The events courantologiques
The chalky series contains levels of hardened chalk or Hard-grounds. The premature lithification of these chalks by carbonates, phosphates, silicates or hydroxides results from the sweeping of the sea bed by thorough currents which prevent the deposit of sediment spawns and favor the cementation. The duration of action of the currents can be more or less long in the geologic scale and the temporal interval associated with a hard-ground can cover several zones fauniques, which takes away from the notion of event. Worse, an erosion of the sea bed can accompany the hard-ground and the ablation of coats is going to increase the uncertainty of dating of the phenomenon.
In spite of the stratigraphical precautions which they arouse, hard-grounds is excellent marks on the ground. The remarkable works of P. Juignet (1974 and later) on Norman Cénomanien showed that hard-grounds has a geographical extension exceeding hundred kilometers. This author counted about ten these levels to which, for the understanding and the comparison, he attributed local names (Bruneval, Rouen, Antiiron ..). The stratigraphical wedging, by ammonites in particular, assures of the simultaneity of every level in various regional cups.
In England, numerous authors, since Barrois (1876), indicated the presence of hard-grounds, sometimes qualified as Sponge Beds or Rocks (Chalk Rock, Top Rock, Spurious Chalk Rock). Mortimore (cf. biblio from 1983 till 2011) widely contributed to describe these levels, to make correlations in all the English-Parisian Pond, and to synthesize their list. The most plentiful are located in lower Coniacien but he exists in all the floors of the upper Cretaceous there.
Except their obvious interest in the factual stratigraphy, hard-grounds asked the question of their integration and their meaning in the sequential stratigraphy. In this frame, the answers are less obvious.
Hard-grounds identified in Pays-de-Caux
Barrois ' sponge Bed / HG Frécotte
HG Veulettes / HG Middeton / Cuckmere SB / HG Juggs Lane
HG Nostrils / HG Warren / HG Belval
Light HGs Point / HGs bar End
HGs Hope Gap
Caburn Sponge Bed / HGs Vévigne
Spurious Chalk rock / HG Ogbourne / Hgs Manneporte
HG lime tree 2
HG lime tree 1
HG antiiron 3
HG antiiron 2
HG antiiron 1
HG Rouen 2
HG Rouen 1
HG Bruneval 3
HG Bruneval 2
HG Bruneval 1
HG saint Jouin