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Abstract:

The Permian Unidad Roja Superior (URS) of the Aragón-Béarn Basin in the western Pyrenees contains the thickest (some tens of m) carbonate units, and the only gypsum outcrops of all Pyrenean Late-Hercynian basins. These carbonate units are intercalated among clastic red beds. From the analysis of seven URS carbonate units, four facies associations containing 16 separate facies were identified: limestones-siltstones, limestones-gypsum, dolostones-siltstones, and carbonates-black laminated siltstones. Based on evidence from fossils, sedimentology, petrology and geochemistry, we propose a continental paleoenvironment comprising a clastic alluvial plain developed in distal areas of alluvial fans, transitional to small, low gradient margins and shallow, carbonate and saline lakes. Four carbonate-saline lacustrine facies models are envisaged: calcite-precipitating palustrine lake; sulfate-precipitating lake; holomictic dolomite-producing lake, and meromictic dolomite-producing lake.

In the calcite-precipitating palustrine lakes, hypersaline conditions were occasionally achieved in marginal littoral areas leading to intrasediment precipitation of gypsum, subsequently to be replaced by calcite. Palustrine carbonate facies show a distinctive chemical imprint—Mg2+ and Fe2+ enrichment, Sr2+ depletion and heavier δ18O composition (about –7‰ PDB)—caused by pedogenic and vadose modifications and increasingly evaporative conditions. Several lines of evidence suggest that the paleohydrology of the sulfate-precipitating lakes was controlled by SO4-rich groundwater recharges in lake marginal springs. These are: (1) the presence of travertine fabrics and calcite-rafts in the limestones, (2) the relatively heavy δ13C composition (–5‰ PDB) caused by high organic productivity stages, and (3) the absence of well-developed evaporative sequences consistent with the relatively light values of δ18O (–10‰ PDB).

Primary precipitation of dolomite, related to increasing evaporative conditions, is proposed in the holomictic, dolomite-producing lake model. This is supported by the micritic textures, the Sr-enrichment, the absence of replacement evaporite textures and a broad suite of pedogenic and vadose features (especially, calcite–cementing ones) postdating dolomitization. Dolomite also formed in the organic-rich, anoxic and highly concentrated environments of the meromictic model.

This variety of lakes in the same basin, without forgetting the importance of global-scale factors like tectonics and climate, illustrates the fundamental role of depositional subenvironments and paleohydrology in the genesis and evolution of small lakes in arid, closed basins.

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