International Geologiical Congress - Oslo 2008

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CGC-13 Fjords: climate and environmental change

 

Scotland's fjord landsystem

 

Martyn Stoker, British Geological Survey (United Kingdom)
Tom Bradwell, British Geological Survey (United Kingdom)
John Howe, Scottish Association for Marine Science (United Kingdom)
 

 

Scotland's fjords have received minimal attention in terms of their marine geology, geomorphology and glacial history. In 2005, the British Geological Survey (BGS) undertook sea bed mapping of the Summer Isles region of NW Scotland. During the Last Glacial Maximum (LGM), this area was a major convergence zone for several valley glaciers that resulted in a broad coastal embayment that contains several overdeepened fjord basins. The sea bed data revealed an undisturbed marine landscape comprising basins with a multi-phase infill, erosional megagrooves, recessional moraines, outwash fans, pockmarks, sediment drifts and slide scars. Collectively these features represent a Late Devensian (Weichselian)?Holocene fjord landsystem.
In more detail, the basins and megagrooves represent focused subglacial erosion during successive ice-sheet cycles. This is linked to ice-stream development, when this region acted as a fast-flow corridor or tributary of The Minch palaeo-ice stream, a major artery of the British Ice Sheet, during the LGM (about 26?28 ka BP). By way of contrast, the suite of recessional moraines and the differential pattern of basinal sedimentation chart the stepwise retreat of a marine-terminating ice sheet at the end of the LGM (c. 15?18,000 ka BP), following collapse of the ice stream. Up to 40 sea floor moraines are preserved in the Summer Isles region, spanning about 40 km from The Minch to inner Loch Broom. These are up to 10 km long, 10?20 m high and display regular spacings of about 100 to 1000 m. Many of the moraines display intricate crenulated morphologies typical of recessional push moraines, whereas some of the ridges cut straight across topography as De Geer moraines.
The associated sequence-stratigraphic record of sedimentation within the fjord basins further documents the change from an ice-sheet dominated environment with a complex stratigraphic style (including sheet drape, onlap, ponded and chaotic seismic facies), to a topographically controlled tidewater- to valley-glacier setting characterised by onlapping and wedging basin-fill patterns. Sea bed scouring and sediment drift accumulation reflect an increasingly open-water system as tidal currents penetrated into the fjords, particularly during the Holocene. The release of shallow gas from organic-rich proglacial sediments is manifest by the widespread distribution of pockmarks. Finally, slumps and slide scars demonstrate slope evolution following the retreat of the fjord glaciers. This slope instability, both onshore and offshore, continues to the present day.
In 2006 and 2007, the BGS, together with the Scottish Association for Marine Science, acquired a number of sediment cores that reveal a variety of sediment facies, including subglacial till, debris-flow deposits, fluvioglacial sands, and glacimarine and marine muds. Work is currently underway to determine the age of the sediments in order to measure rates of change within the fjord landsystem during deglaciation.

 

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