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Iceberg plough marks in the northeast Atlantic
Palaeogeography, Palaeoclimatology, Palaeoecolog3~, 13 (1973): 215-224 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
ICEBERG PLOUGH MARKS IN THE NORTHEAST
ATLANTIC
R. H. BELDERSON, N. H. KENYON and J. B. WILSON
National Institute of Oceanography, Wormley, Surrey (Great Britain) (Accepted for publication November 30, 1972)
ABSTRACT Belderson, R. H., Kenyon, N. H. and Wilson, J. B., 1973. Iceberg plough marks in the northeast Atlantic. Palaeogeogr., Palaeoclimatol., Palaeoecol., 13: 215-224.
A criss-cross pattern of large furrows on the sea floor has been observed on sonographs (side-scan sonar records) west of the British Isles, from the Faeroe Islands to Porcupine Bank. By analogy with present-day examples from the Canadian Arctic, this is attributed to the ploughing action of drifting icebergs. The "iceberg plough marks" occur in the sediments of the outer shelf and upper continental slope and around shoal areas of the deep sea at depths mainly between 140 and 500 m. The implications of these limits are discussed. The plough marks should also be found in many other areas previously traversed by Glacial and Late Glacial icebergs, as suggested by the widespread distribution of glacial erratics in the oceans. INTRODUCTION Little attention has been paid to iceberg action as an erosional agent since the 19th century when at one time most glacial "drift" and striae were incorrectly attributed to floating ice action (Charlesworth, 1957, p p . 6 1 9 - 6 2 2 ) . The same author ( p p . 5 8 7 - 5 9 0 ) summarises observations on the geological action of sea-ice. Marks made on the sea floor by present-day floating ice have hitherto been observed, together with submarine pingos, on sonographs (side-scan sonar records) in the Beaufort Sea, Canadian Arctic, obtained using 120 kHz side-scan sonar from the C.S.S. "Hudson" by Shearer et al. (1971, fig.2). An example from that locality, furnished by Dr. B. R. Pelletier of the Bedford Institute of Oceanography, has been described by Belderson et al. (1972, fig.83). Those furrows were ploughed into a muddy sediment by floating ice, with a more or less random orientation, but occasional parallel grouping. An order of superimposition of the plough marks could be deduced by inspection of intersections. The marks are up to about 25 m across and 5 m deep and traceable for distances up to at least 2 kin. The rims are frequently raised a metre or so above the surrounding floor, presumably due to the shoving aside of sediment by the moving iceberg. With increasing side-scan sonar coverage (Fig. 1) a similar feature has been found
216
R. H. BELDERSON,N. If. KENYONAND J. B. WILSON I
"-'
I
I
'F
62~
Fig.1. Ship's tracks along which sonographs (side scan sonar records) have been taken. Locations of Fig.3-6 are shown.
extensively on the continental shelf and upper slope of the northeastern Atlantic (Fig.2). This paper delineates and discusses the present known limits of this distinctive pattem and describes its main characteristics. These results have been deduced from sonographs (using 36 kHz and 48 kHz side-scan sonar from R.R.S. "Discovery" and R.R.S. "John Murray", respectively); from 190 grab and dredge samples, collected on M.V. "Surveyor"
ICEBERGPLOUGHMARKSIN THENORTHEASTATLANTIC
217
B~LtB~ <5 ETLANDS
RKNEYS
Fig.2. Known occurrences of iceberg plough marks, as detected on sonographs (thick lines), and probable minimum extent of area of occurrence (stippled patches). Additional sonar runs are expected to increase this, both on the contental shelf and around the edges of the deep-sea hanks. S o f H represents the Sea of the Hebrides. The extension of the stippling shown at the southwest of Rockall Bank is based on echo soundings taken on M.V. "Surveyor" in 1970. Depth contours axe in metres.
in 1970 and 1971 and R.R.S. " J o h n M u r r a y " in 1972, at 72 stations located along sonograph lines w h i c h indicated the presence o f the pattern and 174 samples at 90 additional stations w i t h i n the probable area o f occurrence o f the pattern; as well as f r o m some observations f r o m a deep submersible, and u n d e r w a t e r p h o t o g r a p h y .
