Red Crag Formation

The Red Crag Formation outcrops in south-eastern Suffolk and north-eastern Essex. The name derives from its iron-stained colouration and an East Anglian word for shells, crag. It is part of the Crag Group, a series of notably marine strata which belong to a period when Britain was connected to continental Europe by the Weald–Artois Anticline, and the area in which the Crag Group was deposited was a tidally dominated marine bay.[1] This bay would have been subjected to enlargement and contraction brought about by transgressions and regressions driven by the 4x104y Milankovitch cycles.

The sediment in the outcrops mainly consists of coarse grained and shelly sands that were deposited in sand waves (megaripples) that migrated parallel to the shore in a south-westward direction.[2] The most common fossils are bivalves and gastropods[3] that were often worn by the abrasive environment.[1] The most extensive exposure is found at Bawdsey Cliff, the site of which is designated a Site of Special Scientific Interest (SSSI);[4] here a width of around 2 kilometres (1.2 mi) of Crag is exposed.

The Red Crag Formation at depth in eastern Suffolk clearly has one member, the Sizewell Member, a coarse shelly sand with thin beds of clay and silt.[5] It was interpreted as having been deposited in large scale sand waves where the sea bed was deeper. The overlying Thorpeness Member, was provisionally assigned to the Red Crag based on its lithology but there is more evidence to suggest that it is part of the Norwich Crag.

The Red Crag is considered to have started in the late Pliocene and to have possibly extended up into the early Pleistocene, but there is disagreement on more precise dating. It has been proposed that the oldest Red Crag deposits are at The Naze SSSI described by Natural England as the type site for the Waltonian stage of the UK.[6] and usually correlated with the final Pliocene Reuverian Stage in the Netherlands.[7] According to the British Geological Survey,[8] the Red Crag sits within a segment of time from about 3.3 to 2.5 mya although the British Geological Survey's Lexicon,[9] contradicts this by stating that it falls within the Thurnian Stage (around 2.2 to 2.1 mya). There are clearly difficulties in reconciling how the Red Crag equates with international chronological stages. In particular, the start and end dates are poorly defined due to the general paucity of age-diagnostic stratigraphic indicators and the fragmentary nature of the geology. It can also be difficult to separate the Red Crag from the overlying Norwich Crag.

References

  1. 1 2 Lee, Woods & Moorlock (2015), pp. 110-111.
  2. ↑ Dixon, R. G. (1979). "Sedimentary facies in Red Crag (Lower Pleistocene, East Anglia)". Proceedings of the Geologists’ Association. 90 (3): 117–132. doi:10.1016/s0016-7878(79)80014-0.
  3. ↑ Dixon, R. G. (1977). Studies in mollusca of the Red Crag (Pleistocene, East Anglia) (PhD). University of London. Retrieved 18 April 2018 – via British Library.
  4. ↑ "Designated Sites: Bawdsey Cliff" (PDF). Natural England. Retrieved 5 April 2018.
  5. ↑ Zalasiewicz, J. A.; Mathers, S. J.; Hughes, M. J.; Gibbard, P. L.; Peglar, S. M.; Harland, R.; Nicholson, R. A.; Boulton, G. S.; Cambridge, P.; Wealthall, G. P. (19 December 1988). "Stratigraphy and palaeoenvironments of the Red Crag and Norwich Crag formations between Aldeburgh and Sizewell, Suffolk, England". Philosophical Transactions of the Royal Society B. 322 (1210): 221–272. doi:10.1098/rstb.1988.0125.
  6. ↑ "Designated Sites: The Naze" (PDF). Natural England. Retrieved 5 April 2018.
  7. ↑ "Global Chronostratigraphical Correlation Table for the Last 2.7 Million Years". Subcommission on Quaternary Stratigraphy. University of Cambridge. 2011. Retrieved 5 August 2016.
  8. ↑ Lee, Woods & Moorlock (2015), pp. 92-93.
  9. ↑ "The BGS Lexicon of Named Rock Units: Red Crag Formation". British Geological Survey. Retrieved 5 April 2018.
Bibliography
  • Lee, J. R.; Woods, M. A.; Moorlock, B. S. P., eds. (2015). British Regional Geology: East Anglia (5th ed.). British Geological Survey. ISBN 978-0-85272-823-9.


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