Plate 27

Large-scale cross-bedding


This section, cropping out in the mini state of San Marino, included in northern Italy, is comparable with plate 26 C, and shows cemented calcareous sands made of skeletal debris (biocalcarenites) of Miocene age. The outcrop size is insufficient to show the whole geometry of the bed form that originated the cross-bedding. However, judging from the set thickness, in excess of 1 meter, we are dealing here with a large-scale form. Three unconformable bedsets can be discerned with variable dip direction; this should indicate that the section is not cut parallel to the paleocurrent (longitudinally) but at some angle (compare with plate 26C, where all foresets dip the same way).

Miocene "foramol" platforms of western Mediterranean, where many benthic organisms (Bryozoa, Echinids, Algae, Fora minifera, Mol luscs, etc.) lived in shoals and banks, were locally swept by strong tidal currents. These carbonate environments, characterizing temperate climatic zones, were later split by tectonic movements of the Apenninic orogen, and their pieces were displaced toward the foreland as allochtonous slabs  floating on top of a peculiar thrust sheet  (Ligurian).

Monte Fumaiolo Formation, Epiligurian Sequence, San Marino, northern Apennines.


In terms of scale, subaqueous dunes  must be distinguished from both eolian dunes and sediment waves. They are smaller, with a relief in the order of a few decimeters (below 5 cm, similar forms are regarded as ripples), and length in the order of decimeters or meters at most. In stratigraphic sections, it is convenient to use only one parameter, thickness,  to define the scale. When you find a set of inclined laminae thinner than 5 cm, you are dealing with small-scale  structures (ripples, ripple cross-lamination). When the set thickness (normal to its bounding surfaces, not to foreset planes! ) is in the range 5-30 cm, you get medium scale , i.e., normal, dune-size cross-lamination (or cross-bedding). The upper limit is quite arbitrary, and others might be equally proposed (for example, 50 cm or 1 meter). Anyway, set thicknesses exceeding this limit define large-scale  structures.

Photo: G. Piacentini 1970.


Sedimentographica