The fracturing of the chalk
On the scale of the height of the cliff or on a reduced outcrop, we observe that the chalk is broken. Generally, fractures are only simple joints without visible rejection. However, it is very rare to travel one kilometer along cliffs without crossing a weakness with metric rejection. No weakness of a rejection upper to 10 meters is directly visible. The vertical rejection hectométrique of the weakness of Fécamp-Lillebonne is arrested only by the gap of coats on both sides from the beach of Fécamp.
The types of fractures met (Duperret and al ., on 2012) are:
-Normal, very sloping weaknesses, rejection centimétrique in metrics;
-Weaknesses décrochantes, sometimes with one streak subhorizontales;
-Master's degrees-joints, fractures affecting all the cliff;
-Joints, small-scale fractures, without visible rejection;
-Fractures filled with flint or of calcite.
The modern studies of the fracturing rest on certain methods of geometrical measures and their treatment notions of which are approached for example on the site "stylolite" of G. André.
Main directions of fracturing
The studies made within the framework of Interreg IIIa (recoil of cliffs) showed that there were 6 main directions of fracturing:
- N 0
- N 30-40
- N 50
- N 100
- N 120-130
- N 150-160
Fractures present a pendage from 70 to 90 °.
Stratigraphy of the fracturing
Mortimore (numerous articles from 1978 till 2010) clearly highlighted that the style and the orientation of the fracturing were partially dependent on the stratigraphy. He noticed:
- Oblique conjugated fractures in:
-The Chalk of Holywell [unity lithostratigraphique English, between marls in plenus and the main marl Gun Gardens],
-The Chalk of New Pit [between the main marl Gun Gardens and the marl Glynde 1],
-The Chalk of Newhaven [over marls Buckle],
-The coats of Beautiful Everything in the Chalk of Seaford;
The term of slickensided shear joints" is often used. He indicates polished surfaces of friction sometimes streaked in the direction of movement and sometimes coated with fibers (slickenfibres) having believed during the movement and indicating the sense of this movement.
- Sets of vertical joints in:
-The coats of Cuckmere [between flint Seven Sisters and the flint columnaire of Bedwell],
-The coats of Haven Brow [between Bedwell's flint and marls Buckle],
-Culver's chalk [sleep campaniennes not represented here].
The networks of fractures forced between two stratigraphical levels are sometimes qualified of "strata-bound".
By announcing mechanical differences which can exist between various chalks (being able to play on the style of fracturing), it remains that the orientations are different and thus that the tensor of constraint varied during the deposit of the chalk. However, we can legally wonder why the fracturing does not extend in depth and what it remains confined in a stratigraphical slice. Why is it in the superficial slice, least lithifiée, that fractures are going to express themselves? This peculiarity is not discussed by the partisans of this interpretation. Another interpretation, seismic this time, would better report the superficial character of the fracturing.
Fractures are sometimes épigénisées by the silica and train sheet-flints (flint in blades).
According to S Vandycke (2003), the studies of ground indicate that the chalk is usually affected by sets of combined weaknesses occurring during the sedimentation and the diagénèse. So that it is likely that every successive state of paléocontrainte created new sets of combined weaknesses. The hypothesis is that for every state of paléocontrainte, the recently trained weaknesses are much more plentiful than those inherited. This hypothesis seems to us however debatable: why coats lower than the set "strata bound", normally more strengthened by the diagénèse, they do not register a more consequent fracturing under a continuous field of constraint? For us, this fracturing could be due to immediate constraints passed on by seismic waves of surface.
Duperret, Vandycke, Mortimore and Genter (2011) led a study of the fracturing of chalks in Sussex and Normandy bringing them to distinguish 5 successive phases of contemporary fracturing of the chalk:
- Phase I: extension Was born - SW
- Phase II: extension NW-
- Phase III: extension NNE-SSW, compression ESE-WNW
- Phase IV: compression N-, extension E - W
- Phase V: extension E - W
The directions of fracturing in Normandy, according to these authors, are not identical according to the stratigraphical position. Here are their data:
Mortimore (2010) also underlines the presence of weaknesses of growth (growth faults), described mainly in the Chalk of Newhaven. These weaknesses are slightly curved and are connected downward with a surface subhorizontale. The rejection of these weaknesses eases and tends to nullify upward.