The Upper Cretaceous and Paleocene stratigraphy of Turtle Mountain, Manitoba

Abstract

The Turtle Mountain study area is situated on the northeastern rim of the Williston Basin, a negative structure on the craton. During the last Cretaceous marine transgression and regression, the Riding Mountain and Boissevain Formations were deposited. In the Paleocene, after a short erosional interval, deposition of Turtle Mountain Formation occurred. The Riding Mountain Formation consists of three members. The Millwood Member, the lowest unit, is a soft, greenish-brown, bentonitic, slightly silty clay. The Odanah Member, the middle unit, is a hard, grey, siliceous, clay shale. The Coulter Member, as proposed in this paper, is the upper unit, a light grey to buff, bentonitic, fine-grained clayey silt. The Riding Mountain Formation is correlated with the Bearpaw Formation of Saskatchewan and the upper part of the Pierre Shale of North Dakota. All of these formations were deposited in less than 200 feet of water. The overlying Boissevain Formation is composed of a thick lower unit of crossbedded, buff, quartz-rich, medium-grained, "salt and pepper" sand, and a thin upper unit of massive, white kaolinitic, fine-grained silt or clay. The Boissevain Formation is equated to the Fox Hills Formation of North Dakota and the Eastend, Whitemud, and Battle Formations of Saskatchewan. Deposition of the Boissevain Formation occurred at the mouths of rivers which emptied into a basin. An easterly direction of sediment transport is indicated by crossbedding measurements in the Boissevain Formation. The overlying Turtle Mountain Formation consists of two members, as proposed in this paper. The Goodlands Member, a lower assemblage of bentonitic, lignite-bearing sands, silts and clays, is correlated with the Hell Creek and Frenchman Formations of North Dakota and Saskatchewan, respectively. This member was deposited in a lagoonal environment. The Peace Garden Member, an upper assemblage of grey silty clays with minor greenish sand and silt beds, was deposited, for the most part, in a shallow water marine environment during readvance of the sea in the Paleocene. These marine beds are equivalent to the Cannonball Formation, a part of the Fort Union Group of North Dakota. The Peace Garden Member, as a unit, is correlated with the Fort Union Group and the Ravenscrag Formation of Saskatchewan.