Abstract: Linking Pattern to Process: Along-Strike Variations in Mid-Cretaceous Clastic Wedge Architecture
GARDNER, MICHAEL H., Department of Geology and Geological Engineering, Colorado School of Mines, Golden CO 80401
A regional stratigraphic hierarchy relates changes in stratal architecture to eustatic and tectonic controls on middle Cretaceous deposition in the central Western Interior foreland basin. Three temporal and spatial scales of stratigraphic cyclicity are recognized. One long-term cycle (600 m thick, 4.5 m.y.) contains four intermediate-term cycles (300 m thick, 1 to 2 m.y.). Each intermediate stratigraphic cycle consists of two to eight short-term cycles (up to 40 m thick, 0.3 m.y.). The long-term stratigraphic cycle spans Turonian through middle Coniacian stages and consists of an upward-coarsening succession of marine and nonmarine deposits bounded by deposits formed during eustatic transgressions.
The architecture of the four intermediate-term cycles change as a function of local rates of accommodation relative to supply in foreland subbasins comprising the western margin. These stratigraphic cycles were deposited under low accommodation to 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 cycle). Where sediment accommodation relative to supply rates are lower, stratigraphic cycles 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 are related to southward migration of depocenters and increases in accommodation recorded as more conformable stratal successions containing higher proportions of deeper water deposits. Basinwide variations show that a stratigraphic hierarchy may be resolved where stratal patterns are not consistent. Correlating turnarounds in sedimentological energy of stratigraphic cycles across foreland subbasins links dissimilar stratal patterns within chronostratigraphic units recording the same record of relative sea level change, but of varying magnitude and with changing contributions from primary controls (i.e., subsidence, sediment supply, and eustasy).
AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah