Abstract

The sub-Watt Mountain unconformity occurs near or at the top of the Middle Devonian Elk Point Group. This unconformity has slight topographic relief and changes character from place to place in northern Alberta and District of Mackenzie. In the northern part of the Elk Point embayment the unconformity overlies a paleoregolith, in the crestal part of the Tathlina Uplift it overlies a limestone breccia, and along the seaward margin of the Presqu’ile Barrier the unconformity is an erosional surface or depositional hiatus. This situation indicates that the erosional unconformity is the result of a relative drop in the waterlevel of the Elk Point embayment, during which paleotopographic high areas (Peace River Arch, parts of Tathlina Uplift) were emergent and eroded. Because of differential compaction during the period of low-stand the area in northern Alberta underlain by Elk Point evaporites subsided more than the area underlain by carbonates and the Tathlina Uplift. The result was a reversal in paleotopography and the formation of a very shallow basin in northern Alberta. In this shallow basin the paleoregolith is overlain by relatively thick brackish water deposits and evaporites of the Watt Mountain and Fort Vermilion formations.

An analysis of the sedimentation pattern across the Presqu’ile Barrier suggests that the Sulphur Point Formation may include reefal beds equivalent to the Upper Elk Point evaporites and reefal beds that prograded northward during the pre-Watt Mountain hiatus. The analysis explains why the sub-Watt Mountain unconformity changes character along the seaward edge of the Presqu’ile Barrier.