Detailed sedimentological and petrological investigations of the Cambro-Ordovician shallow-marine facies of western Newfoundland, northwest Scotland, and central East Greenland reveal arresting similarities in their histories. Faunal, stratigraphic, sedimentologic, geochemical, and diagenetic comparisons of the thick sandstone, shale, and carbonate successions indicate contemporaneous development on a contiguous broad marine shelf bordering the western margin of a “proto-Atlantic” basin. Sedimentation dominated by intertidal to shallow subtidal environments is indicated by tidal sand bodies, ichnofossils, desiccation phenomena, oölitic lithologies, and stromatolitic algal structures. Complex diagenetic processes probably account for anomalous potash concentrations in correlative Lower Cambrian shale horizons and demonstrably effect replacements by feldspar, silica, dolomite, and calcite in the sandstones and carbonates. Parallel trends of recrystallization, dolomitization, silicification, calcitization, and dolomitization of the oölitic carbonates in each of the three study areas suggest correspondent post-depositional diagenetic environments. Independently the similarities are inconclusive, but collectively they are compelling in their suggestion of common depositional and diagenetic histories for the Cambro-Ordovician successions of western Newfoundland, northwest Scotland, and central East Greenland, ostensibly on the western shelf of a “proto-Atlantic” basin.