ABSTRACT

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 and chemically analyzed, in order to reveal the factors controlling their formation. Mn-rich layers are composed of micas/clay minerals, 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 weights are observed for Fe₂O₃, MnO, and CaO. This factor accounts for the competition between the terrigeneous component, the authigenic carbonate phases accumulating Mn and Fe which likely formed during paucity of detrital supply. The negative weight of CaO and MnO in this factor, the higher Ca contents in the Mn-rich layers compared to shales, and the lack of calcite, 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 form in equilibrium with bottom seawater. Thus the resulting sediment should be a mixing between the detrital component and the authigenic one. Assuming Al₂O₃ as an index of the detrital component, it is clearly envisaged that in the Al₂O₃/MnO vs. Al₂O₃ diagram the carbonate-rich samples fall on the mixing curve having as end members the average shale and the richest MnO sediment. This supports the idea that carbonate-rich samples formed through precipitation of carbonate minerals in the pore waters of the terrigenous detritus accumulating at the sea bottom. Further the REE distribution of unaltered marine carbonates is expected to be representative of ambient seawater where carbonates precipitated. Carbonates normalized to fine-grained siliciclastic sediments, have typical HREE enrichment, negative Ce-anomaly, and lower total REE. In our case, the carbonate-rich samples normalized to the average composition of the interbedded free-carbonate shale, show HREE enrichment, lower total REE contents, and the lack of negative Ce-anomaly, due to the anoxic environment of formation for Mn- and Fe-carbonate. Finally was observed that the mineralization is enhanced if the site of accumulation is protected from dilution by clastic sediment input. The alternation between Mn- and Fe-carbonate silts and carbonate-free shales along the studied sedimentary succession, were likely controlled by eustatic sea-level oscillations which are well documented in the western Tethys during Middle and Late Triassic.