The region is underlain by an assemblage of sedimentary rocks consisting mainly of continental margin and shelf facies rocks. This assemblage was deposited on and to the west of the ancestral North American craton. These sedimentary rocks, for the most part typical miogeoclinal facies, range in age from Hadrynian to Upper Cretaceous. Structurally these rocks are part of the Foreland Thrust and Fold Belt of the North American Cordillera.

In this region phosphatic beds are commonly found in Upper Paleozoic to Lower Mesozoic rocks. These are exposed to the west of a major thrust fault which has thrust these rocks over younger, mainly Cretaceous strata. The Cretaceous strata is exposed to the east.

A 1.3 metre thick phosphorite bed occurs near the base of the Whistler Member of the Sulphur Mountain Formation, Spray River Group. Host rocks for the phosphorite are siltstone, calcareous siltstone and minor limestone.

Two thin phosphatic siltstone beds occur in the upper part of the Vega-Phroso member of the Sulphur Mountain Formation. Phosphate occurs as fluorapatite.