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A2.2MG2 Marine & Environmental Geology Lecture 4

A2.2MG2 Marine & Environmental Geology Lecture 4. THE UK CONTINENTAL SHELF. Contents. Introduction Quaternary history of the UK shelf Present day conditions Area studies The northwestern shelf and margin The northern North Sea The southern North Sea Concluding overview. INTRODUCTION.

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A2.2MG2 Marine & Environmental Geology Lecture 4

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  1. A2.2MG2 Marine & Environmental GeologyLecture 4 THE UK CONTINENTAL SHELF

  2. Contents • Introduction • Quaternary history of the UK shelf • Present day conditions • Area studies • The northwestern shelf and margin • The northern North Sea • The southern North Sea • Concluding overview

  3. INTRODUCTION

  4. This lecture is concerned with the seabed and shallow geology of the UK continental shelf. • This area poses many issues of scientific, economic and conservation importance.

  5. Such issues include: • Georesource extraction and the engineering infrastructure to support this • Bioresource exploitation for benthic species • Marine habitat classification and protection • Sediment movement and residence times • Role of sediments in transport and storage of pollutants

  6. It is important to recognise that most sediments on the continental shelf are relict • This means they are the product of past conditions and not present day ones. The most significant events took place during previous glacial periods. • At the present, there is little new sedimentation over most of the shelf. For the most part, the surface layer is just being reworked by tidal currents.

  7. QUATERNARY HISTORYOF THE UK SHELF

  8. The relevant part of the Quaternary for our study is the period during and after the last glacial maximum, ie the last 25,000 yrs or so. • Although earlier Quaternary sediments are to be found, they are largely covered by later deposits. • Only in parts of the southern North Sea do earlier sediments (mainly Anglian glaciation) form a significant part of the seabed.

  9. Within this time period, there are three distinct phases: • The Last Glaciation (Devensian) during which glacial and glaciomarine deposits were produced. • The Lateglacial (ca.14,000 to 10,000 yrs BP [before present]). This was a transitional period dominated by rising sea-level. • The Holocene (10,000 yrs BP to present). This is largely a period of sediment reworking.

  10. Consider first the glacial phase. • Sea level was around 150m lower than today. • The Scottish ice sheet extended over the western shelf and eastwards into the northern North sea. • The Scandinavian ice sheet extended westwards into the northern North Sea from the east. • The southern North Sea was an exposed periglacial plain with tundra vegetation. • The Rhine and Thames drained northwards across this plain into a marine embayment.

  11. Reconstruction of the glacial maximum ice sheet 22,000 yrs BP

  12. Quaternary environments of the UK shelf Northwestern shelf mostly glaciated Northern North Sea a marine basin Southern North Sea a tundra plain

  13. Now consider the Lateglacial period: • The ice sheets steadily diminished (with a small readvance around 11,000 BP). • Sea level rose slightly due to the loss of ice • Land levels rose in the north and sank in the south due to isostatic adjustment. • This initially caused more of the shelf to become exposed. The effect was greatest in the northern North Sea. The western shelf was mainly flooded.

  14. During the Holocene period sea levels continued to rise, especially post 7,000 BP when the North American ice sheet melted. • There was a major flooding of the southern shelf, combined with the breach of the Dover Strait. • Most of the southern UK coastline and estuaries became established in their present form during this period.

  15. Detailed 8,000 BP reconstruction of palaeoenvironments

  16. Detailed 7,000 BP reconstruction of palaeoenvironments

  17. Detailed 6,000 BP reconstruction of palaeoenvironments

  18. PRESENT DAY CONDITIONS

  19. At the present day the seabed geology is dominated by tidal currents. • These have: • distributed sediments along transport pathways • produced accumulations of particular sediment types in some areas along these pathways • produced distinctive bedforms such as sand waves and sand ridges • Some of the latter are no longer active and are thus relict from earlier, lower sea level.

  20. Surface tidal velocities in cm/s

  21. Net sediment circulation in the North Sea

  22. Sediment movement around the northwestern shelf is dominated by the oceanographic circulation along the shelf margin • This transports surface water northwards along the shelf edge and across the shelf • There is a deeper return flow southwards below the shelf edge (a contour current).

  23. THREE STUDY AREAS

  24. We now proceed to the study of three areas with contrasting histories and geology. (1) The northwestern margin (2) The north North Sea (3) The south North Sea

  25. STUDY AREA 1The Northwestern Margin

  26. Quaternary environments of the UK shelf Northwestern shelf mostly glaciated

  27. The northwestern margin has a relatively simple history. • At the glacial maximum ice extended west to the shelf edge and deposited glaciomarine sediments down the slope into deep (1000m+) water • This built out a prograding wedge of sediment, that lay on top of similar wedges from earlier glaciations. • The shelf edge has advanced about 50 km by this process.

  28. Stoker et al. 1993

  29. The shelf itself is covered by subglacial sediment such as till. There is evidence of an active ice morphology (drumlins etc) below the superficial deposits • At the shelf edge, where the ice was stationary, there is a series of grounding line moraines. • At several points the sediment has built up from the ice front into very large submarine fans, fed by distinct transport paths.

  30. Paul et al. 1998

  31. On the upper slope, beyond the margin, there are numerous iceberg plough marks down to a water depth of ca.400m. • Below this depth the slope is smooth and appears coated with long run-out grain flows (from GLORIA images) • Many sediments contain iceberg dropstones.

  32. Image courtesy of BP

  33. Vibrocore 60-05/51

  34. Thus we have a simple conceptual model of discharge down the continental slope • subglacial deposits at the crest • disturbed glaciomarine deposits on the upper slope • mass flow deposits lower down. • These can all be associated with detailed seismic facies and core samples.

  35. Seismic facies A subglacial till on outer shelf

  36. Seismic facies Bglaciomarine sediment on upper slope

  37. Seismic facies C mass flow on lower slope

  38. Seismic facies D nose of mass flow at slope foot

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