Abstract

The build-and-fill model for mixed carbonate–clastic systems was originally developed to explain thin, widespread, icehouse depositional sequences that maintain more-or-less equal thickness over wide areas despite complex internal architecture that includes both topography-building and topography-filling depositional phases. Lithologic heterogeneity in each small-scale sequence is driven largely by carbonate deposition providing localized relief above surrounding sediment and the eventual infilling of that relief by siliciclastics or a combination of carbonates and siliciclastics. “Phylloid-algal” carbonate mounds are a critical component of these small-scale sequences (cyclothems) during the Late Paleozoic Ice Age (Pennsylvanian–Permian) and have been ascribed to both relief-building and relief-filling phases of build-and-fill, which hinders the use of this model as a predictor of lithofacies distribution and geometry and limits its broader application to other carbonate mound settings.

This study includes detailed remapping of Upper Pennsylvanian outcrop and subsurface strata over roughly 3,000 km2 of northeastern Kansas, which includes the type strata for the Paleozoic example of the original build-and-fill model. The dominant depositional motif for the phylloid-algal lithofacies is that of relief builder with only moderate relief filling as carbonate mounds switched from aggradation to progradation during continued regression. Phylloid-algal lithofacies in the Iola, Wyandotte, Farley, and Plattsburg cyclothems (upper Missourian Stage) were predominantly aggradational and developed mound geometries (relief building) during regression as the base of the photic zone fell to a position low enough to encourage red- and green-algae-dominated, typically micritic bafflestone lithologies to develop on pre-existing topographic highs. As accommodation space continued to decrease, mound development switched from aggradational to progradational before further regression brought nearshore shales into the basin, which filled in bathymetric lows produced between adjacent mounds.

The revisions to build-and-fill presented here serve to expand the model and highlight the influence that the dominant relief-building mechanism (frame building/binding vs. baffling), sequence stratigraphic position (transgression vs. falling stage), and interplay between antecedent topography and rate of sea-level rise and fall can have on interpretation of these mixed carbonate–clastic depositional systems. Relief building can occur during both transgression and regression. Relief filling is largely limited to regression. The timing of relief-building within each sequence (TST vs. FSST) is a result of the interplay between the dominant relief-building organisms, basin and/or slope geometry, pre-existing accommodation space (i.e., water depth vs. antecedent topography), and the rates of relative sea-level rise and fall.

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