The Horn Plateau Formation, composed of isolated reefs, is part of the Devonian strata that formed in the Mackenzie Basin in the Northwest Territories, Canada. The reefs stretch over a ∼350 m northeast–southwest trend and are dominated by tabulate and rugose corals in the northeast, near exposed Canadian Shield rocks, and stromatoporoids further out in the basin. Detailed facies analyses of each reef type shows distinct differences in their biological makeup, energy regimes, and carbonate sedimentation rates. Geochemical analyses (stable isotopes and rare-earth elements) set against established paleogeography in the Mackenzie Basin reveal that the coral-dominated and stromatoporoid-dominated reefs grew under different ecological conditions. Separations in the data imply that the coral-dominated reefs grew in waters that were relatively enriched in nutrients and the stromatoporoid-dominated reefs, further down the ramp, were in oligotrophic conditions. With no current established method to directly measure Paleozoic nutrient levels or to detect where they were sourced from, it is unclear why the coral-dominated reefs experienced higher nutrient levels. The paleogeography of the Mackenzie Basin could have affected the apparent stratification of nutrients on the carbonate ramp. Possible nutrient sources in the area are from coastal upwelling from the open ocean northwest of the ramp, or locally sourced nutrients from runoff on the adjacent exposed Canadian Shield rocks.

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