Clay-Ironstone Concretion Preserving Fabrics Due to Progressive Compaction

Abstract

Fabrics arising from preferred orientation of clay minerals are presented in a sequence of samples taken from the center of a diagenetic siderite concretion into the enclosing shale sediment. Compactional histories, as deduced directly from estimated porosity values before cementation with siderite, and as deduced from the rotation of mineral flakes to produce the observed fabric, confirm each other. It is concluded that the particular carbonate concretion studied started to grow within the pore space of a flocculated clay sediment soon after deposition. An early period of rapid growth is distinguished from a second, protracted phase which extended effectively until the sediment achieved its present degree of compaction.