ND ISOTOPIC CONSTRAINTS ON THE PROVENANCE OF SYNOROGENIC FORELAND BASIN SEDIMENTS IN THE CANADIAN CORDILLERA

The Cordilleran Foreland sequence preserved in Alberta exposes a relatively complete and well-studied record of Late Jurassic to Paleocene sedimentation. Constraints on the provenance of the basin sediment through time can be acquired using geochemical and petrographic analyses, including Nd isotopic analyses, and may help to elucidate the timing and degree of orogenic uplift and erosion, the affinity and location of synorogenic magmatism, and paleogeomorphology.

Nd isotopic results confirm the presence of an isotopically juvenile component derived from a source that contributed significant amounts of detrital material during Cretaceous time. New Nd data from the Jurassic offshore and shallow marine Fernie Formation through the Paleocene-Eocene continental Paskapoo Formation in southwestern Alberta indicate two distinct trends in epsilon Nd values as a function of time. One trend consists of ubiquitous and relatively homogenous values that range between –8 and –13; the other consists of two distinct “spikes” of increasingly less negative (isotopically more juvenile) values extending to 0. This juvenile component is related to immature clastic sediment delivered during two major pulses of foreland deposition in Early and Late Cretaceous time. The trend toward less negative epsilon Nd values is associated with only a small increase in the Sm/Nd ratio, indicating that the source of the juvenile component is felsic in character.

Whereas petrographic provenance studies (e.g., Mack and Jerzykiewicz, 1989) suggest that the foreland basin has been filled with sediment derived largely from the fold-and-thrust belt and adjacent hinterland to the west, Cretaceous igneous rocks that occur there do not have juvenile Nd isotopic signatures. Moreover, nearly all of the Mesozoic volcanic rocks presently exposed in the Rocky Mountain and Omineca Belts are intermediate or mafic (not felsic) in character. Thus, a more distant source seems to be indicated for the juvenile volcanogenic component. Subsequent study by the authors and their collaborators will include additional Nd isotopic analysis, REE and other trace element analysis, U-Pb and Ar/Ar dating of detrital minerals, and petrographic study to determine the source of this juvenile component.