The Hadrynian Horsethief Creek Group in the northernmost Purcell Mountains and adjacent Selkirk Mountains is subdivisible regionally into grit, slate, carbonate, and upper clastic divisions in upward succession. The grit division represents a submarine fan assemblage and the slate division hemipelagic muds probably deposited in intermediate depths. The carbonate division comprises an interval of discontinuous lenses representing "bahamian" carbonate bank and off-bank assemblages, and the upper clastic division is a heterogeneous clastic wedge, which shows some evidence of northerly and westerly increasing depositional depths. Feldspathic quartz pebble conglomerate beds intercalated with the carbonates in both bank and off-bank facies indicate tectonic activation of granitic source areas like those from which similar rocks in the upper part of the Miette Group of the Rocky Mountains were derived.The upper part of the slate division, which can be differentiated in western localities as a distinct semipelite–amphibolite unit, and the upper clastic division each expand in thickness northwestward to dominate the Horsethief Creek outcrops in the Selkirk Mountains. These thickness variations, the increase of amphibolite northward in the semipelite–amphibolite unit, and the loss of grit beds northward in the slate division suggest deposition in a depocentre that received coarse sediment from southerly and easterly directions, and that became the site of mafic igneous activity.