Along the edge of the Archean Slave Craton, a 1.1 kilometre thick wedge of carbonates formed the Rocknest Formation, which thins to the east.
[2] Geologists have inferred ancient sea floor spreading in the western part of the province from dikes and mafic lava flows, overlain by deep ocean turbidite deposits.
In the Coppermine River Province, tholeiite flood basalts emplaced at the same time over a rapid span of five million years.
Within the Mackenzie Basin, tabulate and rugose corals grew formed the Horn Plateau Formation—a group of isolated reefs from the Devonian fed by nutrients from eroding Canadian Shield rocks and offshore upwelling in the ocean.
[7] The Selwyn Basin, which now spans into the Yukon Territory formed at the same time and accumulated graptolite fossils and bitumen.
[8] The siliclastic, fossiliferous wackestone and mudstone of the Ordovician Bad Cache Rapids Formation record a shallow shelf environment on Southampton Island.
[9] In the Mesozoic, kimberlite pipes intruded Archean basement rock in places beginning around 75 million years ago and continuing into the Cenozoic in the Ekati area.
On Banks Island, manganese spherulites with rhodochrosite, iron-manganese oxides and dolomite mark the boundary between the Christopher and Kanguk sedimentary formations from the Cretaceous.