ABSTRACT

Geodes of great beauty and variety are abundant in the lower member of the Warsaw Formation in its type area. Early diagenetic calcite concretions were transformed into geodes as follows: (1) a shell of chalcedony replaced calcite at the rim of the concretion, (2) chalcedony spherulites and quartz euhedra replaced calcite inside the concretion, (3) a cavity formed by acidic solution of the recrystallized calcite concretion core, and (4) crystals of several minerals were deposited in the cavity. Specimens in all stages of transformation from concretion to geode occur together at some places.

Distribution of concretions and geodes is controlled lithologically and stratigraphically. Concretions grew during early diagenesis in calcilutites, but not in intimately associated biocalcarenites. Concretions grew beneath the water-sediment interface at the floor of the sea, while the calcilutites were plastic and non-indurated. Organic matter decaying in calcilutites produced anaerobic conditions of negative Eh. The products of anaerobic decay raised the pH locally to the point where calcite precipitated in concretionary form.