PRESERVATION POTENTIAL

Question 2: What is the preservation potential of landscapes on the continental shelf over the Quaternary period?

The data presented in Chapter 6 demonstrate that on a paraglacial continental shelf, there is a bias towards preservation of landforms and sediments corresponding to the last glacial- interglacial cycle. Preservation of deposits associated with preceding stages is low as the continental shelf environment becomes a zone of sediment recycling over multiple sea- level cycles. In a subsiding basin, e.g. the southern North Sea, the stratigraphic record of the Quaternary has a greater preservation potential and has been widely used in palaeoenvironmental reconstructions (Sejrup 1987; Busschers et al., 2007; Meijer and Cleveringa, 2009; Hijma and Cohen, 2010). In an area experiencing gradual uplift, formation of terrace staircases preserves a fragmented record of palaeoenvironmental change. Locally, in the eastern English Channel, bedrock terraces are stepped and a relative sequence of events can be determined. Where sediment overlies these terrace surfaces, establishing a depositional age using OSL provides a framework to constrain erosion events (Chapter 6). However, not every erosional period is represented by vertical incision. On a relatively broad flat alluvial plain, such as the exposed continental shelf, erosion through lateral planation creates a bedrock strath terrace (e.g. Lewin and Gibbard 2010) that is the product of multiple phases. Separating a composite erosion surface into a series of discrete phases is problematic. Where sediments overly a distinct morphological feature, e.g. alluvial deposits within a channel meander, a link between erosion and deposition can be made. However, sediments overlaying terraces on the continental shelf are susceptible to reworking over several sea-level cycles creating a more complicated sedimentary record, with significant hiatuses, than envisaged in classical studies of fluvial response to changes in climate and sea level (e.g. Blum and Törnqvist, 2000).

A single OSL age of 176.6 ± 20 ka from sediments preserved in the eastern English Channel (Table 6.3) demonstrates that there is the potential to preserve remnants from preceding

Page| 133 stages. In this case, the sediment was preserved as an alluvial terrace at the margin of a deep palaeovalley (SB Palaeovalley) that was incised during the last glacial period. Preservation of such deposits appears to be rare. However, identification using stratigraphic concepts alone is difficult due to the high degree of variability and erosional truncation observed in both seismic and core records. Despite using the best available data to develop the stratigraphic framework presented in this thesis, to fully understand the preservation potential of landforms and sediments on the continental shelf, 3D seismic data combined with a wealth of chronometric data would be required.

Thus far it has been demonstrated that predicting the preservation potential of landscapes at the Quaternary timescale can be problematic. Using Hastings Bank as an example, the preservation potential of coastal facies over shorter timescales (103 yrs) can be assessed. Typically, coastal facies preserved in the rock record are the product of prograding shorelines during regression (e.g. Storms and Swift 2003; Hampson and Storms 2003; Armitage et al., 2004; Jackson et al., 2010). However, there is increasing evidence to suggest coastal facies, particularly those associated with barrier systems, can be partially preserved during transgression (e.g. Forbes et al., 1991; Roy et al., 1994; Storms et al., 2008; Hijma et al., 2010; Chapter 4). Present understanding of what determines preservation of transgressive coastal deposits offshore of a highstand shoreline is based on numerical simulations that consider the interactions between accommodation created by relative sea-level rise and sediment availability (Storms et al., 2002; Cowell et al., 2003; Storms and Swift, 2003). The stratigraphy preserved at Hastings Bank, constrained with OSL ages, presents an opportunity to test these models using geological data.

Using the landscape evolution model presented in Chapter 4, it is proposed that preservation of barrier deposits during transgression is a function of style of shoreline retreat, i.e. overstepping vs. rollover. This is conditioned by the rate of transgression, sediment availability and accommodation created by rising sea-levels and inherited bathymetry. At this stage there is no chronometric data available to constrain the timing of retreat. An age of 5.3 ± 0.5 ka was calculated for washover sediments providing a minimum age for barrier overstepping. However, according to relative sea-level data (Fig. 7.1) the barrier was submerged in water depths >10 m at this time suggesting overstepping occurred during an earlier phase of transgression. Assuming wave base was similar to that observed today at the coast of south-east Britain, i.e. <1 m (Grochowski and Collins, 1994), according to relative sea-level data (Fig. 7.1) the barrier was beyond the limit of reworking

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by waves after ca. 7.5 ka. This indicates that the reliability of the age determined from washover sediments is questionable (see section 7.4).

