Submerged landscapes on continental shelves archive drainage networks formed during periods of sea-level lowstand. The evolution of these postglacial drainage networks also reveals how past climate changes affected the landscape. Ice-marginal and paraglacial drainage networks on low-relief topography are susceptible to reorganisation of water supply, forced by ice-marginal rearrangement, precipitation and temperature variations, and marine inundation. A rare geological archive of climate-driven landscape evolution during the transition from ice-marginal (ca. 23 ka) to a fully submerged marine environment (ca. 8 ka) is preserved at Dogger Bank, in the southern North Sea.
In this study, our analysis of high-resolution seismic reflection and cone
penetration test data reveal a channel network over a 1330 km
The timing of channel formation lacks chronostratigraphic control. However, the proglacial rivers must have formed as the ice sheet was still on Dogger Bank, before 23 ka, to supply meltwater to the rivers. Ice-sheet retreat from Dogger Bank led to reorganisation of meltwater drainage and abandonment of the proglacial rivers. Palaeoclimate simulations show a cold and dry period at Dogger Bank between 23 and 17 ka. After 17 ka, precipitation increased, and drainage of precipitation formed the second set of channels. The second set of rivers remained active until marine transgression of Dogger Bank at ca. 8.5–8 ka. Overall, this study provides a detailed insight into the evolution of river networks across Dogger Bank and highlights the interplay between external (climate) and internal (local) forcings in drainage network evolution.
Postglacial drainage patterns in the North Sea have become a focus of interest in recent years since the growth in archaeological exploration of the submerged landscapes of the northwest European continental shelf (Bailey et al., 2017; Coles, 1998; Flemming et al., 2017). The adoption of seismic reflection data acquired for oil and gas exploration by the archaeological community has allowed mapping of extensive terrestrial drainage networks throughout the southern North Sea (Fitch et al., 2005; Gaffney et al., 2007, 2009; Hepp et al., 2017, 2019; van Heteren et al., 2014; Prins and Andresen, 2019; Tappin et al., 2011). These subsurface mapping projects focused on rivers as they are likely sites of human occupation. Core- and sediment-based palaeoenvironmental research has augmented seismic mapping studies (Brown et al., 2018; Gearey et al., 2017; Tappin et al., 2011) and put human–landscape interaction into the wider context of Late Quaternary landscape evolution of the North Sea during a period of changing climate (Bicket et al., 2016; Bicket and Tizzard, 2015; Phillips et al., 2017; Tizzard et al., 2014).
Previous explorations of submerged landscapes have used low-resolution 2D or
3D seismic reflection surveys designed to target deeper oil and gas
reservoirs (Fitch et al., 2005;
Gaffney et al., 2007, 2009) or combine oil and gas datasets with sparse
high-resolution 2D seismic reflection data (Coughlan et al.,
2018; Hepp et al., 2017, 2019; Prins and Andresen, 2019). Whilst this
enables drainage networks to be identified, there is little information to
constrain sedimentary and geomorphic processes and therefore the controls
on landscape evolution. The availability of new, high-resolution datasets
from wind-farm site investigation allows more detailed investigation of
shallower submerged landscapes (Cotterill et al.,
2012, 2017a). Dogger Bank is covered by a large (1500 km
The evolution of the terrestrial landscape of Dogger Bank over 10 kyr timescales, during a period of marked climate and far-field base-level change, has mainly focused on glacial (Cotterill et al., 2017b; Emery et al., 2019a; Phillips et al., 2018) and coastal stratigraphy (Emery et al., 2019a). Prins and Andresen (2019) established a transition from subglacial channel to terrestrial drainage in a study area 150 km northeast of Dogger Bank, but detail of the terrestrial landscape evolution at Dogger Bank during and after ice-sheet retreat has yet to be established. Furthermore, the link between external, climatic changes at Dogger Bank and the internal processes of drainage reorganisation, such as river piracy (Bishop, 1995; Shugar et al., 2017) and landscape evolution, is explored for the first time in this study with the integration of palaeoclimate model simulations.
In this study, our aim is to describe in detail the timing and processes of formation of channel networks observed in the seismic reflection data using stratigraphic relationships, alongside palaeoclimate model simulations, to identify changes in temperature and precipitation at Dogger Bank. We identify, for the first time, the evolution of a low-gradient terrestrial drainage network on the low-relief topography at Dogger Bank under changing climate and global mean sea-level rise. We explore the role of ice-sheet meltwater and subsequent precipitation changes in forming a well-developed channel network on a land surface with low topographic relief. This regional picture of changing drainage patterns in the North Sea during the Late Pleistocene and Holocene has implications for human populations and migration during this period of climatic warming and global mean sea-level rise.
