Fig. 82. Carbonate mounds seen on the ORAT- sonograph and subbotom profile record. Low backscattering bands sweeping to be sand sheets
IOC Technical Series No. 52 Page 123 The next 8 km of sidescan record display a complex morphology and set of reflectors and backscattering surfaces, There are more than 30 patches and areas of very high backscatter in this region, with a variety of sizes, up to 1 km in length and typically 5-700 metres in width. They present a distinctly linear morphology, oriented SSW/NNE, with a clear crest. Shadow shape suggests that they have very steep sides and a relatively flat, multi-peaked top, demonstrated also by hyperbolic reflectors from the profiler record when it crosses these features. Smaller features are more conical, and up to 30 metres high. Large mounds are 150-200 metres high. The mounds show rings of high backscatter at their base, most prominent to their north and east sides; the profiler demonstrates that these are deeps, the ‘moats’ described elsewhere, but not completely surrounding the mounds. The sediment surface is higher by 20-30 metres on the SW side. Inter-mound areas are of generally medium backscatter, with areas of pronounced low backscatter that indicate a flow pattern. These low backscatter areas wind between mounds, and are also sourced from the western and eastern margins of mounds, curving and streaming in a northerly direction (Fig. 82).
The final 3.5 km represents a return to the mottled medium backscatter seafloor at the beginning of the profile, with the associated very small spots of high backscatter. The bottom profile in this region is flat and offers zero penetration. There is one small circular mound feature to be found at the end of the line, with associated shadow. The final feature to mention is a potential iceberg ploughmark, to be seen between 06:20 and 06:40 on the downslope side of the line at the depth of about 700 m. It is an irregular linear feature with a wavy plan shape. It is an area of low backscatter and shows high backscatter edges.
Line ORAT 11
ORAT I1 covers the lower half of the slope and was run to complement a part of the seismic line which had been collected previously (PSAT-14). Orientation of ORAT 11 is roughly SE-NW, and the length is approximately 27 km. The line was collected on the 3rd August 1997, between 05:09 and 14:05 GMT. The profiler shows that the slope has an overall convex form steepening at 2000 metres depth and then terminating sharply at the transition from slope to basin plain.
The first 9.8 km from 05:09 to 09:15, depth 1500 to 2000 metres, is a uniform, low backscatter surface, with extremely small (<20m), randomly placed specks of higher backscatter. The specks are more common on the upper part of the line. There is also a faintly seen, curving, relatively high backscatter region between 05:38 and 06:30; it is approximately 1 kilometre in width, and 3.5 kilometres in length and strikes N-S, seeming to correlate with a low (-20 m high) bulge on the seabottom, seen on the profiler record at approximately 1650 metres depth.
The next 15.6 km is an area of generally medium to high surface backscatter, with large (500 metres to 1.5 kilometres) regions of high backscatter. These regions are concentrated between 08:48 to lo:12 and 11:22 and 13:32. Between lo:12 and 11:22, there is an area of medium to high backscatter with no features within it, other than a broad (I km’) patch of high backscatter on the south-western side, at roughly 10:40. The character of the profiler record differs significantly between these high backscatter areas and the rest of the surface, being parabolic (hence topographically rough), and giving a more prolonged return where it crosses these regions. This suggests a solid (rocky?) seabed. In both sections, areas of high backscatter appear to be surrounded on either side by sections of relatively low topography (moats), and in the downslope area, the mounds are of conical cross- section (Fig. 83). The upslope regions of very high backscatter demonstrate no obvious plan-view shape, but display indications of shadowed lineations (ridges) trending SW-NE on their surface. A ridge of high backscatter at the upslope margin of the area, extending for some 2 km and trending W- E is also apparent.
The downslope region of high backscatter (11:22 to 13:32) shows three concentrations of high backscatter, at 11:28, 12:05 and 12:50. The first two are approximately circular and conical, and are 500 metres and 1.5 kilometres in diameter and 100 and 200 metres high respectively. Their surface texture is again rough, although there are no signs of lineation in this lower region in the sonar data.
The final 1.4 km, from 13:32 is an area of uniform medium backscatter, with no apparent surficial features. The profiler shows a completely level and slightly undulating surface, and offers some penetration into the sediment below. Internal reflectors show basinfill. Depth is almost exactly 2900 metres.
-- I
IOC Technical Series No. 52 Page 124
inferred volcanic edifices
1 km
SSE NNW
1t1 km--w
Fig. 83. Highly backscattering conical mounds crossed by line ORAT-I 1 which are likely to be volcanic in nature
Line ORAT 12
This line was collected on 4th August 1997, between 05:34 and 11:57 GMT, and runs at about 45” to the downslope direction. The profile is on the lower part of the slope, west of ORAT 11.
It crosses an area which, on GLORIA data, is characterised by arcuate features resembling slump escarpments. This area is different in character again from the mound structures seen at the base of the slope further north and east and from those upslope. Overall slope morphology steepens downprofile, being steepest at the mid-section, before the slope gently descends to the flat plain at its foot. Once again, therefore, overall slope morphology is convex for the most part.
The uppermost part of the line shows a featureless region of medium backscatter with some very occasional, very small specks of higher backscatter. The profiler record shows some penetration of apparently stratified material here.
Further downslope there three regions of high backscatter which cross-cut the profile along the slope (Fig. 84). They are narrow (3-400 metres), linear features, 2.0 km or more long with a rough texture. These have a narrow (-1-200 metres) region of medium-to-high backscatter downslope.
Three negative topographic features with a steep downslope and gentler, concave upslope shape are seen. The downslope drops 150, 100 and 50 metres respectively, and the base of the depressions appears to contain a sediment package. It is this latter, which is apparently imaged as the band of medium-to-high backscatter on the sonar.
The lowermost part of the profile from 09:OO is an area of very variable backscatter. It shows a 1 km* region of low backscatter on the northern side of the profile, with a neck feature upslope. The profile here is relatively level and smooth and there is some penetration into the sediments. This feature appears to be a downslope flow, possibly of sand because of the very low backscatter. At the
base of the slope are two subcircular high backscatter features of rough surface texture. One is crossed directly and shows a hummocky, peaked profile.