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Transtensional and Transpressive Basins
Stellarton Graben: An Upper Carboniferous Pull-Apart Basin in Northern Nova Scotia
The principal fault systems of northern Nova Scotia include the Cobequid Fault, which forms the southern boundary of the Cobequid Highlands, and the Hollow Fault, which forms the northwestern boundary of the Antigonish Highlands. Rapid facies changes and coarsening of clastics toward these faults, and localization of thick conglomerates adjacent to them, indicate that the faults were a major control on Carboniferous sedimentation. The faults subtend one another to form the Stellarton Graben.
Structural analysis of the Hollow and eastern Cobequid faults confirms speculation that Late Carboniferous movement on both was dextral. Displacement was between 20 and 35 km. In response to this, the Stellarton Graben developed as a synsedimentary rhomb graben immediately northwest of the Antigonish Highlands during Westphalian C-D time.
During Westphalian B time, conglomerates and sandstones of the Cumberland Group were shed northward as alluvial fan deposits along the Cobequid-Hollow Fault system.
Within the Stellarton Graben, the Cumberland Group is disconformably(?) overlain by the Stellarton Formation (Westphalian C-D). In contrast to the fluvial sandstones and shales of the time-correlative Pictou Group, the Cumberland comprises mostly lacustrine and deltaic, grey and red shales and sandstones, as well as oil shales and economic grade coals. Predominance of fine grained clastics, except near the basin margins, suggests that subsidence was gradual, with relatively little relief developing. Redbed and greybed members reflect changes in the water table due to variable rates of subsidence. Three phases of basin subsidence and filling recognized in the Stellarton Formation suggest three synsedimentary pulses of faulting.
Stellarton Graben is one of an array of grabens within the Maritimes Carboniferous Basin, the Hercynian foreland basin complex of the northern Appalachians. It clearly exemplifies the control of regional tectonics on local sedimentation patterns.
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