The shape of submarine canyon heads revealed by Asdic

Deep-Sea Research, 1969, Vol 16, pp 103 to 104

P e r g a m o n Press

Printed m Great BrUam

The shape of submarine canyon heads revealed by Asdic R H BELDERSON* and A H STmOE*

(Received 1 August 1968) Abstract--An integrated picture of canyon heads, provided by side-scan Asdlc, allows direct location of assocmted gulleys and slumps It also emphamses the contrast between active and partmlly buried canyon heads on the continental slope between Ireland and West Africa CANYON

HEADS

IT IS difficult to obtain an integrated picture of submarine canyons by normal echo-sounder procedures because of the inevitable dmtance between profiles, navtgahonal ddticulties and confusmg side echoes, although a narrow, multiple beam side-scan system directed up-slope has some advantages (TtJCKER 1961) However, the mare problems can be largely overcome if the sound Is directed obhquely down-slope In this case (Fig la) one slde-lobe provides a vertical profile of the sea floor, 1, beneath the ship at 0, while the next side-lobe and. the edge of the main beam provldeobhque sections at 2 and 3, on wtuch the main elements of relief can be followed down slope from beneath the ship For the ground " i l l u m i n a t e d " at a low angle by the main beam, between 3 and 4, large and smaller scale features of relief are indicated m almost plan view, the acoustic records resembling oblique aerial photographs (the width of ground " i l l u m i n a t e d " will vary with the gradient) Traverses with ttus system along the upper part of the continental slope between Ireland and West Africa have provided new information about the shape of canyon heads, including active and burred ones The shape of an active canyon head resembles bad-land topography (Figs la and lb) The continuity of canyon axes is shown by the vertical section 1 and oblique sections 2 and 3 m Fig la, while m the oblique view between 3 and 4 it can be seen that small lateral gulleys converge on the axes at a small angle In addition, there appear to be slumps of two sizes Those occttrrlng m the relatively und~ssected inter-canyon areas are probably small wave-hke slump trains, whose crests extend along the continental slope At right angles to these are small slumps of secondary ongm which are present m the sides of the canyons and are well shown m vertical section The large area of rough ground, lying below the large, scar-hke cliff, is interpreted as a slump extending for at least 0 5 km down slope and about 1 km along the slope The evolution of the relief Is probably complex, in that the large slump probably moved sideways into a pre-existing canyon and ~s now being dissected into younger gulleys The second type of canyon head is remarkable for its smoothness (Fig 2), as shown m the vertical and oblique profiles 1 and 3 and by the absence of small scale relief between 3 and 4 From this it is inferred that older erosional canyon heads have been partially buried by later deposmon although the burial may only be temporary, as nearby canyon heads do not appear to be affected

Acknowledgement--Tbe authors wish to thank their colleagues for their able assistance at sea and Mrs C DARTER for drawing the block diagram

REFERENCE

TUCKER M J" (1961) Beam identification m multiple beam echo-sounders lnt hydrogr Rev, 38, 25-32 *National Institute of Oceanography, Wormley, Surrey, England

103

Fig lb

A block d i a g r a m gJx m g an mapression o f the relief s h o w n by Fig la Gulleys are p r e s e n t In the walls o f the c a n y o n s , small s l u m p - h k e features are visible o n flatter inter-canyon g r o u n d a n d a larger s l u m p lies below a cliff

Fig l a A n Asdlc record (area a b o u t 7 4 k m 1 k m ) o b t a i n e d n e a r to the top o f the continental slope at t h e western edge o f the Celtic Sea (mid-point o f record at 48°46 ' N, 1 0 q 6 ' W), m which, contrary to usual presentation, g r o u n d w t u c h Is acoustically " i l l u m i n a t e d " a p p e a r s white a n d a c o u s t i c " s h a d o w s "[are black R o u g h c a n y o n h e a d s o n the left contrast m a r k e d l y with the a l m o s t u n d l s s e c t e d g r o u n d in t h e right h a n d part o f the figure This c o n t r a s t is s h o w n by the vertical profile 1 (referring to the ship at 0), together with o b h q u e sections at 2 a n d 3 a n d by w h a t a p p e a r to be gulleys in t h e c a n y o n walls a n d s l u m p g r o u p s visible o n the flatter g r o u n d between 3 a n d 4 All lines or g r o u p s o f lines m a r k e d with a n x result f r o m t h e s u p e r - p o s i t i o n o f n o i s e f r o m o t h e r acoustic e q u i p m e n t C, is a c a n y o n axis

~r

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factng p

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71g lb (left)

Fig la above)

F

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/ p

D

Fig 2 An Asdlc record (area about 5 8 km × 1 kin) obtamed near to the top of the continental slope west of Portugal (m~d-pomt of record at 39 34'N, 09 37' W) In this figure acoustic "' shadows" appear white The smooth walls and floor of a canyon head is shown by a profile of the sea floor at 1 (referring to the ship at 0), together with an obhque section at 3 and by the lack of roughness w~thm the ground dlumlnated by the mare beam between 3 and 4 F, indicates echoes Iblack) from fish shoals, W, is a wreck (black) with a '" s h a d o w " (white) behind it, and D, indicates the characteristic appearance of no~se made by dolphins The obhque section, 2, as dl shown on th~s rezord and is not labelled