Calcitization of Mg-Ca carbonate and Ca sulphate deposits in a continentalTertiary basin (Calatayud Basin, NE Spain)

Diagenetic carbonates formed through early calcitization of evaporite (gyps um, anhydrite) and/or magnesium carbonate (dolomite, magnesite) deposits ha ve been identified throughout the Tertiary Calatayud Basin (NE Spain). The diagenetic carbonates consist in all cases of low magnesian calcite and are related to main sedimentary discontinuities, which record episodes of sign ificant subaerial exposure in the basin. Early calcite replacements predomi nate in sediments of the Intermediate Unit in central areas of the basin. T wo types of diagenetic carbonates have been distinguished: (I) laminated di agenetic carbonates in which the original structure of sequences formed of laminated magnesite, dolomite and primary gypsum is preserved, though trans formed in calcite; (2) brecciated diagenetic carbonates, which resulted fro m extensive dissolution of lenticular gypsum macrocrystals accompanied by d edolomitization processes of dolomite host rock. The replacement by calcite resulted from rapid transformation of evaporite deposits either at the con tact of these deposits with overlying freshwaters or by percolating freshwa ter at very shallow burial depth. Stable isotope (carbon and oxygen) analys es of the diagenetic carbonates support influence of meteoric-derived carbo nate-rich groundwater for their formation. The meteoric isotope signature i s supported by its comparison with the stable isotope values determined fro m diagenetic calcites formed after magnesite in evaporite sequences during the Late Pleistocene and Holocene. The diagenetic carbonate bodies formed a s a result of a progressive change from highly to moderately concentrated s aline lake waters. This gradual evolution was related to a climatic shift f rom dry and hot to cooler and more humid conditions throughout the Miocene, a trend that has been also recognized in other large Tertiary basins of th e Iberian Peninsula in this period. (C) 2001 Elsevier Science B,V. All righ ts reserved.