The Blow River Formation was deposited in the Blow Trough, a north-trending, middle Cretaceous downwarp near the junction of the Western Canada Seaway and the Brooks and Sverdrup Basins. At Rapid Creek, the Blow River Formation comprises a 380 m cyclic succession of thick, silty, siderite-phosphate pellet packstone and intraformational boulder-pebble conglomerate lenses interbedded wtih grey, laminated siltstone and sideritic siltstone. These rocks rest conformably on grey shale with thin ferruginous siltstone beds, and are overlain conformably by grey siltstone with chert pebbles and siderite concretions. To the west, the formation thickens dramatically, as the ironstone-bearing member interfingers with and overlies dark grey shales and thin sandstones. To the east, it thins, passing into 160 m of interbedded shale and sideritic shale at Boundary Creek. Facies changes indicate a clastic source to the west. Early Albian ammonites and charcoal fragments are locally common. The widespread iron and phosphorous-rich hemipelagic sequence is an important regional marker, and is best knwon for the variety of secondary phosphate minerals (MINFILE 117A 027) found as nodules or along fractures related to Eocene folding.
The Blow River Formation contains about 7 X 10^8 tonnes of P2O5. This major deposit (MINFILE 117A 027) falls between the peak global episodes of phosphogenesis in the Callovian and Campanian, and, like them, corresponds to the late stage of a global anoxia. Unlike most phosphorites, this is a high latitude deposit.
