A biostratigraphically complete but intensely bioturbated Cretaceous/Tertiary (K/T) boundary section was taken during drilling at Ocean Drilling Program Leg 113 Site 690 on the Maud Rise (65°S) in the Weddell Sea off East Antarctica. The boundary, which is contained in a relatively undisturbed core, has been delineated by lithostratigraphic, paleontological, and geochemical methods. The first occurrence of the calcareous nannofossil Biantholithus sparsus is used to biostratigraphically estimate the boundary horizon, and a distinct color change between dark brown, clay-rich Tertiary sediments and light-colored Cretaceous chalks is used to more precisely delimit the boundary between 41.5 and 41.8 cm in section...