Chemical characteristics and origin of Ordovician K-bentonites along the Cincinnati Arch

Abstraet--K-bentonites of the Middle Ordovician High Bridge Group along the Cincinnati arch are char-acterized by interstratified illite/smectite (I/S) clays with rectorite-type ordering. Approximately 20 % of the layers are expandable. They are structurally similar to I/S formed at temperatures xceeding 100~ during burial diagenesis, however stratigraphic evidence and a color alteration i dex of < 1.5 for conodonts in associated carbonates reveals they have never been deeply buried or subjected to temperatures greater than 80~ Whole-rock samples of K-bentonites contain-8 % K~O and-4 % MgO, whereas the <0.1-/xm size frac-tion contains 6-7 % K20 and 5 % MgO. By comparison with a hypothetical parent ash, these values represent a net gain of K and Mg and a net loss of Si, Fe, Ca, and Na during post-depositional alteration. K-fixation is accounted for by a layer charge imbalance arising primarily out of octahedral substitution ofMg +z for AI +3, indicating that the interstratification evolved from a montmorillonite precursor. The chemical char-acteristics of I/S layers in K-bentonites developed early during the alteration of volcanic ash to montmo-rillonite. Relatively high contents of K and Mg probably reflect both seawater and parent material com-position at the time of formation. The composition and ordered stacking in K-bentonites was determined by the composition of the original smectite rather than by the pressure-temperature conditions of burial diagenesis