The fossiliferous freshwater limestones of the Northern Sperrgebiet accumulated in epikarst depressions that formed in Proterozoic dolomites of the Gariep Group. Geological mapping and stratigraphic superposition reveals that they are younger than the Pomona Quartzite, but older than the Blaubok Conglomerate and many other superficial deposits of Palaeogene and Neogene age, including the Klinghardt Phonolites. Age determinations of phonolite cobbles from the Gemsboktal Conglomerate and from occurrences of lava in the Klinghardt Mountains and at Swartkop indicate that volcanic activity spanned the period 42 – 37 million years. It is concluded from the stratigraphic and radio-isotopic evidence that the freshwater limestones were deposited during the middle Lutetian, ca. 47 - 45 Ma. Keywords: Palaeogene, Namibia, radio-isotopic age, phonolite, limestones.