Controls on lithofacies variability in Late Devonian, Cynthia Basin reefs, Nisku Formation, Western Canadian Sedimentary Basin
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The Late Devonian (Frasnian) Nisku Formation in the Western Canadian Sedimentary Basin is a homoclinal ramp composed of four stacked carbonate cycles with reefs in both shallow-ramp and downslope settings. Nisku Formation deposits in the Cynthia Basin, a westward facing sub-basin, contain two distinctly different reef types. One is composed of well-developed coral communities, stromatoporoids, synsedimentary cement and few calcimicrobes whereas the other is composed of calcimicrobes, wafer stromatoporoids and little cement. Deposits along the northeastern rim of the Cynthia Basin, the focus of this study, contain diverse calcimicrobes including Girvanella, Rothpletzella and Renalcis. Twelve lithofacies are documented and consist primarily of calcimicrobe-dominated reef-building communities which typify the first type of Cynthia Basin reefs. These deposits lack the reef-building fauna of corals and massive stromatoporoids common in southern Cynthia Basin reefs and older successions. The difference between coral-stromatoporoid and calcimicrobial communities is interpreted to be a result of increasing nutrient levels during basin restriction and shelf progradation. The nutrification can be linked to relative sea-level change as well as basin geometry and is thought to have been sourced primarily from terrestrial runoff along the northern margin of the basin. This study provides evidence that trophic resource variation could have contributed more to the development of calcimicrobe-dominated lithofacies in Devonian systems than has been previously documented.