SEDIMENTOLOGY AND GEOCHEMISTRY OF CARBONATES FROM LACUSTRINE SEQUENCES IN THE MADRID BASIN, CENTRAL SPAIN

Lacustrine and alluvial carbonate facies have been investigated in Mid dle Miocene successions of the western side of the Madrid Basin in ord er to evaluate paleoenvironments in which carbonates formed. Carbonate facies are varied and include: (1) calcrete and dolocrete; (2) pond d eposits; (3) lake margin dolostone; (4) mudflat carbonate; and (5) ope n-lake carbonate facies. The dominant mineralogy of these is dolomite and/or low-Mg calcite. No high-Mg calcite or aragonite have been detec ted in any sample. delta(18)O- and delta(18)C-values range from -8.20 to -1.80 parts per thousand PDB and -10.25 to -0.70 parts per thousand PBD, respectively. More negative delta(18)O- and delta(13)C-values co rrespond to predominantly calcite calcretes and to carbonate deposited in ponds at the foot of arkosic alluvium. Higher delta(18)O-values ar e from both lacustrine carbonate and dolocrete. This latter lithofacie s has strong geochemical similarities to dolostones deposited in a lak e margin environment. Mudflat carbonate, deposited on shallow platform s subject to lake water oscillation, shows great heterogeneity in both stable isotope value and trace-element content. The mineralogy of the se carbonates is dominated by calcite and the limestones contain molds of gypsum. Occurrence of calcitized dolomite textures in these facies suggests the influence of fresher water during expanding lacustrine c ycles or further interaction with less saline groundwater. Trace-eleme nt contents are considered to be potential indicators of the different carbonate facies types, thus aiding the paleoenvironmental interpreta tion. However, discrimination among carbonate facies on the basis of t race-element contents appears to be dependent on the statistical metho d utilized for treatment of data. More information is needed to ascert ain their use as paleoenvironmental indicators.