End Frasnian calcimicrobial-stromatoporoid carbonate reefs, Western Canada Sedimentary Basin

Abstract

The Late Devonian was a significant period in Phanerozoic reef evolution. Reef complexes reached their acme in the Middle Devonian and then declined in numbers and complexity thereafter. This change was accompanied by a shift in reef composition characterized by an increase in calcified microbes in the reef milieu. Late Devonian Nisku Formation reefs in the Cynthia Basin at Meekwap, Alberta are composed of calcimicrobes (Renalcis, Epiphyton, Girvanella, Rothpletzella, and Wetheredella), stromatoporoids, and corals. Accessory organisms include brachiopods, benthic foraminifera, molluscs, sponges, bryozoans, and crinoids. Calcimicrobes generate dorsal or ventral crusts on stromatoporoids, and form oncolites in open marine facies. Dorsal crusts are either Girvanella and Rothpletzella, or non-calcimicrobial, spongiostromate or cemented calcisilt encrustations. Ventral crusts are cryptic and composed of Renalcis and Epiphyton. Oncolites comprise layers of Girvanella and Rothpletzella and occur in lagoon and upper-foreslope facies. Girvanella and Rothpletzella are thought to have been photosynthetic because of their preference for interpreted well-lit settings (oncolites and dorsal surfaces) and possible competition for light as expressed by alternating layers of Girvanella and Rothpletzella as well as domal, accretionary growths of either Girvanella or Rothpletzella in oncolites. Renalcis and Epiphyton are viewed as non-photosynthetic or light sensitive because they are rare on upper surfaces and are instead found in cryptic environments. The abundance of calcimicrobes in Meekwap limestones is interpreted to have reflected elevated nutrient levels. Nutrients were likely terrestrially sourced and brought onto the shelf via fluvial runoff and submarine groundwater discharge. Geometry of the Cynthia Basin, as well as the presence of local nutrient sources at Meekwap is thought to have enhanced more regional nutrification via ocean upwelling. The change in the composition of reefs throughout the Late Devonian is attributed to paleoenvironmental changes, such as a colder climate and falling ocean temperatures, as well as increasing nutrient levels, prior to the Late Devonian mass extinction.