Lateral Variability of Sequence Stratigraphic Framework in the Campanian and Lower Maastrichtian of the Western Interior Seaway
L. F. Krystinik, Beverly Blakeney DeJarnett, 1995. "Lateral Variability of Sequence Stratigraphic Framework in the Campanian and Lower Maastrichtian of the Western Interior Seaway", Sequence Stratigraphy of Foreland Basin Deposits: Outcrop and Subsurface Examples from the Cretaceous of North America, J.C. Van Wagoner, G.T. Bertram
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The Cretaceous Western Interior Seaway of North America is the focus of many sequence stratigraphic analyses because of extensive outcrops and abundant subsurface data. Although most of these studies are local in scope, someworkers have been tempted to correlate stratal stacking patterns and bounding surfaces across large areas based on lithostratigraphic similarities rather than age equivalence. We have integrated biostratigraphic and lithostratigraphic data to create a chronostratigraphic framework for the Campanian and lower Maastrichtian of the Western Interior Seaway. This chronostratigraphic framework allows comparison of coeval stratal stacking patterns and key surfaces along the western margin of the basin and documents some of the complex sequence stratigraphic relationships that can occur within foreland basins.
Basinwide progradational units occur at the base of the Campanian and the lower Maastrichtian, but wide variability in the stratal stacking patterns within time-equivalent deposits exists along strike for most of the Campanian. Sequence boundaries and maximum flooding surfaces have been documented by workers for local areas within the Western Interior Seaway, but in most cases the regional extent and significance of these surfaces have not been addressed. In addition, recently published eustatic curves do not compare favorably with most of the lithostratigraphic/chronostratigraphic profiles in this study. This suggests that tectonic activity during the Campanian and early Maastrichtian probably played a major role in creating the complex relationships observed in the Western Interior Seaway. Local uplifts and basinal downwarps were a dominant control on relative sea level histories for most of the western margin of the basin. Major sequence-bounding unconformities, with hundreds of meters of erosion, are the direct time equivalents of maximum flooding events in other parts of the basin, reflecting the tectonic complexity typical of foreland basins.
In contrast, in Central Montana and southern Alberta, two areas apparently subjected to only moderate tectonic activity during this time, depositional patterns similar to the gross trends predicted by global eustatic curves do occur. Although the importance of structurally induced perturbations to relative sea level history was acknowledged in early papers on sequence stratigraphy, some practitioners of sequence stratigraphy tend to underestimate the effect that local structural influences may have upon the response of sedimentary systems.
The variability along the western margin of the Western Interior Seaway for the Campanian and early Maastrichtian should serve as a cautionary note for stratigraphers working in tectonically active basins. Detailed lithostratigraphic and biostratigraphic control is critical in order to constrain regional correlations of major bounding surfaces and stratal stacking patterns in any basin. This is especially important in a basin like the Western Interior Seaway where tectonic activity caused substantial variations in sediment supply and accommodation rates along the western margin of the seaway.