Origin of the peripheral rim, Redwater Reef, Alberta
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AbstractThe 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 feet. 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 feet above the periphery of the complex. Secondary downwarp of the lagoon relative to the rim thus totals 150 feet. 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 feet of differential downwarp. The remaining 55 feet 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 upon 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% while the lagoonal carbonates underwent 24% volume reduction. In terms of feet, the rim compacted by 174 feet and the lagoon by 319 feet. The resultant 145 feet of measured differential thickness reduction through stylolitization is thus essentially consistent with the observed 150 feet of differential downwarp. Previously proposed methods of secondary lagoonal downwarp (e.g. downfaulting of the lagoon) are not required.
Bibliography: p. 77-81.