ABSTRACT

The Boundary Member, an erosional remnant of a more-extensive unit, is associated with an unconformity in the upper part of the Charlie Lake Formation. The member, the reservoir of the Boundary Lake oil field, is a stromatolitic limestone and dolomite complex in a red-bed sequence of Triassic age. Underlying the member is a thick-bedded anhydrite unit, 70 ft thick, which contains well-defined, widespread marker beds and appears, at least in part, to be a subaqueous deposit. Overlying and surrounding the member is a unit of red and grey-green siltstone and mudstone. This unit, which appears to be mainly of shallow-water or intertidal origin, overlies the post-Boundary unconformity.

The isopach map () of the interval from the erosion surface to a marker about 50 ft up in the siltstone shows the paleotopography of the post-Boundary erosion surface. The isopach map of the interval from the above marker in the siltstone to a marker down in the anhydrite below the erosional surface shows the structure of the anhydrite marker at the time of the siltstone deposition. Because marker beds in the anhydrite are nearly parallel to those in the boundary, this may also show the configuration of Boundary Member paleo-structure.

The Boundary Member is preserved in a paleostructural low, but its external configuration is related both to structural movements and to erosion that occurred before deposition of overlying rocks. Stratigraphic trends within the member do not conform to its outline but are truncated at the zero edge. Although the thickness of the Boundary is a function of both paleotopography and paleostructure, there is no reason to expect thickness trends to be continuous, and interpolation between thickness data points is unreliable. A reliable isopach map of the Boundary Member cannot be obtained directly, but must be generated by subtracting the 'paleotopographic' map from the 'paleostructure' map. A combination of isopach maps allows prediction of six Boundary Member outliers in the study area. Because of the uncertainties inherent in the derivation, the predictor map is considered a qualitative likelihood map.