International Geologiical Congress - Oslo 2008

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EIE-07 High-resolution geophysical imaging of geological structures and processes in environmental studies

 

Seismic Sequence Stratigraphy from a Decimetre Resolution 3D Seismic Volume acquired in Lake Windermere, UK

 

Mark Vardy, National Oceanography Centre, Southampton (United Kingdom)
Luke J. W. Pinson, National Oceanography Centre, Southampton (United Kingdom)
Jonathan M. Bull, National Oceanography Centre, Southampton (United Kingdom)
Justin K. Dix, National Oceanography Centre, Southampton (United Kingdom)
Timothy J. Henstock, National Oceanography Centre, Southampton (United Kingdom)
 

 

Here we present the results of a survey acquired in Lake Windermere, UK, using the 3D Chirp high-resolution sub-bottom profiler. 3D Chirp provides unparalleled imaging of coastal and in-shore seabed and sub-seabed structure by combining the known, highly repeatable source waveform of Chirp profilers, with the coherent processing and interpretation afforded by true 3D seismic volumes. Comprising 60 hydrophone groups arranged around four Chirp transducers, 3D Chirp permits the acquisition of a true 3D volume with decimetre horizontal resolution and centimetre vertical resolution, providing the perfect base for shallow-water geological studies.
Lake Windermere is a glacially over-deepened lake located in the south-eastern Lake District, UK. Recently acquired seismic reflection surveys have documented two basal sub-basins, each more than 6 kilometres long and up to 1 kilometre wide, extending more than 100 metres below sea level, and with up to 100 metres of unconsolidated sediment infill. The oldest seismic stratigraphic sequence (SSS I, following the terminology of Eyles and Mullins) is localised between bedrock highs and demonstrates the high seismic velocities typical of an outwash deposit/glacial till. This is overlain by a thick (up to 80 metres) package of cyclical reflectors (SSS II), representing early lacustrine deposition during a period of high sedimentation rates in the Late Quaternary. SSS III is a unit of variable thickness (less than 1 metre to 10 metres), containing discontinuous reflections and bounded by continuous, high amplitude reflectors. The youngest deposit (SSS IV) is a thin (up to 10 metres) package of Holocene gyttja and varved clays.
With dimensions of 150 metres by 300 metres, the study area was located 250 metres from the western shoreline and 800 metres north of Belle Isle, Northern Basin, and provided an opportunity to explore the interaction between syn- and post-glacial sediments with the glacially scoured lake margin. Package distributions, thicknesses, and internal structures are mapped in 3 dimensions. Particular attention is paid to a sub-aqueous slide that cuts through SSS II, and what appears to be a younger debris flow, possibly of terrestrial origin, sitting above SSS III. Attempts are made to compare the SSS with sequence stratigraphy obtained from limited existing cores, showing strong similarities between the internal sub-units of SSS IV and observed transitions between varved clays, gyttja, and recent ooze.

 

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