The ichnofaune of the chalk
The chalk is a rock strongly bioturbée. His uniform white tint masks the reorganizations, but we can visualize them artificially by diverse treatments (the simplest being to coat the surface of the sample by some lubricating oil).
A fossil track, or better an assembly of fossil tracks defines an ichnofaciès. This ichnofaciès allows to characterize the environment of deposit: bathymetry, oxygenation, sedimentation speed, etc.
In chalks of the Pond Anglo-Parisien, the mainly described tracks are Thalassinoides, Planolites, Zoophycos, Chondrites, Bathichnus, Teichichnus and Trichichnus.
Thalassinoides A branchy den (branches in the shape of T or of Y) is. These branches are arranged in horizontal network (hexagonal motive plan), oblique or vertical (wooden forms of deer), with extensions in the junctions of branches (rooms of about-turn). Walls are smooth. It is mixed dens, nutrition and house. We often associate them with arthropods decapods or bryozoaires. The dimension of dens can reflect the level of energy of the environment. They form in some decimeters of the surface of the bottom.
Chondrites A branchy, vertical den in horizontal, dendritic is. We can imagine him as an opposite tree: the trunk communicating with the interface water-sediment and branches being subdivided by sinking. It is the den of nutrition that we associate with badly oxygenated substrata. We attribute him generally to a worm which moves forward and behind, every branch being one piece of news bums around of exploration.
Zoophycos A horizontal den with oblique is. We can imagine him from a den U in a vertical plan (type) (Rhizocoralium). Between both vertical branches is situated a blade (= spreite) with the successive tracks in meniscus of the digging (= track of the backfill or filling defers). Another strategy than digging in a vertical plan can be adopted by the animal, it is the one of Zoophycos. The successive spreiten moves laterally of certain angle. Their envelope describes a surface of rolling-up around an axis with an angle of variable immersion. The animal can so investigate a more compact volume. The external edge of the spreite can take a helical shape if the gap angular takes place in the same sense. It is a den of systematic nutrition, the animal tests the sediment by a sweeping there U which it repeats by turning in a sense or in an other one. In section vertical line, we recognize generally Zoophycos by the vertical axial handle (if the cup crosses her) and in the sub-horizontal sections of the spreiten which train greyer nets.
The previous explanation is only one among other hypotheses on the ethology of the body responsible for the Zoophycos. Löwemark and al. (2004) summarize clearly 5 possible hypotheses.
Planolites represents the back filling of a short-lived, simple den, méandriforme, oblique or horizontal. It is a den of nutrition made by an animal vermiforme. The filling of sediment can be different of him taking. Dens can cross itself but are not branchy.
Bathichnus A particular track in the chalk is. Indeed, this shape is systematically épigénisée by the flint and we shall also mention them in the analysis of particular flints. Thus it is not the initial imprint but the strongly dilated object. It is supposed to be an organic fossil said track Bathichnus paramoudrae. Precise in descriptions chalks cénomano-coniaciennes of the region of Etretat were made by Breton (2006). The platier of Grainval, near Fécamp, at the level of the Beautiful marl quite 2 (average Coniacien) is a place favored to be made an idea in 3D of the track.
She contains a horizontal part made of a slightly sinuous den being able to reach around thirty meters in length and a vertical part 80 cm in height 1 m, this handle being connected by a butt with the horizontal section. Until now, we did not observe handle in both extremities.
This track is apparently organic, the animal having dug at first the mud vertically on a meter, to progress then horizontally on a big distance. The generalization of this model in all the raised paramoudras seems inappropriate and we shall examine this case with flints.
The fossil tracks, as Thalassinoides, can be produced by animals with hard test (Bivalves, Echinides, Arthropods as Callianasses). The protection led by the burying often allows them to reduce the calcification of their skeleton. Other tracks were produced by animals with soft body conservation of which was not assured.
The ichnofaciès Thalassinoides is most plentiful and enough ubiquiste although associated with deposits shallow rich in macrofauna. A large number of flint result from the épigénie of these dens. The HST with alternations flints-chalks show this ichnofaciès (lower Cénomanien under the HG Bruneval, Turonien lower: flint St Nicholas' Day, upper Turonien: flint Lewes).
The ichnofaciès Planolites is also ubiquiste, especially confined in the superficial slice of unconsolidated sediments and thixotropiques.
The ichnofaciès Zoophycos is associated with white and fine, probably deeper chalks. He finds itself in particular levels, either that the filling of the den remains marly, or that it is épigénisé in flint: chalk "tiger" of Crimea and "Bänderkreide" of Campanien of Germany of the NW, Zoophycos Cuilfail under hard-grounds navigation, Zoophycos Beachy Head under the bentonite Shoreham.
The ichnofaciès Chondrites is considered as the deepest and developing in a mud dysoxyque. We find him in particular in current chalks of "deep sea" with Zoophycos and Planolites.
For Pemberton and MacEachern, 1995, the ichnogenre Zoophycos is situated below the maximal surface of action of waves.