Calcareous K-bentonite deposits in the Utica Shale and Trenton Group (Middle Ordovician), of the Mohawk Valley, New York State

Abstract

Numerous cemented and partially cemented K-bentonite beds occur in the long, complex succession of altered ash beds with the Middle to Late Ordovician Trenton Group and Utica Shale of New York State. Petrography and trace element geochemistry suggest a generally felsic-intermediate, calc-alkaline composition of the volcanic component. Cementation by carbonate is expressed as nodular concretionary masses within K-bentonite clay, or as a complete transformation of ash layers to hard, tabular ledges. Cementation typically imparts a sparry texture to K-bentonites, which is distinct from that of normal concretionary micrite layers observed at many levels. Cemented K-bentonites display remarkably well-developed glass shard textures, suggesting that carbonate cementation sometimes occurs prior to alteration of glass to clay minerals. The presence of cross-lamination in several cemented K-bentonites, dramatically shows that post-depositional hydraulic reworking or bioturbation of tuffaceous material was an important process on the Ordovician seafloor. Preliminary observations show the potential for detailed petrographic textural characterization of certain ash layers where cementation occurred soon after burial. Given that ashes are often associated with sedimentary condensation and frequently occur above submarine discontinuities, petrographic study of cemented ash should reveal if, and to what degree, ash layers record multiple depositional events. REE patterns show enrichments in light REE and moderate to pronounced Eu anomalies consistent with a felsic-intermediate igneous signature. Individual Calcareous ash bed trace element patterns and ratios – for example, REE and Hf/Zr, Th/U, K/Rb – may persist laterally in the Mohawk Valley, allowing their use as correlation tools.