The Wash-Fenland Embayment : sediment sources and supply during the Holocene.
For appropriate management of any stretch of coastline, it is desirable to understand (and
ultimately to be able to predict) the exchange, transfer and storage of material at the landocean
boundary. To this end, the multidisciplinary Land-Ocean Interaction Study (LOIS)
was set up by the UK Natural Environment Research Council (NERC), focusing chiefly on
eastern England as an example. Distinction between short-term fluctuations and long-term
trends is particularly important when devising predictive models of coastal change. Within
LOIS, the Land-Ocean Evolution Perspective Study (LOEPS) provided data on a Holocene
time scale using evidence from the sediment record of east coast sinks including the Humber,
the Fens and the North Norfolk coast.
As part of LOEPS, the present study centred on the Wash-Fenland Embayment, the
largest accumulation of Holocene tidal sediments in Britain. Since initial marine inundation
of the area around 7,000 years BP, approximately 24 km' of sand and mud have been
deposited by vertical and lateral accretion under varying rates of sea-level rise. However, in
the last 50 years, parts of the shoreline have started to erode, raising questions over sediment
supply and other conditions necessary for maintaining the position of the intertidal zone.
The main aim of the work presented in this thesis was to ascertain the main sources of
sediment supplied to the Fenland and adjacent Lincolnshire coast. Selected tidal sediments
from early Holocene to recent age were compared with a range of potential source
lithologies. The latter included sections of eroding coast between Holderness and North
Norfolk and local strata exposed within the embayment and its river catchments.
In the Fens, use of chemical ratios showed the majority of Holocene sediment to be well
mixed and very homogeneous, variations in bulk chemical data mainly being a function of
sediment grade. However, samples with anomalous chemical ratios were identified at the
base of the succession close to the landward margins. Grain-size specific analyses of these
samples suggest that they have a similar composition to lithologies from Fenland river
catchments and the floor of the embayment. For example, clay mineralogy indicated an early
Holocene fluvial input of Oxford Clay at Adventurer's Land. In a similar way, surface area,
mineral magnetic analyses, optical mineralogy and SEM of anomalous sand samples show
quantitatively and qualitatively the links between selected non-marine sources and these
early Holocene sediments.
The bulk of the Fenland sediment appears to have been well mixed in the North Sea
sediment pool prior to deposition. Comparisons of the potential marine source lithologies
showed the fine fraction of the glacial tills to the north and east of the embayment to be
chemically very similar. On the basis of clay mineralogy, subtle differences between
Devensian and Anglian Tills were identified, and Fenland sediment was found to have an
intermediate composition. The sand fraction of most Devensian Till samples could be
distinguished from the more quartz-rich sand fraction of the Anglian Tills, and quantitative
analyses of surface area and magnetic properties of selected Fenland samples suggest till
eroding from the Lincolnshire foreshore is the most likely source.
Particle size analysis, summarised using Principal Components Analysis (PCA), proved
successful in characterising sediments according to depositional environment. Grain-size
data indicated a general fining-upwards trend in the sand fraction. In the embayment as a
whole, vertical accretion was accompanied by lateral progradation indicating that sediment
supply was more than sufficient to keep pace with sea-level rise.
The use of particle size analysis together with PCA seems widely applicable to studies of
modern and Holocene sedimentary environments, estuarine sedimentation history and
consideration of long term sediment budgets. Chemical ratios appear to have potential as
first indicators of changing sediment source.