Dogger Bank, in the southern North Sea (Fig. 1),
is a present-day bathymetric high (15–30 m b.m.s.l. – below mean sea level) surrounded by deeper water (
A large, integrated subsurface dataset of 2D seismic reflection profiles and geotechnical logs acquired for site investigation of Tranche B of the Forewind wind-farm project (Fig. 1) was used in this study.
A dense, 2D grid of shallow, single-channel seismic reflection data was
available to this study, totalling 17 000 line kilometres in Tranche B
(Cotterill et al., 2017b); 629 NE–SW-oriented
lines (mainlines) are spaced at 100 m intervals, and 75 NW–SE-oriented lines
(crosslines) are spaced at 500–1000 m intervals. Two ships were used,
employing the same 1.6 kJ sparker source. Data were recorded in StrataView,
then imported to ProMAX for processing, where a bandpass filter with a 100 Hz low cut and an 800 Hz high cut was applied, followed by F–k filtering and time migration, then exported to SEG-Y. The sparker source has a maximum
vertical resolution of approximately 1.25 ms, which gives a vertical
resolution of
Seismic reflection data were interpreted using IHS Kingdom Suite. Maps were
generated from interpreting seismic grid lines in two-way time (TWT). P-wave
velocities derived from local geotechnical data were used to convert TWT to
depth (Cotterill et al., 2017a). Seawater velocity was
taken to be 1505 m s
Seismic interpretation was undertaken by identifying distinct seismic facies and major bounding surfaces between them. Seismic facies were identified and named based on Mitchum et al. (1977), with interpretation of glacial sediments using terminology based on Emery et al. (2019a). Seismic facies were correlated to sedimentary facies interpreted from geotechnical logs to establish a seismic stratigraphic framework for the study area. This framework was used to identify the transition from glacial to terrestrial to marine and to map the major bounding surfaces between the different sedimentary environments.
Eighty-three cone penetration tests (CPTs; Fig. 3), up to 50 m below seabed, were acquired throughout Tranche B (Cotterill et al., 2017b). These tests
provide cone resistance (qc) measurements that were used, uncorrected, as a
grain-size proxy through the sediments, with low resistance corresponding to
clay and high resistance corresponding to sand, as used by Emery et al. (2019a). CPTs were used to calibrate sedimentary information to seismic
facies observed in the seismic reflection data to constrain sedimentary
environment. CPT depth was converted to TWT by using the sediment velocity
of 1600 m s
Channel networks were digitised from the gridded seismic horizon interpretation to polygon shapefiles in QGIS by mapping channel forms where underlying reflections are truncated. Each channel or channel network was ascribed an individual shapefile. Centre lines for each of the channels were digitised as line shapefiles. The shapefiles were projected in WGS84/UTM 31N. Centre-line shapefiles were then imported into a Python script (Grieve, 2020) that measured centre-line length and the straight-line distance from the shapefile start to end from UTM coordinates. These two lengths were used to calculate sinuosity. Centre-line shapefiles were also used to extract the channel base long profiles from the depth-converted seismic horizons. As the seismic horizon was gridded at 10 m long profile points were automatically extracted by the QGIS Profile Tool plugin every 10 m, then visually smoothed to remove effects of seismic line mistie and any interpolation bias.
Palaeoclimate simulations provide an estimate of changing climate at Dogger
Bank since the Last Glacial Maximum. These equilibrium-type simulations were
run with the coupled ocean–atmosphere–vegetation general circulation model,
the Hadley Centre Coupled Model version 3 (HadCM3; Gordon
et al., 2000; Pope et al., 2000; Valdes et al., 2017). They broadly follow
the protocol described by Ivanovic et al. (2016), opting for a melt-uniform
freshwater scenario that conserves water across the deglaciation in
accordance with the prescribed ice sheets. Two sets of simulations were
analysed; one uses the global ICE-6G_C ice sheet
reconstruction (Peltier et al., 2015) performed
at 1000-year intervals from 26 to 21 ka and 500-year intervals from 21 to 0 ka (the same simulations are described in more detail by
Morris et al. (2018) in their Supplementary
Materials and Methods), and the other uses the GLAC-1D global ice-sheet
reconstruction (Briggs
et al., 2014; Ivanovic et al., 2016; Tarasov et al., 2012, 2014; Tarasov and
Peltier, 2002) at 500-year intervals from 26 ka to present that are
otherwise identical to the ICE-6G_C set. From these
simulations, we extracted 50-year climate means for annual precipitation,
total annual evaporation, and annual temperature for the
Three seismic stratigraphic units were established based on previous investigation of the stratigraphic architecture of the study area (Cotterill et al., 2017b; Emery et al., 2019a, b). The basal unit (basal seismic unit) is separated from the younger two units (channel-fill unit, upper seismic unit) by a major unconformable surface (Horizon Z) that is mapped across the study area.