218
R_H. BELDERSON, N. H. KENYON AND J. B. WILSON
ICEBERG PLOUGH MARKS NEAR THE BRITISH ISLES In general the best developed plough marks ( F i g . 3 - 5 ) are found on the shelf edge and upper slope to the north and west o f the Shetlands, and Orkneys, to the west o f Scotland and Ireland, in the vicinity o f the Faeroe Islands, on the perimeter o f Rockall Bank, and, closer inshore, in parts of the Sea of the Hebrides. Possible poorer examples occur over broad areas on the outer shelf west o f Scotland. The depth o f occurrence ranges from about 140 to 500 m; except in the case o f The Minches and the Sea o f the Hebrides where they may be found at depths as shallow as 100 m. Shorewards o f the p a t t e m there are indications o f Pleistocene drowned valleys overdeepened by ice action (Ting, 1937) and ice-carved rock and morainic relief, together with present day forms indicative o f sediment transport and deposition. Approximate average dimensions of the marks as known at present (with maxima in brackets) are: width 20 m (100 m) and depth 2 m (10 m). The limited range scale on the sonographs precludes an estimate o f average length, but the longest mark found so far is 5.5 km. The feature appears to have its greatest relief in the greatest depths in which it is found. The relief (for example, Fig.6), and particularly that o f the raised rims, is
1.4km Fig.3. Sonograph showing iceberg plough marks. Dark tones represent relatively coarse material including boulders, and light tones relatively fine material. A time relationship can be seen in the crossing at right of centre, in that the edges of one mark appear to c u t across an earlier one. Sea-floor relief is up to 3 m. Water depth is 180 m. In this and subsequent sonographs P is the profile of the sea floor directly beneath the ship and the arrow gives direction of "view" from the ship. Location is given in Fig. 1.
Fig.5. Sonograph showing plough marks in coarse material (dark tone) undergoing progressive 3urial beneath muddy sand (light tone). The single black line across the sonograph is an overprinted :ime mark. Water depth is 1 4 0 - 1 6 0 m. Location is given in Fig.1.
7km
•ig.4. Sonograph showing a long plough mark which is crossed at various angles by numerous other 91ough marks. The single black line across the sonograph is an overprinted time mark. Water depth s 175 m. Location is given in Fig.1.
2.6km
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3"3 r,
220
R. H. BELDERSON, N. H. KENYON AND J. B. WILSON
Fig.6. Precision Echo Sounder profile across the small-scaleirregular relief of plough marks. Location is given in Fig.1. generally not as sharp in appearance as in the Beaufort Sea example described above. However, the maximum dimensions appear to be greater in the present examples, perhaps because they are produced by much bulkier icebergs than the sea ice responsible for the Canadian Arctic plough marks. Samples and underwater photography show the darker toned strips on the sonographs to be composed of boulders, cobbles and pebbles with some sand and mud in various admixtures. The boulders from the Scottish shelf, some of which are striated, and cobbles are generally sub-rounded and consist of a wide range of unrelated rock types typical of glacial erratic assemblages, whilst those from Rockall Bank are mostly basalt. The lighter toned strips consist of sandy or muddy material. It seems likely that the original sediment into which the pattern was gouged was largely glacial marine in origin with a wide range in textural composition. The more muddy continental slope sediments may have been more readily and deeply ploughed. Nowhere do structural features associated with rock outcrops appear to be responsible for the pattern - nor could they be confused with the longitudinal tidal scour furroWs described by Stride et al. (I972). There is a superficial resemblance to trawl marks (Belderson et al., 1972, fig. 147) but the iceberg features are normally much larger. Bridger (1972) has observed the relatively slight disturbance of the seabed due to the passage of a beam trawl, even on soft, muddy floors. On Rockall Bank deep living coral [Lopheliaprolifera (Pallas)] has been dredged in abundance from various localities within what is now recognised as iceberg ploughed ground (Belderson et al., 1972, fig.86) and it seems possible that the coral is growing preferentially on coarse material originally thrown up to either side of the furrow, while the furrow itself is now floored or partly filled with sand or mud. Observations from a manned submersible have been made during dives at two localities within or close to the pattern (Eden et al., 1971). The first, near the top of the continental slope from a depth of 3 8 0 - 1 6 0 m, indicated mainly sand (occasionally rippled) in the deeper water, giving way upslope to irregular patches of coarser material
