- GeoRef, Copyright 2008, American Geological Institute. Reference includes data from Bibliography and Index of North American Geology, U. S. Geological Survey, Reston, VA, United States
The cone structures are of two types: those in concretions and calcite-cemented sandstone, composed of plumose aggregates of fibrous calcite, and those in lenses of cone-in-cone, composed of microgranular calcite. Origin of the first is due to the growth and precipitation of fibrous calcite during early diagenesis, and the microgranular texture is due to the recrystallization of the fibrous calcite by diagenesis. Recrystallization probably was induced by anaerobic decay of organic material in underlying fossiliferous strata. The consistent association of calcite, marcasite, and carbonaceous debris in the cone-in-cone implies a unique set of physico-chemical conditions while the sediments were still plastic. The diagenetic environment contrasts with that in which concretions of impure siderite grew, chiefly in the availability of Ca ion and reduced sulfur.