STABLE-ISOTOPE CHARACTERIZATION OF THE MIOCENE LACUSTRINE SYSTEMS OF LOS-MONEGROS (EBRO BASIN, SPAIN) - PALEOGEOGRAPHIC AND PALEOCLIMATIC IMPLICATIONS

The lacustrine carbonate facies from three allostratigraphic units (N- 1, N-2 and N-3) of Miocene age in the Los Monegros region (Ebro Basin) are characterized isotopically. The dominant facies are marls, lamina ted limestones, stromatolitic limestones, massive limestones and biotu rbated limestones. These are associated with lacustrine sulphate depos its and distal alluvial facies. Palaeohydrological reconstructions of the study area are supported by the isotopic and the mineralogical com position of the carbonates. Massive and bioturbated limestones occupy a very similar compositional domain and have the lowest isotopic compo sitions of all the facies (-9 parts per thousand<delta(18)O(PDB) < -4. 5 parts per thousand and -6.4 parts per thousand < delta(13)C(PDB) < - 0.4 parts per thousand), reflecting a short residence time of the wate r and variable, commonly high influence of biogenic CO,. Laminated lim estones and stromatolites define a comparatively enriched domain with delta(18)O values ranging from -6 to 0 parts per thousand and delta(13 )C values ranging from -3.5 to 0 parts per thousand. These values indi cate stronger evaporation and enhanced C-12 assimilation due to intens e biological activity during periods of longer water residence time. M arls have isotopic values intermediate between these two domains and c orrespond to periods of lacustrine dilution. Dolomite-bearing samples, mainly laminated and stromatolitic facies, show a Delta(18)O(DOL-CAL) = 7.5 parts per thousand with respect to calcite from the same facies , while delta(13)C values show very little enrichment (up to 1 parts p er thousand). The positive correlation (r= 0.85) between O-18 and dolo mite contents suggests that dolomite resulted from the progressive eva porative concentration of a single water mass and not from the mixing of waters of different compositions. The dolomite is thought to be pri mary or, at most, the product of very early diagenetic processes. The isotopic composition of the facies and their spatial and temporal vari ations depend on the depositional environment and were constrained by changing regional paleogeography. Isotopic trends from units N, to N, (Upper Agenian-Vallesian period) display a regular depletion in heavy isotopes that indicates a climatic change towards wetter conditions.