ABSTRACT

The Peace River Arch did not become an obvious tectonic element until Middle Devonian time. During the Late Silurian and Early Devonian (Pridolian to Siegenian), the western part of the North American craton was a large landmass except for thick peritidal deposition along its margin. In late Early Devonian time (Emsian), the western craton started to subside and was covered by carbonate shelves and evaporite basins. However, the southern part (south of 58°N) largely remained a landmass until late Eifelian (early Middle Devonian) when it assumed the form referred to as the Peace River Arch and West Alberta Ridge. Subsidence of the surrounding areas (Elk Point Basin to the east and Golden Embayment to the west) continued into early Givetian (late Middle Devonian).

The lower Paleozoic carbonate-dominated sediments on the Peace River Arch and West Alberta Ridge were gradually eroded, with the

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result that Lower and Middle Devonian sediments transgressed across increasingly older units to the east. The lower Paleozoic cover was only partly removed from the west Alberta Ridge, whereas it was entirely stripped from the Peace River Arch by late Eifelian time as documented by Granite Wash in Elk Point sediments immediately adjacent to the Arch.

During the mid-Givetian, a strong regressive event (Watt Mountain hiatus) affected most of the western craton. For this brief interval, the Peace River Arch became an active source of detritus, forming the up to 240 km wide Gilwood Delta Complex. Following this positive pulse, the Peace River Arch remained a passive high ringed by fringing reefs during latest Givetian and early Frasnian, and was gradually buried during late Frasnian and Famennian.

The Devonian succession in the Rocky Mountains to the west shows no influence of the Peace River Arch. In fact, siliciclastics of Middle Devonian age, when the Arch was identifiable as a tectonic entity, were consistently transported from a western or southwestern direction, suggesting a western source area. Radiometric work in progress on detrital zircons from these siliciclastics may elucidate the age of the western source area.

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