Sampling of the bentonites included in the Upper Cretaceous Bearpaw Formation of southern Alberta and adjacent areas has provided material for a geochronological investigation of this marine sequence of strata. K-Ar dating of biotite and sanidine included in the bentonites has indicated that the Bearpaw Sea invaded most of the southern Alberta Plains 72-73 m.y. ago. The transgression of the sea was probably rapid and the base of the formation may be isochronous over most of the area, with the possible exception of areas in southern Saskatchewan and northern Montana where the sea might have transgressed somewhat earlier. The regressive upper boundary of the Bearpaw Formation is set at 68 m.y. in the westernmost Plains and at 66 m.y. further to the east in the Cypress Hills region. The geochronological picture is compatible with the paleogeography of the Bearpaw.

The bentonites intercalated with the normal sediments represent ashfalls produced by relatively remote volcanic eruptions. Study of the pheno-crysts in the sand-size fraction, provided that contamination by detrital material has been negligable, has indicated that most of the bentonites are remarkably uniform in petrologic type and prevailingly andesitic. A source area is suggested in the eastern Cordilleran belt of northern Montana, where strong volcanism throughout most of the Late Cretaceous accompanied the gradual emplacement of the Boulder batholith. The andesitic nature of the Bearpaw bentonites is compatible with the granodioritic magmatism in the postulated source area.

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