Diagenesis, compaction strain and deformation associated with chert and carbonate concretions in organic‐rich marl and phosphorite; Upper Cretaceous to Eocene, Jordan

Abstract

ABSTRACTThis paper presents an integrated petrographic–geochemical–geomechanical study of the growth mechanisms of carbonate and chert concretions observed at outcrop and core from the Upper Cretaceous to Eocene organic‐rich carbonate mudrocks, central Jordan. It provides evidence for displacive and replacive concretion growth from the analysis of primary lithological characteristics, compaction strain and deformation structures associated with concretion growth. Concretions were analysed to determine the primary lithological controls on their development and the measurement of strain in the host rock to develop a method for constraining the growth mode and their paragenesis. Concretions exhibit either a replacive or displacive growth mode largely dependent on the original host lithology. Displacive concretions exhibit irregular shapes and semi‐fibrous internal structures in contrast to regular shapes and microcrystalline textures observed for replacive concretions. Cement fraction is high in both carbonate concretion types, indicating early formation in high‐porosity sediments at shallow burial depths. The strain field around displacive concretions is vertically asymmetrical. Conversely, it is symmetrical with uniform differential compaction for the replacive concretions. Evidence for displacive growth comes from triangular areas of chert at the lateral margins of some carbonate concretions, interpreted as areas of reduced strain. Another indicator is the forced asymmetrical folding of heterolithic host rocks around displacive concretions, with displacive carbonate units separated by trace laminae of the original (chert) beds. Enveloping chert beds exhibit early‐formed radial silica fractures with increased aperture size in the areas of maximum curvature. Carbon isotopic signatures of carbonate concretions show a strong correlation between concretion centres and host rock, suggesting a relatively shallow depth (first few tens of metres) of initial growth. Carbonate concretions are interpreted to have formed at shallow depths in the presence of alkaline pore waters rich in dissolved organic carbon in the presence of Mg2+ ions, available organic matter and redox‐sensitive metals such as U and Mo. A paragenetic history for the different concretion types is presented.