Why it still possess great porosity and why it is not affected by subsequent diagenesis. At first glance it should be "easy" to change the chalk by diagenesis?
Zbyszek
Chalk possess great porosity and softness under two conditions: if it was never buried deep enough to be compacted, or if the pore system retained an above-normal formation pressure in spite of a deep burial.
The first case is seen where the chalk always remained (since Cretaceous times) relatively near the surface. In this case the rock is fairly friable and can be used as a writing tool for blackboards. This originates the confusion between the UK usage of the word (writing tool), also in French, and its translation in South European languages ("gesso" in Italian, which however in mineralogical terms is gypsum, not calcite).
The second case is seen in certain oil fields in the North Sea (Ekofisk, and similar fields). In this case the original high porosity is preserved by overpressure in the pores. Incidentally, producing oil from this reservoir causes the formation pressure to decrease, and the rock to compact (there is severe sea bed subsidence at Ekofisk for this reason).
There are areas where chalk underwent some degree of burial, and the rock compacted to one half of its original thickness at the time of deposition. This type of chalk is an indurated rock which was subject to significant diagenesis. For example, at the Melton Bottom chalk quarry (East Yorkshire, UK), in 1984 (before the quarry abandonement), I collected a piece of chalk showing a U-shaped burrowing trace of approx. 1/2 inch diameter, burrowed in chalk and filled with chalk of slightly different colour. The burrow cross section was perfectly circular in the two vertical tracts of the U-shaped burrow, while it was 1/2 inch wide and 1/4 inch tall in the horizontal part of the burrow. This indicates 50% compaction since sediment deposition, and it is consistent with the induration of the chalk at this locality. Such compaction represents significant diagenesis, due to some burial of the rock (followed by uplift to surface).
Zbyszek:
Massive thickness of Cretaceous CHALK found in Central Graben and other places in North Atlantic terrain normally remains soft, unless subjected to appreciable compaction. Plausible explanation lies in Nannofacies and pore characteristics: see link-
http://onlinelibrary.wiley.com/doi/10.1002/9781444304855.ch7/summary
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Syed
Hi Zbyszek,
The physico-chemical transformation depends on the circumstances and the succession of events after the chalk has been deposited; The initial porosity of the Chalk is subject to change, for example, if necessary, the filling of the pores by another substance such as silica, in case of arrival and infiltration of a solution rich in silica, The tardive silicification process takes place; Also, an increase in the volume of the pores may occur as a result of the dissolution of the calcite due to the infiltration of acidic waters.
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Slimane.
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