Stratigraphy of cominco potash shaft no. 1 Vanscoy, Saskatchewan
Authors: Price L L , Ball N L 
Published: November 2013

A detailed description is given of the stratigraphic succession exposed in the Cominco Potash Division No. 1 Shaft west of Vanscoy, near Saskatoon, Saskatchewan. The shaft reaches a depth of 3,750 feet (1,140 m), more than 200 feet (61 m) below the mining level
in the Middle Devonian Prairie Evaporite Formation. This formation is an evaporitic unit within a dominantly carbonate sequence of Middle and Upper Devonian rocks overlain uncomfortably by nearly 2,000 feet (610 m) of Cretaceous clastic strata, in turn overlain by Pleistocene till. The stratigraphy
is similar to that of the Duval shaft 12 miles (19 lun) to the northeast (Price and Ball, 1971), but differs in details controlled by regional trends of sedimentation. The Devonian rocks above the Prairie Evaporite salt consist mainly of carbonates that range from marine limestone to anhydritic
dolomite. Intervals of mudstone and anhydrite identify a succession af cyclic clastic-carbonate-evaporite sequences. The thick Prairie Evaporite salt represents the culminating phase of an early cycle in a succession of cycles that become thinner upward and cease at the end of Middle Devonian time.
It includes in its upper part three sylvite-rich zones that provide the large reserves of potash ore in Saskatchewan. Cominco ore is drawn from the uppermost zone, which alone is well-developed in the Saskatoon region. The entire cycle that follows the Prairie Evaporite is included in the Dawson Bay
Formation, consisting of basal clastic red beds, middle marine limestone and upper evaporite composed of anhydrite. The Souris River Formation, with three members, overlies the Dawson Bay. The lowest, or Davidson Member, consists of a basal clastic unit, the "First Red Beds", a middle argillaceous
limestone unit and an upper, intertidal and supratidal sequence of dolomite and anhydrite preceding the final beds of halite. The overlying Harris and Hatfield Members each contain possibly incomplete cycles, or alternations, of marine limestone or dolomite and anhydrite, without intervening clastic
beds. The lowest evaporite bed in the Harris Member contains halite. The Upper Devonian Duper ow Formation is distinguished by an alternation of argillaceous and non-argillaceous limestones that forms the basis for subdivision (from base to top) into the thin Saskatoon and Elstow Members and the
more massive Wymark and Seward Members. Sediments deposited on aggrading very shallow or intertidal surfaces characterize the Wymark, while diastems between same marine limestone beds of the Seward are represented by scoured surfaces. The silty lower member of the Birdbear Formation overlies the
Duperow, followed by a well-developed upper member of porous dolomite which has been permeated by calcite. Calcareous siltstone above the Birdbear, assigned to the Big Valley Formation of the Three Forks Group at the top of the Devonian, is part of a distant northern outlier beneath the
sub-Cretaceous surface. The Cretaceous sequence begins with thick-bedded sandstone and claystone of the Cantuar Formation, followed by the finer grained Pense Formation of interlaminated sandstone and shale. Dark grey shale of the Ashville Group with marine fossils overlies the basal sandstones and
the Ashville is followed by kerogenous white-speckled shale of the Vermilion River and Favel Formations in the basal part of the Upper Cretaceous. Blocky shale of the Lea Park Formation that overlies the "white specksmp;gt;" contains lithic fine sand and silt. An abundance of ammonites distinguishes
the Lea Park beds. The overlying sandstone of the youngest Cretaceous Belly River Formation is noted for copious plant carbon and the predominance of lithic sand. The succession thus evolves from basal sandy beds above the sub-cretaceous unconformity, in the form of the aggrading fluvio-deltaic
Cantuar Formation, through fine laminated sand of the intertidal or shallow subtidal Pense Formation, to shallow marine Ashville Group shale. In the lower part of the Upper Cretaceous above the Ashville, organic material accumulated on surfaces without current bedding, features of poorly aerated
bottom sediments. This condition is terminated with the influx of fine lithic-grained material that with the Lea Park marine muds and which, with filling up of the interior basin, becomes the dominant sediment in the nonmarine sandstones of the Belly River Formation.