Abstract

The Upper Cretaceous non-marine Whitemud Formation of south-central and southwestern Saskatchewan and southeastern Alberta consists of kaolinitic, metamorphic lithic sands and silts, and kaolinitic clays. The sands and silts are not highly feldspathic as was originally thought. The major constituent is metamorphic lithic grains with minor kaolinitic clay and vermicular kaolin, clear angular quartz, chert, muscovite, and minor volcanic lithic grains and feldspar. The upper part of the Upper Cretaceous Eastend Formation, which conformably underlies the Whitemud Formation, consists of non-marine sands, silts, and clays. Kaolin is very rare. The bulk of the sands are composed of volcanic lithic grains with minor metamorphic lithic grains, clear angular quartz, chert, feldspar, muscovite, and biotite.The contact is characterized by the following changes from the Eastend Formation upward into the Whitemud Formation: an abrupt decrease in volcanic lithic grains and increase in metamorphic lithic grains; the appearance of kaolin and the disappearance of biotite and apatite; a slight increase in clear angular quartz and muscovite and a decrease in feldspar; a general increase in metamorphic heavy minerals; and an increase in the percentage of ilmenite (both as solitary grains and intergrown with magnetite), which is altered to leucoxene.On the basis of mineralogy, the Whitemud Formation is definitely a correlative of the Colgate Member of the Fox Hills Formation in Montana and North Dakota.The upper Eastend and Whitemud Formations were derived from Upper Cretaceous volcanic rocks, Precambrian and Paleozoic metamorphic rocks, and Paleozoic carbonates all situated in Montana. Upper Eastend sediments represent fast mechanical weathering of mountains of freshly extruded volcanic rocks, whereas the Whitemud sediments represent slow chemical weathering and leaching, which predominated once the mountainous volcanic rocks were worn down. This deep chemical weathering altered the volcanic tuffs and flows into kaolinitic clay at the source area; the kaolin of the Whitemud Formation is not derived from the weathering of feldspars at the site of deposition.It is suggested that the Frenchman and Ravenscrag Formations were also derived from Upper Cretaceous and Lower Tertiary volcanic rocks in Montana.

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