Concretions ranging from 10 to 700 μ in diameter form a marker zone between the Christopher Formation (Albian) and the Kanguk Formation (Cenomanian to Maastrichtian) in three wells in western Banks Island. The concretions contain in excess of 30% rhodochrosite (MnCO3), plus minor quantities of dolomite, and iron and manganese oxides. Quartz sand and silt, clay, and sparry dolomite comprise the matrix between the concretions. The concretion zone ranges up to 40 ft (12 m) in thickness and is tentatively assigned to the Kanguk Formation.Manganese was probably derived by decomposition of contemporaneous volcanic rocks, possibly located offshore west of Banks Island. The metal was concentrated by ionic or molecular diffusion processes acting immediately below the sediment–water interface.Subsequent diagenetic recrystallization allowed for further manganese concentration and the development of a strong radial-fibrous crystal texture as the surrounding sediments were passively replaced. Concentric laminations were caused by further partial expulsion of impurities, probably including organics and iron and manganese oxides.