A wedge of Proterozoic strata in the Blackwater Lake area of the Northwest Territories, Canada, is preserved below the sub-Cambrian unconformity in a half-graben associated with a large normal fault. Reflection seismic data define the shape of the wedge, but neither it nor drilling information is adequate for identifying the strata preserved within the half-graben. Although these data permit either Tweed Lake Assemblage (density D = 2730–2780 kg/m3) or Mackenzie–Shaler Assemblage (D = 2580 kg/m3) as the half-graben fill, gravity and (to a lesser extent) magnetic modeling show Tweed Lake Assemblage (∼1270 Ma) to be the more likely candidate. Inclusion of any appreciable amount of Mackenzie–Shaler Assemblage in the half-graben produces too low a gravity response and is therefore rejected. The character of the Fort Simpson Magnetic Anomaly (FSA) differs north and south of the Liard Line (∼N61°). North of the line and south of Great Bear Lake, the FSA consists of a collection of strong, short-wavelength features superimposed on a weak, long-wavelength magnetic trend. Modeling of one of the higher amplitude, short-wavelength anomalies shows that the sources lies in the upper Proterozoic section immediately below the sub-Cambrian unconformity and therefore is not directly related to a basement terrane boundary. The weaker and longer wavelength component of the FSA may, however, mark a terrane boundary, as is commonly interpreted. Neither the shallow (short-wavelength) component nor the deep (long-wavelength) component has been sampled by drilling north of the Liard Line. Where the FSA crosses the track of the Liard Line, it undergoes a left-lateral offset. South of the Liard Line, the FSA is stronger with more distinct boundaries but lacks the overprint of shorter wavelength features. There, two drill holes have determined the source of the anomaly to be a shallow granite intrusion whose age has been established by U–Pb analysis as 1845 Ma. The number and magnitude of the differences in the FSA bring into question the validity of carrying the feature northward across the Liard Line.