Growth Dynamics of Neoproterozoic Calcimicrobial Reefs, Mackenzie Mountains, Northwest Canada

Abstract

ABSTRACT Neoproterozoic buildups of the Little Dal Group grew in a deep-water epicratonic basin. These kilometer-scale reefs display aggradational and progradational geometries comparable to those described from Phanerozoic reefs. Four phases of reef growth correspond to four regional shale-to-carbonate packages in laterally equivalent off-reef strata. The lower, shaly part of each package is interpreted to reflect transgressive to highstand deposition. The upper carbonate part of each package reflects carbonate precipitation in the water column as a result of postulated basin restriction and increased salinity during sea-level lowstand. Reefs nucleated at the beginning of the first major transgressive event. Reefs typically aggraded during transgressive intervals, but could also prograde or contract, likely depending on the rate of relative sea-level rise versus reef growth rate. Reefs prograded during regressions, probably owing to reduction of accommodation space. They shed talus at lowstand in response to increased erosion and/or progradation over unstable substrates. Reef growth ended with return of shallow-water conditions. The overall architecture of aggradation and progradation demonstrates that the Neoproterozoic calcimicrobial reef ecosystem was capable of responding to environmental changes in the same way as the more extensively studied, ecologically complex, and faunally diverse buildups of the Phanerozoi .