INTRODUCTION

Many examples of landscapes that were submerged and preserved by post-glacial sea-level rise have been identified due to advances in geophysical survey techniques and increasing availability of commercially-acquired data (Browne, 1994; Fitch et al., 2005; Storms et al., 2008; Shaw et al., 2009; Kelley et al., 2010; Timmons et al., 2010). These drowned landscapes and their component landforms are important sedimentological and geomorphological archives that are the basinward equivalent of deposits preserved on land. Study of these preserved landscapes can be a means for better predicting coastal landscape response to future climate change by providing important constraints on our understanding of how systems respond to abrupt environmental change. When continental shelves emerge in response to relative sea-level fall in ice-marginal settings, fluvial and periglacial processes become the principal factors shaping the landscape. Commonly, the deposits preserved on the continental shelf in these settings are limited to former river valleys and their associated interfluve/terrace surfaces (Velegrakis et al., 1999; Lericolais et al., 2003; Proust et al., 2010). Preservation of coastal landforms on continental shelves is rare due to their susceptibility to reworking and erosion as the shoreline migrates landward during trangression (Shaw, 2005). Recent exploration of continental shelves, however, has uncovered an increasing number of early Holocene coastal landscapes (Gaffney et al., 2007; Storms et al., 2008; Hijma et al., 2010) suggesting that the conditions required for preservation of these landscapes are not as unique as once assumed. Comprehensive stratigraphic and sedimentological analysis of such submerged landscapes provides the necessary data with which to constrain the conditions, i.e. rate of sea-level rise, basement topography and sediment supply, under which coastal landforms may become submerged.

The English Channel continental shelf is a fluvial basin that has been periodically flooded by the sea during the Quaternary (Lagarde et al., 2003). Consequently the shelf floor is dissected by a complicated network of palaeovalleys (Shepard-Thorn, 1975; Smith, 1985; Bridgland, 2002; Antoine et al., 2003; Lericolais et al., 2003; Gupta et al., 2007). The present day coastline of the north-eastern English Channel is fringed by a range of coarse clastic barriers and associated back-barrier environments (e.g. Dungeness (Hey, 1967; Greensmith and Gutmanis, 1990; Long et al., 2006; Plater et al., 2009); Start Bay (Hails, 1975); The Crumbles Shingle (Jennings and Smyth, 1987); and Chesil Beach (Bennett et al., 2009; Carr

Page| 33 and Blackley, 1973)). The presence of early Holocene barrier deposits offshore of Start Bay (Hails, 1975) indicates conditions were suitable in this setting for preservation of submerged coastal deposits. Preservation of deposits offshore of other barrier systems on the English Channel coast have not yet been documented.

Figure 4.1: (a) Classification of landforms and sedimentary environments associated with a gravel-dominated barrier complex. MSL is mean sea-level. (b) Conceptual model of barrier retreat through rollover. As the shoreline retreats continuously from a position at MSL 1 to MSL 2 the shoreface and barrier beach is reworked and no deposits are preserved offshore of the new shoreline. (c) Conceptual model of barrier retreat through overstepping. As the shoreline retreats discontinuously from MSL 1 to MSL 2, disequilibrium between the conditions forcing barrier retreat encourage shoreline displacement and back-barrier deposits are preserved offshore. For both a and b, the dashed line shows the position of former shorelines, an erosion surface is represented by the red line and white lines are time equivalent depositional surfaces.

The term ‘barrier’ refers to a range of coastal landforms that have evolved under the predominant influence of waves and net sediment transport pathways (e.g. Plater and Kirby, 2012). A barrier complex or system is defined according to the presence of an attached barrier and/or barrier island that is separated from the mainland by backbarrier

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lagoons/tidal inlets (Oertel 1985). Principal morphological and sedimentological elements of a gravel-dominated barrier complex, classified according to Otvos (2012), are summarised in Fig. 4.1a, and include shoreface, barrier, washover and back-barrier tidal inlet/lagoon landforms. Barrier formation at the most rudimentary level can be explained through the interactions between wave energy, sediment supply (and transport vector), accommodation space and rates of relative sea-level change (Roy et al., 1994). The preservation potential of transgressive barrier deposits is a function of the style of retreat and considered to be relatively low in comparison to deposits associated with regressive coasts (Storms and Swift 2003). During sea-level rise, barriers retreat landwards either through continuous landward migration (rollover) where there is complete reworking of barrier deposits offshore (Swift and Moslow, 1982; Leatherman et al., 1983) (Fig. 4.1b) or through a process of overstepping where there is a landward displacement of the shoreline and partial preservation of back-barrier deposits offshore (Rampino and Sanders, 1980; Rampino and Sanders, 1982; Leatherman et al., 1983; Forbes et al., 1991; 1995; Storms et al., 2008; Hijma et al., 2010) (Fig. 4.1c). Gravel-dominated barriers are more resilient to relative sea-level rise than their sandy counterparts (Long et al., 2006), therefore, continental shelves offshore of present-day gravel-dominated coasts, such as the English Channel, are potential field sites for exploring the conditions required for drowning and preservation of coastal landscapes, with the findings providing an important evidence base for future coastal resource planning.

