This is the author accepted manuscript. The final version is available from the Geological Society of London via https://doi.org//10.1144/M46.9

Dowdeswell, Julian

Ottesen, D

English

Apollo (Cambridge)

3D-seismic imagery of deeply buried iceberg ploughmarks in North Sea sediments  J. A. DOWDESWELL1* & D. OTTESEN2 1Scott Polar Research Institute, University of Cambridge, Cambridge CB2 1ER, UK 2Exploro AS, Innherredsveien 7, N-7014 Trondheim, Norway *Corresponding author (e-mail: jd16@cam.ac.uk)  Where the keels of icebergs and sea ice touch the seafloor, deformation and reworking of the sedimentary substrate may take place. Point impacts produce pits in the seafloor, whereas continuing ice drift results in the formation of linear to curvilinear depressions referred to as ploughmarks, furrows or scours (Woodworth-Lynas et al. 1991). Sea-ice keels only exceptionally exceed about 20 m below water, where pressure-ridging takes place. By contrast, the keels of icebergs can range from tens of metres for small irregular bergs produced from grounded tidewater glaciers, to over 500 m where calving takes place from major ice-sheet outlet glaciers (Barnes and Lien 1988; Dowdeswell et al. 1993; Dowdeswell & Bamber 2007). The presence of large iceberg ploughmarks in the geological record therefore indicates that ice was present at sea-level to release bergs into the adjacent ocean waters. 3-dimensional (3D) seismic data are required, however, in order to identify these features on buried shelf surfaces (Fig. 1).  Description     Two 3D-seismic cubes from the central North Sea Basin (57°N 2°E) were examined (Dowdeswell & Ottesen 2013), as part of a wider study of the sedimentary architecture and basin-fill of the North Sea during the past 2.7 million years (Ottesen et al. 2014). The cubes were C08 (62 by 50 km and 3,100 km2) and E08 (less regular in shape and 1,900 km2) of the Petroleum Geo-Services (PGS) North Sea MegaSurvey (Fig. 1f). The cubes had a thickness of Quaternary sediments of between 600 and 900 m, calculated using a sound-velocity varying between 1850 and 1900 m s-1 across the cubes. A series of clinoforms, indicating the presence of palaeo-shelves in the Early Quaternary seismo-stratigraphic record of the central North Sea, is shown in Figure 1e. The Base Naust-equivalent reflection represents the base of the glacier-influenced sedimentary package. Examination for the presence of linear and curvilinear features on buried surfaces within the cubes took place through analysis of horizontal time slices by stepping down from the seafloor every 10 ms through each cube (Dowdeswell & Ottesen 2013).     Linear to curvilinear features, often distributed in a chaotic pattern, were observed between about 270 and 900 ms within the Early Quaternary sediments of the two 3D cubes in the central North Sea (Fig. 1a-d). The mean width of the features was approximately 50-60 m and average length of about 3 km. The features are depressions, typically a few metres deep relative to the general level of the palaeo-shelf surface in which they appear. They are sometimes fringed by berms on one or both sides (Fig. 1b). Abrupt shifts in feature orientation are sometimes observed, occasionally accompanied by the occurrence of rounded pits a few tens of metres in diameter (Fig. 1d).   Interpretation    The buried linear to curvilinear depressions are very similar in morphology to seafloor landforms interpreted to be produced by the ploughing action of iceberg keels through soft sediments in both bathymetric and earlier side-scan sonar imagery of modern high-latitude shelves (e.g. Barnes & Lien 1988; Dowdeswell et al. 1993; Dowdeswell et al. 2010). The icebergs producing these Early Quaternary ploughmarks are thought to be derived from a full-glacial Scandinavian Ice Sheet to the north (Ottesen et al. 2014). Similar buried landforms, also interpreted as iceberg ploughmarks, have been identified in, for example, the Dutch sector further south in the North Sea (Kuhlmann & Wong 2008).    