British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3 LA ... depths between 100--150 m and known as the Witch Ground Basin (Fig.
Marine Geology, 61 (1984) 85--93 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
85
Letter S e c t i o n A RELICT ICE-SCOURED NORTH SEA
EROSION
SURFACE
IN THE CENTRAL
M.S. STOKER and D. LONG
British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3 LA (U.K.) (Received March 30, 1984; revised and accepted June 25, 1984)
ABSTRACT Stoker, M.S. and Long, D., 1984. A relict ice-scoured erosion surface in the central North Sea. Mar. Geol., 61: 85--93. The interpretation of shallow seismic records from the central North Sea has revealed the existence of an irregular erosion surface within late Pleistocene sediments. A morphological study of this surface has identified two main types of relief: (1) an irregular, rough topography with depressions varying in depth from 1 to 6 m, and in width from 25 to 300 m; and (2) a much smoother topography with relatively few depressions. On a palaeobathymetric map the rough topographyextends from ca. 110 to 160 m below sea level (OD), while the smoother topography extends beyond the 160 m below OD contour. This surface is interpreted as an ice-scoured erosional feature formed by the grounding of sea ice in a shallow shelf environment. The stratigraphie position of the ice-scoured surface shows it to be a relict late Weichse]ian feature formed at ca. 17,000-18,000 B.P.
INTRODUCTION The area of study extends from 57°30 , to 58°50'N, and from 0 ° to 1°30'E (Fig. 1), and this work forms part of the British Geological Survey (BGS) regional Quaternary mapping programme of the UK continental shelf. During the course of regional mapping studies, a distinct erosion surface within late Pleistocene sediments has been identified on shallow seismic records. A morphological study of this surface, combined with a knowledge of the local Quaternary geology has enabled us to establish the palaeoenvironmental conditions responsible for its formation. This study is based on the interpretation of shallow seismic records collected from the study area in the summers of 1980 and 1981. The locations of the geophysical survey lines axe shown in Fig. 1. The survey equipment included a Huntec deep-towed boomer (6 kV) and a UDI 50 kHz side-scan sonar. The present-day bathymetry (Fig. 2) is based on echosoundings.
0025-3227/84/$03.00
© 1984 Elsevier Science Publishers B.V.
86
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Fig. 1. Location of study area and geophysical survey grid. GEOLOGICAL
SETTING
The area of study is a basinal depression at present located at water depths between 100--150 m and k n o w n as the Witch Ground Basin (Fig. 2). Overconsolidated middle to late Pleistocene sediments form the rim and floor of the basin, while normal to slightly overconsolidated late Pleistocene to Holocene sediments partially infill the depression (Fig. 3). The irregular erosion surface dealt with in this paper occurs within the late Pleistocene to Holocene sediments and separates an upper, seismically well-layered unit from a lower, seismically structureless unit (Figs. 4 and 5). The upper layered unit is characterised by an upwards transition from glaciomarine to temperate marine sediments, and can be seismostratigraphically correlated with the Fladen and Witch Deposits of Jansen (1976). These sediments have been dated as late Weichselian (ca. 17,000--18,000 B.P.) to Holocene (ca. 8400 B.P.) in age (Jansen, 1976; Jansen and Hensey, 1981). The lower structureless unit comprises glaciomarine sediments which were deposited prior to ca. 17,000--18,000 B.P. and during and/or subsequent to the late Weichselian glacial m a x i m u m (unpubl. B G S data). DESCRIPTION OF THE EROSION
SURFACE
The present,lay seabed bathymetry is illustrated in Fig. 2. which also indicates the k n o w n extent of the erosion surface. It should be noted that
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Fig. 2. Bathymetry of the Witch Ground Basin and known extent of the ice-scoured surface. parts of this surface locally crop out at the present seabed. By removing the upper layered sedimentary unit, a palaeobathymetric map has been compiled for the erosion surface (Fig. 3). The relief of the erosion surface can be divided into two main morphological types: (1) Type-A relief: this type of relief is illustrated in Fig. 4A and 5 and is characteristically rough with U- or V-shaped depressions. These depressions vary from 1 to 6 m in depth, and from ca. 25 to 300 m in width. Narrower features may exist but this is beyond the limits of resolution on the boomer records. This type of relief occurs mostly between 110 and 160 m below sea level (Ordnance Datum = OD) being most irregular in the
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Fig. 3. Palaeobathymetry of the Witch Ground Basin (ca. 17,000--18,000 B.P.) showing the distribution of the two types of surface relief and the trends of exposed ice-scour
marks.
shallower parts of the palaeobasin, particularly on palaeohighs and around the margin of the basin. It is difficult to gauge ,the d e n ~ t y of these features on areal scale due mainly to the variable quality of the seismic data. However, up to 15 scours per km were noted locally. The irregularity of this surface is clearly expressed in the lower part of the overlying sediments (Upper Layered Unit) i n which the seismic layering mimics the surface outline. This implies that these sediments were deposited from suspension in a low-energy environment, and this is probably a major reason why the erosion surface has been so well preserved. This erosion surface locally crops out at the present-day seabed surface,
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?ig. 4. Boomer records (two-way time) showing Late Pleistocene to Holocene seismic facies, separated by (A) a highly irregular surface, md (B) a much smoother surface. The dark line above the seabed in B is a reflectivity marker.
