The sub-Cretaceous unconformity is an important regional surface across the Alberta Basin, and represents a significant period of non-deposition and erosion initiated after the deposition of upper Jurassic/lowermost Cretaceous sediments of the first foreland basin clastic wedge. In the investigated area, with the exception of the upper Jurassic Fernie strata, this major unconformity surface separates the basin stratigraphy into sequences assigned to distinct depositional settings: an early passive margin basin and a subsequent foreland basin.
The topography of the sub-Cretaceous unconformity surface was an important control on the deposition and preservation of the overlying Cretaceous succession.
The study area is located in the Peace River-Slave Lake region and covers NTS map sheets 83N, 83O, 84C, and 84B.
Log correlation was the primary method for mapping the unconformity surface. Geophysical wireline logs for 4,918 wells were examined to assign picks identifying the depth of the unconformity and to determine the underlying subcropping formation. An additional 7,884 picks were made to identify the position of underlying stratigraphic units. The unconformity surface was picked using a minimum density of three wells per township. Where subcrop boundaries, complex structural or erosional features were present, additional wells were picked to aid in the delineation of these features.
The sub-Cretaceous unconformity picks were interpolated using ArcGIS Geostatistical Analyst to model the unconformity surface at a 500 metre grid cell spacing. The paleotopographic surface is derived by subtracting the dominant southwest dipping trend from the modelled sub-Cretaceous surface. The stratigraphic units underlying the sub-Cretaceous unconformity were geostatistically modelled and then integrated into a three-dimensional (3D) geocellular model using Petrel 2013. The 3D model was truncated by the sub-Cretaceous unconformity surface to identify the subcrop geometry of each stratigraphic unit. These results were cross-validated with additional well control and subcrop edges were adjusted where required. Refer to Alberta Geological Survey (AGS) Report 90 for additional information on the sub-Cretaceous unconformity, sub-cropping unit geology, or the modelling methodology.
Peterson, J.T. and MacCormack, K.E. (2015); Paleotopography of the sub-Cretaceous unconformity, Peace River - Slave Lake region, Alberta (NTS 83N, 83O, 84C and 84B); Alberta Energy Regulator, AER/AGS Map 578.