The Pethei Group, deposited between 1,795 and 1,872 m.y. ago, undergoes an abrupt facies change from the Slave craton into the Athapuscow aulacogen, an intracratonic trough. The cratonic sequence contains a platform-shoaled facies (heterogeneous laminar and columnar stromatolites associated with ooid and intraclast grainstones) and a platform-submerged facies (homogeneous, poorly laminated columnar stromatolites in places conically laminated). The trough sequence contains a basin-slope facies (limestone rhythmite with bedded slump breccias), a basin-floor facies (siliciclastic mudstone with poorly laminated calcareous columnar stromatolites), and closely interstratified tongues of graywacke-turbidite facies. The outer edge of the platform-shoaled facies has a belt of tran verse stromatolitic mounds with intervening tidal channels filled by intraclast grainstone. The basin-slope facies is present between the mound-and-channel belt and the basin floor. The lateral transition from platform-submerged to basin-floor facies is gradational and lacks a basin-slope facies. Although the overall facies zonation resembles Phanerozoic platform-to-basin facies changes, the stromatolites have a much greater environmental range, extending from peritidal to basin floor.