The basal seismic unit comprises three main seismic facies, with minor contributions of other seismic facies. Generally, the area east and southwest of the study area is characterised by high-amplitude, varying frequency reflections and asymmetric and symmetric serrate, inclined, patchy and sporadic reflections (see Emery et al., 2019b, for description of terminology), termed sub-unit 1 (Fig. 2a). The central study area is dominated by high-frequency, medium-amplitude parallel reflections that onlap or drape previous stratigraphy, infilling a depocentre (Fig. 1), termed sub-unit 2 (Fig. 2a). The northwest of the study area mainly comprises low-amplitude, low-frequency variable to transparent seismic facies, termed sub-unit 3 (Fig. 2a). The basal seismic unit is bounded above by Horizon Z.
Horizon Z is present across the study area and truncates the underlying basal seismic unit and, therefore, represents an unconformity (Figs. 1 and 2b). Channel forms mantle Horizon Z and incise the basal seismic unit. Commonly, Horizon Z is coincident with the seabed (Figs. 1, 2, and 5b). The depth to Horizon Z, relative to mean sea level, is shown in Fig. 3. In seismic section, Horizon Z is generally identified by a continuous, medium- to high-amplitude reflection, especially where coincident with the seabed. In the north of the study area, Horizon Z loses reflectivity and can only be interpreted by differences in seismic facies above and below. In some areas, Horizon Z is high-amplitude and overlain by a thin unit of further high-amplitude reflections (e.g. centre of Fig. 3b).
The channel fills above Horizon Z comprise varying seismic facies. Channel forms vary in size and morphology, as described further in Sect. 4.4. Larger channel fills often cause acoustic blanking of the underlying seismic reflections (Fig. 9). Channel-fill architecture is variable. The dominant channel-fill seismic facies comprise high-frequency, high-amplitude, continuous reflections that are generally subparallel to the base of the channel, or horizontal, which varies in thickness between and along channels (Figs. 4b and 5b). High-amplitude reflections can also be mounded externally with parallel to tangential oblique reflections internally (Fig. 4b). Typically, above this high-amplitude fill is a low-amplitude to transparent, low-frequency fill with parallel and horizontal, draped, or sigmoidal reflections (Figs. 4b and 5b and c). In some cases, the fill is divergent in the high-amplitude section and comprise stacked channel fills (Fig. 4b). Prograding fill is observed in the low-amplitude, low-frequency section of some of the channel-fill seismic facies (Fig. 4c). In other channels, the fill pattern is divergent in the basal high-amplitude reflections, which are overlain by low-amplitude to transparent fills (Fig. 5b and c).
The horizon separating the channel-fill seismic unit from the overlying upper seismic unit is variable. Typically, this horizon comprises a single medium- to high-amplitude horizontal reflection (Figs. 3b and 4b) but can also be draped over the partial channel fill (Figs. 4c and 5b and c). Where channels are absent, this horizon is coincident with Horizon Z.
The youngest seismic unit is present between the seabed and Horizon Z and sometimes partially fills the channels (Figs. 1b, 3b, 4c and 5b and c). This upper seismic unit comprises low-amplitude to transparent seismic facies. In central and northern parts of the study area, where the upper seismic unit is thickest (Fig. 2b), low-frequency, low-amplitude, west- or southwest-dipping sigmoidal-to-tangential oblique and shingled reflections are present. The upper seismic unit is absent where Horizon Z is coincident with the seabed.
Two large, elongate features, oriented approximately NNW–SSE, up to 2.5 km wide and 15 km long, are also present in the centre of the study area that incises into Horizon Z and the basal seismic unit (Fig. 2). In the north of the study area, the largest elongate feature can be observed to incise through the channel-fill unit and into the basal seismic unit (Fig. 9c), suggesting that these erosive features are younger than the channel-fill unit.
