DIAGENESIS OF LATE MIOCENE MICRITIC LACUSTRINE CARBONATES, MADRID BASIN, SPAIN

Mud-grade lacustrine limestones of Miocene age from the Madrid Basin, Spain, reveal varied and complex diagenetic histories. Microporous mic rites occur as well as micrites with anhedral mosaics. The micrites ha ve recrystallized from a metastable precursor, in part under sub-oxic meteoric diagenetic conditions probably before extensive karstificatio n in the Pliocene. The absence of aragonite relies, their mainly micri tic microfabric, low Sr content and covariant trend of Sr and Mg sugge st that the micrites formed from mainly high-magnesian calcite muds. L acustrine micrites can be compared to marine forms and criteria used t o assess the composition of the marine precursor muds can be applied t o lacustrine limestones to complement other techniques to identify ori ginal lake water compositions.