Print this pageControls on Platform Evolution and Carbonate Sedimentation in the Late Devonian: Case Study of a Ramp-Situated Reef System
Alex J. MacNeil1 and Brian Jones2
1 University of Alberta, Edmonton, AB
2 Department of Earth & Atmospheric Sciences,
Edmonton, AB
The Late Devonian (Frasnian) Alexandra Formation, located in the Northwest Territories, is formed of two reef complexes (< 40 m thick) that accumulated on a gently sloping ramp. The second reef complex developed basinward of the first, after sea level fell ~ 20 m, as part of a regressive reef system. Development of the second reef complex in the lowstand system was punctuated by intense storm activity that deposited significant amounts of detritus from the reef-front at its margin. Renalcid boundstones and stromatolites in these deposits, and in the reef-front, are volumetrically significant and played an important role in framework lithification. Green muds, believed to be derived from distal ramp deposits, frequently buried the top surfaces of stromatoporoids in the outer reef-front, as storm activity waned. Transgression marked a significant decline in microbial carbonate, but dense stromatoporoid growth in water depths of 25 – 30 m. Reefal facies at the top of the complex were dominated by stromatoporoids and significant volumes of clean carbonate sediment, without renalcids or stromatolites.
Evolution of the regressive reef system that forms the Alexandra Formation, and the types of carbonate production, were controlled through a complex interplay of sea level change with the platform geometry, the paleobiology of reef builders, and variations in trophic resources. Microbial sedimentation may have been due to elevated nutrient flux that followed storm disturbance of the muddy, possibly hypoxic, deposits on the distal ramp. This dynamic, not likely to affect carbonate shelves with steep, narrow margins, may be an important control on the evolution of gently (e.g., < 4°) sloping carbonate platforms.
AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005