GROWTH DYNAMICS OF NEOPROTEROZOIC CALCIMICROBIAL REEFS, MACKENZIE MOUNTAINS, NORTHWEST CANADA

Neoproterozoic buildups of the Little Dal Group grew in a deep-water e picratonic basin, These kilometer-scale reefs display aggradational an d progradational geometries comparable to those described from Phanero zoic reefs. Four phases of reef growth correspond to four regional sha le-to carbonate packages in laterally equivalent off-reef strata, The lower, shaly part of each package is interpreted to reflect transgress ive to highstand deposition, The upper carbonate part of each package reflects carbonate precipitation in the water column as a result of po stulated basin restriction and increased salinity during sea-level low stand. Reefs nucleated at the beginning of the first major transgressi ve event. Reefs typically aggraded during transgressive intervals, but could also prograde or contract, likely depending on the rate of rela tive sea-level rise versus reef growth rate, Reefs prograded during re gressions, probably owing to reduction of accommodation space. They sh ed talus at lowstand in response to increased erosion and/or progradat ion over unstable substrates. Reef growth ended with return of shallow -water conditions. The overall architecture of aggradation and prograd ation demonstrates that the Neoproterozoic calcimicrobial reef ecosyst em was capable of responding to environmental changes in the same way as the more extensively studied, ecologically complex, and faunally di verse buildups of the Phanerozoic.