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Contrasting deep-sea depositional systems in the Hering Sea

N.H. Kenyon1 andJ. Millington2

'Institute of Oceanographic Sciences, Deacon Laboratory, Brook Road, Wormley, Godalming, Surrey GU8 5UB, UK

'Depanment of Geology, University of Leicester, Leicester LEI 7RH, UK

lntroduction GLORIA long-range sidescan sonar records and 3.5 kHz seismic profiles of the Bering Sea basin floor show remarkable patterns of acoustic backscatter. The most recently active depositional systems are those from the northern slopes of the Aleutian volcanic chain rather than from the deeply canyoned slopes bordering the Bering shelf. Tributary canyons/channels, with steep axial gradients and low sinuosity, feed extensive, presumably sandy, channel-mouth Iobes.

There is a contrast in style and size between the systems. The Umnak system has deep channels and a channel-lobe transition zone within which are zones of possible longitudinal and transverse scours, implying that this is a detached lobe. Depositional zones include a braid-like pattern and extensive sheets of unknown composition, up to 10m thick. Flows are relatively unconfined and believed to be relatively rapid. The Pochnoi system has a shallower feeder channel and a lobe with a branching pattern of small channels, implying that this is an attached lobe. Flows are relatively confined and believed to be relatively slow.

Areas of overall high acoustic backscatter from beyond many deep-sea channels have been mapped by the GLORIA long-range sidescan sonar. The first such area to be described was on the Orinoco Fan and showed a braid-like pattern with a relief that was too low to be measured by the high resolution profilers used at that time (Belderson et al. 1983). 1t was considered to be a sandy channel-mouth lobe. Of the many probable sandy Iobes that have since been

mapped by sidescan sonar a few have been looked at in more detail, as for instance on the Monterey Fan (Gardner et al. 1991) and on the Mississippi Fan (Twichell et al. 1992). One of the most spectacular looking of such regions is described here from the GLORIA coverage of the Bering Sea basin. This coverage was obtained during the project to map the deep seabed off the USA, undertaken by the USGS and lOS. The survey has a track spacing of c. 30 km in the deeper water and coverage is complete, apart from the area near to the centre of each swath. A digitally enhanced mosaic has been made (Bering Sea EEZ­SCAN Scientific Staff 1991). High resolution seismic profiles, taken with a 3. 5 kHz profiler, have been correlated with the sonographs.

Bering Sea basin The Bering Sea basin is a back arc basin between the volcanic islands of the Aleutian chain and the broad Bering shelf. The Bering shelf margin is dissected by some of the largest known canyons. The Zhemchug Canyon, for instance, is up to 1600 m deep and 100 km wide (Carlson and Karl1988). The GLORIA data show that these canyons are not at present connected to the depositional areas of the deep basin. Some of the channels beyond the Bering shelf canyons are at present apparently hanging above the Bering Channel, which drains slopes north of the Aleutian Islands.

The most recent flow paths to the deep basin are from the slopes of the Aleutian Islands (Fig. 27.1). The Aleutian Island arc is by far the more seismically active of the two margins, with an earthquake above 6.3 occurring every year or so (International Seismological Centre 1975). The paths head towards the deepest part of the basin, where there is a basin plain near the foot of the Bowers Ridge with a depth of c. 3750 m.

Feeder systems The underwater slopes of the volcanic islands have a well-developed system of tributary canyons and gullies with a high order of branching (Figs 27.2 and 27.4(a)). The canyons are less than 800 m deep. The Umnak

Atlas of Deep Water Environments: Architectural style in turbidite systems. Edited by K.T. Pickering, R.N. Hiscott, N.H. Kenyon, F. Ried Lucchi and R.D.A. Smith. Published in 1995 by Chapman & Hall, London. ISBN 0 412 56110 7.

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Channel is about 400 km long and in its middle part is c. 350m deep and runs along the foot of the volcanic island slopes. The width of the floor of the channel is c. 2-3 km. There is a well-developed levee on its northern (right-hand) side (Fig. 27.5(b)). The backscattering Ievel is low within the channel, apart from some strongly backscattering patches that are found near the channel bends and that may be depositional bars (Fig. 27.4(a)). The sinuosity is low, less than 1.15, and the maximum gradient is c. 1 in 10. The mouth of the channel is trumpet-shaped, widening to 50 km (Fig. 27.4(b)). The width to depth ratio increases from c. 20 at the foot of the slope to c. 1000 near the mouth.

The Pochnoi system is similar in having many tributary channels leading into a single channel with a low backscattering floor and a sinuosity of only 1.1 (Figs 27.2 and 27.6). The channel differs from the Umnak Channel in having a slightly lower maximum axial gradient of c. 1 in 20, in being less than 60 m

ALASKAN SHELF

170°

58°

5Y

51°

Fig. 27.1. Location of the study area in the eastern Bering Sea basin showing the recently active turbidite systems draining to the north of the Aleutian Islands. Supposed sandy Iobes are in grey.

deep, in having less developed levees and in having a thaiweg with a fairly constant width of 1 km. The gently sloping walls of the channel (Figs 27.7(a) and (b)) are highly backscattering. The width to depth ratio of the middle reaches of the channel (with width measured as bank full width) is about 150. At the distal end the channel has no measurable relief (Fig. 27.7(c)).

