Tectonic and Eustatic Controls on the Stratal Architecture of Mid-Cretaceous Stratigraphic Sequences, Central Western Interior Foreland Basin of North America
Michael H. Gardner, 1995. "Tectonic and Eustatic Controls on the Stratal Architecture of Mid-Cretaceous Stratigraphic Sequences, Central Western Interior Foreland Basin of North America", Stratigraphic Evolution of Foreland Basins, Steven L. Dorobek, Gerald M. Ross
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Changes in stratal architecture defining a hierarchy of time-stratigraphic units record eustatic and tectonic controls on deposition in the central Western Interior foreland basin. Three temporal and spatial scales of stratigraphic cyclicity are recognized, each recording base-level changes of different periodicity. One long-term base-level cycle (600 m thick, 4.5 my) contains four intermediate-term base-level cycles (termed stratigraphic sequences, up to 300 m thick, 1 to 2 my). Each stratigraphic sequence consists of two to eight short-term cycles (up to 40 m thick, 0.3 my). A long-term stratigraphic cycle spanning Turonian through middle Coniacian stages consists of an upward-coarsening succession of marine and nonmarine deposits bounded by deposits formed during eustatic transgressions. Long-term base-level fall is recorded by the episodic eastward progradation of shoreface sandstones into the basin. The regressive maximum of the youngest stratigraphic sequence corresponds to a well-documented eustatic drop in the late Turonian.
Stratigraphic sequences of the Western Interior change as a function of local rales of sediment accommodation relative to supply in foreland subbasins comprising the western margin of the central Western Interior seaway. Turonian–Coniacian stratigraphic sequences were deposited under low accommodation and sediment supply conditions across Wyoming, Colorado, northeastern Utah, and northern New Mexico, and in high sediment accommodation to supply settings in western Wyoming, central Utah, and in northwestern New Mexico (upper part of the youngest sequence). Where sediment accommodation relative to supply rates are lower, stratigraphic sequences show: (1) more unconformities, (2) higher magnitude facies offsets across cycle boundaries, (3) seaward-stepping cycle stacking patterns, (4) vertically truncated facies tracts, (5) lower proportions of nonmarine strata, and (6) higher sandstone- to mudstone-ratios.
Along strike changes in sediment supply and accommodation are superimposed on a long-term pattern of westward-thickening and eastward-prograding basin fill. Regional variations in stratal architecture within stratigraphic sequences are related to southward migration of depocenters in successive foreland subbasins, and southward increases in accommodation recorded by more conformable stratal successions and higher proportions of deep-water carbonates and mudstones. Basinwide variations documented here show that a hierarchy of Turonian–Coniacian chronostratigraphic units may be resolved where stratal patterns are not consistent. Correlating base-level rise-to-fall turnarounds across foreland subbasins links dissimilar stratal patterns within chronostratigraphic units recording the same record of base-level change, but of varying magnitude and with changing contributions from primary controls (i.e., subsidence, sediment supply, and eustasy). For example, northward movement of subtropical (Atlantic-Telhyan) water masses and southward migration of depocenters appear to have locally combined in the late Turonian to produce high accommodation and sediment-supply conditions in the central Utah foreland basin. Here, short-term cycle stacking patterns closely resemble parasequence sets of a third-order depositional sequence. This suggests that this particular cycle stacking pattern and sequence model is most applicable to settings where accommodation and sediment supply are high.
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Stratigraphic Evolution of Foreland Basins - A strong case can be made that foreland basins are where the casual links between sedimentation and tectonic events were first recognized, as evidenced by the interpretations of geologists working in classic foreland areas. This Special Publication was derived from a Research Symposium entitled ?Stratigraphic Sequences in Foreland Basins ?held at the AAPG-SEPM joint annual meeting on June, 1992, in Calgary, Alberta, Canada. This volume provides a well-balanced perspective of current research on foreland basin stratigraphy and also serves as another element in the evolving framework that comprises our understanding of foreland basins. Given that so many of earth?s resources are found in foreland basins and that foreland basin strata often provide the only preserved record of the tectonic events that led to basin development, the impetus for continued studies of foreland basin strata should remain for many generations of geologists to come.