The Cadomin Formation: Stratigraphy, Sedimentology, and Tectonic Implications

Abstract

ABSTRACT The Cadomin Formation, the basal unit of the Lower Cretaceous Blairmore Group, is one of the most prominent and useful stratigraphic markers in the Rocky Mountain Foothills, yet has received little attention in the literature. The formation ranges from 0 to at least 200 m (660 ft) in thickness, and is composed entirely of conglomerate at some localities but contains abundant interbedded sand and finer clastics elsewhere. Gravel-size clasts are mainly subrounded to well rounded, have an average size of 1 to 3 cm (0.4 to 1.2 in) and a maximum size up to 40 cm (16 in). A long period of pedimentation accompanied formation of the Cadomin, with removal of hundreds of metres of older sediments in eastern areas but possibly with continuous sedimentation to the west. Extensive areas covered by a single thin bed of conglomerate are remnants of the pediment surface, whereas thicker conglomerate and sandstone sequences are of alluvial-fan and fluvial origin, mostly deposited under humid conditions. These fans and rivers fed a trunk stream, the Spirit River Channel, flowing northwestward and bounded on the east by an erosional scarp, the Fox Creek Escarpment. This channel and scarp were formed and maintained during pedimentation, and transported finer detritus northwestward, possibly to be deposited in the Peace River autogeosyncline, the Bowser Basin, and in the Mackenzie Delta area. Thickness and clast size indicate a western provenance, the location of which is uncertain but included parts of the region west of the present-day Rocky Mountain Trench, and probably parts of the Main Ranges of the Rocky Mountains. Coarsest clast sizes occur where the horizontal movements accompanying thrusting encountered areas with histories of vertical movement -- the Peace River Arch and the Sweetgrass-Belt Arches. The mountain front was less bowed than at present and more nearly parallel with the Fox Creek Escarpment. A large alluvial fan in the Smoky River region separates an area to the north with thick coal seams in the post-Cadomin strata from an area to the south with only thin coal beds.