International Field Workshop on 'The Triassic of eastern France'

IV. The Keuper
IV.1. - Geological setting
Classically, two areas of sedimentation have been distinguished in the Keuper of the Paris Basin.
These are an eastern area consisting essentially of halitic or anhydritic coastal sabkha deposits and a western
area dominated by fluvial deposits (Dubois and Umbach, 1974; Courel et al., 1980; Matray et al., 1989). The
Saint-Martin de Bossenay fault divides these two areas (Bourquin and Guillocheau, 1993, Fig. III.2). The
Norian as never been dated in the eastern Paris Basin.
IV.1.1. - Lettenkhole
In the eastern part of the Paris Basin, the first facies attributed to the Keuper are characterized by
dolomites and clays of the Lettenkhole. The age of this unit is well established in the eastern part of the Paris
Basin (Fig. I.1): the Middle Lettenkohle is Upper Ladinan (Kozur, 1972; Adloff et al., 1984) and the
‘Dolomie limite’ is Lower Carnian (Kozur, 1972). These strata are equivalent to the Lettenkeuper deposits of
the German Basin (Gall et al., 1977). The Lettenkohle, which crops out in the eastern part of the Paris Basin,
is composed of dolomitic-claystones and dolomites overlain by an anhydrite bed. It was deposited in a
restricted marine environment (Courel et al., 1980; Duringer and Doubinger, 1985). The upper part of the
Lettenkohle ‘stage’ is characterized by lagoonal-marine facies (Ainardi, 1988), followed by more marine
dolomitic claystones which accumulated during maximum water depth.
IV.1.2. - Marnes irisées
The ‘Marnes irisées’ Group is divided in three parts: the ‘Marnes irisées inférieures’, ‘Marnes irisées
moyennes’ and the ‘Marnes irisées supérieures’ (Table 1 and Fig. I.1).
In the east of the Paris Basin, the ‘Marnes irisées inférieures’ are made up of evaporite coastal sabkha
deposits (Fig. I.1): the ‘Couches à pseudomorphoses’ (anhydritic shales), ‘Formation salifère’ (halite and
shale) and ‘Couches à esthéries.’ The base of the ‘Formation salifère’ in Lorraine is dated as Lower Carnian
(Kozur, 1972; Geisler et al., 1978). These evaporite series occurred at remarkably similar times throughout
the Triassic basins of western Europe, where halite deposits may be several hundred metres thick. This major
Triassic evaporite ‘crisis’ corresponds to the ‘Unterer-Gipskeuper’ (Grabfeld Formation) in the Germany.
The ‘Marnes irisées moyennes’ are composed of three formations: the ‘Grès à roseaux’, the ‘Argiles
bariolées intermédiaires’ and the ‘Dolomie de Beaumont’ formations (Table 1 and Fig. I.1).
In the Germanic Basin, evaporite sedimentation was suddenly interrupted by the spread of fluvial
deposits forming the most consistent lithostratigraphic marker of the entire basin: the ‘Schilfsandstein’
(Dittrich, 1989) and ‘Grès à roseaux’ of northeast France (Fig. I.1). This unit is dated as Middle Carnian
(Julian) (Kannegieser and Kozur, 1972; Kozur, 1993) and must have been deposited during a relatively short
time (Hahn, 1984). Correlative units have been identified in several other basins: ‘Arden Sandstone’ in Great
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