Investigations of the bottom current sculpted margin of Hatton Bank, NE Atlantic
Introduction
Sediment drifts are irregular sediment bodies that commonly form positive relief features on the seabed. Such features typically develop in areas swept by bottom currents and frequently where there is a change in seabed gradient, for instance at the base of continental slopes (Stoker et al., 1998). Sediment drifts and commonly associated bedforms, such as sediment waves, are widespread phenomena relating to bottom-current activity along the continental slope, and are exemplified on the NE Atlantic margin by large drift complexes such as the Feni, Gardar and Bjorn Drifts (van Weering and de Rijk, 1991, Bianchi and McCave, 2000). Evidence of bottom-current activity along the North Atlantic margins has been reported from rise areas (Heezen et al., 1966), slopes (Laberg et al., 1999) and deep basin floors (Bianchi and McCave, 2000).
The NW Hatton Bank represents a unique opportunity to examine the interaction of bottom currents with complex seabed topography and to analyse the effects of such currents on the morphology and distribution of sediments along the margin. The NW Hatton Bank margin is a slope located remote from any major terrigenous sediment supply and at present is over 360 km from the closest sediment source onshore. Despite being quite an old margin (break-up occurred ~ 56 Ma) it has received little, if any, sediment supply from the shelf-slope break since the Late Eocene when the present day bottom-current system was initiated in the NE Atlantic Ocean.
This paper presents a detailed analysis of sediment features created by alongslope bottom-current activity and downslope sediment transport on a sediment starved margin in the northeast North Atlantic. Sediments deposited, eroded, and redistributed by near-bottom currents have a number of well known physiographic and acoustic characteristics that permit their identification over wide areas of the Rockall-Hatton Area (Roberts et al., 1979). A dense grid of acoustic data on the northwestern periphery of Hatton Bank now provides insights into the sedimentary processes sculpting the margin. The aims of this paper are: i) to describe the main characteristics of both the depositional (waves, drifts) and erosional (moats, furrows) features created by bottom currents and the controls upon them; ii) to understand the association between sedimentological features and the present bottom-current regime, with particular reference to current pathways and velocity; iii) to investigate the relationship between features related to bottom-current activity and those formed by downslope sediment transport.
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Geological and oceanographic setting
The Rockall Plateau is a bathymetric high located to the west of the British Isles, in the NE Atlantic and is bordered by the Rockall Trough to the east and the Iceland Basin to the west. The Rockall Plateau comprises a series of banks and basins that trend generally in a NE–SW direction with Hatton/Rockall Basin separating Rockall Bank on the eastern side from the smaller bathymetric highs of Hatton Bank and Edoras Bank on the western periphery (Fig. 1). This study will focus on the region to
Physiography of the margin
The slope break along the NW Hatton Bank is located at approximately 1000 m water depth (Fig. 1). The study area comprises the middle and lower regions of the slope, at water depths below 1200 m. The regional slope aspect is to the northwest, predominantly with slope angles < 3°. Locally much steeper slopes are present with gradients of > 6° and in some areas rarely up to 44°. The Maury Channel is located at the foot of the slope, on the basin floor. The presence of the Maury Channel at the base
Survey methods and data
The data presented in this study were collected on the research vessel R.R.S. Charles Darwin in 1999 (cruise CD118), for the purpose of determining the outer limit of the legal continental shelf according to the United Nations Convention on the Law of the Sea. Multibeam echosounder bathymetry data was collected using a 13 kHz hull-mounted Kongsberg-Simrad EM12 system to map the margin morphology, in particular the slope and rise regions (Fig. 1). The system comprises 81 individual echo-sounding
Seabed features
From the multibeam bathymetry various seabed morphologies can be recognised and mapped over the study area. These seabed features are predominantly erosional and depositional sedimentary bedforms, which can be classified into six geomorphological classes, based on the interpretation of the multibeam bathymetry and sub-surface datasets. Assessment of whether these bedforms are active at the present day, and if they represent the present day bottom-current regime will be made in the discussion
Bedform derived bottom-current regime
Our current understanding of the bottom-current regime along the NW Hatton Bank is derived from the regional understanding of the Iceland Basin water mass dynamics (Bianchi and McCave, 2000, Read, 2001). However, detailed studies of bottom currents around Hatton Bank were limited because of a lack of suitable datasets. The available oceanographic data indicates the currents entering the Iceland Basin form a cyclonic circulation system with a broad south to north flow direction in our study area
Summary and conclusions
New acoustic profiles and high-resolution swath bathymetry collected across the NW Hatton Bank margin show a variety of acoustic facies and morphological features that indicate these sediments have been sculpted predominantly by along-slope processes and to a lesser extent downslope processes. The main findings of this study are:
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The NW Hatton Bank margin is an area of diverse morphology with variable slope gradients, a range of topographic obstacles such as igneous cones, erosional remnants and
Acknowledgements
The authors would like to acknowledge the Department of Trade and Industry and National Environment Research Council for funding data collection through the CD118 cruises. We thank the British Geological Survey for providing seismic profiles along the Hatton margin. The editor David Piper and the two reviewers are also thanked for their constructive comments and reviews.
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