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The Mesoproterozoic Borden, Hunting-Aston, Fury and Hecla, and Thule basins, collectively known as the Bylot basins, are a series of sub-parallel extensional depressions associated with the Mackenzie igneous event at ∼1270 Ma. In the Borden basin, basal strata of the Eqalulik Group are dominated by tholeiitic basaltic rocks and siliciclastic sandstones previously interpreted as predominantly terrestrial flows and fluvial deposits respectively. New field data indicate that thick basalt flows in the Nauyat Formation contain cryptic pillow structures and typically have fine-grained, fragmental tops. A subaqueous origin for these flows is supported by the presence of stromatolitic carbonate on the top of several flows, and the presence of peperite and sand-injectite structures at the base of others. Associated quartz arenite units are all of marine origin. Although much of the overlying Adams Sound Formation was previously interpreted as fluvial, there is little evidence for this, except locally at the base of the succession near Paquet Bay. Paleocurrents do not exhibit a strong unimodal pattern to the northwest, as indicated in earlier work, but reflect a less restricted basin, with paleoflow oblique to later graben margins. Most of the Adams Sound Formation was deposited on a tide- and storm-dominated shelf, with local development of large sand-wave complexes. A similar, marine-dominated setting can be demonstrated for basal terrigenous clastic units in both the Hunting-Aston, and Fury and Hecla basins. The latter contains thin sheets of fluvial strata only in the Nyeboe Formation on the south side of the basin, and in the basal 3 m of the Whyte Inlet Formation on the northeast margin of the basin. Paleocurrent patterns in all three basins do not reflect confinement between the walls of restricted grabens. This suggests that the early development of the basins may have been related to transtensional extension rather than aulacogen development.<br/>Although uranium, present in basement granites north of the Fury and Hecla Strait, may have been scavenged by fluids well after lithification of the Mesoproterozoic basin fill to produce local vein-type uranium deposits, the potential for extensive unconformity-type deposits, analogous those in the Athabasca basin, is exceptionally low. Most of the sandstone units have early siliceous cement, and low initial feldspar content, both of which would have inhibited development of extensive secondary porosity.