HIDDEN CRETACEOUS BASINS IN NOVA SCOTIA

Early Cretaceous deposits of unconsolidated quartz sand and kaolinitic clay are found in the Carboniferous and Triassic lowlands of Nova Scotia, hidden in narrow fault-bounded basins blanketed by glacial drift. The Cretaceous sediments have been defined as the Chaswood Formation and subdivided into three informal units: the lower, middle and upper members, based on continuous lignite horizons (middle member) and facies changes. The upper and lower members feature cycles of coarse to fine quartz (silica) sand and silt, fining upwards into mottled silty clay and light grey silty clay. The middle member is distinguished from these inorganic units by thick sequences of black and grey lignitic clay and lignite. The Chaswood Formation sediments are interpreted as fluvial and lacustrine. Ferruginous oxisols are common in the fine-grained facies of the upper and lower members. Seismic data indicate that deformation has produced steeply dipping faults and fault-related folds in the Chaswood Formation. An Aptian-Albian age for this tectonic event is inferred from synsedimentary deformation, from the angular unconformity spanning the Late Cretaceous and Tertiary that truncates the Chaswood Formation on land, and from correlation with offshore Cretaceous basins.

The presence of non-marine Cretaceous sediments in faulted basins below present sea-level suggests that the evolution of the Nova Scotia landscape was more complex than a single Tertiary erosion cycle, as had been previously thought. We suggest that Mesozoic tectonics and uplift initiated an exhumation cycle in which a thick cover of Mesozoic sediment (1-2 km) was removed. Uplift and exhumation explain the preservation of outliers and the lack of younger terrestrial or marine sediments in the basins. Cretaceous sediments were lowered into their present topographic setting by regional subsidence during the Tertiary. The discovery of faulted Cretaceous outliers throughout Maritime Canada implies that the topography of the region is largely structurally controlled.

¹ Nova Scotia Department of Natural Resources, P.O. Box 698, Halifax, NS, B3J 2T9, Canada, E-Mail rrstea@gov.ns.ca

² Geological Survey of Canada, 601 Booth Street, Ottawa, ON, K1AOE8, Canada, spullan@gsc.nrcan.gc.ca