The Devonian Gilwood sandstone of the Athabasca River area, Alberta, Canada : a sedimentological study, based on subsurface data

The Gilwood Member of de Watt Mountain Formation (upper Middle Devonian) in the Athabasca River area of central Alberta, Canada consists of a deltaic deposit fringing the emergent Peace River Arch and West Alberta Ridge. Paleotopography reconstruction techniques, trend analysis on the sandstone isolith and detailed well log examination, used to obtain the paleogeography of the deposits, show a system of submerged ridges, which radiate from the uplands into the Elk Point Basin, influencing the depositional pattern in the northern part of the area. They cause the existence of a quiet lagoonal area north of Twp. 60 and west of Rge. 10 W5M. A thick (20 metre or more) sequence of coarse clastic material, belonging to a prograding delta complex, covers the Cambrian and Middle Devonian strata. Finer grained deposits occupy the eastern part of the lagoon. Outside this protected area, an agitated open marine environment has prevented the formation of extensive delta complexes. Tidal currents, sweeping into the Elk Point Basin from the north have possibly built up the barrier bars in which the Nipisi and Mitsue oilfields (Twp. 76-81, Rges. 1-7 W5M and Twp. 67-73, Rges. 3-6 W5M respectively) have been found. A northwesterly. running counter current in the area south of Twp. 60, has transported the available sediments and redeposited them as bars off the coast. Data derived from core study and petrographic investigations reveal that the Gilwood Sandstone originated from two different source areas: the Peace River Arch and the West Alberta Ridge. The Peace River Arch consists of metamorphic and igneous rocks, while the West Alberta Ridge consists of Cambrian sediments. The climate during the Watt Mountain deposition has been inferred as tropical, causing deep weathering of the exposed surface associated with extensive leaching of the weathered zone. The deposition of coarse clastics ceased as the uplands became planated. A following transgression has been interrupted by two periods of stillstand or slight regression. The Watt Mountain Fm. is believed to be diachronous in the vicinity of the Peace River Arch and West Alberta Ridge. Evaporites and carbonates of the younger Fort Vermilion Formation have been deposited in shallow lagoons and embayments on the abandoned Gilwood delta complexes.