Shoreline progradation occurred between 8.4 ka and 7.8 ka, and it is probable that overstepping occurred not long after this phase of progradation when sediment availability remained high enough to prevent reworking of the shoreface. It is not possible to resolve inflections in the rate of relative sea-level rise over this time period, such as those associated with the 8.2 ka event, using the data presented in this thesis. However, by assuming no major changes in the rate of relative sea-level rise over the lifetime of the barrier (ca. 1 ka), it is important to consider the style of retreat and thus preservation potential of coastal facies in terms of ‘coastal resilience’, i.e. the self-organising ability of the coast to respond dynamically to external drivers in a sustainable manner (Klein et al., 1998). This is contrary to common preconceptions where high ‘resilience’ is used to describe an immobile, morphologically robust landform, i.e. one that does not respond to change dynamically (e.g. Long et al., 2006). Here, as an alternative, the term ‘inertia’ is used to define response, in terms of mobility, of coastal landforms to changes in external agents. Prior to initial overstepping (‘sediment surplus’ Chapter 4) the barrier was resilient to change, as demonstrated by breaching and internal reworking of sediment to compensate for accommodation in the back-barrier environment. Despite being resilient, the barrier had high inertia and was static due to obstacles created by basement topography. A morphological threshold was reached when the back-barrier was filled and the barrier switched state to one of low (or dynamic) resilience and low inertia (rollover). However, the barrier did not maintain this state of resilience during retreat and phases of ‘sediment deficit’ overstepping represent periods of relatively higher resilience. Whilst rising sea-level is an absolute requirement for barrier overstepping, changes in the mode of resilience driven by internal response to this forcing agent, can influence the style of barrier retreat and preservation potential of coastal facies.

The preservation of landforms and sediments on the continental shelf in response to repeated sea-level cycles at the Quaternary timescale has been established. However the persistence of these landforms in the future and their preservation potential in the rock record is low in a basin that is not subsiding. A relative sea-level fall would expose the continental shelf and fluvial and terrestrial processes would rework any existing sedimentary records. Given the low preservation potential of sediments from preceding glacial-interglacial cycles, Weichselian to Holocene environments would be removed from

Page| 135 the stratigraphic record or, at best, highly reworked. Where subsidence is negligible, the best means of preserving a sedimentary record is through vertical incision and the creation of terrace staircases, driven by faster rates of uplift or increasingly lower base-levels. Identification of terraces would require a morphological perspective of the erosion surface, which can be achieved in the sub-surface using 3D seismic data. It is expected that Quaternary continental shelves in paraglacial settings will be represented in the stratigraphic record by an erosion surface that encompasses >2 Ma of geological history. This surface would be the product of multiple processes operating over high frequency, high magnitude climate and base-level changes. However, extracting this information will be extremely difficult and a bias towards the sedimentary processes operating during the final stages of formation will always remain.

In conclusion, sedimentary and erosional landscapes preserved on a non-subsiding continental shelf are biased towards the processes operating in last glacial-interglacial cycle due to high degrees of reworking and modification, driven by dramatic fluctuations in sea- level at this timescale. The record is generally fragmented. However, the results demonstrate that it is possible to preserve remnants of preceding stages. Preservation potential is the greatest where valley incision creates terrace staircases. Erosion surfaces are composite and reflect multi-phase processes operating over long periods (104 yrs) of time. Predicting preservation potential is problematic as sedimentary regimes vary spatially across the shelf and can be influenced by local conditions such as topography. The results demonstrate that it is possible to preserve coastal landscapes during transgression thus changing perceptions of coastal response to relative sea-level rise. Hastings Bank may be one of the most exceptionally well preserved examples of a drowned barrier complex identified to date.