Michael T. Whalen¹, James E. Day²
(1) University of Alaska Fairbanks, Fairbanks, AK
(2) Illinois State University, Normal, IL

Abstract: Initial Devonian onlap of the West Alberta Arch, Canadian Rocky Mountains: Implications for paleogeography and sea-level history of western Alberta

We report here evidence of a pre-Flume Formation marine onlap of the West Alberta Arch (WAA) recorded by a new late Givetian unit identified below the isolated Frasnian Ancient Wall carbonate platform in western Alberta. The WAA was a passive tectonic high during the Middle Devonian that was onlapped and buried by carbonate platform sediments during the early Frasnian. The initial stage of carbonate platform construction in western Alberta is marked by development of the Flume Formation carbonate ramp (latest Givetian-early Frasnian). Attached and isolated Frasnian carbonate platforms developed atop the Flume ramp.

The new mixed carbonate-siliciclastic unit underlies the Flume and onlaps unconformity bounded Cambrian rocks along the southeast margin of Ancient Wall. The new unit (nearly 10 m thick) records a single Transgressive-Regressive (T-R) cycle, and consists of three thickening and coarsening upward parasequences. Bioturbated mudstones in parasequence 1 contain a restricted inner shelf brachiopod fauna ( Athyris aff. A. vitatta). Middle shelf deposits of parasequence 2 signify maximum flooding and yield a diverse late Givetian (disparilis Zone) brachiopod fauna (Allanella sp., Shizophoria aff. S. meeki, Desquamatia sp., Cyrtina sp., Cranaena sp., Strophodonta sp.) and ichnofossils of the Cruziana ichnofacies. Low-angle cross-stratified dolomitic sandstones capping parasequence 3 indicate shallow inner shelf conditions and maximum shoaling.

Identification of late Givetian facies has implications for the paleogeography and sea-level history of western Alberta. The new unit is equivalent to pre-Waterways deposits that onlap the Peace River Arch in northern Alberta, and deeper-water shelf facies of the lower "Flume" Formation in northeastern British Columbia. Facies stacking patterns within the unit indicate progradation of mixed carbonate-siliciclastic environments prior to the latest Givetian-early Frasnian sea-level rise of T-R cycle IIb that initiated Flume ramp development. The initial late Givetian onlap recorded by the new unit coincides with T-R cycle IIa-2 previously undocumented along the WAA.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana