Formation of Barite Nodules in Dysaerobic and Anaerobic Zones of Stratified Basins

Sandra H. B. Clark

Barite nodules occur at numerous localities in black and gray shale sequences of Ordovician and Devonian age in the Appalachian basin. The nodules occur generally in rocks deposited in oxygen-deficient marine environments: (a) in stratigraphic positions recording dysaerobic bottom conditions and (b) in association with layers containing calcareous and pyritic nodules that formed in anaerobic to dysaerobic zones. The barite nodules do not generally occur in shales representing aerobic environments, and the nodules generally occur in anaerobic shales only in association with carbonate. The barite nodules are in units that probably represent sedimentation during a shallowing pulse following times of maximum transgression.

Characteristics of the barite nodules and the surrounding rocks,
such as the deformation of laminae in enclosing shales around nodules and the alternation of layers of calcite and barite in some nodules, suggest that the nodules formed during diagenesis when the surrounding rocks were still uncompacted and that they formed in response to fluctuations in chemical conditions. Sulfur and oxygen isotope values for barite nodules have a large range and are considerably heavier than values of coeval seawater. The enrichment of heavy isotopes may have occurred as the result of nodule formation at interfaces where rates of sedimentation were slow relative to the rates of sulfate reduction.

Although the amount of barium in normal seawater is very small, barium is soluble in anoxic seawater where sulfur is present mainly as hydrogen sulfide. Barite could be concentrated at interfaces having more oxygenated sulfate-bearing waters of the dysaerobic zones during early diagenesis, possibly with microbes serving as a catalyst. Barite could also be concentrated in areas of locally increased pH where calcium carbonate was forming in anaerobic environments.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.