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The White Cliffs of Dover are mostly chalk but there are regularly spaced horizontal lines of flint.

White Cliffs of Dover, chalk with lines of flint

I understand how the chalk and flint layers are formed independently (from Wikipedia) but I haven't been able to find any information on why they formed in these regular stripes.

flint layer close up

Would coccoliths alone pile up for a long period of time, then sponges/siliceous plankton pile up for a much shorter time, then back to the coccoliths alone piling up and so on? ... seems very implausible.

Sorry if this is a very basic question but this has been bugging me ever since I visited over 10 years ago! :D

Thanks

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    $\begingroup$ I observed the same kind of flint bands in the chalk cliffs of Møns Klint in Denmark. $\endgroup$
    – njuffa
    Feb 17, 2022 at 3:17
  • $\begingroup$ When I have found flint in limestone has been lumps rather than layers. $\endgroup$
    – blacksmith37
    Feb 17, 2022 at 16:48

1 Answer 1

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Flint formation is complex, and the details still subject of research and debate.

The main factor is not a change in the relative proportion of siliceous and carbonate plankton, but the influence of chemical processes in the sediment after deposition. The flints precipitated where carbonate was dissolved. The dissolution occurred at the boundary between oxygenated water moving from the surface downwards and anoxic water rich in hydrogen sulphide from decomposing organic matter (the redox boundary). The reaction between oxygen and sulphide makes the water acidic at the boundary and this allows the carbonate to dissolve. Silica, which is in solution throughout the chalk, is in a state where it is precipitated from water with high carbonate concentrations, so flint forms at the redox boundary, typically between 5 and 10 metres below the sea bed.

The repetitive layering of bands seen in the cliffs may be influenced by cycles in the rate of sedimentation, with bands forming when sedimentation paused and the position of the redox boundary was stable. A secondary factor may be changes in the chalk matrix that influenced flow rates of water, and hence the redox boundary location. These changes are the result of changes in sediment supply, and bioturbation by burrowing animals at the sea bed.

The process is described in a paper by Clayton, 1986.

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  • $\begingroup$ Thanks for the answer! Your formation explanation makes much more sense. As for layering, what might cause sedimentation to pause? $\endgroup$
    – John Leonard
    Feb 18, 2022 at 6:46
  • $\begingroup$ Changes in chalk matrix sounds plausible. My layman's guess is that there is a tipping point where the redox action is more likely to occur (maybe due to sediment density), flint begins to form and acts as a barrier preventing further upward movement of the hydrogen sulphide. The flint layer can thicken until the sediment below the barrier finishes decomposing, and the chalk can thicken above the barrier until there is enough decomposition to create the conditions for the tipping point again. Total conjecture of course! $\endgroup$
    – John Leonard
    Feb 18, 2022 at 7:06
  • $\begingroup$ The cyclical variation in sedimentation is probably due to changes in the local climate; warmer drier conditions favoured chalk formation, and cooler wetter conditions favoured marls. $\endgroup$
    – Andy M
    Feb 18, 2022 at 16:24
  • $\begingroup$ It should be possible to back-calculate the (theoretical) total height of the cliffs if that carbonate had not dissolved.... $\endgroup$
    – Knob Scratcher
    Feb 18, 2022 at 19:49

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