Marnes irisées
The Marnes irisées are a sedimentary formation of the eastern Paris basin . They were deposited in the Middle Keuper .
etymology
The name Marnes irisées (in German colorful marl ) is derived from the French noun marne (marl) and the verb iriser (iridescent, shimmering in rainbow colors). The latter in its adjective form alludes to the very colorful coloring of the rock.
Occurrence
The Marnes irisées are in the middle and upper section of the Keuperband in the eastern Paris basin. This stretches from Faulquemont in the north via Morhange , Lunéville , Charmes , Vittel to the southeast of Langres . From here it bends east and follows the southern edge of the Vosges to the north of Héricourt . Separated to the south, there are also deposits on the northwestern edge of the French Folded Jura, for example between Poligny and Lons-le-Saunier , on the western edge of the Bresse trench west of Mâcon and in the basement north of Dole .
stratigraphy
The flat-lying, altogether up to 235 meters thick Marnes irisées, French equivalent of the German Grabfeld Formation , Weser Formation and Arnstadt Formation , lie concordantly on the Dolomie limite ( border dolomite ) of the Lettenkohle (Unterer Keuper). They are in turn unconformably by sandstones of the Grès rhétiens ( Rhätsandstein superimposed). The formation is divided into three members (from hanging to lying):
- Marnes irisées supérieures
- Marnes irisées moyennes
- Marnes irisées inférieures
Between the Marnes irisées inférieures and the Marnes irisées moyennes lies concordantly the Grès à roseaux ( reed sandstone ), between the Marnes irisées moyennes and the Marnes irisées supérieures the Dolomie de Beaumont and above the Argiles de Chanville , likewise concordant.
Marnes irisées inférieures
The finely layered Marnes irisées inférieures, equivalent to the German Grabfeld Formation or the Lower Gipskeuper, reach an impressive thickness of up to 200 meters. The sediments are considered to be typical peritidal deposits of a sebkha .
They are made up of several, repetitive, brightly colored (gray, gray-green, red, ocher-colored, violet) series, the thicknesses of which vary in the meter and decameter range. These cyclical series are very long-lasting (confirmed by drilling). They should be correlable with the Milanković cycles . Unfortunately, their respective duration cannot be determined due to shift gaps and a lack of time stamps (fossils).
These large-cycle series are in turn made up of smaller sub-units - the elementary cycles - whose thickness is only a few decimeters. An elementary cycle consists of two sections - a sedimentation section of the marginal brackish water area ( lagoon stage ), followed by a section of the drying out of the sediment with accompanying sediment-internal capillarity effects (sebkha stage).
During the inundation phase, which took place under relatively little water cover, siliciclastics ( silts and clays ), clayey dolomites and finally very fine-layered, light, sulphate-containing dolomites sedimented one after the other .
The drying out phase, which could go hand in hand with the inundation phase locally, affected the higher parts of the respective deposit area and concentrated sulfates and salts in the deeper areas. Further evaporation eventually led to (locally degradable) gypsum or salt precipitations .
Shrinkage cracks formed on the sediment surface . The peritidal polygonal bowls inside bent up like a tent at the edges and the so-called teepee structures were created .
During the dry fall, clayey material could still be deposited in places, which had been accumulated by wind input or surface runoff. The intense evaporation resulted in capillary-related rise of the pore water enclosed in the sediment and the precipitation of its mineral load as concretions (sulphate bulbs). If left dry for a long time, paleo soils with a carbonate crust could develop on the surface ( aridisols ).
Often an erosive contact marks the onset of the next elementary cycle.
Diagenesis and immersion
In the course of the diagenesis, the concrete and crust-shaped gypsum recrystallized to anhydrite . This caused the detritic clay minerals to be displaced and created the characteristic chicken wire structure (mesh structure ) in the anhydrite that grew together. Later dissolution of the sulfate crystals led to the formation of chamber-like cavities. Occasionally the original plaster of paris was also replaced pseudomorphically by anhydrite.
Very characteristic of the Marnes irisées are their rock salt pseudomorphoses , which are mainly to be found at the base of silt layers. The rock salt formed by capillary action was dissolved again in the subsequent inundation cycle and replaced by fine-grained sediments.
In the later Mesozoic Era , the Marnes irisées were covered by up to 500 meters of sediment, which resulted in considerable load pressure. Any pore water that was still present was squeezed out and any remaining gypsum was finally converted into anhydrite. Subsequent erosion of the Mesozoic layer slowly brought the layer structure back to the surface. However, anhydrite is not stable under reduced pressure and under the influence of meteoric surface waters and so gypsum formed again with an increase in volume. As a result, the layer structure was inflated in places and individual layers were bent. In addition, tensile fissures formed in the rock structure under strain, which also mineralized with gypsum.
Marnes irisées moyennes
The Marnes irisées moyennes are only 5 meters thick and lie between the Grès à roseaux and the Dolomie de Beaumont. The sediments are very similar to the Marnes irisées inférieures, but differ in their somewhat more intense coloring.
Marnes irisées supérieures
The marnes irisées supérieures are about 30 to 40 meters thick. They have two dolomitic locations at their base . These gray to white dolomite marls are very compact and hard and disintegrate into angular fragments when they are struck. The actual marls are predominantly gray-green and purple.
Fossils
Dinosaur remains have been discovered in the Marnes irisées supérieures . The prosauropde Plateosurus longiceps was found near Poligny , Lons-le-Saunier and at several other sites . In Lunéville and Saint-Nicolas-de-Port , in addition to Plateosaurus, remains of Thecodontosaurus and teeth of Coelurosauria were found.
Age
The absolute ages of the Marnes irisées are not known. As the lowest member of the Middle Keuper, however, she can be assigned an Endladinic to Lower Carnic age. It covers roughly the period 232.5 to 228 million years BP .