None of the other feeder systems in the area have a well-developed leveed channel.

Presumed sandy channel­mouth Iobes The strongly backscattering areas at and beyond the ends of the channels have a variety of shapes, sizes and patterns. For the most part there is very low relief. The overall gradients are less than 1 in 500.

The Umnak lobe

A number of zones have been mapped that are characterized by different types of acoustic pattern on both sonographs and profiles. The lobe covers an enormaus area, 400 km lang and 120 km across.

Zone 1. Longitudinal bedforms Theseare found in the trumpet-shaped mouth of the channel and beyond (Fig. 27.4(b)). They are up to 15 km lang, very elongate and parallel to the expected flow direction. The profiles show little penetration and some discontinuous sub-bottom layers (Fig. 27.5(b)). It is not known whether depositional bedforms in coarse-grained sediment are present but, because of the proximity to Zone la, erosional bedforms are considered to be likely.

Zone 1 a. Crescentic scour holes These are in an area in the north (right-hand side) of the channel mouth (Figs 27.2 and 27.3). They are seen on the sonographs (Fig. 27.4(b)) tobe relatively small (up to 2 km across) and consistently transverse to and concave down the expected flow direction. The profiles (Fig. 27.4(d)) are like those of Zone 1, apart from showing eroded areas, up to 5 m deep, that are believed to be !arge scour holes.

Zone 2. Braid-like bars The most extensive zone has a braid-like pattern (Figs 27.2 and 27.4(c)). The individual bedforms have a high Ievel of backscatter, are rhomboid or lozenge-shaped and elongate down the expected flow path. There is some down flow asymmetry in plan view, as the upstream ends are in some cases more rounded and wider than the downstream ends. Between the lozenge-shaped features there are narrow strips of weakly backscattering seabed. The profiles show limited penetration and no measurable relief. The pattern is identical to that previously described from the Orinoco Fan (Belderson et al. 1983) and is considered to be possible braid bars in sandy sediment.

Zone 3. Thick sheets Beyond the area of the braid-like pattern with no measurable relief there are high backscattering sheets that have a thickness of between c. 3 and 10m (Figs 27.2 and 27.3). The sheets arealso braid-like in shape. Same of the individual sheets are very extensive and all are longitudinal. There is an ornamentation at the surface that is noticeably crescentic and concave down the expected flow path. The narrow strips of low backscatter between the sheets are negative in relief, i.e. channel-like.

The Pochnoi lobe

The area of high backscatter near the end of the Pochnoi Channel is roughly circular with a diameter of c. 60km. Channel-like features of low backscatter splay out over a sector of c. 120° (Figs 27.2 and 27.6). The 'channels' become narrower and shallower and eventually pinch out distally. Bordering each of them is ground with higher backscattering Ievels on GLORIA sonographs. Each 'channel' appears to have formed at a different time. There are also subcircular to ragged­shaped patches of low backscatter that are surrounded by the channels and their associated borders of high backscatter (Fig. 27.6). Individuallenses of sediment (Fig. 27.7(c)), that are usually up to 5-lüm thick, are believed to correspond on the sonographs to both the low backscattering 'channels' and their higher backscattering borders.

The basin plain All of the !arger depositional systems head towards the deepest and flattest part of the Bering Sea basin. GLORIA backscattering here is uniform and of a low Ievel. The 3.5 kHz profiles show relatively deep penetration of c. 60 m, with continuous sub-bottarn layers.

Discussion The volume of sediment input into these depositional systems is probably fairly low because the subaerial drainage basins of the Aleutian Islands are relatively small, less than 1000km2 for the islands near the Umnak system. The sediments in these depositional systems have been little sampled. The presence of turbidite sands in four cores from the area of Fig. 27.2 has been noted by Gardner et al. (1982). They are probably volcaniclastic turbidites because of the provenance of the flows.

The channels of the Umnak and Pochnoi systems arerather similar in style. With low sinuosity and steep gradients the channel systems are typical for ones in which bed load is the dominant component of the flows (Clark et al. 1992). The main difference is the greater depth of the Umnak Channel and its trumpet­shaped mouth. The strongly backscattering patches at the bends of the Umnak Channel (Fig. 27.4(a)) are in the expected position and have the right shape to be depositional bars.

