Mineralogy of Bearpaw sediments in the South Saskatchewan River Valley

Abstract

The Cretaceous Bearpaw formation in the South Saskatchewan River valley is a 900-foot sequence of mudstones interlayered with several thin sandstone units. The diversified mineralogy of these sediments reflects a complex depositional history, in which sedimentary, volcanic, and metamorphic source rocks all supplied detritus. A sedimentary source is suggested by the abundance of detrital chert and by the occurrence of rounded grains of tourmaline and zircon in the sandstones. Volcanic rocks supplied montmorillonite, sanidine, and hexagonal biotite flakes. Further evidence of a volcanic source is found in the recurrence of thin bentonite seams in the succession of sediments. A metamorphic source is indicated by a distinctive suite of metamorphic minerals in the heavy mineral residues. The postulated source areas of the sediments are all to the west of the Bearpaw depositional basin. The sedimentary source rocks are believed to have occupied the site of the Purcell and Selkirk Mountains in British Columbia. Active volcanoes in western Montana probably supplied the volcanic detritus. The metamorphic source area was most likely the Shuswap terrane in British Columbia. Transportation of detritus from the source areas to the site of deposition was mainly by water, but, almost certainly, some of the volcanic material was wind-borne. The results of the mineralogical studies have proved to be of little use in stratigraphic zoning of the Bearpaw formation.