Late Cretaceous-Palaeogene nannofossil biostratigraphy of Challenger No. 1 well (Challenger Formation type section), offshore Perth Basin, Western Australia

Late Cretaceous-Palaeogene calcareous nannofossil assemblages from the mainly carbonate section in Challenger No. 1 well, above the 780m level, have some similarities with the recently described succession of carbonates in the Perth Canyon, south Perth Basin. The continuity of the marine Palaeogene in these two sections contrasts with the single marine unit (the hemipelagic Kings Park Formation) known in the onshore Perth Basin. In both offshore sections, it is impracticable to subdivide the Palaeogene carbonates into mappable lithostratigraphic units, and identification of known Paleocene and Eocene hemipelagic rock units is evidently not possible although their age equivalents (or partial equivalents) can be detected biostratigraphically. Marine Oligocene sediments, recently identified (for the first time in the Perth Basin) in the Perth Canyon, are shown to occur in Challenger No. 1. The age of the type section of the Challenger Formation is revised (from Late Eocene) to Middle Eocene-Oligocene. Sediments immediately underlying the type Challenger Formation in Challenger No. 1 are Middle Eocene in age, and consequently are not the Kings Park Formation as previously reported. They probably equate with the basal type section of the hemipelagic Porpoise Bay Formation, suggesting a possibly significant age overlap between the type sections of the Challenger and Porpoise Bay Formations; the latter is probably a more terrigenous facies of part of the former. Lower Eocene beds similar to those discovered recently in the Perth Canyon also occur in Challenger No. 1. The Late Cretaceous assemblages identified in Challenger No. 1 are probably representatives of both the upper Campanian Lancelin beds (lower part) and the upper Santonian-lower Campanian Gingin Chalk. Three major disconformities (or condensed sections) are detected in Challenger No. 1: at the Eocene/Oligocene boundary (lowermost Oligocene is missing) within the upper part of the Challenger Formation; at the Cretaceousrrertiary boundary (Maastrichtian and Lower Paleocene are missing), and within the Campanian section. The Late Cretaceous-Palaeogene section in Challenger No. 1 is more complete than coeval sections at nearby Deep Sea Drilling Project sites in the Perth Abyssal Plain and on the Naturaliste Plateau.