FAULT OFFSET OF THE ATLANTIC UPLANDS PENEPLANE: NEW LOOK AT AN OLD CONCEPT

The prevailing concept of landscape development in eastern Canada is that of a regionally extensive low-relief surface, or peneplane, that was uplifted and tilted to the southeast, then set in relief by erosion of weaker rocks. J. W Goldthwait envisioned this upland surface (Atlantic Uplands Peneplane -AUP) as co-planar, cut across rocks of different ages and erodability. Later, more detailed work found that the AUP consisted of facets with differing tilts, enough to cast doubt on the peneplane concept. Douglas Grant observed that the AUP was offset along the Cobequid-Chedabucto Fault System (CCFS) and then suggested a Cretaceous age for the AUP based on offshore correlations. Stea and Pullan (2001), using high resolution seismic data, found that unconsolidated Early Cretaceous (EK) outliers were deformed and truncated by basin margin-faults that extend into the Carboniferous and older basement rocks. They suggested that the valleys containing the outliers are structural features, rather than features formed by differential erosion.

The age of the AUP can be constrained by mapping it in relation to known faults in Nova Scotia and the EK outliers. Using a GIS-generated series of topographic profiles we found that the offsets on the SE -dipping erosional surface were not confined to the CCFS but also occurred on the subsidiary fault systems bounding EK outliers. The AUP also exhibits an inflection point associated with major fault systems, changing to a north-dipping surface. Assuming that these surfaces across the inflection point are coeval, the age of the AUP is younger than Early Jurassic, as it truncates rocks of this age. The AUP is clearly pre-faulting, but can we establish the age of faulting from the stratigraphic relationships in the outliers? The lack of Late Cretaceous and Tertiary strata in these structural basins implies that offset-deformation occurred soon after the EK, thus bracketing the AUP between the Late Cretaceous and Early Jurassic. The EK deposits can then be restored to the peneplane surface along the basin margin faults. When this reconstruction is made the mature, quartz arenite and kaolin sedimentary deposits characteristic of the EK outliers can be linked to the AUP. We envision that the AUP formed by subareal fluvial processes, weathering and denudation during the tropical Early Cretaceous.