Resumen

SELLES-MARTINEZ, José. Implicancias mecánicas de la diferenciación entre septarias polares y ortogonales. Rev. Asoc. Argent. Sedimentol. [online]. 1996, vol.3, n.1, pp.11-16. ISSN 1853-6360.

Septarian nodules display different fracture networks whose significance has not yet been fully explored. Although Richardson (1919) pointed out that many septarians did not follow the classical definition of "radiating cracks uniformly widening towards the center, with a series of concentric cracks usually added, the combination producing a spider-web appearance", but instead, they displayed "the bulk of the interior divided into irregular polygons (...) they really correspond to columns of polygonal section which run vertically through the nodules", his statement did not succeed to produce much investigation on the subject. Research in the following years concentrated in physico-chemical conditions of concretionary growth and whether or not the cracks reflected desiccation, expansion or synaeresis, but not in the significance of the different geometries of fracture systems in the septarian. Surprisingly, most of published definitions of septarians are in agreement with the one Richardson considered unsatisfactory. Illustrations in text books and current use of the term in papers involve several different possibilities that have not been discussed and incorporated to the description and  interpretation of environmental conditions during formation of these structures, although they are surely meaningful on the subject. This paper aims in interpreting the mechanical implications of the two geometries discussed by Richardson (1919) and illustrated in Comite des Techniciens (1966, Figs. 156 and 157) and proposes that their geometrical differences reflect those in the stress field during septarization of the concretion. It is important to underline that fracture sets in "polar" septarians do not reach the outer surface of the concretion, which is usually spheroidal, while those in an "orthogonal" septarian, that is generally ellipsoidal, always cross cut the whole body of the concretion, a feature that is relevant to interpret to relative influence of isotropic internal stress field and anisotropic (vertical flattening, compactive) one. Polar septarians, with two fracture networks, one concentric (spheroidal or polyhedral) and the other planar (defining pyramids with a common apical point at the center of the concretion), are here interpreted to be the result of contraction of the material in the nucleus of the concretion under a highly isotropic stress field. Complete isolation from the external field (represented by overburden load and lateral constriction) may be achieved by at least three mechanisms: isolation by a rigid selvage, isolation because the host is too rigid, or, on the contrary, because it is enough plastic to induce an hydrostatic stress field on the concretion (thus introducing no modifications on the relative magnitude of the stresses in the core). When the stress field is not hydrostatic, compaction goes on before the concretion is completely rigidized and the rheological contrast between concretion and host is relatively important, tough flattening of the concretion will result in tensional stresses. These tensional stresses, externally imposed to the internal isotropic field, develop parallel to layering, being mutually perpendicular and normal to the compactional stress that is vertical. As a result, two sets of fractures develop normal to each other that cross cut the whole concretionary body in a structure known as "chocolate tablet".

Palabras clave : Diagenesis; Concretions; Rheology; Stresses.

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