Abstract: Patterns of Calcite Cementation at the Outcrop Scale in Tertiary Sandstones, Northern Apennines, Italy

E. F. McBride, Kitty Milliken, William Cavazza, Ubaldo Cibin, Daniela Fontana, M. D. Picard, G. G. Zuffa

Calcite cement in six stratigraphic units (four units are turbidite deposits and two are marine shelf deposits) of Tertiary age in the northern Apennines is distributed very inhomogenously at the outcrop scale. Calcite cement occurs chiefly in concretions that make up to 30% of a bed but also in completely cemented beds. Concretion shapes include spheres (<1 m diameter), oblate and prolate spheroids (<1.5 m), tabular forms (to 8 m), and irregular forms. Distribution patterns of concretions within beds are remarkably varied and include both random and uniform spacing; selectivity in places for either the top, middle, or bottom of beds; selectivity for faults that cut bedding at a high angle; and local selectivity for mudstone rip-up clasts. There is no selectivity of concretions or shell-rich layers Some cement patterns are specific to particular formations, whereas some formations have different patterns at different outcrops. Most formations have more than one cement pattern in an outcrop.

Petrographic and stable isotopic evidence implicate detrital carbonate particles (CRFs and marine skeletal debris) in both sandstones and intercalated shales as the main sources of cement components. IGV, thermal data, and oxygen isotopes indicate that cementation took place at or near maximum burial chiefly by water enriched in ¹⁸O relative to sea water and which was probably derived from underlying rocks by compaction.

The lack of strong textural (grain size, graded bedding) or compositional controls on the localization of calcite cement suggests the preeminence of hydrologic factors in determining the spatial distribution of authigenic calcite. Spherical concretions grew by diffusive supply of components, whereas prolate and elongate concretions grew dominantly by advective supply. Faults apparently served as fluid conduits and were selectively cemented. In general, only sandstones intercalated with shale are totally cemented. This indicates that shales were a major source of cement components for the latter sandstones at least.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994