Climatic vs. tectonic control on facies and salinity changes in an Eocene rift lake, Upper Rhine Graben, Central Europe
Résumé
The Upper Rhine Graben (URG) is a complex rift and wrench basin at the borders of Germany, France and Switzerland about 300 km long, 35 to 50 km wide and filled with up to 3500 m of Eocene to Quaternary sediments. In its early rifting stage, normal and transtensional block faulting created an elongated, land-locked rift valley possibly hundreds of metres deep. Middle to Late Eocene lacustrine and saline deposits cover an area of c. 7500 km² of the central and southern segment of the URG. Thicknesses vary from a few metres of condensed palustrine limestone on tectonic highs to more than 1000 m of halite-bearing marls in the Wittelsheim depocentre. Lithofazies associations include bedded halite-bituminous marl alternations with salic paleosols (central halite zone), laminated marls with poor freshwater fauna, partly with intercalated vertic, calcic or gypsic paleosol horizons (Lymnaea marl facies zone), poorly bedded marls with vertic paleosols and subordinate sandstone beds (Pechelbronn facies), and palustrine, partly rooted or brecciated limestone with pisolites and freshwater gastropods (Melania limestone marginal facies). Due to post-Eocene uplift of parts of the basin margins, lake shore deposits are rarely preserved. Where they are, lacustrine sediments with pedogenic horizons pass over to cyclic deposits of channelized alluvial conglomerates, sandstones and rooted overbank mudstones. This alluvial facies belt is typically only a few kilometres wide and rapidly replaced by block conglomerates of steeply sloped fan-delta deposits close to the rift margins (conglomerate zone). There is a first-order change in the position of facies boundaries in the Eocene succession, indicating rapid transgressive drowning of the internal parts of the basin in the Lutetian and successive progradation of fluviolacustrine environments during the Priabonian. Paleosol facies and fossil content do not show significant changes in the overall climate regime over this transgressive-regressive cycle, spanning almost 10 Ma. Thus, we attribute this first-order cycle trend mainly to early syn-rift tectonics creating accommodation space and its subsequent autocyclic fill-up. In any of these facies there are also high-frequency cyclic variations in lake level and salinity, however, e. g. by alternations of halite deposits and fossiliferous freshwater marls, recurring pedogenic overprint on laminated hypolimnion deposits, and subaqueous green clay intercalations within successions of palustrine carbonates. They suggest rapid lake level oscillations between deep freshwater lake phases and intense droughts, reducing the open-water surface to a small and shallow salt lake in the deepest parts of the Graben. The time scale of these oscillations is poorly constrained. According to the maturity of the paleosols, time intervals seem to represent cycles well within the Milankovich frequency band, i. e., 10e4 to 10e5 yr, and thus suggest a climatic control on these small-scale cycles.