Modelo de desarrollo y formación de concreciones carbonáticas en los sedimentos lutíticos anóxicos (Coniaciense-Campaniense), de la Formación La Luna en Venezuela Occidental

Abstract

The carbonate concretions that characterize the anoxic-dysoxic calcareous shales deposits of Late Cretaceous La Luna Formation in Western Venezuela, represent a non-conventional model of growth and development of carbonate concretions. Its evolutionary pattern is closer to that of the geodes. A hard outer layer cone in cone cement protects the nucleus consisting of friable and humid carbonate mud associated with fenestral structures composed of Mg-calcite and framboidal pyrite. The migration of fluids determines the characteristics of the textural facies as well as their typical internally laminated structure. The size and shape of the concretions vary within the stratigraphic sequence, following a pattern that seemed to be linked to the oxygen variation in the basin. The evaluation and the modelling proposed respond to a systematic and detailed petrographic evaluation (SEM-BSEM) and isotopic analyses (oxygen and carbon isotopes)