Ten CPTs intersect the channel fills of Horizon Z (Fig. 3). Low cone resistance correlates to clay, and high cone resistance correlates to sand
(Robertson, 1990). CPT facies of each of these 10
geotechnical logs, plus two logs that did not penetrate channel fills, were
interpreted for each seismic unit by correlating CPTs to the seismic
reflection data. This correlation was made by converting CPT depth to TWT
using the sediment velocity of 1600 m s
The upper seismic unit has high cone resistance values, as seen in CPTs H, I, K, M, V, and W (Fig. 3), which implies a sand-rich unit. The basal seismic unit is highly variable depending on which sub-unit is encountered. Sub-unit 1 has mostly low cone resistance values, suggesting it is clay-rich (CPTs H and O) but can also be interbedded with more sand-rich layers (CPT N; Fig. 3). Sub-unit 2 is dominated by clay-rich sediments with rare thin layers of coarser material (CPTs I, K, P, V, and W; Fig. 3). Sub-unit 3 is variable but dominated by high cone resistances intercalated with intermediate responses, implying silty and sandy sediments (CPT R; Fig. 3, Robertson, 1990).
The channel-fill signatures differ between each CPT, varying from clay-rich to sand-rich, and are commonly interbedded. In CPT W, the channel fill is clay-rich at the top, between 6 and 14 m, and sand-rich to the base of the channel fill at 19 m. This correlates to the difference in seismic facies within the channel-fill seismic unit, where the clay-rich unit correlates to low-frequency, transparent seismic facies, and the sand-rich unit correlates to high-amplitude, parallel reflections (Fig. 3). In contrast, CPT V contains no clay-rich layers, solely comprising intermediate cone resistances, implying a silty to sandy fill. CPTs I, L, M, and P comprise low cone resistance responses, implying clay-rich facies. CPTs H and N are also dominated by clay but have irregularly spaced, 10–40 cm thick, intercalated silts. The general trend within CPT N shows fining upwards. CPTs K and O are highly variable in cone resistance throughout the channel-fill seismic unit, with a sandy base, a clay-rich middle, and a sandy top.
The three seismic units and Horizon Z, when correlated to CPT log facies, suggest a transition between three distinct sedimentary environments.
The wide range of seismic and CPT facies within the basal seismic unit implies a complicated depositional setting. Sub-unit 1 has varying seismic facies that include serrate, patchy, inclined, and sporadic reflections and correlates to fine-grained, clay-rich facies with siltier interbeds and some sand, such as in CPTs H, O, V, and W (Fig. 3). The nature of the reflections implies deformation of this sub-unit, which is interpreted to be glaciotectonic compression of subglacial and glacial outwash sediments deposited at the margin of an ice sheet (Cotterill et al., 2017b; Emery et al., 2019a; Phillips et al., 2018). The rhythmic, parallel, high-frequency reflections within sub-unit 2 correlate to clay-rich CPT facies in CPT I, P, L, V, and W (Fig. 3). The fine-grained, rhythmic deposits suggest deposition in a low-energy environment, and the basin-filling geometry of the sub-unit supports an interpretation that these are lake-fill sediments. The third sub-unit is characterised by low-amplitude reflections whose geometry implies aggradation at low angles. CPT R (Fig. 3) has a variable but generally high cone resistance, implying interbedding of sand and siltier units. Sub-unit 3 has varying thickness; it is thicker in the west of the study area, thinning to a lobate geometry to the east, where it overlies sub-unit 1, and is onlapped by sub-unit 2. Sub-unit 3 is interpreted to be glacial outwash deposited subaerially in outwash fans during ice-sheet retreat. A full description of these glacial and proglacial sedimentary environments and their implication for landscape evolution during ice-sheet advance and retreat is provided in Emery et al. (2019b).
Horizon Z is an unconformable surface, into which channel forms have
incised. These channel forms vary in length and width but form a connected
network of channels (Fig. 6). There are three main
channels, which generally have smaller tributary channels joining them. The
high aspect ratio channel fills (
Map of channel network interpreted from Horizon Z showing the
three main channels, seven smaller streams, and their tributaries. Numbers
correspond to the sinuosity of each individual channel. The outline of the
proglacial lake-fill sediment subcrop is shown. IC
Horizon Z is interpreted to represent a composite terrestrial surface that
formed after retreat of the ice sheet and infilling of the proglacial lake
(Cotterill et al., 2017b; Emery et al., 2019a). The channels might have originated as a tunnel valley network,
such as that interpreted to the east of Dogger Bank (Prins and
Andresen, 2019). Tunnel valleys are generally much deeper than the channels
observed incising Horizon Z, with lower aspect ratios (
The CPT logs have mixed responses within the channel-fill seismic unit, implying different infill histories. Sandy and silty channel-fill sediments are frequently encountered towards the base of the channels (CPTs K, N, O, V, and W), implying a moderate- to high-energy sedimentary environment. Fining upwards is also apparent in CPTs K, N, and W, which is characteristic of bar deposits in channel fills. Clay-dominated facies (CPTs H, I, L, M, P, and the upper section of CPT W) suggest a low-energy sedimentary environment. The clay-dominated facies could also represent brackish or marine deposition during marine transgression. Without detailed sedimentary information provided by cores, and palaeoenvironmental analyses, such as microfossil assemblages, it is not possible to confirm the depositional environment of these clay-rich facies.
