OCCURRENCE AND MORPHOLOGY OF CARBONATE CONCRETIONS IN THE BEULAH-ZAP COAL BED, WILLISTON BASIN, NORTH-DAKOTA

Carbonate concretionary bodies were encountered during mining of the B eulah-Zap lignite seam in the Coteau Properties' Freedom mine, Mercer County, North Dakota. Preliminary studies show that areal and vertical distribution of the concretions are variable. All concretions examine d are composed almost entirely of calcite. They occur as thin tabular bodies, as more or less elliptical forms, or as tear shaped bodies, an d may occur individually or as clusters of buff-colored, poorly consol idated to solidly crystalline material. The carbonate masses vary in s ize from a few millimeters to tens of centimeters. Bedding in the lign ite may display some compactional folding over dense spheroidal to ell iptical concretions, indicating formation of the concretions prior to compaction. Internal morphology of the concretions is complex, and inc ludes cone-in-cone structure, cross-cutting calcite veinlets, and mult iple generations of calcite. Carbon isotope values suggest the concret ions are composed of biogenic carbonate, probably related to early dia genesis and decomposition of organic matter(peat); oxygen isotope valu es are light, and consistent with a freshwater origin.