Repetitive sequences of carbonate deposits, occurrence include in the lower part of the Paleogene Carbonate Unit (northeast border of the Madrid Basin), have been studied, defining regressive lacustrine sequences and early diagenetic processes.Binocular microscopic examination, scanning electron microscopic/energy dispersive X-ray observations, and X-ray diffi-action analyses joint to isotopic studies dolomite precipitating -waters. The cherts included in palustrine limestones show j180 values of quartz that record more evaporated waters than those of the general stages of silicification.

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The Palaeogene lacusttine chalky carbonates of the Madtid Basin are a peculiar type of very soft and friable carbonate facies with high porosity despite being covered by more than 800 m of sediment. Similar physical propetiies to those desctibed in marine chalk reservoirs emphasize the interest in analysing and characterizing these carbonate facies within a lacustrine depositional system. Lithologically, they are calcitic and/or dolomitic poorly cemented carbonate muds with no significant amounts of skeletal debris. Clay minerals such as illite, smectite and palygorskite are present between the carbonate crystals. Palygorskite is the most common, coveting the carbonate crystals and forming sheets between them. These lacusttine chalky carbonates were formed in the basinal areas of the lake as the result of inorganic carbonate precipitation and/or detrital sedimentation related to episodic reactivation of the adjacent fan systems. Their petrological, geochemical and physical properties indicate that few textural and compositional modifications occUlTed during diagenesis. Their main physical properties are a very low dry bulk and grain density (1.6-2.2 and 2.62 g/cm3, respectively) and medium to high porosity (10-40%) due to micropores « 2 flm, 70%) and macropores (>2 flm, 30%). The convergence of lacustrine sedimentation dynamics (rapid sedimentation), the Oliginal mineralogy of these calcareous lacusttine muds (relatively stable low-magnesian calcite and dolomite), the early formation of the palygorskite cement of these muds, and the retention of Mg-enriched fluids in the pore system, were decisive in the partial inhibition of calcite cementation, compaction and recrystallization. The chalky carbonates are also intercalated between impetmeable littoral carbonate facies that impeded fluid flow through their pore systems.

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