricupon whicl-rthe provinces were identified was the shapeof
the rnosr comrroo positive relief feature in each province. In the Gulf of Alaska Seamount Provincc these tearureswere large, isoiared,conical seamounc having summits
a few hundred fathoms beneathlhe water surface. In the Ridge and Trough Province,
on the other hand, the most common posirive relief fearureswere clusters of quasiparallel ridges supporting a few peiks that extendedalmost to the seasurface. A more
gcnctic disrinction was made by assigning a volcanic origin to the features in the
seamount ptovince and a fold or fault mountain origio to those in tl-re Ridge and
Trough Province. The iatter province rvas considereda submarinecontimration of the
Pliocele-Pleistoceneorogenic zone in California and southern Alaska.
A later and n-rorecomplete description by Menard (1955a) of the characteristics
of lbese two provinces based the distinction between them on the greater age of the
SribmarinePhysiographyin Gulf of Alaska
1f9
provinc€s
identifiedby MenardandDiet, (1951) andMenard(i955a).
Figure2. Physiographic
The provincesate sepatated
on the criteriaof type, age,and origin of the largepositiverelief
features.
topoglaphy in the Gulf of Alaska SeamountProvince,where a thick apron of sediment
extendsour from the baseof the continental slop€,uuncated seamountshave sunk, and
recenr seismic and volcanic activity is unrecorded. In contrast to this, the Ridge and
Trough Province was considereda more youthful topography, as evinced by seismic
aod volcanic activity, the proposed block-fault origin of the ridges, and the numerous
searnountsrising to within oniy a few farhoms of the sea surface. A thick apron of
sediment,extendingoutward fiom the baseof the contioentalslope off the northwestern
United States,however, indicates the elapseof sufficient time fo! the formarion of a
thick sequenccof terrigenoussedimenr. This smooth aPronlike paft of the Ridge and
Trough Provincewas delirnitedby Menard ( 1955b) as a seParatearea,the Great Trough.
The developmentof the Great Trough was thought to have been causedby the damming
effect of the rough topography in the Ridge and Trough Province to the west, which
prevented the terrigenolrsmaterial from continuing as turbidity currentsinto the deeper
part of the North Pacific.
Gibson (1960) also recognized rhe Ridge and Trough Province and the Great
Trough, but he subdivided the Gulf of Alaska SeamountProvince into the compolent
rangcs of submarine mouotains (Figrre 3). The topographic data used by Gibson
(1960) representsoundingstaken by U.S. Coast and Geodetic Survey ships during the
petroA 1925-7957. His more recent data were cootinuous echo-soundingrecords,and
rhe horizonral positioning of the ship by this time was based on the more accurate
systemof LORAN. Still, the emphasiswas on the large positive relief features.
120
Dean A. McManus
Figure l. ,.Physiographic
divirjonspropoge!by Gibsoo(1960). E_mphasis
is placedon rhe 1ar.se,
posrtrveteliel leatures,
whi.h aregfoupedinto nofthwest-trending
fracturezones,
The significance of the provinces identified by Gibson q/as thought to be rheir
expressionas,or relation to, fracture zonesin the seafloor. The large east-westfracrure
zonesin the easternPacific, such as the Mendocino Fracture Z,one,had been described
by Menard (1955a). Gibson considered norhwest-southeas.fracrure zones in rhe
Nocheast Pacific to be representedby the submarine mountain systemswhich he
assumedwere formed of volcanic material from fissuresor isolated vents. The northnortheastedytrending featureswithin the Ridge and Trough Province and Mid-Gulf
Zone were consideredto be significantly different from rhe northwestedy trending
features forming the three seamountareasand to lepresent a different stresspattern
in the earth's cmst. Gibson also discussedthe Aleutian Treoch as a geosynclineand
rhe outer ridge, which he referred o as rhe Aleurian Trench fuse, as an anticline.
No emphasis, however, was placed on the areas of smoorh floor or minor relief.
PrecisionEcho Sounding
In the early 1950's the inadequacyand errors of the standard,small-size,conrinuorlsrecording echo sounder were overcomeby the developmentof an expanded-scale,
precision recorder (Luskin el al,, 1914). The precision recorderspermitted far grearer
resolution of the bottom topography. As a resulg l tge areas of ocean bottom covered
with abyssalhills of a few tens of fathoms relief were surveyed.Also recolded on .his
high-resolution instrument, however, were smooth, flat floors, which were delimired
as areascalled abyssalplains and explained as representingaccumulationsof rurbidity
curfent deposits (Heeze|LeNal., L954).
Submarine Physiography in Gulf of Alaska
121
F i g u r e 4 . P h y " i o g r a p h i c p r o v i n c e s i d e n i i l i e d b y H u r l c ) (1960) on the basis of the extert of
emphasis on the plains of
Iow-relief Dlains and hills. lmoroved instrumentarioD permifted
presumed iurbidity current origin.
The new instrumen$ and the empltasis on featutes of low relief were aPPlied to
the Gulf of Alaska by Hurley ( 1960). The suessingof the imPoltanceof abyssalplains
in a classificationof submarine physiographic provinces and the sffessingof the significance of rurbidity currents to the development of abyssal plains relegated the seamounts ro secaodarystatus as a basis for delimiting provinces. Consequently, the outlines
of the provinces identified by Hudey (1960) are very different from those drawn by
previous workers (Figure 4). The abyssalhills are consideredto rePlesentthe original
topography not coveredby turbidity current depositscrmting the abyssalplains.
