A new Cretaceous-Tertiary boundary site at Flaxbourne River, New Zealand: Biostratigraphy and geochemistry

An exceptionally complete rock sequence across the Cretaceous-Tertiary (K-T) boundary has been discovered near the Flaxboume River, Marlborough Province, South Island, New Zealand. The boundary is marked by a large Ir anomaly (21 ng/g on a decalcified basis), with an integrated abundance of 134 ng/ cm2 after correction for background. Above the boundary there is a 30 cm transition zone, in which a few Cretaceous foraminiferal taxa such as Hedbergella monmouthensis and Guembelitria cretacea survived, though with reduced abundance and size (only ca. 1 2 to 1 4 of normal), apparently reflecting environmental stresses. INAA and ICP analyses show that, in addition to Ir, the boundary clay is also enriched in Cr and Ni, mainly from meteoritic material, and As, Co, Cu, Sb, and Zn from terrestrial sources. Volcanic sources, even when scaled to the 10 7 km 3 volume of the Deccan basalts, fail by three orders of magnitude to account for the Ir and As at the K.-T boundary and by even larger factors for Sb, Zn, Cu, etc. Comparison of our data with those from six other K-T boundary sites shows that the Zn Sb , As Sb , and Zn As ratios generally fall between crustal and oceanic values, suggesting contributions from both sources. Mass balance calculations show that As and Sb could be derived from only 300–500 m of ocean water or also from modest amounts (20–36 g/cm 2) of average crustal rock. Copper and Zn, on the other hand, can only be derived from crustal or mantle rock (5–15 g/cm 2), presumably impact ejecta. Such an amount of ejecta is fairly close to the global fallout of boundary clay (2–5 g/cm 2).