ICEBERG PLOUGH MARKS IN THE NORTHEAST ATLANTIC
221
with, in places, a discontinuous elongation roughly parallel to the depth contours, and finally to a higher proportion of cobbles, often well-rounded, including low cobble ridges up to nearly a metre across and 20 cm high. Notable at the second locality, south of the island of Barra in the Sea of the Hebrides at depths from 85-105 m, were two boulder ridges each about 5 m high and 10 m across which were separated by a trench about 30 m wide. The boulders are largest and most abundant towards the tops of the ridges where they are up to 1 m across, while the trench is floored with muddy sand. It was suggested by Eden et al., (1971) that at the first locality the cobble ridges resulted from wave action at a time of lower sea level and at the second that they were morainic accumulations. The nature of the features is nevertheless compatible with an origin as put forward here. DISCUSSION
Reworking and burial It is perhaps not surprising that the iceberg plough marks featured here (even taking account of the lower resolution of the sonar used) do not exhibit the clear-cut lines and secondary grooves sometimes seen on the furrow floors of the Canadian Arctic occurrence. Although occasional icebergs have been sighted in the northeast Atlantic (IceAtlas of the Northern Hemisphere, 1946), it is presumed that the pattern was at least largely imposed during Glacial times. An attempted reconstruction of Glacial and Late Glacial conditions in the north Atlantic is given by Manley (1951, fig. 6, 7) in which he indicates the extent of the southwards displacement of the Polar Front and its associated icebergs. Since that time the effect of reworking and/or deposition has been to smooth off the ridges and fill in the furrows. A possible exception to this are those areas where coral growth on the edge of the furrow has kept pace with any infllling of the trough. Thus it is reasonable to expect all gradations in effacement from a relatively fresh appearance to a vague and ill-defined "palimpsest" pattern due to superimposition of present-day linear sedimentary features.
Shallowest occurrence It is interesting to note that, with the exception of the Sea of the Hebrides example, the pattern is restricted to depths greater than about 140 m. This may bear some relation to an ancient shoreline or to the edge of the ice cap or ice shelf which would at that time have prevented icebergs from drifting further inshore. The Sea of the Hebrides occurrence might then be explained as a later stage product, after the continental glacier has retreated from this area. However, if present areas of rock outcrop, recent deposition, and reworking and sand transport (tidal currents of 1 knot or more) are included together, it is possible to explain the lack of the pattern over much of the shallower shelf area either it was never imprinted in the first place (in the case of a rock floor), or it has
222
R.H. BELDERSON, N. H. KENYON AND J. B. WILSON
since been either buried or erased. West of the Shetlands (Fig.5) and in the Sea of the Hebrides area there are good examples of progressive burial of the pattern beneath muddy sand and mud respectively, and sub-bottom profiles show similar relief buried beneath up to 10 m of mud in about 90 m of water on the northwest Irish shelf at around 55°35'N 8°30'W. Deepest occurrence
Smith (1931) remarks that a common height for large bergs off Greenland is about 80 m, and that the ratio of height to draught varies widely, from 1 : 1 to 1: 5. This would give a general maximum draught of about 400 m. The deepest observed relict occurrences are at about 500 m, and after allowing for a fall in sea level of 100 m or so, would suggest formation by icebergs with a similar maximum draught. Therefore, the icebergs of Glacial times may not have been significantly larger than the largest presentday ones. With sufficient crossings of the lower limit of occurrence it might also be possible to estimate relative variations in post-glacial isostatic movement both along the shelf edge and between the Scottish shelf edge and the outlying deep sea banks. A more absolute value might be obtained if it is assumed that these latter lacked a continentaltype glaciation and were thus unaffected by ice-loading, and that the same maximum size of iceberg drifted over all areas. At the present time there are too few such observations (unfortunately many of the sonar lines indicated on Fig. 1 stop short of the lower boundary of the feature), but there is nevertheless a suggestion that the deepest marks on the continental slope west of Scotland are 100 m or more shallower than those around the deep sea banks. Dominant trends
