Mn– and Fe–carbonate rich layers in Meso–Cenozoic shales as proxies of environmental conditions: A case study from the southern Apennine, Italy

Mn-rich layers and interbedded shales from a well exposed natural section of the Northern-Calabrian Unit (Late Jurassic–Early Oligocene) in the surroundings of the Terranova del Pollino village, southern Italy, have been mineralogically andchemically analyzed, in order to reveal the factors controlling their formation. Mn-rich layers are composed of micas/clayminerals, rhodochrosite, siderite, chlorite and quartz whereas shales are formed by micas, clay minerals, chlorite, quartz,and feldspars. The MnO abundances in the Mn-rich layers, which are depleted relatively to the UCC in SiO2, TiO2, Al2O3,Na2O, K2O, and P2O5, are in the range of 11.01 18.41 (wt. %). R-mode Factor analysis indicate that SiO2, Al2O3, TiO2,Na2O and K2O have high positive weights in the first factor (59.8% of the total variance) whereas high negative weightsare observed for Fe2O3, MnO, and CaO. This factor accounts for the competition between the terrigeneous component, theauthigenic carbonate phases accumulating Mn and Fe which likely formed during paucity of detrital supply. The negativeweight of CaO and MnO in this factor, the higher Ca contents in the Mn-rich layers compared to shales, and the lack ofcalcite, suggest the presence of a mixed Mn–Ca carbonate rather than pure rhodochrosite. It is generally retained that Ca–rhodochrosite precipitates within the pore waters of reducing sediments since neither rhodochrosite nor siderite can formin equilibrium with bottom seawater. Thus the resulting sediment should be a mixing between the detrital component andthe authigenic one. Assuming Al2O3 as an index of the detrital component, it is clearly envisaged that in the Al2O3/MnOvs. Al2O3 diagram the carbonate-rich samples fall on the mixing curve having as end members the average shale and therichest MnO sediment. This supports the idea that carbonate-rich samples formed through precipitation of carbonateminerals in the pore waters of the terrigenous detritus accumulating at the sea bottom. Further the REE distribution ofunaltered marine carbonates is expected to be representative of ambient seawater where carbonates precipitated. Carbonatesnormalized to fine-grained siliciclastic sediments, have typical HREE enrichment, negative Ce-anomaly, and lowertotal REE. In our case, the carbonate-rich samples normalized to the average composition of the interbedded freecarbonateshale, show HREE enrichment, lower total REE contents, and the lack of negative Ce-anomaly, due to theanoxic environment of formation for Mn– and Fe–carbonate. Finally was observed that the mineralization is enhanced ifthe site of accumulation is protected from dilution by clastic sediment input. The alternation between Mn– and Fe–carbonatesilts and carbonate-free shales along the studied sedimentary succession, were likely controlled by eustatic sea-leveloscillations which are well documented in the western Tethys during Middle and Late Triassic.