Concretion morphology, classification and genesis
A discussion of the most relevant morphological features of concretionary bodies and the different classifications, and the criteria involved in these classifications is presented, together with suggestions for improvements to the various classification schemes. The meaning of syngenetic, diagenetic and epigenetic related to the relative timing and environment of growth of concretionary bodies is also reviewed and discussed. The replacement of mixed morphogenetic classifications, which lead to conflicting results, by classification categories based on textural features is proposed. Because the identification of the genetic environment of a concretionary body tells little about its history, I recommend defining growth patterns in which the successive steps associated with changes in composition and/or texture and the development of new structures are recorded. A new path is presented that accounts for the contradiction between textural and isotope features, which suggests respectively syngenetic and diagenetic (or even epigenetic) signatures. This new path is characterized by the preservation of large porosities over very long time spans and down to depths that are somewhat greater than expected, due to the inhibition of normal compaction caused by early overpressuring. This overpressuring is the result of the development of hydraulic seals since approximately the syngenetic phase times. The concepts of “force of crystallization” and “displacive growth” are also reviewed. It is being suggested to discard some controversial interpretations of their actual importance for true concretionary displacive growth under epigenetic conditions. In accordance with other authors, the conclusion is reached that displacive growth is only possible if the shear strength of the host is very low and the stress field within the host is almost hydrostatic. A new model (the Symcompactional Concretionary Growth Model) is proposed which explains how a concretionary body, generally a nodule, can grow while its host thins down by compaction. This overpressured-undercompacted model will be useful for the interpretation of an assemblage of features such as injection dikes, hydraulic breccias, cone-in-cone structures, among others that are thought to be representative of the former presence of overpressured horizons. These overpressured horizons could serve as detachment planes in paleotectonically active basins. Seals that could have controlled the movement of brines and hydrocarbons during the diagenetic evolution of the basin can also be assessed; this is relevant for the identification of maturation conditions. The potential development of secondary fracture permeability due to hydraulic fracturing in buried areas of the basin can also be evaluated based on the identification of formerly overpressured horizons in outcrops. “The origin of concretions is generally a geological puzzle” Clifton (1957).