Four Lower Ordovician buildup categories occur in carbonate platform strata in the Canadian Arctic Islands: outer and inner shelf-margin buildups and outer and inner shelf-interior buildups. The outer shelf-margin buildup in the Bulleys Lump Formation consists mainly of calcimicrobial biolithite and early marine cement, forming a structure that probably had a depositional relief of more than 1.5 km over contiguous basinal chert and mudrock. The length of the outer shelf-margin buildup is probably more than 1400 lan, although it is discontinuously exposed. Inner shelf-margin buildups are smaller, mounded structures, attaining thicknesses in access of 100 m and widths of several kilometres. A representative inner shelf-margin buildup comprises four distinct microbial facies; the stratigraphically lowest and deepest water facies contains up to 4 m-thick thrombolite mounds surrounded by Conophyton-like stromatolitic pinnacles. The two middle facies consist of numerous, aligned ovoidal thrombolites with thromboidal or columnar stromatolitic interiors and stromatolitic margins. Thrombolites were built mainly by benthic microbial communities, but fleshy and locally well-sclerotized sponges were probably important constructors as well. These thrombolites are overlain by the shallowest water and youngest buildup facies, consisting of cerebroform stromatolites and thrombolites, but which lack the distinctive ovoidal thrombolites of the underlying facies. Zoned inner shelf-margin microbial buildups such as these are previously undocumented. Shelf-interior buildups are composed of thrombolites and stromatolites, forming isolated, 1 to 5 m-high mounds and coalesced mounds in buildups more than 2.0 km wide. The distribution of the four buildup categories implies a shelf-margin embayment in the central part of the Arctic Islands.