Black mud canyon

Deep-SeaResearch,1962,Vol. 9, pp. 457 to 464. PergamonPressLtd. Printedin GreatBritain

Black Mud Canyon T. J. G. FRANCIS Department of Geodesy and Geophysics, Cambridge University

(Received 5 August 1962) Abstract--A prominent canyon in the continental slope 150 miles west of Brittany has been followed from its origin at the edge of the shelf to a depth of 2000 fro. The bathymetric chart covers an area of approximately 5 by 30 nautical miles and shows contours at 100 fm intervals. Profiles across the canyon are shown with vertical exaggeration of 4 : 1, and a profile along its axis with an exaggeration of 10 : 1. A discussion on the canyon's origin takes into account recent seismic and geological work in the area. INTRODUCTION THE CONTINENTALmargin to the west of Brittany is broken up by a complex pattern of submarine canyons. The ruggedness of the continental slope in this area is apparent from the bathymetric chart of the NE. Atlantic published by HILL (1956). DAY (1954) has studied the continental margin between Brittany and Ireland in greater detail, in particular, an area 200 miles long and 80 miles wide west of Brittany and reveals a dendritic pattern of submarine canyons between the continental shelf and the Biscay abyssal plain. In similar detail to Day's work the continental edge from the west of Ireland to Cape Finisterre has been contoured by BERIHOIS and BRENOT in France, their results being presented in a series of twelve charts published in 1960 on a scale of approximately 1 in 290,000. To learn more about their formation, a detailed knowledge of individual canyons is required. The prominent canyon immediately to the east of Meriadzek Terrace (named by DAY) was therefore surveyed by R.R.S. Discovery I I during late September and early October 1960. It appears on Chart No. 6 of the French series mentioned above.

SURVEY An area approximately 5 miles wide and 30 miles long has been traversed by eastwest tracks typically half a nautical mile apart (FIG. 1). The survey occupied the ship for five consecutive nights, about 50 hours total time, representing at an average speed of 8½ knots a total distance steamed of 425 miles. For a night's survey, the ship's track was determined by fixing on a moored danbuoy by radar. To extend the survey the following night, another buoy was laid and fixed relative to the first before that was lifted. The process was repeated on subsequent nights. The buoys carried radar reflectors enabling them to be detected at up to about 4 miles range. Thus the greater part of practically all tracks was within radar range of a buoy, and with the exception of the northernmost, all buoys were fixed simultaneously relative to each other, by radar. For the northernmost buoy a run of about four 457

