FROM THE FORELAND BASIN TO THE CRATONIC INTERIOR: STEPS TOWARD REFINING HIGH-RESOLUTION SEQUENCE STRATIGRAPHIC MODELS FOR USE ON THE CRATON
The Paleozoic foreland basin fills of North America show broadly similar patterns. Numerous high-resolution case studies of these foreland basins demonstrate that 4th-order sequences (~400 ky) are typically the most prominent scale of cyclicity. In general, 4th-order sequences on the thrust-loaded margin are thick, dominated by coarse siliciclastics. In the basin center, they tend to be slightly thinner and shale dominated. The mixed carbonate-siliciclastic cratonic margin contains the thinnest sequences. The systems tracts there are dominated by a) paleosols and estuarine and lagoonal facies (LST); b) thick pelmatazoan grainstonerudstones and reefs (TST); c) shales, calcisiltites, and wacke-packstones (HST); and d) silty argillaceous calcarenites that have multiple internal discontinuity surfaces (FSST). This variation in lithologies largely reflects the antithetical effects of changes in accommodation on siliciclastic influx and carbonate precipitation.
Tracing 4th-order sequences onto the craton reveals similarities to cratonic foreland basin margins as described above, especially the TSTsdifferences include fewer current-induced sedimentary structures, greater micrite/shale proportion, higher ichnofabric indices, and thinner, erosionally incomplete systems tracts. Thinning in cratonic platform sequences is primarily the result of erosion at sequence boundaries. Above these surfaces, distinctive LSTs are typically absent; in many cases, TSTs of consecutive sequences are stacked directly atop one another with intervening HSTs and FSSTs truncated. In adjacent cratonic basins, FSSTs and HSTs re-emerge under sequence boundaries; HSTs tend to be greatly thinned and micrite rich.
Thus, sequence stratigraphy can be applied to the craton, although successions may be stratigraphically incomplete. Studies should be conducted at a regional scale to aid in recognizing missing systems tracts.