Tubular Calcareous Nodules from the Pennsylvanian Minturn Formation of Colorado

Unusual tubular calcareous nodules occur in reddish-brown siltstones in the upper 150 m of the Pennsylvanian Minturn Formation in northwestern Colorado. These nodules have an inner core extending the length of the specimens and consisting of sparry calcite and/or siliciclastic material. This core is surrounded by several concentric zones of crudely laminated micritic limestone with variously shaped small cavities filled with clear calcite. The outer parts of the nodules generally consist of clotted micrite with some siliciclastic material incorporated near the contact with the surrounding sediment. Several nodules were later fractured, some with the cracks partially filled with micrite and others filled with sparry calcite. These nodules may be microbialites that encrusted small reed-like plants that grew in a salt marsh, a shallow coastal lake, or a pool on a muddy tidal flat. These plants later decayed allowing access of siliciclastic sediment into the hollow core.