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The continental shelf on the western margin of the Cretaceous Interior seaway was a muddy surface which bore abundant northwest-southeast trending sand bodies, up to 20 m (65 ft) thick and many km long (Medicine Hat, Mosby, Shannon, Sussex, Duffy Mountain, and Gallup Sandstones). These features resemble the storm-built or tide-built sand ridges reported from the modern Atlantic continental shelf, or from the Southern Bight of the North Sea. However, whereas modern sand ridges may rise from the Holocene transgressive sand sheet through overlying Holocene mud deposits to be exposed at the present sea floor, no modern examples are known where sand ridges are completely encased in mud, as the Cretaceous examples seem to have been.
Hydrodynamical theory suggests that special circumstances may make it possible to build sand bodies from a storm flow regime whose transported load consists of sandy mud. Under normal circumstances, such a transport regime would deposit little clean sand. The sea floor is eroded as storm currents accelerate, but erosion ceases when the boundary layer becomes loaded with as much sediment as the fluid power expenditure will permit (flow reaches capacity). Deposition of the graded bed occurs as the storm wanes; the resulting deposit is liable to consist of a sequence of thin shale beds with basal sand laminae. However, slight topographic inequalities in the shelf floor may result in horizontal velocity gradients so that the flow undergoes acceleration and deceleration in space as well as in time. Fluid dynamical theory predicts deceleration of flow across topographic highs as well as down their down-current sides. The coarsest fraction of the transported load (sand) will be deposited in the zone of deceleration, and deposition will occur throughout the flow event. Relatively thick sand deposits, 20 to 50 cm (8 to 20 in.) can accumulate in this manner. Enhancement of initial topographic relief results in position feedback; as the bed form becomes higher, it extracts more sand from the transported load during each successive storm. Individual storm beds may tend to fine upward (waning current grading), but the sequence as a whole is likely to coarsen upward, reflecting increasing perturbation of flow by the bed form as its amplitude increases.
Stability theory suggests that the end product of these processes should be a sequence of regularly spaced sand ridges on the shelf surface. However, sand bodies are localized in stratigraphic position and lateral distribution within Cretaceous shelf deposits. Upward-coarsening sequences are a widespread phenomenon in the Western Interior Cretaceous System, and the sand bodies appear to constitute localized sand concentrations within more extensive sandy or silty horizons. Especially widespread upward-coarsening sequences appear to be due to the close coupling between activity in the overthrust belt to the west and sedimentation in the foreland basin. In the proposed sequence of events, a thrusting episode increases relief in the source terrane as well as the load on the crust. Sedime tation at first dominates over subsidence, and initially the shelf on the western margin of the basin becomes shallower. As it does so, intensified wave scour on the shelf floor increases the amount of bypassing, which results in the deposition of increasingly coarser sediment, culminating in a sandy horizon. As relief in the hinterland wanes, subsidence overtakes sedimentation and the shelf subsides. Renewed thrusting begins the cycle anew.
In a second mechanism for the formation of upward-coarsening sequences, tectonic uplift affects parts of the shelf as well as the hinterland. The initiation of Sevier or Laramide structural elements beneath the shelf, and the remobilization of other, older structures, creates submarine topographic highs. These highs cause slight sand enrichment over broad sectors by means of the process described above. The development of sand-enriched areas on the shelf floor by both mechanisms leads to the flow-substrate feedback behavior that builds large scale, elongate bodies of clean sand.
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