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miles was required between losing the echo of the adjacent buoy and the appearance of its own echo on the radar screen. The positions of the buoys on the Earth's surface were determined by astronomical and Decca fixes. Depth measurement was made with an N.I.O. precision echo-sounder, calibrated for a standard velocity of 800 fm/sec and providing an 11 in. record from which depths could be read to an accuracy of _~: l fm. For a moving ship a small reflecting surface on the bottom produces a hyperbola on the echo-sounder record which is a function of the ship's speed and the water depth. Templates of the hyperbolae produced at depths of 500, 1000 and 1500 fm with a ship speed of 8½- knots were made so that the records could be more fully appreciated. The contour chart has been made with the soundings corrected for the depth of the transducer, but uncorrected for the difference between 800 fm/sec and the true velocity of sound in sea water. To build up the contour chart, profiles were drawn for each east-west track, and on each night's track chart all I00 fm crossings, peaks and valleys marked. The separate night surveys were then contoured with the help of the profiles (FIG. 2) and fitted together to form the complete contour chart (Fl(;. 3). The overlap between separate surveys allowed fits to be obtained so tirol the positions of adjacent buoys agreed within half a mile of their relative positions as fixed by radar'. In 1000 fm of water, a buoy moored with 10 per cent excess of wire is free to move in a circle of half a mile radius. Work of HILL and MASON (1961) has shown that, in a light, steady wind, a buoy of this type drifts only about 0.1 miles from its mean position. The strong and variable winds experienced during the Black Mud Canyon surveys (force 3-7, direction southerly between W. and S.E.) almost certainly made the buoys drift over considerably larger areas than this. The fitting together of the separate charts indicates the maximum size of the errors involved in the work. In general the relative position of any two prominent features has probably been plotted to within half a mile of their true relation. Within the scope of one night's survey, the accuracy will be better than this, and along a single track, better still. The position of the canyon on the Earth's surface, determined by Decca Navigator and astronomical fixing of the buoys, is unlikely to have been recorded to much better than a mile, The position is near the limit of the S.W. British Decca system, though these fixes showed greater consistency than the astronomical ones, which were impaired by the generally overcast weather. The two Decca and two astronomical fixes of buoy No. 5, for which most fixes were obtained, were spread over ~ mile of latitude, and 3 miles of longitude. The next buoy to the south was similarly fixed better in latitude than in longitude, and this is, therefore, probably true of the canyon as a whole. In June, 1961, a small area over the canyon in the region of the 1100 fm contour was re-surveyed. To obtain a good fit the small survey had to be moved 1~ miles to the S.W. on the large chart and somewhat surprisingly rotated clockwise through about 7L An error ira orientation of this magnitude is unlikely, even though the later work used a ship having only a magnetic compass, and is probably more an effect of the normal navigational errors. The discrepancy in absolute position is not more than to be expected, probably involving more error in the small survey, positioned by fewer fixes on Decca alone. The name given to the canyon derives from the results of dredging operations performed in it in 1960 and 1961. An underwater camera was also used on the 1960 cruise. The conclusions of these studies have yet to be published.

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462

T . J . G . FRANCIS DISCUSSION

Canyons of the type described are a common feature of the continental slope between Ireland and Spain. especially in the area immediately west of Brittany. Nearly all start at, or near, the shelf edge, many can be followed to the abyssal plain, but only the Cap Breton canyon in the S.E. corner of the Bay of Biscay makes a large indentation in the 100 fm line and extends close to the present coastline. The longitudinal profile of Black Mud canyon (FIG. 4) has been drawn along the axis as indicated in the bathymetric chart. To a depth of 800 fm the profile resembles that of a graded river, indicating, perhaps, that the upper part was subaerially eroded. If this is true to a depth of 800 fm then d o , n-warping must have followed for this part of the canyon to be at its present depth, l~r the most xxidely accepted estimate of the lowering of sea level during the ice ages is only about 50 fm (KuENEN, 1950). 800 fin seems an excessive amount of dov~n-uarping: on the other hand, the Palaeogeographic atlas of W[u.s (195 I) suggests that the continental shelf area off the mouth of the English Channel has been more often land then sea since Palaeozoic times.

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The generally convex appearance of the profile probably has no bearing on tile formation of the canyon itself. DAY publishes three profiles across the slope in this area showing it to be typically convex in character as far as 2000 fm. Seismic work has been done nearby (BtJLkARD and GASKELL (1941 ~): HILL and LAVGHTON (1954), DAY e t a / . (1956), the most recent of which suggests that the sedimentary structure of the shelf stops near the edge with little downwarping of the bedding to follow the continental slope (BuLLARD 1959). A post-Palaeozoic westward movement of America can explain an abrupt termination of the Palaeozoic sediments at the edge of this part of the continental shelf. HILL and LAUGHTON summarize their results with a profile running E.N.E.-W.S.W. from south of Start Point to well out on the Biscay abyssal plain. This crosses the slope about 100 miles west of Black Mud canyon. DAY et aL show a contour map of the supposedly Palaeozoic floor in the Western Approaches of the Channel which extends as near as 50 miles to the 100 fm line. Extrapolating this structure to the canyon itself implies that the steepening of the profile which is noticeable at about 1000 fm depth may correspond with the refracting boundary which has been identified as the Palaeozoic floor. Alternatively it is perhaps the level of the first ' kink ' in the longitudinal profile which corresponds

