ABSTRACT

Cretaceous black shales bearing complex authigenic carbonates are common in some Penninic nappes of the western Alps. In this paper, we discuss the petrography, mineralogy, chemistry, and origin of a typical occurrence of these diagenetic carbonates in the pre-Senonian Serenne Zone of the Embrunais-Ubaye area. At least three types of carbonates with the following average compositions were identified by microprobe analyses: Mn, Fe-rich dolomite: Ca_1.02 Mg_.42 Fe_.24 Mn_.32 (CO₃)₂; Ca-rich rhodochrosite: Ca_.30 Mg_1.2 Fe_.07 Mn_.51 CO₃; and Mn, Mg-rich siderite: Ca_.07 Mg_.19 Fe_.54 Mn_.20 CO₃. These carbonates occur as lami ae, nodules, and layers. Petrographic results combined with X-ray diffraction suggest that Mn, Fe-rich dolomite is a relatively early precipitate, occurring as fine-grained, unevenly distributed cement which incorporates zones of less-compacted shales. Its abundance, relative to Mn, Mg-rich siderite, increases towards the center of the diagenetic layers. Mn, Mg-rich siderite is a late cement, as suggested by large, evenly distributed rhombs occurring in lower concentrations but with increasing abundance toward the margins, relative to Mn, Fe-rich dolomite. The presence of Carich rhodochrosite could not be established with certainty, as its (104) XRD peak is located, in most samples, in the shoulder of the dolomite peak. Compositional trends suggest equilibrium precipitation between Ca-ri h rhodochrosite and Mn, Fe-rich dolomite. Isotopic compositions of carbon and oxygen vary between -6.2 and -11.0 (¹³C_PDB) and between -4.4 and -9.6 (¹⁸O^PDB), the oxygen being somewhat heavier for Mn, Mg-rich siderite (-6.1) than for Mn, Fe-rich dolomite (-8.8). The presence of iron in the authigenic carbonates and the carbon isotopic composition suggest that precipitation occured b low the lower limit of diffusion of sulfate from seawater, after the onset of methane production. Absence of sulfate and high metal and CO₃^-2 activities favored initial precipitation of Mn, Fe-rich dolomite, which was followed by formation of Mn, Mg-rich siderite when an unknown value of (Fe + Mn + Mg)/Ca activities was reached. The metals were probably derived from hemipelagic metal-rich sediments accumulated just above an oceanic crust. The accumulation and preservation of high organic matter contents, favored by episodes of an expanded oxygen-minimum layer during the mid-Cretaceous, may have promoted the mobilization of Fe and Mn and their incorporation into complex carbonates.