An integrated dataset of multibeam swath bathymetry and a network of 2D reflection seismic lines calibrated to multiple vibrocores has permitted identification of a previously undocumented, yet exceptionally well preserved, submerged barrier complex at Hastings Bank in the north-eastern English Channel. Here we provide a high resolution stratigraphic framework for deposition at Hastings Bank that is used to reconstruct late Quaternary landscape evolution and explore the interactions between the factors controlling barrier initiation, breakdown, submergence and preservation.

4.2 SETTING

Hastings Bank is a flat area of seabed that presently lies in shallow water depths (<20 m) in the north-eastern English Channel (Fig. 4.2). Part of Hastings Bank is a licensed aggregate extraction area that has been dredged for sand and gravel resources since the early 1990s (BMAPA and The Crown Estate, 2007). The coarse clastic nature of sediment has long been recognised (Hamblin et al., 1992) and the area is referred to on maps as Hastings ‘Shingle’

Page| 35 Bank. However, little is known about the origin and development of the bank, so the term shingle will not be used herein. Hastings Bank is an isolated feature that lies ca. 15 km offshore from the present-day British coastline between Beachy Head and Dungeness (Fig. 4.2). It forms the southernmost extension of a shallowly dipping (<0.2°) planar surface that deepens abruptly (from -20 m to -55 m) at the margin of one of the most prominent morphological features in the English Channel, the Northern Palaeovalley (Smith, 1985). Seafloor morphology in the eastern English Channel is characterised by a complicated network of palaeovalleys (Dingwall, 1975; Smith, 1985) that, in part, formed through fluvial incision when present-day rivers extended offshore in response to base-level fall during Quaternary cold phases (Bellamy, 1995). Formation of the Northern Palaeovalley has been linked to catastrophic drainage through the Straits of Dover during the Middle Pleistocene (Smith, 1985, 1989; Gupta et al., 2007). The English Channel continental shelf has been periodically flooded by post-glacial sea-level rise during the Quaternary leaving behind a complicated sedimentary record that is an amalgamation of fluvial, coastal and marine deposits.

Bedrock geology is controlled by Cretaceous strata of the Weald-Artois anticline (Hamblin et al., 1992). Chalk, Lower Greensand and Weald Clay outcrop at cliffs on the present day coast to the north of Hastings Bank. Significant thicknesses of coarse sediment (<30 m) are usually limited to submerged palaeovalleys offshore or fringing the present day coastline of southeast England (Anthony, 2002). Although the distribution and composition of seabed sediments is relatively well constrained (Cazenave, 2007; James et al., 2011), and the overall thickness of Quaternary deposits has been mapped (Hamblin et al., 1992), the sedimentary processes and environments of deposition preserved in the stratigraphy are poorly documented.

In terms of present-day hydrodynamics, the English Channel is a tide-dominated shelf where sediment transport by waves is limited to water depths of only a few metres (Grochowski et al., 1993). In shallow water depths, significant wave height (Hs) during calm conditions is typically <1 m (Grochowski and Collins, 1994), and during storm events with a return period of 1 year, ~4 m (Henderson and Webber, 1978). Waves are principally derived from the Atlantic Ocean and propagate through the eastern English Channel in a SW-NE direction (Bray et al., 1995). The Hastings Bank area has a moderate tidal range of ca. 2-3 m (Anthony, 2002). Large parts of the English Channel, particularly to the west of Hastings Bank, are sediment-starved and bedrock is at or close to the seabed (James et al.,

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Figure: 4.2: Study area (a) location of Hastings Bank in English Channel; (b) seamless land- sea elevation model for north-eastern English Channel highlighting key morphological features (Digital terrain model ©Intermap Technologies. NEXTMap Britain: Digital terrain mapping of the UK. NERC Earth Observation Data Centre, 2011. Bathymetry © British Crown and SeaZone Solutions Limited. All rights reserved. Product license No. 112010.009); (c) high resolution (1 m grid) bathymetry of Hasting Bank (Cazenave, 2007). Thick black line delimits interpolation grid; (d) locations of vibrocore transects (see Fig. 4.8) including dredging area after BMAPA and The Crown Estate (2007); (e) trackplot showing location of seismic reflection profiles and vibrocores. Thick black line delimits interpolation grid.

2011). These areas are coincident with a bedload parting zone where the strongest tidal currents in the English Channel are recorded (Reynaud et al., 2003). At Hastings Bank, sand

Page| 37 is mobile under present day tidal regime (Dix et al., 2007) and is transported north eastwards towards a bedload convergence zone within the Dover Straits (Grochowski et al., 1993).

In the English Channel, the tidal regime is considered more important than wave climates in transporting sediment at the sea-bed (Kenyon and Stride, 1970; Grochowski et al., 1993: Reynaud et al., 2003). However, during the early Holocene when water depths were shallower, the effect of wave climate on large scale sediment transport was more important than observed today (Neill et al., 2009). In comparison to present-day tidal regime, tidal amplitude during the early Holocene is predicted to have been lower (Uehara et al., 2006). In addition, tidal currents reconstructed for 8 ka are predicted to have been three times greater (Uehara et al., 2006) corresponding to the time when Atlantic waters reconnected with the southern North Sea through the Straits of Dover (Shennan et al., 2000).