The generally low-amplitude to transparent seismic facies of the upper
seismic unit imply a relatively homogeneous sediment. The CPT logs that
correlate to the upper seismic unit, such as CPTs V and W, have high cone
resistance values (
Three main channel fills are identified above Horizon Z, whose widths are greater than 400 m and up to 1000 m wide and 15 m deep (Fig. 6). Main channel fill 1 runs from west to east and is located in the east of the study area. Main channel fill 2 runs from north to south in the centre of the study area. Main channel fill 3 runs from northwest to southeast in the west of the study area (Fig. 6). A tributive network of smaller channel fills associated with the large channel fills also exists, whose widths are up to 250 m and depths up to 10 m. Longer, isolated channel fills of a similar scale are also observed within the study area.
Two forms of channel cross section are observed. The first form corresponds to main channel fills 1 and 2, with a wide channel incision that comprises numerous smaller erosion surfaces separated by shallower and generally horizontal sections at its base (Fig. 4). The second form, corresponding to the tributaries, main channel 3, and the isolated channels, is generally U- or V-shaped with a single deep incision (Fig. 5).
The bases of main channels 1 and 2 show cross-channel depth variations
(Fig. 4) from narrow, deep channel sections
separated by wider, flat-topped, mounded shallow sections elongated parallel
to the channel with internal oblique reflections that dip downstream
(Fig. 4c). The deeper sections split and rejoin,
with between one and three deep channel sections across the main channel
width (Fig. 4). The main channels 1 and 2 have low
sinuosity (channel
Long profiles of the three main channels and their longest tributaries were
drawn from the centre lines of the deepest point of the channel base
relative to the channel edge and smoothed to reduce issues of seismic
mistie and interpolation bias (Fig. 7). The
profiles undulate but show overall decrease in elevation towards the east (channel 1), south (channel 2), and southeast (channel 3). The tributary channel bases also decrease in elevation (Fig. 7)
and sometimes become steeper from the tributary head to the confluence with
main channels, such as in main channel 1 (Fig. 7),
implying these channels were cutting down to the main channels. The flow
direction of the rivers is interpreted to be the same as the direction of
decrease in elevation (Fig. 7). Therefore, the
network of channels is a dendritic to subdendritic river drainage network
(Zernitz, 1932) draining from tributaries into main channels,
then out of the study area. The maximum elevation of this drainage network
is
The palaeoclimate simulation outputs for the two model runs using GLAC-1D
and ICE-6G_ C ice sheet reconstructions for the time span of 26 ka to present are shown in Fig. 10. Generally, the climate simulations
show similar trends through the Holocene but differ through the Late
Pleistocene. The climate simulation using GLAC-1D has much higher
precipitation than the equivalent simulation with ICE-6G_C
between 26 and 18 ka, but the climate with ICE-6G_C shows
much higher precipitation than with GLAC-1D between 18 and 11 ka. The
temperature profiles are largely similar between the GLAC-1D and
ICE-6G_C runs, except between 26 and 20 ka, where the
ICE-6G_C run gives temperatures consistently 5
Conceptual landscape evolution model for the study area, showing a single, representative proglacial channel. (1) Initial drainage of meltwater into the proglacial lake. (2) Proglacial lake gradually infilled with fine, draped sediments. Subsequently, proglacial lake accommodation filled, proglacial river channel incises into the fill. (3) Ice-sheet retreat and drainage reorganisation abandons the proglacial river channels. (4) Temperature and precipitation increase, tributaries incised. (5) Marine transgression floods the river channels first. (6) Final inundation, with wave ravinement, followed by deposition of shallow marine sand.
