The Gippsland Basin in southeastern Australia contains Early Cretaceous sandstone and shale of the Strzelecki Group, which are overlain by Late Cretaceous to Early Tertiary sandstone, shale and coal of the Latrobe Group. Around the basin flanks, the groups are separated by a marked angular unconformity dated in previous publications at about 100 Ma. Evidence from seismic sections together with dipmeter logs and palynological reports on samples from offshore petroleum exploration wells indicates that the unconformity developed during deposition of the Latrobe Group and is dated at about 76 Ma (within theNothofagidites senectusspore-pollen zone). The unconformity is probably a breakup unconformity marking initiation of the opening of the adjoining Tasman Sea.