ABSTRACT

The parts of the Lower Cretaceous (Albian) Kiowa Formation that crop out in north-central Kansas contain abundant calcareous cone-in-cone structures occurring as discontinuous lenses and ellipsoidal concretions in shale, and as irregular layers in calcite-cemented sandstone. The cone structures are of two petrographic types: (1) those in the concretions and calcite-cemented sandstone, which are composed of plumose aggregates of fibrous calcite, and (2) those in the lenses of cone-in-cone, which are composed of microgranular calcite. Formation of the various types of cone-in-cone is attributed to precipitation and growth of fibrous crystals of calcite during early diagenesis of the enclosing brackish-water Kiowa sediments. The microgranular texture of the calcite in the cone-in-cone le ses is attributed to diagenetic recrystallization of initially fibrous calcite. Recrystallization probably was induced by anaerobic decay of organic matter in fossiliferous strata underlying the lenticular beds. The consistent association of calcite, marcasite, and carbonaceous debris in the cone-in-cone implies that a unique set of physico-chemical conditions was essential to the development of the cone structures at some unspecified depth below the sediment-water interface while the sediments still were plastic. The diagenetic environment in which the calcareous cone-in-cone structures formed contrasts with that in which concretions of impure siderite grew in Kiowa shale chiefly in the availability of calcium ion and reduced sulfur. The environments in which the shale that now encloses calcareous cone-in-cone structures was deposited, although brackish, probably were less brackish than the environments in which shale that now encloses concretions of impure siderite was deposited.