The Devonian succession in the subsurface of the Great Slave and Great Bear plains, Northwest Territories

Abstract

The Devonian succession in the southern part of the District of Mackenzie thickens to the west and northwest. It onlaps a pre-existing paleotopographic high area in the southern part of the Great Slave Plain. The crestal part of this high area, named Tathlina Uplift, was emergent until the Middle Devonian. The Lower and lower Middle Devonian rocks accumulated in Root Basin, a slowly subsiding basin located in the southern part of the Mackenzie Mountains. During the Middle Devonian rise in sea level, the basin expanded sideways to include an embayment on the north flank of Tathlina Uplift, named Willow Lake Embayment. The episodes of sedimentation in Root Basin and the subsequent progradational infill from the southeast of Willow Lake Embayment and Root Basin are recognizable as three discrete events in the Devonian rock succession. The locations of the facies boundaries between the nearshore, evaporitic, peritidal, lagoonal, and shallow marine deposits of the three individual basin-fill sequences shifted laterally across the basin over considerable distances. The Lower and lower Middle Devonian rocks are separated from overlying Devonian strata by the sub-Headless unconformity. This unconformity truncates the eastern parts of sequences 2 and 3. It may have been the result of local uplift, a worldwide fall in sea level, or from a combination of these two events. The upper part of the Middle Devonian succession onlaps the sub-Headless unconformity and extends southeastward across Tathlina Uplift into the Elk Point Basin. It includes a basal carbonate platform, from which a large number of reef mounds and the Presqu'ile barrier reef complex extend upward. The reefs probably grew upward in response to a rise in sea level and deteriorated during the early Givetian because of a subsequent fall in sea level. The salt and anhydrite deposits southeast of Presqu'ile Barrier, which fill the Elk Point Basin, were deposited by the process of "evaporite drawdown" during the rise in sea level that followed the early Givetian lowstand. The Elk Point evaporite and the carbonate rocks of Presqu'ile Barrier are overlain by the sub-Watt Mountain unconformity, which represents a basinwide regression. Charophyte-rich shale overlies the unconformity and onlaps a part of Presqu'ile Barrier that overlies the crestal part of Tathlina Uplift. It is suggested that this part of Presqu'ile Barrier was a positive feature because of differential compaction. The upper Givetian carbonate rocks of the Slave Point Formation represent a platform that includes a reefal facies along its northwestern margin. This reefal facies overlies Presqu'ile Barrier and extends farther north, or coincides with the seaward edge of the barrier. Apparently, reef growth at the barrier edge was not halted by the sub-Watt Mountain regression. It is shown that reef growth may have moved down the seaward slope of Presqu'ile Barrier during the regression, and may have moved upward and outward during the Slave Point transgression. Dark, bituminous marlstones, containing Givetian fossils, occur between the reef mounds northwest of Presqu'ile Barrier. They are regarded as fondoform deposits equivalent to the reefal and clinoform deposits of both the Presqu'ile Barrier and the Slave Point Formation. Upper Devonian shales and argillaceous limestones overlie the Slave Point platform and reefal carbonates, the Givetian fondothem deposits, and the late Eifelian reef mounds north of Presqu'ile Barrier. These clinoform deposits of stratigraphic sequence 7 prograded westward across the basin after a major relative rise in sea level.