The implications of any preferred linear trend in the pattern for delineating palaeocurrents are interesting, but difficult to interpret. On the shelf west of Scotland the generally poorly developed pattern has a dominant trend more or less parallel to the shelf edge which may indicate palaeocurrent direction. However, this probably also coincides with the trend of sediment distribution by present-day tidal currents (Kenyon and Stride, 1970) so that these may play some part in the preferred orientation. Where it is well developed, the pattern has a tendency on some steeper slopes to become oriented along the contours. For large present-day icebergs Smith (1931) observed that currents are the dominant motive force, but that as the berg melts or calves its increasingly irregular shape and smaller draught give the wind and wind-dominated currents a greater and greater share of control. Therefore it might be reasonable to expect the deepest marks to be most indicative of palaeocurrents. However, there is a second likely cause of a preferred along-slope orientation in that bergs stranded on a slope will on refloating be restrained by the contours, such that the longest marks should be found along contour and the shortest across contour. Occasionally, rounded "blobby" marks have
ICEBERG PLOUGH MARKS IN THE NORTHEAST ATLANTIC
223
been observed. These may be produced by stranded bergs pivoting under the influence of current or wind. Present-day e x t e n t
Glacial erratics have been noted in the Atlantic as far south as 30°N (Pratt, 1961) and floating ice has even been sighted almost to that latitude. Thus, it is possible to predict the existence of iceberg plough marks from south of the British Isles to Portuguese and even northwest African water~. An examination of sonographs from these areas, however, has so far failed to produce any satisfactory examples further south than Porcupine Bank, 53°30'N. Other regions in which they might confidently be predicted to occur in abundance are the shelf and upper slope off Greenland, Norway, Iceland, Labrador, the Grand Banks, particularly around the eastern margin where large present-day icebergs are extremely common (Smith, 1931, fig. 107; lce Atlas o f the Northern Hemisphere, 1946), and Nova Scotia, where a similar looking relief in glacial till is figured (Type III) by King (1967). Preservation in the geological record
Iceberg plough marks should be preserved in some ancient marine glacial deposits, perhaps most clearly in cases of continuing muddy sedimentation when they would be buried without modification by other agencies. They should then appear as intersecting channels with raised rims and much disturbed and grooved sides and floors. ACKNOWLEDGEMENTS The authors are grateful to various colleagues for their help at sea, to Dr. A. H. Stride for critical reading of the manuscript, to Dr. J. S. M. Rusby for the use of sonograph lines in the Sea of the Hebrides, and Mr. D. G. Roberts for running a sonograph line west of Ireland. The Hydrographer of the Navy is thanked for the use of a detailed echo-sounding survey of the Approaches to the North West coast of Ireland. REFERENCES Belderson, R. H., Kenyon, N. H., Stride, A. H. and Stubbs, A. R., 1972. Sonographs of the Sea Floor. Elsevier, Amsterdam, 185 pp. Bridget, J. P., 1972. Some observations on the penetration into the sea bed of tickler chains on a beam trawl. Int. Counc. Explor. Sea, C. M. 1972/B: 7, 6 pp. Charlesworth, J. K., 1957. The Quarternary Era. Edward Arnold, London, 1700 pp. (2 vol.). Eden, R. A., Ardus, D. A., Binns, P. E., McQuillin, R. and Wilson, J. B., 1971. Geological investigations with a manned submersible off the west coast of Scotland 1969-1970. Inst. Geol. Sci., Rep.. 71(16): 49 pp. Ice Atlas of the Northern Hemisphere, 1946. Hydrogr. Off. U.S. Navy, No. 550, Washington, 106 pp. Kenyon, N. H. and Stride, A. H., 1970. The tide-swept continental shelf sediments between the Shetland Isles and France. Sedimentology, 14: 159-173.
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R.H. BELDERSON, N. H. KENYON AND J. B. WILSON
King, L. H., 1967. Use of a conventional echo-sounder and textural analyses in delineating sedimentary facies: Seotian shelf. Can. J. Earth Sci., 4 : 6 9 1 - 7 0 8 . Manley, G., 1951. The range of variation of the British climate. Geograph. J., 117: 4 3 - 6 8 . Pratt, R. M., 1961. Erratic boulders from Great Meteor Seamount. Deep-Sea Res., 8: 152-153. Shearer, J. M., Macnab, R. F., Pelletier, B. R. and Smith, T. B., 1971. Submarine pingos in the Beaufort Sea. Science, 174: 8 1 6 - 8 1 8 . Smith, E. H., 1931. The Marion expedition to Davis Strait and Baffin Bay. Scientific results, Part 3. Arctic ice, with especial reference to its distribution to the North Atlantic Ocean. Bull. U.S. Coast Guard, 19:221 pp. Stride, A. H., Belderson, R. H. and Kenyon, N. H., 1972. Longitudinal furrows and depositional sand bodies of the English Channel. M~rn. Bur. Rech. Gdol. Mini~res, 79: 233-240. Ting, S., 1937. The coastal configuration of Western Scotland. Geogr. Ann., 19: 6 2 - 8 3 .