The northward retreat of the ice sheet from Dogger Bank left a landscape of glaciotectonites, glacial outwash, and proglacial lake-fill sediments (Cotterill et al., 2017b; Emery et al., 2019a; Roberts et al., 2018). The resulting landscape surface is likely to have been modified where the seabed and Horizon Z are coincident, and therefore reconstructing the original topographic template is challenging, although it is likely that the topography was low relief, as part of this land surface beyond the channels is planar (Fig. 3). This is in contrast to the landscape exposed in Tranche A of the Dogger Bank Forewind wind-farm project, to the west of this study area, which had an undulating surface of moraine highs and drainage channel lows (Cotterill et al., 2017b; Phillips et al., 2018). During this period of exposure, the land surface would have been a periglacial tundra with limited vegetation (Cotterill et al., 2017b), resulting in desiccation and overconsolidation of the sediments (Cotterill et al., 2017b; Emery et al., 2019a; Mesri and Ali, 1999).
The morphology and low sinuosity of main channels 1 and 2 reflects modern
proglacial braided river channels (Carrivick and Russell, 2013), such as Icelandic glacial outlet rivers, e.g. Jökulsá á Fjöllum (Alho
et al., 2005; Bristow and Best, 1993; Carrivick et al., 2007; Maizels, 1989;
Marren, 2005; Marren and Toomath, 2014; Vandenberghe, 2001). Braided river
channels often form in cold climates, such as proglacial settings, with a
high sediment throughput, where there is little vegetation and the channels
are unconfined (Bristow and Best, 1993; Marren, 2005). The individual Dogger Bank channels within the main channel body are separated by shallower sections, interpreted as braided channels separated by mounded braid bars with internal cross-bedding implying downstream accretion (Fig. 4c). Given the similarities in morphology to modern systems, we interpret that these
channels formed in a proglacial setting, with meltwater containing a high
sediment supply from the retreating ice sheet to the north, leading to
erosion of tundra-plain surface (Fig. 8, stages 1 and 2). However, the width of the braid plains (400–1000 m) is modest and
remains constant, which is in contrast to unconfined braid plains from modern
day settings, such as Skeiðarársandur, Iceland, which are generally
wider (
The location of the proglacial channels was influenced by antecedent
topography. The location of channel 1 parallels a subtle topographic high
formed by a tunnel valley fill overlain by proglacial lake-fill sediments
(Fig. 9). The tunnel valley fill has a
Palaeoclimate model outputs showing temperature and effective
precipitation for GLAC-1D and ICE-6G_C model runs. The interpretation of the palaeoenvironmental conditions at Dogger Bank is a distinct transition from cold and dry to warmer and wetter at approximately 17 ka. Also shown is the relative sea-level curve from Kuchar et al. (2012) and the sea-level index point from Shennan et al. (2000), plotted alongside the NGRIP ice core climate record (Andersen et al., 2004). RSL
The braided proglacial rivers must have formed prior to the retreat of the
ice sheet down from Dogger Bank. Retreat of the ice down the retrograde,
northern slope of Dogger Bank lowered the ice-sheet basal elevation from
Discharge variability fundamentally controls the geomorphology of river
channels (Fielding et al., 2018; Nicholas et al., 2016). The coefficient of variance (CVQ
The isolated channels and tributaries, and main channel 3, have different morphologies to the two proglacial river channels (1 and 2). The isolated channels are all very similar and are therefore interpreted to have the same origin as tributaries that joined the main channels outside of the study area. The higher sinuosity (Fig. 7) of these smaller channels suggests formation under different conditions to the proglacial channels. The direction of drainage of the tributaries and streams is often perpendicular to the flow of the main channels (Fig. 6), following pre-existing slopes, such as the valley to the main proglacial channels (Fig. 3d). These smaller channels also have heads within the study area, unlike the proglacial channels, which suggests that they did not form due to meltwater. The long profiles of these tributaries show them to cut down to the base of the main channels (Fig. 7). The subdendritic pattern of these smaller channels, combined with their smaller size and higher sinuosity, and that they steepen into the main channels, suggests they formed later (Fig. 8, stage 4). The increase in sinuosity is interpreted to represent a warmer climate, with a more erodible substrate no longer bound by permafrost. The large, flat areas of high seismic amplitude (e.g. centre of Fig. 3b) are interpreted to represent marshy areas with the same seismic character as areas from which marshy plant macrofossils have been recovered (Wessex Archaeology, 2014). These marshy areas mainly occur over the proglacial lake-fill sediments, implying a low-permeability substrate that would have prevented groundwater flow of rainwater (Fig. 8). This in turn led to the development of the subdendritic drainage network, which is most developed and best preserved over the proglacial lake-fill sediments (Fig. 6), except for main channel 3, which developed over basal sub-unit 1, which consists of glaciotectonised and overconsolidated clays.
