Abstract: Sequence Stratigraphy: Working Model or Work in Progress? Or, When is a Flooding Surface a Sequence Boundary?

L. F. Krystinik, B. Blakeney Dejarnett, D. A. Leckie

Block diagrams illustrating sequence stratigraphy typically show key surfaces and facies distributions at a single point in a basin and assume that sea level changes while all other variables remain constant. These variables were clearly defined by the original Exxon workers, but often are not fully integrated into the interpretations of working practitioners. The Campanian and lower Maastrichtian fill of the Cretaceous Western Interior seaway illustrates the variability in the sedimentary record produced by lateral variations in subsidence and sediment supply.

Basin-wide progradational units occur at the base of the Campanian and of the lower Maastrichtian, but wide variability in the stratal stacking patterns within time equivalent deposits exists along strike for most of the Campanian. Although sequence boundaries and maximum flooding surfaces have been documented for parts of the basin no basin-wide surfaces have been noted.

Because of local tectonics most lithostratigraphic/ chronostratigraphic profiles in the Western Interior seaway do not correspond well with published "eustatic" curves. Major unconformities, with hundreds of meters of erosion, are time equivalents of maximum flooding events. Accordingly, sequence boundaries and lowstand systems tracts coexist with time-equivalent transgressive successions and maximum flooding surfaces along depositional strike. The risks of extrapolating from local observations underscore the importance of biostratigraphic control in regional correlation.

Regional similarities of key surfaces and stacking patterns require regionally consistent depositional systems (e.g., wave regime, climate, stream discharge), sediment supply, and tectonic history. Variability in any of these components can produce radically different sedimentary signatures and stratigraphic interpretations.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994