International Field Workshop on 'The Triassic of eastern France'

layers, with frequent mud-cracks that, laterally, either grade into intraformational breccias or are totally
eroded by sandstones. They are interpreted as deposition from settling in topographic hollows, located at the
bottomset, after the cessation of migration of fluvial barforms, during periods of either low water level or
aridity. The floodplain deposits associated with these channel deposits are characterized by laminated
silstones, or fine sandstones, along with red mudstones. These deposits are thinly layered (a few cm to up to
40 cm in thickness), and very sandy, with some weak bioturbations in places. They are composed of
interbedded fluvial overbank, aeolian, and flooded interdune deposits (overbank-interdunes). This facies
association, characteristic of arid environments, replaces classic floodplain facies (Langford & Chan, 1989).
The occurrence of 3D dunes and barforms, as well as the unimodal paleocurrents and scarcity of floodplain
deposits, indicate braided channels. The lack of root traces, and the occurrence of aeolian deposits associated
within the floodplain provide evidence of an arid climate. Moreover, the channels underwent periods of
cessation in their activity that could indicate the ephemeral character of some watercourses. During flood
periods, the formation of ponds allowed the ephemeral development of subaqueous life (bioturbated facies).
The fluvial conglomerate facies association of the ‘Conglomerat principal’ Formation shows vertical
arrangements of trough cross-bedded, and rarely planar cross-bedded, gravel to coarse sandstone lenses (dm
to m scale) that does not exceed 3 m in thickness. This facies contain abundant very rounded gravels,
sometimes of dm size, of extremely mature (exclusively siliceous) composition: quartz, quartzite, lydite.
Paleocurrent measurements indicate unimodal directions, oriented to the NNE, that characterize low
sinuosity channels. These channel deposits seem to be not markedly different from those described below.
However, the lack of channel-fill ends (facies Sr), as well as the predominance of conglomeratic facies,
suggest a period of transit and/or granulometric segregation. At outcrop, this formation has a maximum
thickness of 20 m, and shows a great lateral extent towards the eastern (Durand, 1974; Bourquin et al.,
2006).
The arid climate prevailing in the sedimentation area is demonstrated by the presence of many wind-
worn pebbles and cobbles (ventifacts), not only concentrated in the conglomeratic layers, but also scattered
here and there throughout the ‘Grès vosgiens’ Formation. (Durand et al., 1994).
40