ICEBERG PLOUGH MARKS IN THE NORTHEAST
ATLANTIC
R. H. BELDERSON, N. H. KENYON and J. B. WILSON
National Institute of Oceanography, Wormley, Surrey (Great Britain) (Accepted for publication November 30, 1972)
ABSTRACT Belderson, R. H., Kenyon, N. H. and Wilson, J. B., 1973. Iceberg plough marks in the northeast Atlantic. Palaeogeogr., Palaeoclimatol., Palaeoecol., 13: 215-224.
A criss-cross pattern of large furrows on the sea floor has been observed on sonographs (side-scan sonar records) west of the British Isles, from the Faeroe Islands to Porcupine Bank. By analogy with present-day examples from the Canadian Arctic, this is attributed to the ploughing action of drifting icebergs. The "iceberg plough marks" occur in the sediments of the outer shelf and upper continental slope and around shoal areas of the deep sea at depths mainly between 140 and 500 m. The implications of these limits are discussed. The plough marks should also be found in many other areas previously traversed by Glacial and Late Glacial icebergs, as suggested by the widespread distribution of glacial erratics in the oceans. INTRODUCTION Little attention has been paid to iceberg action as an erosional agent since the 19th century when at one time most glacial "drift" and striae were incorrectly attributed to floating ice action (Charlesworth, 1957, p p . 6 1 9 - 6 2 2 ) . The same author ( p p . 5 8 7 - 5 9 0 ) summarises observations on the geological action of sea-ice. Marks made on the sea floor by present-day floating ice have hitherto been observed, together with submarine pingos, on sonographs (side-scan sonar records) in the Beaufort Sea, Canadian Arctic, obtained using 120 kHz side-scan sonar from the C.S.S. "Hudson" by Shearer et al. (1971, fig.2). An example from that locality, furnished by Dr. B. R. Pelletier of the Bedford Institute of Oceanography, has been described by Belderson et al. (1972, fig.83). Those furrows were ploughed into a muddy sediment by floating ice, with a more or less random orientation, but occasional parallel grouping. An order of superimposition of the plough marks could be deduced by inspection of intersections. The marks are up to about 25 m across and 5 m deep and traceable for distances up to at least 2 kin. The rims are frequently raised a metre or so above the surrounding floor, presumably due to the shoving aside of sediment by the moving iceberg. With increasing side-scan sonar coverage (Fig. 1) a similar feature has been found
216
R. H. BELDERSON,N. If. KENYONAND J. B. WILSON I
"-'
I
I
'F
62~
Fig.1. Ship's tracks along which sonographs (side scan sonar records) have been taken. Locations of Fig.3-6 are shown.
extensively on the continental shelf and upper slope of the northeastern Atlantic (Fig.2). This paper delineates and discusses the present known limits of this distinctive pattem and describes its main characteristics. These results have been deduced from sonographs (using 36 kHz and 48 kHz side-scan sonar from R.R.S. "Discovery" and R.R.S. "John Murray", respectively); from 190 grab and dredge samples, collected on M.V. "Surveyor"
ICEBERGPLOUGHMARKSIN THENORTHEASTATLANTIC
217
B~LtB~ <5 ETLANDS
RKNEYS
Fig.2. Known occurrences of iceberg plough marks, as detected on sonographs (thick lines), and probable minimum extent of area of occurrence (stippled patches). Additional sonar runs are expected to increase this, both on the contental shelf and around the edges of the deep-sea hanks. S o f H represents the Sea of the Hebrides. The extension of the stippling shown at the southwest of Rockall Bank is based on echo soundings taken on M.V. "Surveyor" in 1970. Depth contours axe in metres.
in 1970 and 1971 and R.R.S. " J o h n M u r r a y " in 1972, at 72 stations located along sonograph lines w h i c h indicated the presence o f the pattern and 174 samples at 90 additional stations w i t h i n the probable area o f occurrence o f the pattern; as well as f r o m some observations f r o m a deep submersible, and u n d e r w a t e r p h o t o g r a p h y .
