Abstract

The Kaskapau Formation is mudstone-dominated, ranges in age from Late Cenomanian to Middle Turonian, and records a phase of rapid flexural subsidence of the Western Canada foreland basin. The Late Cenomanian portion of the formation that lies above the Dunvegan Formation and below a regional beveling unconformity (the 'K1' surface) is here divided into three informal allostratigraphic units bounded by regional transgressive surfaces. Each unit comprises several upward-shoaling allomembers, bounded by transgressive or flooding surfaces. The units in ascending order are named the A–X, Doe Creek and Pouce Coupe; only the latter two are discussed here. The Doe Creek unit is a wedge that thins eastward from approximately 115 m to less than 5 m over approximately 400 km. The Doe Creek unit was deposited in a broad, very shallow embayment, open to the southeast. Waters were initially brackish, but became more marine over time. In the west, a series of prograding sandy deltaic shorelines and intervening transgressive mudstones can be traced for tens of kilometres into the basin. About 200 km to the east, Doe Creek strata deposited on the eastern margin of the embayment are much muddier but also contain evidence of emergence. In the central part of the embayment, allomembers shoal upward and locally are capped by laminated to bioturbated, lenticular sandstone bodies up to 9 m thick and tens of kilometres long. These sandstones, which preserve no subaerial facies, are detached from shoreline deposits to both west and east and are interpreted as basinally-isolated lowstand delta-front deposits.

The Pouce Coupe unit is a wedge that thins eastward from approximately 105 m in the Foothills to zero near the Alberta-British Columbia border. In the east, lower allomembers are bioturbated mudstones and muddy sandstones; higher allomembers are dominated by clean fine-grained marine sandstone that form N–S elongate bodies up to 9 m thick, and tens of kilometres long, arranged in a westerly-prograding stacking pattern. In the west, the Pouce Coupe unit is mudstone-dominated. The Pouce Coupe unit was deposited in a N–S elongate marine basin, bounded in the east by a hinge zone located at about 120°W longitude; the western margin of this basin now lies beneath over-thrust rocks. To the east of the hinge lay an extensive, low-relief land area where upper allomembers of the Doe Creek unit (and possibly older rocks) were subaerially exposed and eroded. The detritus was transported westward and incorporated into the Pouce Coupe unit. The hinge zone appears to coincide with a tectonic domain boundary in the Precambrian basement, which may have localized and amplified flexural deformation during Late Cenomanian time.