Mineralogy and geochemistry of early-formed deep marine dolomite in the Castagnola Formation (Oligocene-Miocene, northern Italy)

A dolomitic bed of the Castagnola Formation (Aquitanian, Northern Italy) with on average 75 weight % dolomite is interbedded with marine siliciclastic turbidites and hemipelagites with very low dolomitic content. A detailed sampling of the bed (22 samples, 1 cm spacing) enabled the characterization of mineralogical and geochemical features of the dolomitic bed. The main mineralogical peculiarities of the dolomite within the studied bed lie in the cell parameters, in the low crystallographic ordering and in the thermal behaviour (TG, DTG, DTA) that strongly differ from well crystallized dolomites. The high contents of Fe, Na, Ba and Sr are also peculiar. The concentration of some accessory minerals is also anomalous for the Castagnola Formation basin, mainly because of the enrichments in Ca-phosphates and pyrite. Even though pyrite is abundant, the texture of framboidal aggregates indicates the oxic character of the water column, so that a post-depositional formation of sulphides can be supposed. The occurrence of diatom remnants and of high Si/Al ratio in some samples indicate variable carbonate/siliceous productivity, suggesting variations in the climate or water circulation, because the energy of the environment was rather constant, according to the grain size of detrital quartz and feldspars. Since the (super 87) Sr/ (super 86) Sr ratio of dolomite is typical of the Aquitanian seawater, the formation of dolomite long after sediment deposition during deep burial is unlikely, as well as direct precipitation, because the burrowing is pervasive. An early and rapid formation of dolomite after burial agrees with the small grain size of dolomite crystals and with the chemical and mineralogical features observed. Such dolomite crystallization is interpreted as the response of the system to a bloom in biogenic sedimentation; in this case the system is represented by early buried sediments, deposited in a restricted marine basin.