Abstract
The Middle Eocene and Oligocene basins in northwestern half of Mallorca are filled by detrital rocks (conglomerates, calcarenites and lutites) and carbonates deposited in alluvial, fluvial, palustrine and lacustrine settings. The sediment supplied to these continental basins was transported by alkaline, bicarbonate-rich waters. As a consequence, these non-marine carbonate deposits display extensive developments of diverse kinds of organosedimentary facies. The sedimentary record from three localities (Peguera, Alaró and Sineu) enabled one to characterize well-preserved Eocene tufa deposits as well as Eocene and Oligocene stromatolites and oncolites. A total of ten carbonate facies are described and interpreted. The facies related to tufa deposits are diverse types of boundstones, rudstones and packstones of bryophytes, in situ stems s.l., phytoclasts, etc. and associated oncolite rudstones and bioclastic mudstones-to-packstones. These facies deposited in littoral lacustrine and fluvio-lacustrine environments. Oncolites and stromatolites present a great variety of shapes and sizes (oncolites from centimetres to several meters in diameter) that somehow were conditioned by environmental conditions within shallow lacustrine and fluvial settings. Three types of lamination apparently dependent on the type of microorganisms responsible for calcite precipitation are distinguished (fan-shaped and bush-shaped grouped filaments and isolated to loosely grouped filaments). The δ 13C and δ 18O composition of these deposits agrees with carbonate precipitation in freshwater, hydrologically open systems with similar carbon sources throughout the Eocene and Oligocene. However, Eocene and Oligocene samples are set apart by the δ 18O. Oligocene facies present lower δ 18O values, which might indicate deposition from isotopically less evolved waters, perhaps under slightly greater precipitation/evaporation conditions. Tufa facies have the highest δ 13C composition and show some positive covariation of δ 13C and δ 18O, primarily reflecting the effects of physical and biological CO 2 outgassing, and maybe also minor evaporation.
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