ABSTRACT

The cone-in-cone concretions examined in this study occur as two hemispheres of cones separated by a layer of bedded siltstone. Microscopic examination reveals both a bladed and radial extinction pattern in the equigranular calcite constituting the concretion. The cones are disrupted by two systems of fractures: those termed major form the boundary of the cones, those termed minor penetrate the cones and are associated with the formation of clay rings which encircle the cones.

X-ray analysis shows all of the CaCO₃ in the concretions to be calcite. The predominate clay minerals are illite and chlorite. The concentration of mixed layer clay is greater within the concretions than in the adjacent sediments.

The cone-in-cones are thought to have originated from a syngenetic concretion of fibrous aragonite. The conical structures are believed to have formed while the surrounding clay materials were still plastic, and that clay was introduced along the major fractures as the cone-in-cone structure developed.