Sedimentology of an open-marine platform of late Fransnian Age : the Simla Formation, West-central Alberta, Canada
Talon, Véronique Bernadette
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The fossiliferous limestones and dolostones of the Simla Formation were deposited 365 Ma ago in the present day areas of west-central Alberta and east-central British Columbia. This carbonate succession represents the last Frasnian depositional phase of the Western Canada Sedimentary Basin. The Simla Formation consists of a 60-85 m-thick succession of medium-bedded carbonates locally bearing biostromal patch reefs, and covers an area of about 80,000 km2 in the Canadian Rocky Mountains. The Simla Formation passes laterally to the east in the Alberta subsurface into the fossiliferous and partly anhydritic limestones and dolostones of the Blue Ridge Member of the Graminia Formation, illustrating the shallowing platform to the east. A revision of the stratigraphic nomenclature proposes use of the term Simla Formation for the latest Frasnian carbonate units in the outcrop areas and the subsurface deposited north of the Jasper Basin. The sedimentology ofthe prograding Simla carbonate shelf reveals a shallow subtidal, open-marine depositional setting in the late Devonian. This study comprises detailed petrographic descriptions of the microfacies of six Simla outcrops in the Canadian Rocky Mountains, which are complemented with the lithofacies analysis of seven Blue Ridge cores and three time-equivalent strata deposited on nearby platforms on the southern shelf. Five microfacies have been recognized in the Simla Formation (and four lithofacies in the 40 m thick Blue Ridge Formation) representing low-to moderate-energy depositional environments in the shallow subtidal zone. The fossil assemblage, including crinoids, brachiopods, corals and stromatoporoids, is characteristic of an open-marine biota. Biostromal patch reefs developed locally on the platform most likely upon inherited lowrelief paleotopography. A shallowing-upward trend has been observed, indicating a relative sea-level fall or a stillstand. Alternative correlations between outcrops over a 250 km distance indicate interfingering of shallow subtidal microfacies, illustrating a poorly differentiated platform interior. Factors such as paleotopography, storm and tidal activities, and nutrient levels likely had a dominant influence on carbonate production. The Simla platform sedimentation in the late Frasnian contrasts markedly with the underlying mid-Frasnian reefal platforms (Leduc, Nisku/Southesk). The abundance of calcareous particles such as foraminifera, calcareous algae, moravamminidae, and calcimicrobes, indicates a diverse Simla benthic community which was often dominated by calcareous green algae. No erosional surfaces were observed at the top of the Simla Formation. The similarity between the Frasnian Simla and the overlying Famennian Palliser microfacies suggests a rather transitional sedimentary environment across the Frasnian-Famennian boundary. This boundary is distinguished with regards to a faunal depauperization in the Famennian, relative to the Late Devonian mass extinction.