KILOMETER-SCALE MICROBIAL BUILDUPS IN A RIMMED CARBONATE PLATFORM SUCCESSION, ARCTIC CANADA - NEW INSIGHT ON LOWER ORDOVICIAN REEF FACIES

Four Lower Ordovician buildup categories occur in carbonate platform s trata in the Canadian Arctic Islands: outer and inner shelf-margin bui ldups and outer and inner shelf-interior buildups. The outer shelf-mar gin buildup in the Bulleys Lump Formation consists mainly of calcimicr obial biolithite and early marine cement, forming a structure that pro bably had a depositional relief of more than 1.5 km over contiguous ba sinal 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, attaini ng thicknesses in access of 100 m and widths of several kilometres. A representative inner shelf-margin buildup comprises four distinct micr obial facies; the stratigraphically lowest and deepest water facies co ntains up to 4 m-thick thrombolite mounds surrounded by Conophyton-lik e stromatolitic pinnacles. The two middle facies consist of numerous, aligned ovoidal thrombolites with thromboidal or columnar stromatoliti c interiors and stromatolitic margins. Thrombolites were built mainly by benthic microbial communities, but fleshy and locally well-scleroti zed sponges were probably important constructors as well. These thromb olites are overlain by the shallowest water and youngest buildup facie s, consisting of cerebroform stromatolites and thrombolites, but which lack the distinctive ovoidal thrombolites of the underlying facies. Z oned inner shelf-margin microbial buildups such as these are previousl y undocumented. Shelf-interior buildups are composed of thrombolites a nd stromatolites, forming isolated, 1 to 5 m-high mounds and coalesced mounds in buildups more than 2.0 km wide. The distribution of the fou r buildup categories implies a shelf-margin embayment in the central p art of the Arctic Islands.