Cadomin Conglomerate of Western Alberta -- A Result of Early Cretaceous Uplift of the Main Ranges

ABSTRACT A petrographic and sedimentologic study of the Lower Cretaceous Cadomin Conglomerate was made at nine localities. The Cadomin is about 11 m in thickness, and consists primarily of chert and quartzite pebbles, and quartz sandstone. Approximately 5 per cent of chert pebbles are fossiliferous. Foraminifera in silicified fossiliferous clasts are of Lower Carboniferous age. The cherts were probably derived from upper Paleozoic formations only, and the orthoquartzites from the Lower Cambrian Gog Group. A few shale and siltstone clasts were probably derived from Triassic and Jurassic strata. The sheet-like areal distribution, the well-rounded pebbles, good sorting, clay-free matrix, poor stratification but good pebble alignment and relative uniformity in composition along strike of the conglomerate favour a pediment deposit overlain by an alluvial-plain or distal alluvial-fan environment for the area studied in the central Foothills. The Cadomin represents different depositional environments at various localities. Coarse, thick deposits depicting proximal alluvial fans grade eastward in other areas into alluvial plains. The distinct pebble lithologies, especially carbonates, cherts and orthoquartzite clasts, indicate a sedimentary source exposing a wide variety of stratigraphic levels ranging from Mesozoic to Cambrian. The most probable sources were thrust sheets within or near the eastern part of the Main Ranges of the Rocky Mountains. The Main Ranges are estimated on the basis of pebble sizes and transport mechanism to have been a maximum of 60 to 80 km and a minimum of 10 to 32 km west of the present most westerly Cadomin outcrops in the central Foothills. The Omineca geanticline west of the Rocky Mountain Trench can be ruled out as a source area on the basis of lack of metamorphic and volcanic clasts, pebble sizes, rate of thickening of the clastic wedge, and known ages of structures (Main Ranges are Early Cretaceous and Omineca structures are Jurassic and earlier). In addition, palinspastic structural reconstructions suggest that more than 95 per cent of the Mesozoic clastic wedge may have been derived from erosion of the Main Ranges and western Front Ranges.