218
R_H. BELDERSON, N. H. KENYON AND J. B. WILSON
ICEBERG PLOUGH MARKS NEAR THE BRITISH ISLES In general the best developed plough marks ( F i g . 3 - 5 ) are found on the shelf edge and upper slope to the north and west o f the Shetlands, and Orkneys, to the west o f Scotland and Ireland, in the vicinity o f the Faeroe Islands, on the perimeter o f Rockall Bank, and, closer inshore, in parts of the Sea of the Hebrides. Possible poorer examples occur over broad areas on the outer shelf west o f Scotland. The depth o f occurrence ranges from about 140 to 500 m; except in the case o f The Minches and the Sea o f the Hebrides where they may be found at depths as shallow as 100 m. Shorewards o f the p a t t e m there are indications o f Pleistocene drowned valleys overdeepened by ice action (Ting, 1937) and ice-carved rock and morainic relief, together with present day forms indicative o f sediment transport and deposition. Approximate average dimensions of the marks as known at present (with maxima in brackets) are: width 20 m (100 m) and depth 2 m (10 m). The limited range scale on the sonographs precludes an estimate o f average length, but the longest mark found so far is 5.5 km. The feature appears to have its greatest relief in the greatest depths in which it is found. The relief (for example, Fig.6), and particularly that o f the raised rims, is
1.4km Fig.3. Sonograph showing iceberg plough marks. Dark tones represent relatively coarse material including boulders, and light tones relatively fine material. A time relationship can be seen in the crossing at right of centre, in that the edges of one mark appear to c u t across an earlier one. Sea-floor relief is up to 3 m. Water depth is 180 m. In this and subsequent sonographs P is the profile of the sea floor directly beneath the ship and the arrow gives direction of "view" from the ship. Location is given in Fig. 1.
Fig.5. Sonograph showing plough marks in coarse material (dark tone) undergoing progressive 3urial beneath muddy sand (light tone). The single black line across the sonograph is an overprinted :ime mark. Water depth is 1 4 0 - 1 6 0 m. Location is given in Fig.1.
7km
•ig.4. Sonograph showing a long plough mark which is crossed at various angles by numerous other 91ough marks. The single black line across the sonograph is an overprinted time mark. Water depth s 175 m. Location is given in Fig.1.
2.6km
,..2
.4
-]
-]
¢)
7
3"3 r,
220
R. H. BELDERSON, N. H. KENYON AND J. B. WILSON
Fig.6. Precision Echo Sounder profile across the small-scaleirregular relief of plough marks. Location is given in Fig.1. generally not as sharp in appearance as in the Beaufort Sea example described above. However, the maximum dimensions appear to be greater in the present examples, perhaps because they are produced by much bulkier icebergs than the sea ice responsible for the Canadian Arctic plough marks. Samples and underwater photography show the darker toned strips on the sonographs to be composed of boulders, cobbles and pebbles with some sand and mud in various admixtures. The boulders from the Scottish shelf, some of which are striated, and cobbles are generally sub-rounded and consist of a wide range of unrelated rock types typical of glacial erratic assemblages, whilst those from Rockall Bank are mostly basalt. The lighter toned strips consist of sandy or muddy material. It seems likely that the original sediment into which the pattern was gouged was largely glacial marine in origin with a wide range in textural composition. The more muddy continental slope sediments may have been more readily and deeply ploughed. Nowhere do structural features associated with rock outcrops appear to be responsible for the pattern - nor could they be confused with the longitudinal tidal scour furroWs described by Stride et al. (I972). There is a superficial resemblance to trawl marks (Belderson et al., 1972, fig. 147) but the iceberg features are normally much larger. Bridger (1972) has observed the relatively slight disturbance of the seabed due to the passage of a beam trawl, even on soft, muddy floors. On Rockall Bank deep living coral [Lopheliaprolifera (Pallas)] has been dredged in abundance from various localities within what is now recognised as iceberg ploughed ground (Belderson et al., 1972, fig.86) and it seems possible that the coral is growing preferentially on coarse material originally thrown up to either side of the furrow, while the furrow itself is now floored or partly filled with sand or mud. Observations from a manned submersible have been made during dives at two localities within or close to the pattern (Eden et al., 1971). The first, near the top of the continental slope from a depth of 3 8 0 - 1 6 0 m, indicated mainly sand (occasionally rippled) in the deeper water, giving way upslope to irregular patches of coarser material