Only the proglacial river channels show evidence for aggradation of sediment within the channels (Fig. 4), with little evidence in the tributaries or overbank deposits (Fig. 5). These smaller river channels are only partially infilled by alluvial sediments, with the rest of the infill being shallow marine sand (Fig. 5). Models of relative sea-level rise suggest that inundation of the North Sea basin began around 16 ka (Brooks et al., 2011; Kuchar et al., 2012), resulting in a base-level rise for the drainage network (Fig. 10), which should result in aggradation within the drainage network. The lack of aggradation may be due to low discharge and sediment flux, with only a small local supply, or due to the drainage network being distant from the base-level rise, draining into a local depocentre, i.e. the previously abandoned proglacial river channels.
Marine transgression occurred in the study area between 9.5 and 8.5 ka (Cotterill et al., 2017b; Emery et al., 2019b; Shennan et al., 2000), inundating the incisional channel network first (Fig. 8). The small size and limited drainage basin area would not have been sufficient for aggradation under a rising base level to outpace the inundation by marine waters. This may explain the fine-grained channel-fill sediments observed in some CPTs (e.g. CPT L, CPT W; Fig. 3), as marine transgression would have modified the sedimentary environment in the sheltered estuaries to low-energy tidal mudflats, as observed elsewhere in the North Sea during Holocene marine transgression (Coughlan et al., 2018; Gaffney et al., 2009; Hepp et al., 2017, 2019; Prins and Andresen, 2019). The final stage of regional landscape evolution was continued marine transgression, with associated ravinement of the pre-existing topography (Figs. 3 and 8; Cotterill et al., 2017b; Emery et al., 2019a). The large, elongate features that incise into Horizon Z, the channel fills, and underlying basal seismic unit are interpreted to have formed at this stage as large tidal scours (Fig. 9c). Continued relative sea-level rise and the transport of sediment, shown by the broadly west to east dip direction of sigmoidal to oblique reflections in the upper seismic unit (Fig. 4c), resulted in the deposition of shallow marine sand that completed the infill of the channels and the tidal scour features.
Between deglaciation of Dogger Bank (
Two palaeoclimate simulations using the GLAC-1D and ICE-6G_C ice-sheet models were run, giving differing results. The GLAC-1D ice-sheet model uses the DATED-1 chronological database for the Eurasian Ice Sheet (Hughes et al., 2016), which gives a realistic reconstruction of the ice sheet and palaeogeography of the British Isles, and thus provides the more up-to-date chronology for Eurasian Ice Sheet evolution. However, the DATED-1 database shows Dogger Bank to be glaciated until 19 ka, as opposed to deglaciated by 23 ka (Emery et al., 2019a; Roberts et al., 2018). Therefore, the climate evolution simulated in the Dogger Bank area may be biased and simulated as too young during this early time window (23–19 ka). Nevertheless, it should provide a more faithful representation of climate thereafter.
Effective precipitation (precipitation minus evaporation) trends from the
GLAC-1D simulations show a general decrease in precipitation from 26 to 17 ka, whereas the ICE-6G_C runs show an increase from 22 ka to a maximum at 14 ka (Fig. 10). The
fluctuating, high precipitation outputs from GLAC-1D may be related to the
local presence of a modelled ice sheet during this time, when it should have
been largely deglaciated. During the same time period, mean annual
temperature (MAT) increased from
We interpret the time period from deglaciation at 23 ka to MAT reaching
0
Numerous studies have identified Late Pleistocene to Holocene channel networks of a similar stratigraphic position to those in this study (Busschers et al., 2007; Fitch et al., 2005; Gaffney et al., 2007, 2009; Hepp et al., 2017, 2019; Hijma and Cohen, 2011; Prins and Andresen, 2019). During this period, main channels 1 and 2 were active as proglacial channels draining the margin of the Eurasian Ice Sheet into the Late Weichselian North Sea lake, a large proglacial lake proposed to have existed to the south of Dogger Bank (Becker et al., 2018; Hjelstuen et al., 2017; Jansen et al., 1979; Murton and Murton, 2012; Roberts et al., 2018; Sejrup et al., 2016; Toucanne et al., 2010). The proglacial channels would have drained directly into this lake (Fig. 11), until the ice retreated off the topographic high at ca. 23 ka to cut off the meltwater and sediment supply. The Late Weichselian North Sea lake drained rapidly ca. 18.7 ka through the Elbe Palaeovalley mouth (Becker et al., 2018; Hjelstuen et al., 2017), leaving the Oyster Ground subaerially exposed. After lake drainage, there were two drainage outlets to the ocean: (i) the Fleuve Manche system draining south and westwards (Bourillet et al., 2003; Gibbard et al., 1988; Mellett et al., 2013; Toucanne et al., 2010, 2015), which drained the Rhine–Meuse and Thames, and (ii) the Elbe Palaeovalley, which drained the Elbe, Weser, and Ems rivers (Figge, 1980; Gibbard et al., 1988; Hepp et al., 2017; Toucanne et al., 2015) into the Norwegian Trough (Fig. 11). A third outlet opened after marine transgression inundated the lower-elevation areas north and west of Dogger Bank, eventually inundating the Outer Silver Pit between 12 and 10 ka (Brooks et al., 2011; Shennan et al., 2000; Sturt et al., 2013).
