Hardgrounds (or hard-grounds) results from the evolution of an ordinary chalk subjected to phenomena diagénétiques premature and which affect him unless a meter of the surface of the sea bed. These phenomena lead to a cementation and to a hardening. A hardground is the ultimate evolution of a stop of the sedimentation which begins with the transformation in a chalk noduleuse.
Hardgrounds shelters a flora and a specific fauna adapted at the bottom hardened. The bodies (oysters, bivalves fossorial animals, toweling) can live fixed to the substratum or drill in the mud cemented to shelter there.
We can follow various phases in the realization of a complete hardground:
- Stop of the sedimentation
- Cementation diagénétique beginning with a network of nodules,
- Fouissage by the bodies (Thalassinoides) which meet more and more evil,
- Filling of dens
- Invasion by faunae perforantes or encroûtantes (Trypanites and Gastrochaenolites),
- Replacement of the calcite by fillers of minerals authigènes
- Glauconie (green, silicate in which the ion Fe is in the ferrous state)
- Phosphate (brown),
- Iron oxide (red),
On surface or on the walls of dens,
- Physical or chemical erosion on a certain depth.
Why a stop of the sedimentation?
If a hard-ground corresponds well to a stop of the sedimentation, the cause of this stop is not however unique. Two general explanations can be moved forward:
- Thorough currents forbid any deposit and even go as far as removing coats of beforehand put down chalks. This explanation seems the best adapted, if we observe a soil erosion associated with the hard-grounds, for example those of lower Coniacien (NNavigation, Hope Gap) ) or of upper Coniacien (HHG Veulettes / Middleton) ) .Les thorough currents have various origins:
- The currents of tide:La Sea of the chalk was opened on the Atlantic Ocean and the Thétys. Thus she must be swept by currents of tide, susceptible in leading channels in the least deep parts. Such an interpretation is the one of Quine and Bosence (1991) for the basins of Etretat. It is in this case forms lengthened in the direction of the current. But, the currents of tide may also cause hydraulic dunes the extension of which perpendicular, this time, in the direction of the current.
- The currents of outline:Il involves currents géostrophiques parallel to isobathes, being able to train "veins" (as, for example, the Vein of current Mediterranean Water, HANQUIEZ V., on 2006) and responsible for deposits qualified as" wrinkles of outline "or" contouritic drifts". This hypothesis is supported by E.V. Esmerode and F. Surlyk (2009) for Danish chalks or those of the Pond of Paris. We subscribe rather to this theory.
- A rise of the sea level can also starve in sediment a platform which does not act any more then in "factory with carbonates" (carbonate factory). It is an interpretation stemming from the sequential stratigraphy. In the sequential terminology, the hardground is a surface of erosion representing a conflict (unconformity) limiting a set of deposits. We shall find preferentially a hardground at the level:
- Of the SB (sequence boundary, the fastest decrease of the relative marine level),
- Of the MFS (maximum - flooding surface, the fastest rise of the relative marine level).
Such an interpretation is presented by Mortimore (2010) for hard-grounds cénomaniens Rouen and Antiiron and hard-grounds turoniens lime tree. It is a question of hard-grounds extensive in horizontal surface.
Main things hard-grounds or surfaces of erosion
Distinguished on the coast normano-picard
HG Veulettes (Veulettes E)
HG Conteville (Saint Valéry W)
HG Gulls (Veulettes W)
HG Anvil (Small Paving stones)
HG Etigue (Val Saint Martin)
HG Hope Gap
HG navigation (Big Paving stones)
HG Bondeville (Eletot)
HG Senneville 2 (Eletot) / HG Vévigne 2
HG Senneville 1 / HG Vévigne 1
HG Manneporte 2 / HG "nameless 2?
HG Manneporte 1 / dolomite Etretat / HG "nnameless 1? (Mass of fallen rocks Val de Mer)
HG lime tree 2
HG lime tree 1
HG Fagnet 2
HG Fagnet 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