ABSTRACT

The Upper Devonian Redwater atoll of the Alberta subsurface exhibits a pronounced peripheral raised rim around its entire circumference. The relief of the rim relative to the inset central lagoonal area averages 110 ft. Isopachs from some overlying shale horizons to the reef, and paleoecologic studies, indicate that at the time of deposition the lagoonal area stood about 40 ft. above the periphery of the complex. Secondary downwarp of the lagoon relative to the rim thus totals 150 ft.

First-order trend-surface analysis on Devonian and Cretaceous horizons above the reef show that the peripheral rim developed in two discrete episodes. The first is reflected only in horizons below the pre-Cretaceous unconformity and accounts for about 95 ft. of differential downwarp. The remaining 55 ft. of relief between rim and lagoon developed as a result of events late in the Cretaceous. Both of these episodes of differential depression are related to sediment loading and its effect on stylolitization in the reef carbonates.

Data on stylolite amplitudes and distribution, collected through the analysis of Redwater borehole cores, indicates that compaction of rim carbonates averages 13 per cent while the lagoonal carbonates underwent 24 per cent volume reduction. In terms of feet, the rim compacted by 174 ft. and the lagoon by 319 ft. The resultant 145 ft. of measured differential thickness reduction through stylolitization is thus essentially consistent with the observed 150 ft. of differential downwarp. Previously proposed methods of secondary lagoonal downwarp (e.g., downfaulting of the lagoon) are not required.

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