Abstract

The Sydney Basin, a large depocentre about 350 by 150 km in present-day area, was active from mid-Devonian to Permian times. It contains a relatively undeformed fill 4 km thick that rests upon continental crust of the Appalachian Orogen. Mid-to Upper Devonian terrigenous strata and volcanics (McAdam Lake Formation) are present locally. The Carboniferous to Permian succession is composed of two fining-upward megasequences, which are separated by a major hiatus. The lower megasequence (Horton, Windsor and Canso groups) accumulated over about 27 My, and contains fanglomerates that can be related to contemporaneous strike-slip faulting. Subsidence to below sea level allowed a prolonged marine incursion, and a subsequent lacustrine phase was followed by uplift of the basin floor, gentle folding and faulting, erosion and karst weathering.

The upper megasequence (Morien Group and overlying redbeds) accumulated over about 25 My, and indicates renewed subsidence and largely inactive local faults. Alluvial strata containing coals, deposited from a long-lived drainage system, record stable tectonic conditions within the basin and the surrounding uplands. Subsidence is attributed to contemporaneous motion on distant fault systems, especially the Hollow, Long Range and Minas Geofracture systems.

The basin originated as a rhomb-shaped extensional depocentre where the Minas Geofracture, a major transcurrent fault oriented parallel to the Appalachian Orogen, generated a series of divergent splay faults. The position of the basin also coincided with a major, long-lived dislocation that ran into the continental margin from offshore. The 25 to 30 My duration of the megasequences may approximate the duration of phases of activity on the major fault systems. Correlation with sequences elsewhere in Atlantic Canada suggests that the basin history represents a regional response to continent-wide tectonic events, especially the docking of the Meguma Terrane to North America along the Minas Geofracture, and the North American/Gondwana collision. The hiatus between the megasequences represents a phase of renewed tectonic activity and uplift in the Meguma Terrane, with the development of a major sediment source and subsequent alluvial sedimentation across the Atlantic region. The increase in abundance of redbeds in Stephanian to Permian times may be attributed, in part, to an increasingly continental climate following landmass suturing and the termination of accretion.