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Fig. 5. Boomer record (two-way time) showing the irregular erosion surface cropping out at the present-day seabed on a topographic high in the east and becoming gradually buried to the west. (N,B.: Horizontal scale: 1,75 km between vertical fix lines. The variable spacing of these lines is due to changes in the ship's speed.) The clark line above the seabed is a reflectivity marker.
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Fig. 6. Side-scan sonar record from the area indicated on Fig. 5. showing exposed icescour mark. A second feature was probably just obscured as the fish was lowered.
91 and although the depressions have been slightly modified and 'smoothed', the irregular nature of the surface is still discernible (Fig. 5). Moreover, in these areas the irregular surface can also be identified on side-scan sonar records (Fig. 6) as faint linear features. While it is impossible to get a measure of their length, their trends can be calculated and these are indicated on Fig. 3. Around the northern margin of the palaeobasin the majority of these features trend sub-parallel to the contours while over intra-basinal highs a general NE--SW trend is observed. This irregular surface has also been noted by other workers in the Witch Ground Basin. From a dense grid of BGS boomer records, which cover a small area in the south of the basin, several buried furrows, 1--2 m deep and up to 200 m wide have been identified and a NE--SW trend was established (A. Judd and M. Hovland, pets. commun.). (2) Type-B relief: This type of relief is illustrated in Fig. 4B and in contrast to the type-A relief the erosion surface is much smoother in appearance. Nevertheless, minor depressions do still occur; these are generally U-shaped and less than 1 m in depth. The relative smoothness of the surface makes it difficult to measure the width of the depressions. In general, this type of relief is restricted to areas within the palaeobasin that exceed 160 m below sea level. MODE OF FORMATION AND PALAEOENVIRONMENTALIMPLICATIONS It is suggested that the irregular erosion surface was formed by ice-scouring on the shelf prior to the depositon of the overlying sediments. This idea is based on a comparison of the morphology of the erosion surface with similar rough undulatory surfaces described from known areas of marine ice-scoured terrain. Echo-sounder and high-resolution boomer and sparker profiles from the Alaskan and Canadian Beaufort Sea (Reimnitz et al., 1972; Reimnitz and Barnes, 1974; Hnatiuk and Brown, 1977), Labrador Shelf (Harris, 1974; Barrie, 1980), Western Grand Banks and Laurentian Channel (King, 1976), Western Lake Superior (Berkson and Clay, 1973), Northeast Atlantic (Belderson et al., 1973) and the Norwegian Continental Shelf (Belderson and Wilson, 1973; Lien, 1983} all show markedly irregular, jagged seabed or palaeo-seabed surfaces. Ice-scour marks from these areas vary from 1 to 10 m in depth, and from 5 to 300 m in width; the wider features probably represent zones of multiple gouging. Moreover, the scour marks appear to be most abundant on topographic highs and shallow banks while their trends are often governed by the local topography. While our data are consistent with an ice-scoured origin for the erosion surface it is further necessary to deduce the ice-type involved, in an attempt to shed some light on the palaeoenvironment. In contrast to the deep Norwegian Trough, where ice-scour marks down to 500 m below sea level imply large-draught icebergs (Lien, 1983), the central North Sea was probably relatively shallow. The stratigraphic location of the erosion surface indicates
92
that it was formed at ca. 17,000--18,000 B.P. Jansen et al. (1979} suggest that sea level at this time may have been as much as 100 m below the present level. If we assume a sea level of - 1 0 0 m OD at 17,000--18,000 B.P. then water depths in the palaeobasin would have had a maximum of ca. 80 m which clearly indicates a shallow shelf environment. This kind of environment is similar to the present-day setting of the Beaufort Sea (Reimnitz et al., 1972, 1977; Reimnitz and Barnes, 1974) and the Bering Sea (Thor and Nelson, 1980; Hunter et al., 1982). In these areas the ice-scoured relief has resulted from the interaction of sea ice with the seabed. Various workers have shown that movement of the sea ice due to wind and tidal currents causes zones of compression and the development of shear and pressure ridges which force ice keels deeper into the water, enough to plough the seabed (Reimnitz et al., 1972, 1977; Thor and Nelson, 1980; Hunter et al., 1982). Pressure ridges with keels extending from 10 to 47 m below the sea surface have been reported from the Beaufort Sea (Hibler et al., 1972; Reimnitz et al., 1977; Hnatiuk and Brown, 1977}. Consequently, the most abundant ice-scoured relief occurs in water depths of 10--50 m, although present maximum activity occurs in the 10--30 m depth range, suggesting that the scouring process is time-transgressive. By analogy with the shallow shelf environments of the Beaufort and Bering seas, we propose that the ice-scoured relief in the palaeo-Witch Ground Basin was formed dominantly by the interaction of sea ice with the seabed. While this process is favoured in preference to large-draught iceberg/seabed interaction, we cannot rule out the possibility of small, locally derived, shallow
