Prospect B is one of the largest Cretaceous sag-phase buildups yet identified along the outer high of Namibia’s Atlantic volcanic-rifted margin. These enigmatic buildups constitute a potential new carbonate play offshore Namibia and South Africa. However, no unambiguous carbonate geometries have been reported to date, and they sit atop a highly volcanic sedimentary sequence. In the absence of well data, it is thus prudent to examine these buildups carefully using all available data and analogues, to test their carbonate versus igneous origin and therefore their potential as hydrocarbon reservoirs. This study used three-dimensional seismic data to extract detailed depositional information for Prospect B. The analysis included assessment of the buildup’s external morphology and internal seismic facies, measuring the dip and dip direction of inclined reflectors, making horizon slices, mapping internal surfaces onto which seismic attributes were extracted (root mean square, amplitude, spectral decomposition), and creating thickness maps to show buildup evolution through time. These data were then evaluated against known and published observations made on volcanic and carbonate systems (continental and marine). Architectural elements like vents, igneous flows, and complex clinoform geometries suggest that a large part of the buildup is likely volcanic in origin. Though it has carbonate-like features, no definitive carbonate geometries were identified. Thus, Prospect B is more likely to be dominated by igneous materials such as hyaloclastites. Contrary to existing interpretations, Prospect B and its equivalents probably represent a late, waning phase of regional volcanism and are an important bathymetric record of the South Atlantic’s formation.