The Cretaceous rocks of the Orpheus Graben, offshore Nova Scotia

Abstract

The Orpheus Graben is an offshore Mesozoic basin along the trace of the Cobequid-Chedabucto fault system, along strike from the Lower Cretaceous Chaswood Formation of central Nova Scotia. Orpheus Graben contains up to 10 km of Upper Triassic and Lower Jurassic strata, including the Argo salt, overlain by a relatively thin succession of Upper Jurassic to Lower Tertiary strata. The Mississauga Formation is divisible into its Middle and Upper members and the Logan Canyon Formation is divisible into the Naskapi, Cree, Sable and Marmora members in the Orpheus Graben. The sediments are most likely those of delta plain to shoreface environments, which experienced two episodes of open marine conditions. Two volcanic intervals and one pyroclastic unit are also observed.

All observed detrital lithic clasts of both formations in Orpheus Graben could have been derived from the Meguma and Avalon terranes.

At least five unconformities are observed in the Orpheus Graben, the last three of which are thought to correspond to the last three erosional events that make up the multi-phase "Avalon Unconformity", formed by the progressive rifting of Iberia and the Grand Banks and movement along the Cobequid-Chedabucto-SW Grand Banks transform fault system. (Abstract shortened by UMI.)