Heterogeneous distribution of calcite cement at the outcrop scale in Tertiary sandstones, northern Apennines, Italy

Calcite cement derived intraformationally in seven stratigraphic units of marine origin is distributed heterogeneously at the outcrop scale. Sandstone beds intercalated with calcareous shale older than Pliocene tend to be completely cemented, whereas stacked sandstone beds that lack shale interbeds have calcite cement in the form of tightly cemented concretions that make up only 10-30% of a bed. Patterns of concretions within beds are remarkably varied and include both random and uniform spacing. There is no preference of concretions for shell-rich layers. The lack of strong textural or compositional controls on the localization of calcite cement suggests the preeminence of highly localized hydrologic factors in determining the spatial distribution of authigenic pore-filling calcite. Faults apparently served as fluid conduits and were selectively cemented. In general, only sandstones intercalated with shale are totally cemented. -from Authors