The Iobes of the two systems are very different in appearance. The zones of presumed erosional

bedforms in and beyond the Umnak channel-mouth are in keeping with this being an area of detachment (Mutti and Normark 1991). The nature of the extensive thick deposit beyond is unknown. It may be the product of a single flow or of multiple flows. The downpath arrangement of the bedform zones is in keeping with their being in equilibrium with decreasing peak flow speeds, the flow expanding and the peak flow decreasing as in the zonation observed at the orifice of a jet. The pattern of the Pochnoi lobe is very similar to that of the Iobes on the Mississippi Fan mapped by medium-range sidescan sonar (O'Connell et al. 1991; lobe 9 of Twichell et al. 1992). As on the distal Mississippi Fan, and the Navy Fan (Normark and Piper 1983/1984), it is possible to distinguish sublobes, with each being fed from a different and newly avulsed channel. The channels have been mapped into the middle of the Iobes and there is no indication of any extensive zone of erosion, unlike in the Umnak system. Thus these Iobes are taken as being relatively attached to the main feeder channel.

There is as yet no full explanation as to why these two systems have such a cantrast in sedimentary geometry and consequently in flow type. The Umnak flow(s) must have been very much !arger and of a higher velocity than those of the Pochnoi system.

Heferences Belderson, R.H., Kenyon, N.H., Stride, A.H. and Pelton, C.D.

1983. A 'braided' distributary system on the Orinoco Deep-sea Fan. Marine Geology, 56, 195-206.

Bering Sea EEZ-SCAN Scientific Staff 1991. Atlas ofthe Exclusive Economic Zone, Bering Sea. US Geological Survey Miscellaneous Investigations Series 1-2053, 145 pp.

Carlson, P.R. and Kar!, H.A. 1988. Development of !arge submarine canyons in the Bering Sea, indicated by morphologic, seismic, and sedimentologic characteristics. Geological Society of America Bulletin, 100, 1594-1615.

Clark, J.D., Kenyon, N.H. and Pickering, K.T. 1992. Quantitative analysis of the geometry of submarine channels: implications for the classification of submarine fans. Geology, 20, 633-636.

Gardner, J.V., Dean, W.E., Klise, D.H. and Baldauf, J.G. 1982. A climate-related oxidizing event in deep-sea sediment from the Bering Sea. Quaternary Research, 18, 91-107.

Gardner, J.V., Field, M.E., Lee, H., Edwards, B.E., Masson, D.G., Kenyon, N.H. and Kidd, R.B. 1991. Ground-truthing 6.5-kHz side scan sonographs: What are we really imaging? Journal ofGeophysical Research, 96, 5955-5974.

International Seismological Centre 1975. World Earthquakes,

1964january-1970]une. Bartholomew and Son Ltd,

Edinburgh. Mutti, E. and Normark, W.R. 1991. An integrated approach to

the study of turbidite systems: In: Weimer, P. and Link, M.H. (eds) Seismic Facies and Sedimentary Processes of Submarine Fans and Turbidite Systems. Frontiers in Sedimentary Geology, Springer-Verlag, pp. 75-106.

Normark, W.R. and Piper, D.J.W. 1983/1984. Navy Fan, California Borderland: growth pattern and depositional processes. Geo-Marine Letters, 3, 101-108.

O'Connell, S., Ryan, W.B.F. and Normark, W.R. 1991. Evolution of a fan channel on the surface of the outer Mississippi fan: evidence from side-looking sonar. In: Weimer, P. and Link, M.H. (eds) Seismic Facies and Sedimentary Processes of Submarine Fans and Turbidite Systems. Frontiers in Sedimentary Geology, Springer­Verlag, pp. 365-381.

Twichell, D.C., Schwab, W.C., Nelson, C.H., Kenyon, N.H. and Lee, H.J. 1992. Characteristics of a sandy depositional lobe on the outer Mississippi fan from SeaMARC 1A sidescan sonar images. Geology, 20, 689-692.

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Fig. 27.2. Line drawing of the patterns of acoustic 54 ° backscatter on the GLORIA sonographs. Lower Ievels of backscatter on otherwise strongly backscattering channels and Iobes are here shown as black (as on the GLORIA sonographs). Some of the boundaries are not easy to determine as there is in reality a wide range of ~ tones.

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Fig. 27.6. GLORIA sonographs of the distallobe of the Pochnoi system (higher Ievels of acoustic backscatter are white). The position of two of the profiles in Fig. 27.7 are shown.

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Fig. 27.7. 3.5kHz profiles from the Pochnoi system. (a) and (b) The channel has a steep-sided thaiweg with a flat floor, flanked by broad, gently sloping and possibly terraced slopes. The bedded horizons outside the channel are believed to be overbank deposits. (c) A crossing of five sublobes with a thickness of up to about 10 m each, corresponding to the sharp, narrow bottarn echoes. On the GLORIA sonographs the sublobes are seen as areas of low backscatter with higher backscattering borders. Ground truth from the Monterey Fan lobe (Gardner et al. 1991) suggests that such features may consist of massive sands or sandy debris flow deposits. The sublobes are separated by areas with prolonged bottarn echoes that, by analogy with the Monterey Fan, may consist of sandy turbidites.