Permian Saline Lakes in the Aragón-Béarn Basin, Western Pyrenees
Published:January 01, 1994
Blas L. Valero Garces, Josep Gisbert Aguilar, 1994. "Permian Saline Lakes in the Aragón-Béarn Basin, Western Pyrenees", Sedimentology and Geochemistry of Modern and Ancient Saline Lakes Models, Robin W. Renaut, William M. Last
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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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Sedimentology and Geochemistry of Modern and Ancient Saline Lakes Models
Sedimentology and Geochemistry of Modern and Ancient Saline Lakes - This volume of papers grew out of a four-day symposium entitled ?Sedimentary and Paleolimnological Records of Saline Lakes? held at Saskatoon, Canada in August, 1991. The aim of this Special Publication is to bring together selected papers from this conference that deal specifically with the sedimentological, inorganic geochemical, and hydrological aspects of salt lakes and their stratigraphic records. This volume is divided into four sections. The first section contains papers that deal with modern saline lakes. The second section contains papers dealing with sedimentation and diagenesis of late Quaternary salt lakes. The third and fourth sections contain papers devoted to ancient (pre-Quaternary) Lacustrine sequences.