Abstract
Stephanian to Early Autunian lacustrine series are exposed in the Aragón-Béarn Basin (western Pyrenees) and comprise two main lithostratigraphic units that represent the unfilling of a closed lake located in a transtensional setting accompanied by extensive pyroclastic deposition. The Lower Lacustrine Unit overlies epiclastic conglomerates, sandstones and siltstones and it is mainly formed of massive and laminated grey limestones, laminated carbonates and grey, organic-rich, tuffaceous siltstones. A freshwater, probably stratified, lake with frequent depth fluctuations is proposed for this first stage. Limestones show very light δ 18O values (−10 to −14% PDB) believed torepresent a primary isotopic signal which is consistent with an intracontinental setting of the basin and the influx of 18O-depleted meteoric waters derived from the adjacent Hercynian mountains. A wide-spread pyroclastic flow deposit marks the limit with the Upper Lacustrine Unit which comprises the following lithofacies: pisolitic dolostones, stromatolites, massive and laminated dolostones and limestones and variegated siltstones. This final stage of the lake infilling is interpreted as a shallow, brackish environment. The transition between both units is marked by nodular limestones and dolomitization of some limestone layers, but the stable isotopic composition remains similar to the lower unit. Strong evaporative concentration, shallow well-mixed waters and re-equilibration with the atmospheric reservoir are suggested by the heavier δ 18O (−2to −10) and δ 13C (0 to −4) values in the dolomites and the heavier δ 13C values in the limestones (−1.5 to +2). The lacustrine episode ended with the progradation of an alluvial plain. Because many documented lacustrine deposits in volcanic settings are poor in carbonates, the Stephanian-Autunian series provide a case study for concomitant carbonate and pyroclastic deposition. High volcanic inputs are implied by the tuffaceous facies, the ubiquitous presence of volcanic and pyroclastic components in both detrital and carbonatic facies, and by the numerous intercalated pyroclastic layers. Several criteria are applied to discriminate subaerial and sublacustrine pyroclastic fall-out and flow deposits. Similar lake sequences are found in other Stephanian-Autunian basins in Western Europe suggesting that sedimentation in the Variscan continental basins reflects the dominant climate of the time.
Published Version
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