Mapping Doggerland 61 6 Salt tectonics in the Southern North Sea: controls on late Pleistocene-Holocene geomorphology. Simon Holford, Ken Thomson and Vincent Gaffney 6.1 Introduction During the Upper Permian (c. 260 to 251 million years BP) over 1000 m of marine evaporites (the Zechstein Supergroup) accumulated in the North Sea (Cameron et al. 1992). Their subsequent burial promoted mobility on geological timescales resulting in thickness variations from less than 50 m, in regions of salt withdrawal, to more than 2500 m in some of the major salt diapirs (Cameron et al. 1992). Today, the deformed salt deposits encompass a wide range of structural morphologies (Jenyon 1986) and, in places, the crests of salt structures are within 100 m of the seabed (Cameron et al. 1992). This proximity to the present-day depositional surface suggests that the uplift and penetration of the overburden by Upper Permian evaporites may well have influenced the topography and hence depositional systems in this region in the recent geological past. Measured uplift rates of emergent and immediately subsurface salt diapirs range from 2 to 7 mm yr -1 (Bruthans et al. 2006). This invariably leads to the deformation of the overlying rock layers and hence exerts an important control on synkinematic sedimentation patterns, and hence geomorphic processes. Topographic relief produced when salt approaches the land surface in continental settings can vary from 45 m (Al Salif, Yemen; Davison et al. 1996) to up to 1500 m (Zagros Mountains, Iran; Talbot and Alavi 1996), whilst in offshore settings e.g. the Mississippi Delta; topographic relief varies between 100 and 240 m (Jackson et al. 1994). However, a more appropriate analogy for the influence of salt tectonics on landscape evolution in the SNS during the late Pleistocene-Holocene is that of the Five Islands, south central Louisiana. Located on a low-relief landscape near the western boundary of the Mississippi River delta plain, the Five Islands comprise five salt domes aligned in an approximately north west-south east trend, which have pierced and uplifted overlying late Pleistocene meander belt deposits (Autin 2002). The domes are all nearly circular in plan, surrounded by lowland Pleistocene and/or Holocene delta plain marshes, and attain maximum elevations which range from c. 23 m asl on Jefferson Island to c. 52 m asl on Weeks Island (Autin 2002). The geomorphic impact of the salt domes is most clearly exemplified by Avery Island, where proximal fluvial channels, although modified by engineering in places, show an overall sub-concentric pattern, encircling the salt dome and following the topography closely. 6.2 Relationships between salt structures and late Pleistocene- Holocene fluvial systems. By comparison with the adjacent onshore landscapes of East Anglia and Continental Europe (Belgium, Denmark) which are characterised by similar subsurface geology, the late Pleistocene-Holocene landscape of the SNS is likely to have been defined by a low-relief, relatively flat land surface. Once Holocene modifications due to sediment accretion (e.g. sandbanks; Stride et al. 1982) or erosion (e.g. by tidal scouring) are factored into consideration, the present-day bathymetry of the SNS reveals a relatively flat surface with water depths of only 20 to 40 m in the study area (Cameron et al. 1992). It is difficult to reconcile the present-day bathymetry to any distinct structural control by salt tectonics, but it is likely that the subaerial late Pleistocene-early Holocene landscape may well have been influenced by near-surface salt bodies e.g. in the form of relative topographic highs above salt cored anticlines. There is evidence to support this hypothesis from 3D seismic data from the north of this study area. Figure 6.1 presents a series of timeslices (amplitude, Hilbert transform and phase, 0.1 seconds) from the south of the project area. Near the centre of the timeslice a series of broadly concentric reflectors, diagnostic of a salt diapir (Stewart 1999) are clearly visible (Figure 6.1b). An arbitrary seismic line through the 3D seismic volume (Figure 6.1e) confirms the structure is a diapir and hence likely to have been expressed by a relative topographic high on the late Pleistocene-Holocene land surface prior to early Holocene marine transgression. There is some faulting associated with the southern flank of the salt diapir (Figure 6.1b). To the south west of the salt structure an approximately west north west - east south east trending sinuous feature is identified from the seismic time slices (Figure 6.1a-d). This feature is interpreted as a fluvial channel. To the southwest of the presumed fluvial channel there appears to be another, more elongate salt structure. The fluvial channel can therefore be interpreted as occupying a relative topographic low, or even a shallow valley, within the late Pleistocene-Holocene land surface, between two relative topographic highs cored by actively upwelling salt. The topographic low may result from the withdrawal of salt in the subsurface, thereby causing the overburden and land surface to downwarp. Several apparent meander loops can be identified within the fluvial channel, and one meander appears to coincide with one of the radial faults associated with the salt diapir, suggesting a further tectonic control on channel geometry and evolution.