Plate 49

Hummocky cross-bedding: rhythmic pattern

The vertical alternation of less and more cemented beds emphasizes the rhythmic stratification style and its concave-convex geometry in this outcrop of Pliocene sandstones in the northern Apennines. The difference in cementation is a diagenetic character but reflects primary textural variations within the sediment. The sand is slightly coarser and cleaner in cemented beds: more pore space was available for chemical cement in it. In softer beds, on the other hand, some matrix  (collective term for fine particles) is admixed with the sand and occludes part of the pores: less cement was precipitated there, also because the sediment was less permeable to percolating water.

Cemented beds have also a sharply defined base, whereas their top grades into matrix-rich sand. One can assume, to explain these features, that storm waves scoured a sandy bottom removing both sand and finer particles. Relatively sorted sand was redeposited first, while the fines were kept in suspension. With waning wave energy, fines were increasingly added to the sand and the upper member of the couple was accumulated, thus completing the storm layer. Hummocky and swaley laminae are preferentially preserved in cemented beds, but not visible from this distance (the cliff is an inaccessible river trench, incised during a rejuvenation phase of the Apennines chain). The presence of the structure, however, gives the bedding its peculiar configuration, with broadly curved, laterally discontinuous surfaces.

Pliocene Intra-apenninic Basin, Zena Valley, northern Apennines.