The identification of iceberg ploughmarks buried beneath hundreds of metres of Quaternary sediments in the central North Sea Basin is only possible where 3D seismic data are available. Even relatively dense 2D seismic grids are usually insufficient to allow the recognition of these often chaotically distributed sets of landforms.     The occurrence of iceberg ploughmarks buried deep within the Quaternary sedimentary record is a clear indicator of the presence of ice sheets to sea level. In the same way that iceberg-rafted debris found within fine-grained marine sediments in ocean basins has been used as an indicator of the inception of an Ice Age (e.g. Jansen & Sjøholm 1991), the presence of iceberg ploughmarks can be used to infer the start of a glacial period in the Earth’s past record (Dowdeswell et al. 2007). The long-term preservation of ploughmarks in the kilometre or so of Quaternary sediments often present on high-latitude continental margins requires, however, that former continental shelves are preserved within the geological record, rather than being eroded by advancing ice sheets during the successive glacial-interglacial cycles that make up a whole Ice Age.   References BARNES, P. W. & LIEN, R. 1988. Icebergs rework shelf sediments to 500 m off Antarctica. Geology, 16, 1130-1133. DOWDESWELL, J. A. & BAMBER, J. L. 2007. Keel depths of modern Antarctic icebergs and implications for sea-floor scouring in the geological record. Marine Geology, 243, 120-131. DOWDESWELL, J. A. & OTTESEN, D. Buried iceberg ploughmarks in the early Quaternary sediments of the central North Sea: a two-million year record of glacial influence from 3D seismic data. Marine Geology, 344, 1-9. DOWDESWELL, J. A., VILLINGER, H., WHITTINGTON, R. J. & MARIENFELD, P. 1993. Iceberg scouring in Scoresby Sund and on the East Greenland continental shelf. Marine Geology, 111, 37-53. DOWDESWELL, J. A. OTTESEN, D., RISE, L. & CRAIG, J. 2007. Identification and preservation of landforms diagnostic of past ice-sheet activity on continental shelves from three-dimensional seismic evidence. Geology, 35, 359-362. DOWDESWELL, J.A., JAKOBSSON, M., et al. 2010. High-resolution geophysical observations of the Yermak Plateau and northern Svalbard margin: implications for ice-sheet grounding and deep-keeled icebergs. Quaternary Science Reviews, 29, 3518-3531. JANSEN, E. & SJØHOLM, J. 1991. Reonstruction of glaciation over the past 6 m.y. from ice borne deposits in the Norwegian Sea. Nature, 349, 600-603. KUHLMANN, G. & WONG, T. E. 2008. Pliocene paleo-environmental evolution as interpreted from 3D-seismic data in the southern North Sea, Dutch offshore sector. Marine and Petroleum Geology, 25, 173-189. OTTESEN, D., DOWDESWELL, J. A. & BUGGE, T. 2014. Morphology, sedimentary infill and depositional environments of the Early Quaternary North Sea Basin (56°-62°N). Marine and Petroleum Geology, 56, 123-146.  WOODWORTH-LYNAS, C. M. T., JOSENHANS, H. W., BARRIE, J. V., LEWIS, C. F. M. & PARROTT, D. R. 1991. The physical processes of seabed disturbance during iceberg grounding and scouring. Continental Shelf Research, 11, 939-951.      Fig. 1.  (a), (b), (c) and (d) Iceberg ploughmarks in 3D-seismic cube C08 from the central North Sea Basin. The iceberg ploughmarks are shown in four different time slices: (a) 430 ms, (b) 630 ms, (c) 860 ms, (d) 900 ms, with the inset showing two ploughmarks in cross-section. (e) Composite seismic profile through 3D-seismic cube C08, showing the Quaternary seismic stratigraphy of the central North Sea Basin above the Base-Naust reflection (Ottesen et al. 2014). VE x 140. The Upper Regional Unconformity (URU) equivalent (white line) and Base-Naust (black line) horizons mark the stratigraphic boundaries of the Early Quaternary (Ottesen et al. 2014). Dashed blue lines are other major reflections. Red lines are locations of time slices from which the iceberg ploughmarks in (a), (b), (c) and (d) are identified. (f) Location of the study area in the North Sea (red box is 3D-seismic cube; white line is seismic profile; map from IBCAO v. 3.0).  

3D seismic imagery of deeply buried iceberg ploughmarks in North Sea sediments

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3D seismic imagery of deeply buried iceberg ploughmarks in North Sea sediments

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