WAS THE CANADIAN SHIELD COVERED BY SEDIMENTARY ROCKS THROUGHOUT MESOZOIC TIME?

Neodymium isotopic data from Mesozoic sedimentary sequences around the western and northern margins of the Canadian Shield display a distinctive set of epsilon Nd values that was established by a flood of clastic sedimentary material following the Caledonian-Appalachian Orogeny. Variable Nd signatures of Archean and Proterozoic Shield rocks predominated before ~450 Ma, but these cratonic signatures are not seen again in Permian, Triassic, Jurassic and Early Cretaceous rocks of Alberta, British Columbia and the Canadian Arctic. In particular, the Sverdrup basin of the Canadian Arctic Islands, filled with ~9000 meters of Mesozoic clastic material, represents an excellent sampling of provenances from the cratons of Canada to the south and Greenland to the east. Thus the Caledonian-Appalachian Orogen remained the ultimate source of clastic sediment being removed from the craton, even though the Paleozoic mountains were largely eroded and/or covered by late Paleozoic time.

The results suggest that a widespread cover sequence was established during the ~450-350 Ma time frame, and this cover served as the source of clastic sediment through most of the Mesozoic, until the Cordillera began to supply sediment in Late Jurassic time. A simple interpretation of this result would call for essentially complete cover of the Canadian Shield from Late Devonian to at least Early Cretaceous time. This would in turn imply a negative topographic deflection of the Shield for ~300 m.y. Alternatively, a covered Canadian Shield could be the more normal condition, similar to the Russian Platform, so that the pre-450 Ma and present-day emergence of the Shield would then be anomalous in terms of dynamic topography. However, interpretation of the sedimentary provenance data does depend on the question of how much basement could be exposed near to erosional base level, and yet be invisible as a contributor to sediment supply. Models of erosion and sedimentary provenance from such regions of low relief contrast are in their infancy, and need improved observational data.