The sequential stratigraphy
The object of the sequential stratigraphy is to define sedimentary units (sequences) limited by surfaces to temporal value corresponding to particular periods of variations of the relative marine level. There are several orders of variations of the relative level of the sea registered in the sedimentary series, and so many orders of sequences of deposit [see the pages of the SEPM]]
In a pond, the sedimentary series get organized in a logical succession of sequences under the control of the space of accommodation, that is the thickness of the slice of water between the bottom and the surface of the water (or relative depth). The potential of accommodation (or accommodation) is the speed of creation of space of accommodation, thus the first by-product according to the time.
The space of accommodation varies in the time and the space according to three factors:
- The sedimentary contributions (detrital or of haste),
- The tectonics (subsidence or rise of the bottom),
- The eustatisme (variation of the marine level with regard to a fixed datum).
The third factor, the eustatisme, is generally the main factor.
A sequence [of type décamétrique as modelled for Turonien or Cénomanien (Mange, on 1996; Owen, on 1996) and for Coniacien (Grant and al ., on 1999)] ] decomposes into three processions of deposit (= = systems pamphlets)
- A prism of low level (Lowstand Systems Tract or LST),
- A transgressive procession (Transgressive Systems Tract or TST),
- A high-level prism (Highstand Systems Tract or HST).
The variations of the potential of accommodation define temporal surfaces between these three processions, generally noted by their initials (Crankcase and al ., on 1991):
- The base of sequence (Sequence Boundary or SB) between HST and LST.
- The transgressive surface (Transgressive Surface or TS) between LST and TST.
- The surface of maximal flood (Maximum Flooding Surface or MFS) between TST and HST.
What are the sedimentary reactions to a variation of the space of accommodation?
When the depth increases, a transgression takes place on the continent, it is the transgressive interval (TST):
- Grains of glauconie or phosphate form;
- The diet of the currents changes: these currents reduce the sedimentation speed and pull the appearance of surfaces of omission or ablation (hard-grounds);
- Fossils are kept in large number.
When the depth stabilizes, the space of accommodation is raised and the pond fills with a strong rate of sedimentation. We obtain a high-level prism (HST). We shall have:
- White and pure chalks (reduction of the continental surfaces);
- If the environment allows it, levels of well trained flints and regularly spaced out;
When the depth decreases, the sea declines. The platform affects and sediments settle in the deepest zones. We shall have:
- Discreet detrital marly levels;
- Chalks noduleuses, hardgrounds and horizons of erosion;
- Ferruginous surfaces and nodules of pyrite.
Works of Lasseur
Sequences, interpretation Lasseur
Section going from Bracquemont to Diepp e, Eric Lasseur's sequential interpretation.
Section going from Fécamp to Saint-Martin-aux-Buneaux, interpretation E. Lasseur.
Section going from Saint Jouin to the Lime tree, the interpretation E. Lasseur.
Section going from Fécamp to the Cap d' Ailly, the interpretation E. Lasseur.
Section going from Penly to Dieppe, interpretation E. Lasseur.
Summary of sequences distinguished by E. Lasseur, from Cénomanien to Campanien.
Works of Mortimore :
The main levels Transgressive And Regressive (Color) are:
Coat 4 of the marls to Plenus ,
Peak in Mytiloides hercynicus (At the beginning of average Turonien = base of the Chalk New Pit = the main Marne Gun Gardens),
Over marls Glynde,
Over the bentonite Lewes, the levels to Zoophycos,
At the level of the hardground Hope Gap,
Summit of the level in plentiful shells of Volviceramus and Platyceramus, under the flint Seven Sisters,
At the level of the hardground Veulettes / Cuckmere,
Level of appearance of Cladoceramus undulatoplicatus (at the beginning of Santonien) = flint Michel Dean,
-Barrois ' Sponge Bed (at the end of average Santonien),
-The base of the zone in Uintacrinus socialis (Upper Santonien).