4.3 METHODS

4.3.1 Seismic data acquisition and interpretation

The seismic data comprise 619 line–km of shallow, sub-bottom Boomer 2D seismic data collected over three surveys (herein referred to as surveys A, B and C) that have been made available by the Resource Management Association (RMA, comprising CEMEX UK Marine Ltd, Hanson Aggregates Marine Ltd and Tarmac Marine Dredging Ltd) (Fig. 4.2). The surveys used surface-towed boomer sources operating at frequencies of 1-5 kHz that penetrate and resolve unconsolidated sediments up to ca. 50 m below the seabed. Within unconsolidated sediments, this provided a lateral resolution of 1.9 m to 4.2 m and a vertical resolution (1/4 wavelength) of 0.08 m to 0.43 m. Positioning accuracy for each of the surveys was <2 m. An orthogonal grid (interpolation grid) of closely spaced (80 – 120 m), NNW-SSE and NE-SW trending seismic reflection profiles (Fig. 4.2) permits integration and correlation between datasets. Interpretation of seismic reflection profiles at this resolution can resolve the nature, extent and thickness of stratigraphic units. Disturbance of sediment due to dredging activities between individual seismic surveys has led to discrepancies in seismic signatures leading to uncertainty in interpretations within the active dredge zone. Seismic traces from Survey A and Survey B are in the form of analogue paper records and no post- acquisition processing of the seismic pulse was possible. Seismic data from Survey C were logged and processed digitally in the Coda Octopus DA2000 system and were available as

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Coda digital boomer files for post-acquisition processing. Interpretation of seismic stratigraphy is based on Mitchum et al. (1977). High amplitude seismic reflectors separating stratigraphic units have been correlated from line to line across the grid, digitised manually on both analogue and digital records and imported into ArcGIS. Interpolation between cross-lines was carried out using kriging, and isopachs outlining unit geometry (25 m cell size) and thickness (1 m contours) were plotted. Morphology of the bedrock surface was mapped by subtracting unconsolidated sediment thickness from a 1 m grid of the present- day bathymetry. Artefacts of dredging activities and seabed morphology can be seen due to differences in the resolution of interpolated surfaces. Acoustic velocity surveys were carried out in the Hasting Bank region (UTEC, 2005) and typical values for seawater were 1505 ms-1. An acoustic velocity of 1700 ms-1 for subsurface sediments was used to convert two way travel time (TWTT) to depth in metres.

Figure 4.3: Trackplot of seismic reflection profiles and the locations of interpreted seismic line sections presented in figures 4.4 to 4.6. Thick black line delimits interpolation grid.

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4.3.2 Bathymetric data collection and processing

To determine depths to seabed, multibeam swath bathymetric data were collected simultaneously with reflection seismic data during Survey B and Survey C (see section 4.3.1). Full details of bathymetric data collection and processing during Survey B are outlined in Cazenave (2007). A Reson Seabat 8125 multibeam system was used in conjunction with a POS MV precision motion and altitude system to obtain bathymetric soundings during Survey C. Post-processing was carried out using QINSy processing software and relevant corrections and filters were applied including patch test calibration results, removal of outliers with automatic and manual filtering, and post-processing of tidal data. For the purpose of this study, multibeam swath bathymetric data were gridded to 1 m cell size to provide a morphological surface to which all seismic stratigraphic interpolations could be tied. Seazone digital survey bathymetry and digital charted bathymetry (Licence No. 112010.009)were obtained for the eastern English Channel to give a regional perspective of seabed morphology. The data were interpolated using kriging in ArcGIS into a 120 m cell grid size.

4.3.3 Vibrocoring

As part of aggregate prospecting and monitoring surveys, 36 vibrocores tied to seismic lines have been recovered from the Hastings Bank region during surveys in 2003 (RMA vibrocore Fig. 4.2d). The RMA provided data from these surveys in the form of vibrocore logs, photographs and particle size distribution analyses. Targeted positioning of the vibrocores by aggregate companies has led to an irregular distribution of cores, with limited data availability in the central region due to dredging activities. Positioning accuracy is within 2 m of the recorded coordinates. Analysis of vibrocore records has enabled the classification and identification of a number of key lithofacies, however given the geotechnical nature of the reports, any small-scale sedimentological details that are vital for interpreting depositional setting were not recorded. An additional six vibrocores were collected during March 2010 to supplement existing vibrocore records, to ground-truth lithofacies to seismic facies, and to provide material for sedimentological analysis (vibrocore sample location Fig. 4.2d). The cores were collected using a high-powered C-CoreHP vibrocorer and recovered in 85 mm-diameter opaque plastic liners. Maximum penetration of the vibrocorer was 5 m, with a maximum recovery of 2.70 m. Cores were immediately sealed and transported to the laboratory where they were split, logged, photographed and

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sampled. Lithological data collected from each vibrocore dataset forms the basis of lithostratigraphical analysis and allows differentiation of depositional environments.