Shallow and Deepwater Stromatolites in Lower Proterozoic Platform–to–Basin Facies Change, Great Slave Lake, Canada

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 transverse stromatolitic mounds with intervening tidal channels filled by intraclost grainslone. The basin-slope facies is present between the mound-and-channel belt and the basin floor. The lateral transition from platform-submerged to basinfloor facies is gradational and lacks a basin-slope facies. Although the overoll facies zonalion resembles Phanerozoic platform-to-basin facies changes, the stromatolites have a much greater environmental range, extending from peritidal to basin floor.