Topographic highs and lows on the Pre-Cretaceous unconformity of central Alberta are reflected as irregularities on the structure of overlying formations. These highs and lows are many times themselves reflections of changes in the deeper stratigraphy. In some places, the effects of large highs and lows can be seen directly on structure maps of Cretaceous formations. For example, the Leduc reef chain, which itself is up to 1,000 ft (305 m) below the unconformity, causes anomalies in the structure of all overlying formations and its effects can even be seen in the present-day topographic surface.

However, many irregularities on the unconformity are small and their effects are masked by the regional dip of the Alberta basin. Their effects also become more diffuse on the upper formations.

Trend surface analysis on the structure of the overlying formations removes the regional trend from the data, and these more subtle highs and lows can be recognized. They can be seen not only as differences between positive and negative residuals, but also as relative highs and lows within areas of positive and negative residuals.

Advantages of using residual maps of the structure of Cretaceous formations to locate highs and lows on the Pre-Cretaceous unconformity include: (1) showing that some structural and stratigraphic traps are a direct result of irregularities on the unconformity, and (2) despite limited well control to the unconformity, highs and lows can be mapped using the more

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numerous shallow top values.

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