The internal structure of carbonate concretions in mudrocks: a critical evaluation of the conventional concentric model of concretion growth

Abstract

Carbonate concretions have traditionally been viewed as forming concentrically, through the progressive addition of carbonate to the outer edge of the growing concretion. This conventional model of concretion growth is based mainly upon center-to-edge textural and geochemical trends that are consistent with concentric growth. However, a number of recent studies of concretions in mudrocks have documented complex zonation that is not compatible with the conventional concentric model. In some cases complexly zoned concretions contain late-stage cements that precipitated throughout the concretion body. None of the center-to-edge relationships used to support the conventional concentric model exclude more complex modes of origin. As concretions that fit the conventional model may be the exception rather than the rule, high-resolution petrographic analysis of concretions should always be used to characterize internal structure and mode of growth prior to interpretation of geochemical and textural data.