Messinian diatom assemblages of infra-gypsum diatomites in the Sorbas basin (SE Spain).

In the Sorbas basin, the diatomites represent one of the main sedimentary c omponents of the Messinian together with its two marginal carbonate platfor ms and the central gypsum deposits. Several vertical sections were taken in the infra-gypsum diatomites from successively more distal paleogeographic areas, ranging from the reef slopes (Cariatiz) to the basin (Los Yeses, Los Molinos). A total of 88 diatom species were determined: 50 centric and 38 pennate. Characteristic assemblages were defined for each category type, re vealing ecological variations. In the three sections, plankton forms repres ent 90 % of the diatoms, with oceanic forms dominating the meroplanktonic a nd neritic. Nevertheless, some levels clearly indicate a coastal environmen t, attesting to the carbonate platform influence which progrades toward the basin. The diatom assemblages are relatively homogeneous throughout the th ree sections, indicating that, before gypsum deposition, the basin was stil l substantially open to the ocean, with waters sufficiently warm to allow c oral reef development. Nevertheless, the basin might also have experienced episodes of colder Atlantic influences. (C) 2001 Adac/Editions scientifique s et medicales Elsevier SAS.