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Abstract:

A major change in oceanic sedimentation from Uppermost Cenomanian organic-carbon-enriched black shales (Bonarelli Level) to predominantly Lower Turonian oceanic red beds occurred in the Tethys. This paper presents high-resolution inorganic geochemical and mineralogical data on the transitional interval from the Scaglia Bianca to the Scaglia Rossa above the Bonarelli Level of the Vispi Quarry section, Umbria-Marche basin, Italy. Limestones from the Vispi Quarry section have very low Al2O3 concentrations (0.19–1.14 wt%) indicating low input of terrigenous detritus. Elements characterizing lithogenic input, such as Ti, K, Mg, Rb, and Zr, are of similar concentration in both the white limestone and the red/pink limestone and correlate positively with Al2O3, pointing to a homogeneous source area. The sources of terrigenous detrital input did not change throughout the transition from the Scaglia Bianica to the Scaglia Rossa. Geochemical data show that the red limestones were deposited under more oxic conditions, close to the sediment-water interface, as opposed to the white limestones in the Vispi Quarry section. This is supported by: (1) high Fe2O3 values (0.22% in average) and high Fe3+/ TFe ratio (0.58); (2) low values of relative enrichment factor (EF) of redox-sensitive elements U, V, Cr, Co, and Ni in the red limestones, with ratios of Ni/Co <2.5, V/Cr <1.2, V/(V+Ni) < 0.6, Fe3+/TFe > 0.45; and (3) a strongly negative 6Ce anomaly (0.28–0.42). The positive correlation between the peak height of hematite in the diffuse reflectance spectrophotometry (DRS) diagram and redness values (R2 = 0.98) indicates that hematite is responsible for the color of the limestones in the Vispi Quarry section. The DRS data confirm that the red color of the Scaglia Rossa limestones is the result of low concentration (~ 0.1 wt%) of finely dispersed hematite. SiO2(excess) P, and Ba productivity proxies show that there is no significant difference in depositional conditions and paleoproductivity levels between red limestone and white limestone. An increase in dissolved oxygen in bottom waters is the most probable cause of the origin of the red color. We suggest that intensification of bottom circulation with waters having higher content of dissolved oxygen may have resulted in significant oxidation of bottom sediments.

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