Uppermost Precambrian and lowest Cambrian sediments of the Rencontre, Chapel Island, and Random formations in the Fortune Bay area, western Avalon Zone, consist of conglomerates, sandstones, and shales deposited in fluvial, marginal-marine, and open-marine settings. Deposition began in a small fault-bounded basin. Alternation between nonmarine and marine deposition resulted from pulses in fault activity (= uplift and coarse terrestrial clastics), separated by quiescent periods of basinwide thermal subsidence (= transgressive, fine-grained marine units overstepping earlier basin margins). The tectonic regime may have been extensional (rift) or transtensional (strike-slip pull-apart); a strike-slip origin is easiest to defend and is consistent with current tectonic models for the Avalon Zone involving prolonged extension or transtension, but little spreading.The Fortune Bay basin persisted until the beginning of the Cambrian Period, at which time eustatic sea-level rise led to widespread transgression and deposition of platformal sands, shales, and minor carbonates on the Avalon Zone. This event appears to coincide with cessation of important faulting in the western Avalon Zone.