The Labrador Trough is the eroded remnant of a formerly much more extensive Early Proterozoic (Aphebian) geosyncline. The trough axis lies along the boundary between ultrastable Archean craton to the west (Superior Province) and metastable Archean craton to the east (Churchill Province), and defines an eastward-thickening miogeocline. The Labrador Trough sequence (Kaniapiskau Supergroup) consists of two distinct depositional cycles of sediment and volcanic deposition. An erosional unconformity and a possible episode of tectonic disturbance divide the depositional cycles. Smaller-scale cycles within each major depositional cycle represent shallowing-upward siliciclastic-carbonate or iron formation sequences of an early shelf phase. Both depositional cycles are terminated by a basin phase made up of black shale and shelf-derived clastics. The occurrence of platform-margin facies along the deformed eastern trough edge indicates the development of a shelf margin toward the east. A complex of submarine basalt flows and gabbro sills intercalated with basinal sediments represents a relatively static zone of rifting active throughout the deposition of cycles I and II. This igneous zone is preserved mainly by thrusting along the eastern trough edge. The geosyncline was deformed and metamorphosed during the Hudsonian orogeny (1800 Ma). Extensive thrusting produced a fold-thrust structural pattern and the present trough shape.