Black Mud Canyon

463

to the Palaeozoic floor. The seismic results are too few and far between to make more definite conjectures. WHn'TARD (1962) has described dredging operations at depths of about 700 fm on the continental slope in the general area of Black Mud canyon. In particular two dredges to the west of the canyon lead him to predict 'Cretaceous and possibly older Mesozoic strata' intersecting the slope at depths in excess of 1000fm. "fhis prediction does not necessarily contradict the above remarks on the level of the Palaeozoic floor since WHITTARD'Sdredges were closer to the axis of the Palaeozoic trough (contour map of DAY et al.) than is Black Mud Canyon. BLACK (1962) has made a micropalaeontological study of soft limestone dredged 450 fm in the canyon only 10 miles east of Black Mud canyon and dates it as late Tertiary. LAUGHTON (1960), studying the relation between the Biscay and Iberia abyssal plains, finds that their contours point to turbidity currents originating near the edge of the shelf off the western approaches to the Channel and the coast of France. Thus it is probable that the canyons in the Black Mud area, if not actually eroded by turbidity currents, at least have been scoured by them. DAY (1959) believes the scarp (Pendragon scarp) running along the slope at a depth of about 1500 fm just south of the Porcupine Seabight, to be the result of faulting. It is unlikely that such faulting is responsible for the dendritic pattern of canyons situated nearby, and its contribution, if any, could not have been more than to determine the gradients of some of the steeper lower parts of the continental slope. In conclusion, therefore, it appears that subaerial erosion and turbidity currents have been the most important factors in the development of Black Mud Canyon. This seems to be the case for most of the canyons so far studied which are not connected with rivers (SHEPARD, 1948; KUENEN, 1953). Acknowledgments I am indebted to Dr. M. N. HILLwho was primarily responsible for organising the survey and who has provided me with the sounding and navigational data. His encouragement and enthusiasm were invaluable in the rather tedious work of reading and plotting the soundings. I am also grateful to the Director of the National Institute of Oceanography for the facilities provided in R.R.S. Discovery II in the summer of 1960.

REFERENCES BERTHOIS,L. and BRENOT,R. (1960). Institute Seientifique et Technique des P~ches Maritimes charts : Abords du Plateau Continental de l'ouest de l'Irlande aft Cap Finisterre, feuille 6. BLACK, M. (1962). Fossil Coccospheres from a Tertiary Outcrop on the Continental Slope. Geol. Mag., 99, 2, 123-127. BULLARD E. C. and GASKELL,T. F. (1941). Submarine seismic investigations. Proc. Roy. Soc. A. 177, 476--499. BULLARD, E. C. (1959). Forces and Processes at work in Ocean basins. Contribution to International Oceanography Conference, N. Y. 1959. Oceanography, Amer. Ass. Advanc. Sci., Washington D.C. (1961). DAY, k. A., HILL, M. N., LAUGHTON,A. S. and SWALLOW,J. C. (1956). Seismic prospecting in the western approaches of the English Channel. Quart. d. Geol. Soc. Lond. 122, 15-44. DAY, A. A. (1959). The continental margin l;etween Brittany and Ireland. Deep-Sea Res. 5, 249-265. HILL, M. N. and LAUGHTON,A. S., (1954). Seismic observations in the eastern Atlantic, 1952. Proc. Roy. Soc. A. 222, 348-356. HILL, i . N. (1956). Notes on the bathymetric chart of the N.E. Atlantic. Deep-Sea Res. 3, 229. KUENEN, Phil. (1950). Marine Geology. Wiley, New York. KUENEN, Phil. (1953). Origin and clasification of submarine canyons. Bull. Geol. Soc. Amer. 64, 1295.

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LAUGHTON,A. S. (1960). An interplain deep-sea channel system. Deep-Sea Res. 7, 75-88 SnEPARD, F. P. (1948). Submarine Geology. Harper, New York. WrrrARD, W. F., (1962). Geology of the Western Approaches of the English Channel : a progress report. Proc. Roy. Soc. A., 265, 395-406. WILLS, L. J. (1951). A Palaeogeographic Atlas of the British Isles and adjacent parts of Europe Blackie, London.