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 (five submarine-fan deposits and two shelf deposits) is distributed heterogeneously at the outcrop scale. Sandstone beds i ntercalated with calcareous shale older than Pliocene tend to be compl etely cemented, whereas stacked sandstone beds that lack shale interbe ds have calcite cement in the form of tightly cemented concretions tha t make up only 10-30% of a bed. The abundance and distribution of conc retions, with few exceptions, are irregular and unpredictable. Concret ion shapes include spheres (<1 m diameter), oblate and prolate spheroi ds (<1.5 m), tabular forms (to 8 m long), and irregular forms. Pattern s of concretions within beds are remarkably varied and include both ra ndom and uniform spacing; preference for either the top, middle, or bo ttom of beds; preference for faults that cut bedding at a high angle; and localization around shale rip-up clasts. There is no preference of concretions for shell-rich layers. Some formations have cement patter ns specific to that formation, whereas other formations have different patterns at different outcrops. Most formations have more than one ce ment pattern in an outcrop. The lack of strong textural (grain size, g raded bedding) or compositional controls on the localization of calcit e cement suggests the preeminence of highly localized hydrologic facto rs in determining the spatial distribution of authigenic pore-filling calcite. Spherical concretions grew by diffusive supply of intraformat ionally derived components, whereas prolate and elongate concretions g rew chiefly under the influence of advective supply. Faults apparently served as fluid conduits and were selectively cemented. In general, o nly sandstones intercalated with shale are totally cemented. This indi cates that shales were a major source of cement components for these s andstones at least.