Plate 173

Arenaceous concretions

Globular masses often stand out in outcrops of poorly cemented sandstones; they are secondary features, whose occurrence does not depend on the depositional setting. Here, for example, similar forms are shown in littoral (A) and turbiditic (B) deposits. These concretions  are different from the nodules examined in previous plates under a fundamental respect: they are not displacive, but inclusive. In other terms, the host sediment is not pushed apart by the growth of the concretion, but is included in it. Carbonate cement is simply precipitated in the pore spaces.

Whether precipitation was diffused and simultaneous in the concretion volume or expanded radially from a nucleus, it is not clear. If the concretion is split apart, sometimes an object is found in the core (a clay or a wood fragment, a shell or a lens of skeletal debris), sometimes nothing appears. Why, then, cementation is so spotty remains a mystery. Much more so because other examples of selective cementation, more easily explainable, occur in the same sandstone units, for example along bedding or lamination surfaces (see plate 41). Stratiform cementation is particularly developed near permeability barriers, like mudstone partings. It cannot be considered, however, as a real structure.

Whatever their origin, arenaceous concretions are useful for the field sedimentologist, as they emphasize primary structures that would remain scarcely visible in the surrounding sand (see B).

A: Pliocene nearshore deposits of the Intra-apenninic Basin; B: Upper Marnoso-arenacea Formation, Santerno Valley, northern Apennines.