WHEWELLITE IN CARBONATE CONCRETIONS AND ITS GENETIC IMPLICATIONS

Whewellite, a calcium oxalate, is a rare mineral in carbonate concretions but one that helps determine the timing of emplacement of concretion matrix and veins in septaria. In the Late Devonian Huron Shale of north-central Ohio, whewellite exhibits two positions in the paragenetic sequence of the concretions. The early generation occurs in a small (28 cm) non-septarian dolostone concretion and consists of: (1) a group of isolated subhedral short prismatic crystals embedded in matrix, and (2) a lens-shaped mass of fine-grained whewellite in contact with an arthrodire bone and concretion matrix. The bone is not fractured and veins of whewellite in it are absent. The late generation is widespread, occurring sparingly in openings of large (1 to 3 m) septarian dolostone concretions. It consists of coarse-grained granular aggregates with individual grains up to 8 cm in diameter. Most vein whewellite is found resting on barite, magnesian ankerite, and quartz, while rarely crystals of late ferroan dolomite are perched on whewellite. The lack of terminated crystals and rarity of mineral coatings on it indicate that vein whewellite was one of the last minerals to be deposited.

The concretions were emplaced during early diagenesis when the enclosing black shales were still soft and undergoing compaction. Anaerobic decomposition of organic matter in the shales gave rise to oxalate, which migrated to the concretions and precipitated as whewellite. Although the lens-shaped mass of whewellite occurs in contact with an arthrodire bone, this association is atypical. On a global scale, whewellite is associated more commonly with plant matter. None-the-less the presence of whewellite in both the matrix and veins indicates that concretion formation and vein filling occurred relatively early during diagenesis. Carbon isotope analyses of matrix and vein whewellite, dolostone matrix, and veins of early ferroan dolomite are reported.