ICEBERG PLOUGH MARKS IN THE NORTHEAST ATLANTIC
221
with, in places, a discontinuous elongation roughly parallel to the depth contours, and finally to a higher proportion of cobbles, often well-rounded, including low cobble ridges up to nearly a metre across and 20 cm high. Notable at the second locality, south of the island of Barra in the Sea of the Hebrides at depths from 85-105 m, were two boulder ridges each about 5 m high and 10 m across which were separated by a trench about 30 m wide. The boulders are largest and most abundant towards the tops of the ridges where they are up to 1 m across, while the trench is floored with muddy sand. It was suggested by Eden et al., (1971) that at the first locality the cobble ridges resulted from wave action at a time of lower sea level and at the second that they were morainic accumulations. The nature of the features is nevertheless compatible with an origin as put forward here. DISCUSSION
Reworking and burial It is perhaps not surprising that the iceberg plough marks featured here (even taking account of the lower resolution of the sonar used) do not exhibit the clear-cut lines and secondary grooves sometimes seen on the furrow floors of the Canadian Arctic occurrence. Although occasional icebergs have been sighted in the northeast Atlantic (IceAtlas of the Northern Hemisphere, 1946), it is presumed that the pattern was at least largely imposed during Glacial times. An attempted reconstruction of Glacial and Late Glacial conditions in the north Atlantic is given by Manley (1951, fig. 6, 7) in which he indicates the extent of the southwards displacement of the Polar Front and its associated icebergs. Since that time the effect of reworking and/or deposition has been to smooth off the ridges and fill in the furrows. A possible exception to this are those areas where coral growth on the edge of the furrow has kept pace with any infllling of the trough. Thus it is reasonable to expect all gradations in effacement from a relatively fresh appearance to a vague and ill-defined "palimpsest" pattern due to superimposition of present-day linear sedimentary features.
Shallowest occurrence It is interesting to note that, with the exception of the Sea of the Hebrides example, the pattern is restricted to depths greater than about 140 m. This may bear some relation to an ancient shoreline or to the edge of the ice cap or ice shelf which would at that time have prevented icebergs from drifting further inshore. The Sea of the Hebrides occurrence might then be explained as a later stage product, after the continental glacier has retreated from this area. However, if present areas of rock outcrop, recent deposition, and reworking and sand transport (tidal currents of 1 knot or more) are included together, it is possible to explain the lack of the pattern over much of the shallower shelf area either it was never imprinted in the first place (in the case of a rock floor), or it has
222
R.H. BELDERSON, N. H. KENYON AND J. B. WILSON
since been either buried or erased. West of the Shetlands (Fig.5) and in the Sea of the Hebrides area there are good examples of progressive burial of the pattern beneath muddy sand and mud respectively, and sub-bottom profiles show similar relief buried beneath up to 10 m of mud in about 90 m of water on the northwest Irish shelf at around 55°35'N 8°30'W. Deepest occurrence
Smith (1931) remarks that a common height for large bergs off Greenland is about 80 m, and that the ratio of height to draught varies widely, from 1 : 1 to 1: 5. This would give a general maximum draught of about 400 m. The deepest observed relict occurrences are at about 500 m, and after allowing for a fall in sea level of 100 m or so, would suggest formation by icebergs with a similar maximum draught. Therefore, the icebergs of Glacial times may not have been significantly larger than the largest presentday ones. With sufficient crossings of the lower limit of occurrence it might also be possible to estimate relative variations in post-glacial isostatic movement both along the shelf edge and between the Scottish shelf edge and the outlying deep sea banks. A more absolute value might be obtained if it is assumed that these latter lacked a continentaltype glaciation and were thus unaffected by ice-loading, and that the same maximum size of iceberg drifted over all areas. At the present time there are too few such observations (unfortunately many of the sonar lines indicated on Fig. 1 stop short of the lower boundary of the feature), but there is nevertheless a suggestion that the deepest marks on the continental slope west of Scotland are 100 m or more shallower than those around the deep sea banks. Dominant trends