Although Hudey describedfeatures such as the continental rise (the intermediate
gradient between the steep continental slope aod the gentle abyssalplain) and the outer
ridge of the Aleutian Trench (the Aleutian Trench Rise of Gibson, 1960), he did not
distinguish them as physiographictypes differing from the abyssalplains. Alsq although
rhe Ridge and Trough Province was recognizedby Hurley (1960), he assignedto it
no more majo! a role in the classification thao that of aoy other area of abyssalhills.
Since the time of Hurley's work, the main changesin the classificadonhave dealt
with delimiting the continental rise as the wedge of sediment at the foot of the contiqental slope and with the recognition of the Ridge and Trough Province as part of
the worldwide oceanicrise sysrem.The physiographicprovincesdelimited by the writer
are outlined in Figure 5.
The delimitation of the continenral rise has reduced the size of Alaskan Abyssal
Plain and eliminated CascadiaAbyssal Plain. The area forme y consideredas Cascadia
122
Dean A. McManus
Iigure 5. Physiographicproviocesin the Gulf of Alaska as delimited in this paper. The conprovinces.The oceanicrises,fracturezones,
plairs aremair y depositional
tinentalriseandabyssal
origin. Volcanicridgeslnd seamounts
andtrench---outer
ridgeprovinces
areminly of diasttophic
hillsare abundantamongthe abyssal
AbyssalPlain actually tepresentsin its easternpart the coalesceddeep-seafans forming
rhe conrinentalrise, as noted by McManus (1964), ar'd in its western Part it represeots
rhe flank of Cobb Rise.parr of the oceanicrise system.
The Ridge and Trough Province,included by Menard ( 1960) as palt of the oceanic
risc systemand labeled as part of the Xast Pacific Rise, has topography similar to that
on rhe Mid-Atlantic Ridge, including a median valley, as shown by subsequentstudies
of rhe area (McManus, 1!65; Talwani et al., l)65). The rise is separatedinto two
risesby the Blanco FracmreZone ( McManus, 1965) , an arca of large northwest-trending
ridges and deep troughs. Another northwest-uendingfracture zone separaresCobb Rise
from rhe continent.
The presenr classificationof physiographyin the Gulf of Alaska consistsof three
groups of provioces: (1) the enigmatic abyssalhills provinces, (2) the provinces
delimiting arcasof dominant terrigenoussedimentatioo,and (l) the provincesdelimiting areasof dominant diastrophism. The areasof dominant telrigenous sedimentation
include the continental rise, and Alaska, Tufts, and Aleutian Plains. This desigoatioo,
however, d<xs nor imply that preseot sedimentationin these areas is primarily terrigenous, since the surface sediment is principally pelagic as shown by Nayudu and
Enbysk (1964). The importance of the terligetous material is revealed in the cores
from the areas,aod it is to the terligenous sedimentationthat the smoorh bottom is
attributed.
Submarine Physiogaphy in Gulf of Alaska
12t
The provinces in which the physiographyis npparently dominated by diastrophism
are Gorda Rise, Cobb Rise, Mendocino Fracrrre Zone, Blanco Fracture Zone, the
unoamed fracmre zone ne.lr Vancouver Island, Aleutian Trench, and the Outer Ridge.
The signi{icanceof diastrophism in the development of the ridges, troughs, and hills
of the oceanic rises and fracnrre zones is apparent, for the topography is formed on
volcanic or sedimentaryrocks having only a veneer of pelagic sediment. Thicker sections of peiagic sedimenroccur only in the intermontaoebasins. Terrigenoussedimeot
forms the topography locelly in small depressionswhere sediment has entered rhrough
a gap in the hills and ridges and has been ponded. Smoorhingby terrigenoussediment
also has produced a fill deposit in the botom of the Aleutian Trench, and later studies
may rvell identify a trench plain as a separateprovince. This combination of diasrophic
and sedimentationcontrolshas also played a major role in the developmentof the Outer
Ridge, a feaanreconsideredto have a diastrophic origin, but which displaysa smooth,
undulating topography similar to that of the contioental rise.
Summary
Eady stndies of physiographf io the Gulf of Alaska used echo soundersrhat were
adequatetci idendfy only the large positive and negative relief fearuressuch as ridges,
seamountsJ
and trenches.As a result of the limitations of instrumentation,emphasiswas
placed oo the seamounts,ridges,and the trench as determioing the physiographicclassification. The classification,rherefore,was based solely on featuresof volcanic or diastrophic origin. N7ith the later developmentof precision echo-soundingrecordersand
more precise horizontal positioning came an emphasisoo the low relief features such
as abyssalhiils and abyssalpiains. Recognition of abyssalplains also produced emphasis
on rurbidity curcnt deposition of relrigenous sediment as a factor in developing the
submarinesurface.
In the parts of the Gulf of Alaska where the sea floor is relatively smooth, the
physiographyis dependentmainly upon rhe processesdepositing terrigenoussediment;
the resulting provinces are the continental rise and abyssalplains. \fhere the sea floor
is formed of rough topography,however,the physiographyreflects either the surfaceof
the poorly known abyssalhills or the diastrophicand volcanicef{ectsof the development
of oceanicrises,fracture zones,and the trench outer ridge system. The preselr physiographic classificationof featuresin the Gulf of Alaska placesequal significanceon surfaces dcveloped by sedimentation,volcanism, and diastrophism,rather rhan solely on
sedimentationor on volcanism and diastrophism.
Acknowledgments
This researchhas been supporredby the Office of Naval ResearchContractNonr477(37),
ProjectNR 083 012.
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Dean A. McManus
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September 1p, 1)66.
Submarine Physiography in Gulf of Alaska
125