The large areas that remain uncovered by similar datasets, especially in relation to the area formerly covered by the Late Weichselian North Sea lake, make the location of where the rivers in the study area drained challenging to constrain (Fig. 11). Most rivers interpreted from seismic data during the North Sea Palaeolandscape Project (Fitch et al., 2005; Gaffney et al., 2007, 2009) drain into the Outer Silver Pit lake (separate to the Late Weichselian North Sea lake), but it is not known in what direction. The present-day bathymetry of the Oyster Ground shows little topography, with no evidence of transgressed drainage networks expressed at the seabed. The rivers of Dogger Bank may have drained into the Outer Silver Pit lake. Then, it drained northwards into the gradually transgressing northern North Sea via the Wash–Inner Silver Pit and Humber rivers, southwards into the Fleuve Manche system, or eastwards into the Elbe Palaeovalley (Fig. 11). The general direction of palaeoriver flow identified east of Dogger Bank (Hepp et al., 2017, 2019; Prins and Andresen, 2019) is towards the Elbe Palaeovalley, similar to that observed in our study area and the northern area of the North Sea Palaeolandscape Project, south of Dogger Bank (Fitch et al., 2005; Gaffney et al., 2007, 2009). We propose that the proglacial rivers initially drained into the Late Weichselian North Sea lake. After the drainage network began to form at ca. 17 ka, the Elbe Palaeovalley became the mostly likely outlet for the palaeorivers of Dogger Bank (Fig. 11). Further investigation of seismic reflection data over a wider area will permit the postglacial stratigraphic evolution of the drainage networks in the southern North Sea basin to be better constrained, with implications for understanding human interaction and migration through the landscape during the Late Pleistocene.
Investigation of the high-resolution, integrated dataset of the 2D seismic reflection grid lines and CPT logs has revealed an environment in transition from glacial through terrestrial to marine conditions, marked by Horizon Z, a prominent unconformity present across the area. Mapping of Horizon Z revealed a network of channels that incise and therefore postdate glaciogenic and proglacial lake sediments but are buried under shallow marine sand. These channels, along with Horizon Z, are interpreted to represent the terrestrial landscape at Dogger Bank that developed during the period between ice sheet retreat and marine transgression.
Two different types and generations of river channels with distinct
morphologies have been defined. The first channel set comprises two
The data used in this article are not publicly available as they are commercially sensitive site investigation datasets.
ARE undertook the research and wrote the article. DMH, NLMB, JLC, CJC, and CLM provided input on the article and supported the original research. JCR provided input on fluvial geomorphometric methods. RFI designed and undertook palaeoclimate modelling.
The authors declare that they have no conflict of interest.
The authors thank the Forewind wind-farm project for supplying the data. Thanks to Bartosz Kurjanski, Brice Rea, and Nick Schofield at the University of Aberdeen for software support. Thanks to Niall Gandy for NetCDF support. We thank Stuart Grieve for providing the Python script used to calculate sinuosity. Daniel Hepp and Lasse Prins are thanked for their provision of shapefiles of channels in the North Sea. Nigel Mountney and Colm Ó Cofaigh are thanked for their discussions on an earlier version of this manuscript. Carol J. Cotterill publishes with permission of the director of the British Geological Survey. The two anonymous reviewers, Associate Editor Susan Conway, and Editor Andreas Lang are thanked for handling of the manuscript and extensive, constructive reviews, which greatly improved this article.
Andy R. Emery was funded by the Leeds Anniversary Research Scholarship. The contribution from RFI was supported by NERC grant NE/K008536/1 and UKRI grant no. MR/S016961/1.
This paper was edited by Susan Conway and reviewed by two anonymous referees.