8. Chemical and Diagenetic Structures

The last group of structures we are going to examine includes secondary structures,  whose origin is related to diagenetic processes  acting beneath the depositional interface. Various mechanisms, both physical and chemical, operate during diagenesis; moreover, their effects vary in different materials. The review here is very synthetic and gives just a sample of significant categories. Within each category, the variety of phenomena is often very wide, depending on local factors and regional geological settings. A regional bias is, to a certain extent, unavoidable in presenting secondary structures.

It must also be remarked that the products of diagenesis are investigated chiefly in the field of textures, at a microscopic or ultramicroscopic (SEM) scale. At outcrop (mesoscopic) scale, they do not offer the same abundance of forms and geometries that primary structures display.

To be precise, not all structures of chemical origin are secondary; some of them are produced by or during deposition, and are included here, in the first part of the section (plates 166). Primary chemical precipitation, excluding evaporitic basins, is a local phenomenon; it occurs, for example, around springs, in lakes, rivers, etc. Similar products are found in karst environments, i.e., in large secondary voids below the topographic surface. Whether they are to be considered as depositional or secondary (by analogy with chemical cement lining or filling smaller pores) is largely a matter of preferences. Chemical structures in soils have, instead, the same aspect as diagenetic structures in recently buried sediments. In evaporitic strata, representing the bulk of chemical sediments (carbonates must be regarded mostly as biochemical  products), both primary and secondary structures are common.