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Detailed interpretation of 2D and 3D seismic data from the Sable Subbasin (continental margin of eastern Canada) has revealed an extensive polygonal fault system affecting Upper Cretaceous chalk of the Wyandot Formation (the Wyandot Chalk) and Cenozoic mudrocks of the Banquereau Formation. This paper offers the first detailed study of a basin-scale polygonal fault system affecting a chalk-dominated unit. The Wyandot Chalk is up to 300m thick and pervasively deformed by small-scale faults that are morphologically and quantitatively similar to those previously described from polygonal fault systems observed in mudrocks of the North Sea, and elsewhere. This suggests that they may have formed due to a common mechanism. The data presented in this paper point towards a lithological control upon the development of the polygonal fault system and argue that the syneresis hypothesis posed by Cartwright and Dewhurst [Geol. Soc. Am. Bull. 110 (1998) 1242] best explains the distribution and initiation of the analysed fault system.3D seismic attribute maps reveal an orthogonal intersection relationship between polygonal faults and tectonic faults in the study area. This relationship is interpreted to indicate that local stress perturbations around the tectonic faults influenced the propagation of the polygonal faults. [Copyright &y& Elsevier]