CENOZOIC

SEDIMENTOLOGY

Oiselier

Photo 1: “Conglomérats de Porrentruy“ of “Oiselier“.

According to the paleogeographical context at the basin scale, the architecture of the alluvial fill represents a classical lake or estuary margin depositional model. This is a typical alluvial style for the rift basins, as the Rhine Graben. At the site scale of “Oiselier”, the architecture of the alluvial fill represents a vast fan delta in proximal setting. The predominant set of lithofacies (matrix-supported gravel and clast-supported gavel), and the interbedding of gravel bedforms and sediment gravity flows are typical for a braided gravel-dominated river with tendencies to shallowing-up and to gravity flows. The architectural elements and their sedimentary structures suggest multistory channels, avulsions and debris flows. “Oiselier” seems to be a typical “Gravel-Bed Braided River with Sediment-Gravity-Flow Deposits” (fluvial style 1 after Miall 1985, 1996) in a proximal environment, immediately below the upland source area along the southern margin of the Rhine Graben.

Such alluvial environment generally occurs either in arid or semi-arid systems (Miall, 1996). Rivers may be ephemeral, terminating in a sheet flood zone along a playa lake margin, or they form an estuary with tidal flats. The fan distributaries are usually dry, with surface runoff occurring during spring melt and following cloudbursts. Most sedimentological and also biomineralogical features observed at “Oiselier” point to a Mediterranean climate, probably with long dry seasons leading to the desiccation of the floodplain, as previously suggested by Schuler (1990) and Becker (2003).

Beuchille

The “Beuchille” sections present a set of typical lithofacies (sand, silt and marl) and architectural elements (floodplain fines, crevasse splay and crevasse channel), which are interpreted as undifferentiated floodplain deposits or large, laminated, fine sand sheets (Miall 1996). The latter are characteristic of crevasse deposits or sandy bars sprinkled into the crevasse channels. These sets of deposits materialize low energy sedimentation in a distal position of the floodplain (Fig.2).

Fig.2: Sedimentary system of “Beuchille” (modified from Becker et al. 2004)

Beuchille Est

The “Beuchille Est” sections are more diverse than the “Beuchille” sections. The classical deposits of the “Molasse alsacienne” Formation can be observed there, but also some typical marine and coastal deposits of the “Septarienton” Formation. The latter present typical laminated marly and silty lithofacies with marine molluscs.

In the base of “Beuchille Est” deposits, an argillaceous-calcareous lithofacies has also been described, maybe belonging to the “Calcaires inférieurs & Terre jaune” and/or the “Sidérolithique” lithostratigraphic group(s).

Poillat

The “Poillat” deposits display typical lithofacies (horizontally-bedded sand, planar-crossbedded sand, mud pebbles supported lens) and architectural elements (laminated sand sheet, channel, sandy bedform, lateral-accretion form) of a coastal (upper delta face) to alluvial plain environment.

Solé

The “Molasse alsacienne” deposits of the “Solé” locality are similar as the continental deposits of the “Beuchille” locality.

The lithofacies of the “Rote Mergel des Mont Chaibeux” is dominated by a series of massive to laminated mottled silt and mud. These palustrine and overbank deposits suggest a poorly drained alluvial environment, setting up right after the regressive phase of the OMM.