Paleoceanography of the Upper Devonian Fairholme Carbonate Complex, Kananaskis-Banff area, Alberta.

摘要

The Fairholme Carbonate Complex is the southernmost and largest in an extensive stromatoporoid-dominated reef domain that developed in Alberta during Frasnian time. Its western margin is defined, and exhibits a major re-entrant 25 km wide named the Shark Embayment. A Devonian paleoclimatic model suggests that the western margin was subjected to seasonal oceanic upwelling of nutrient-rich and oxygen-poor waters that influenced the development of the carbonate buildups.;Lithostratigraphic units of the Fairholme Group are characterized in terms of a Rocky Mountain (RM) Conodont Zonation. In the western part of the study area, the Flume carbonate platform began to onlap the westernmost slope of the West Alberta Ridge (WAR) as early as latest Givetian time (norrisi Zone). To the east, the Flume platform did not onlap and begin to cover the crest of the WAR until the succeeding RM Zone 1. The overlying Upper Cairn Member (containing a unique coral biofacies) and Perdrix Formation range from the uppermost RM Zone 1 to Zone 4b. The Peechee Member is difficult to date, but is probably within Zones 5a to 5b; the overlying Grotto Member is no younger than Zone 5b. The Arcs Member is within Faunal Intervals (F.I.) 6-7, and the Ronde Member is within Faunal Interval 8 which includes the Frasnian-Famennian boundary near its top. The Mount Hawk ranges from Zone 4b/5a to F.I. 8.;Stromatoporoids were widespread and abundant calcareous benthos living in shallow, tropical, oligotrophic, and agitated marine environments. Thirty-two species representing 5 different orders of stromatoporoids are recognized in this study; thirty-one species comprise a diverse fauna in the Cairn Formation. Five stromatoporoid assemblages are defined in the Flume and Upper Cairn succession, and are correlated with RM conodont zones. Stromatoporoids exhibit mostly domical, but also bulbous, laminar, and dendroid growth forms that were genetically prescribed and only slightly modified by environmental factors. Trace element geochemistry of Devonian stromatoporoids indicates that these sponges originally secreted a calcitic skeleton, in contrast to Ordovician stromatoporoids which secreted an aragonitic skeleton. Evidence suggest that Paleozoic stromatoporoids eventually developed into mixotrophic organisms enabling them to construct large carbonate buildups, but they were also vulnerable to the devastating effects of paleoceanic upwelling of nutrient-rich waters.