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国立大学法人SHIMANE
University

島根大学

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147th Estuaries Open Seminar (Coastal Sedimentology)A A3A 1) %(.1 &)2

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Sedimentology of the fluvial to marine transition zonein Asian river deltas
Dr. Marcello Gugliotta Estuary Research Center
The fluvial to marine transition zones (FMTZs) of the Mekong, Ganges-Brahmaputra, Fly, Irrawaddy, Yangtze, and Dong Nai
river deltas in Asia extend for several hundred kilometers from their river mouths. Here, the interaction of riverine and tidal
processes control channel morphology, sedimentation, and multiple other geological and non-geological aspects. All thesesystems show matching trends in channel width, sinuosity, and bed elevation, which are locally complicated by valley
confinements, bifurcations, and confluences. Channel bed sediment samples are available from the Mekong and Dong Nai andhighlight a link between channel morphology and sediment distribution. The downstream tracts of the channels are largely
controlled by the tidal dynamics and are thus characterized by seaward-widening, straightening, and shallowing channels andby rhythmic or cyclic alternations of sand and mud. The upstream tracts are instead entirely controlled by the river dynamics
and show a more simple channel morphology and sand sediment.

Process response to marine transgression recorded by
the submerged landscapes of the North Sea
Prof. David Hodgson

University of Leeds, UK& Natasha Barlow, Andrew Emery, Stephen Eaton, Claire Mellett
Sea-level rise is one of the greatest threats to the global community, but constraining large-scale landscape response tomarine inudation is restricted to short-duration observational data. By comparison, the palaeo record provides the opportunity
to understand long-term responses of terrestrial and coastal areas to periods of past sea-level rise. During the last ice age, thesouthern North Sea comprised a terrestrial environment beyond the limit of the ice sheets, which extended across much ofEurope. As the ice sheets melted, sea level rose, submerging this palaeolandscape to become the modern North Sea.
Therefore, beneath the modern seabed of the southern North Sea is a terrestrial archive containing a wealth of information
about environmental responses to changing climate and rising sea level. This archive is now more accessible since site
investigations for new windfarms in the southern North Sea have provided a wealth of seismic reflection and borehole data.
Integration of these data are being used to better understand process responses to marine transgression in order to improveforecasts of future coastline response to changing sea level.
The landform assemblage preserved in the North Sea since the Last Glacial Maximum (~27ka) is highly variable, which isrelated to the local sediment supply and rates of relative sea-level change. In the English Channel, there is a highly incisional
unfilled fluvial landscape, with sedimentation concentrated in well-preservation gravel beach barrier systems that indicate rapid
relative sea-level rise. In the southern North Sea, there are extensive peat horizons that form a tributive fluvial network. The
buried peats can be used to show there was widespread erosion and redistribution of sediment during Holocene transgression.
Farther North, the Dogger Bank area originally formed form moraines close to the site of maximum ice advance of the Britishand Fennoscandian Ice Sheets. Here, a rich assemblage of subglacial landforms, proglacial lake-fills with outwash fans, fluvial
networks, and sandy barriers and lagoon-fills have been preserved, marking the evolution from ice marginal through coastal
plain to fully marine conditions.
It is striking that the coastline during most of the transgression was E-W, whereas the present-day coastline is N-S. This
realignment shows how dynamic the position of the coastline is during marine transgression, and the problems involved inaccurately forecasting changes during accelerated sea level rise that are needed to inform policies.

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