The implications of any preferred linear trend in the pattern for delineating palaeocurrents are interesting, but difficult to interpret. On the shelf west of Scotland the generally poorly developed pattern has a dominant trend more or less parallel to the shelf edge which may indicate palaeocurrent direction. However, this probably also coincides with the trend of sediment distribution by present-day tidal currents (Kenyon and Stride, 1970) so that these may play some part in the preferred orientation. Where it is well developed, the pattern has a tendency on some steeper slopes to become oriented along the contours. For large present-day icebergs Smith (1931) observed that currents are the dominant motive force, but that as the berg melts or calves its increasingly irregular shape and smaller draught give the wind and wind-dominated currents a greater and greater share of control. Therefore it might be reasonable to expect the deepest marks to be most indicative of palaeocurrents. However, there is a second likely cause of a preferred along-slope orientation in that bergs stranded on a slope will on refloating be restrained by the contours, such that the longest marks should be found along contour and the shortest across contour. Occasionally, rounded "blobby" marks have
ICEBERG PLOUGH MARKS IN THE NORTHEAST ATLANTIC
223
been observed. These may be produced by stranded bergs pivoting under the influence of current or wind. Present-day e x t e n t
Glacial erratics have been noted in the Atlantic as far south as 30°N (Pratt, 1961) and floating ice has even been sighted almost to that latitude. Thus, it is possible to predict the existence of iceberg plough marks from south of the British Isles to Portuguese and even northwest African water~. An examination of sonographs from these areas, however, has so far failed to produce any satisfactory examples further south than Porcupine Bank, 53°30'N. Other regions in which they might confidently be predicted to occur in abundance are the shelf and upper slope off Greenland, Norway, Iceland, Labrador, the Grand Banks, particularly around the eastern margin where large present-day icebergs are extremely common (Smith, 1931, fig. 107; lce Atlas o f the Northern Hemisphere, 1946), and Nova Scotia, where a similar looking relief in glacial till is figured (Type III) by King (1967). Preservation in the geological record
Iceberg plough marks should be preserved in some ancient marine glacial deposits, perhaps most clearly in cases of continuing muddy sedimentation when they would be buried without modification by other agencies. They should then appear as intersecting channels with raised rims and much disturbed and grooved sides and floors. ACKNOWLEDGEMENTS The authors are grateful to various colleagues for their help at sea, to Dr. A. H. Stride for critical reading of the manuscript, to Dr. J. S. M. Rusby for the use of sonograph lines in the Sea of the Hebrides, and Mr. D. G. Roberts for running a sonograph line west of Ireland. The Hydrographer of the Navy is thanked for the use of a detailed echo-sounding survey of the Approaches to the North West coast of Ireland. REFERENCES Belderson, R. H., Kenyon, N. H., Stride, A. H. and Stubbs, A. R., 1972. Sonographs of the Sea Floor. Elsevier, Amsterdam, 185 pp. Bridget, J. P., 1972. Some observations on the penetration into the sea bed of tickler chains on a beam trawl. Int. Counc. Explor. Sea, C. M. 1972/B: 7, 6 pp. Charlesworth, J. K., 1957. The Quarternary Era. Edward Arnold, London, 1700 pp. (2 vol.). Eden, R. A., Ardus, D. A., Binns, P. E., McQuillin, R. and Wilson, J. B., 1971. Geological investigations with a manned submersible off the west coast of Scotland 1969-1970. Inst. Geol. Sci., Rep.. 71(16): 49 pp. Ice Atlas of the Northern Hemisphere, 1946. Hydrogr. Off. U.S. Navy, No. 550, Washington, 106 pp. Kenyon, N. H. and Stride, A. H., 1970. The tide-swept continental shelf sediments between the Shetland Isles and France. Sedimentology, 14: 159-173.
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King, L. H., 1967. Use of a conventional echo-sounder and textural analyses in delineating sedimentary facies: Seotian shelf. Can. J. Earth Sci., 4 : 6 9 1 - 7 0 8 . Manley, G., 1951. The range of variation of the British climate. Geograph. J., 117: 4 3 - 6 8 . Pratt, R. M., 1961. Erratic boulders from Great Meteor Seamount. Deep-Sea Res., 8: 152-153. Shearer, J. M., Macnab, R. F., Pelletier, B. R. and Smith, T. B., 1971. Submarine pingos in the Beaufort Sea. Science, 174: 8 1 6 - 8 1 8 . Smith, E. H., 1931. The Marion expedition to Davis Strait and Baffin Bay. Scientific results, Part 3. Arctic ice, with especial reference to its distribution to the North Atlantic Ocean. Bull. U.S. Coast Guard, 19:221 pp. Stride, A. H., Belderson, R. H. and Kenyon, N. H., 1972. Longitudinal furrows and depositional sand bodies of the English Channel. M~rn. Bur. Rech. Gdol. Mini~res, 79: 233-240. Ting, S., 1937. The coastal configuration of Western Scotland. Geogr. Ann., 19: 6 2 - 8 3 .