Abstract: Middle Proterozoic Platform-Margin Reefs, Northern Baffin Island, Canada

Noel P. James, Guy M. Narbonne

The platform-to-slope ramp of the Victor Bay Formation (ca. 1200 Ma) is a 10 km-wide belt of large stromatolite reefs in flat-lying strata exposed as exhumed terranes dissected by rivers and fiords. These limestone buildups occur as isolated mounds or chains of coalesced/stacked mounds. Individual structures are up to 130 m high and 1 km in diameter.

Reef structure is interpreted in the framework of one third-order sea level cycle. TST reefs nucleated on grainstones and slumped slope debrites and are buildups of largely horizontally laminated microbial limestone, with little perireefal sediment. Early reefs kept up with rising sea level, and fields of mounds populated the progressively inundated upper slope. These complex buildups are composed of huge decameter-scale domes of subhorizontal and columnar microbialite limestone. The MFS was a period of extensive vertical accretion (up to 80 m) and starved inter-reef sedimentation. The HST is characterized by widespread growth of small domal reefs composed of microcrystalline dolostone that grew in a hypersaline environment; perireefal sediment was shed for the first time. Subsequent ea level fall resulted in subaerial erosion, mass wasting, and extensive karstification.

Reefs were constructed by microbial precipitation of CaCO₃ and extensive synsedimentary cement precipitation. Although these buildups lack skeletal metazoans, they are remarkably similar in paleoenvironment, style, facies, and geometry to many Phanerozoic mounds, especially those of Ordovician and Carboniferous age.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994