Composition isotopique, mineralogique et genèse des dolomies du Bassin de Paris

Abstract

The Tertiary and Cretaceous carbonate sediments of the Paris Basin include several dolomitic limestone and dolostone horizons. Samples of these lithologies were studied employing normal pétrographic procedures and X-ray diffraction methods. The results of these studies were subsequently compared with oxygen and carbon isotopic analyses of these specimens. The abundance of dolomite (which in all cases proved to be well-ordered) was determined by X-ray powder diffraction augmented by chemical analyses and microscopical examination. The present studies indicate that the Cretaceous dolomites were formed by incomplete diagenetic dolomitization of limestone precursors under submarine conditions. Subsequent to uplift a partial dedolomitization of the rocks was effected by percolating meteoric waters which also caused minor isotopic readjustments in non-dolomitic limestones. Lower Lutetian (Middle Eocene) carbonate horizons also demonstrate the same phenomena. Two distinct generations of calcite are distinguishable, namely: “unaltered” limestone calcite and secondary spar. The secondary spar was deposited by meteoric waters and is easily recognised by optical procedures or through isotopic analyses. The calcite of the undolomitised limestones exhibits no signs of recrystallization post-neomorphism and contains approximately 3 permil less O 18 than laterally equivalent dolomites. If the assumption is made that the secondary dolomites will in all cases retain the original isotopic composition of the limestone precursors, several possibilities exist which might account for the isotopic ratios of the undolomitised limestones: (1) The primary limestone precursors of the dolomites may have been deposited under facies conditions different from the ones of the “unaltered” limestones and therefore the two limestone types do not share a common origin. (2) the undolomitised and dolomitised lithologies share a common sedimentary origin but the former rocks suffered a change in isotopic composition due to exchange with percolating meteoric waters. (3) The dolomite is of dual origin, namely, in part a product of primary co-precipitation with calcite and in part a result of secondary diagenesis. The primary dolomite was enriched by approximately 6 permil O 18 with respect to the calcite whereas the secondary dolomite assumed the isotopic composition of the primary calcite which it replaced. This dual origin of the dolomite could thus explain not only the observed isotopic difference between “unaltered” limestone and dolomite but also the coexistence of extremely fine grained dolomite (crystals ⪡10 μ) with dolomite rhombohedra of 40–50 μ diameter. During the Upper Lutetian a highly variable sequence exhibiting rapid facies variations was deposited. The marine sedimentary basin was evidently subjected to alternating periods of strong evaporation and influx of large volumes of continental water. The oxygen and carbon isotopic compositions of the carbonates within the Upper Lutetian sequence were controlled by their depositional environments and thus the isotopic analyses of these carbonate horizons provided valuable clues as to the paleo-environment. Whilst the majority of the dolomite in the Upper Lutetian are of secondary origin, some primary dolomite layers possibly exist. During the Upper Eocene large amounts of gypsum with intercalated argillaceous and dolomitic horizons were deposited. Within these sediments the dolomite is characteristically 4·5–5·5 permil richer in O 18 than the associated calcite and it is concluded that the two carbonates resulted from coprecipitation involving isotopic equilibrium in a continental environment.