Weathered spheroids in a Cretaceous/Tertiary boundary shale at Woodside Creek, New Zealand

Three types of spheroids are found in the iridium-rich shale layer at the Cretaceous/Tertiary boundary at Woodside Creek, South Island, New Zealand. Two of the types (A and B) consist mainly of microcrystalline goethite, whereas the third type (F) contains an appreciable quantity of hematite. There is a strong correlation between the iridium content of the spheroids and their matrices and the concentrations of other siderophiles such as chromium, cobalt, iron, and nickel. A similar pattern was observed for the chalcophiles arsenic, copper, and zinc. It is suggested that spheroids of types A and B are weathering products derived from diagenetic pyrite clumps and that type F, which contains hematite, represents a further stage in the weathering process and was derived from goethite. The original pyritic material could have readily scavenged arsenic and other siderophiles from the sea-water column under organic-rich reducing conditions at the end of the Cretaceous, and the siderophiles including iridium could have been incorporated into the original meteoritic material. It was concluded, therefore, that types A, B, and F spheroids are weathering products and do not represent original meteoritic material.