Are Some Isolated Shelf Sandstone Ridges in the Cretaceous Western Interior Seaway Transgressed, Detached Spit Systems?
Lars Henrik Nielsen, Peter Niels Johannessen, 2008. "Are Some Isolated Shelf Sandstone Ridges in the Cretaceous Western Interior Seaway Transgressed, Detached Spit Systems?", Recent Advances in Models of Siliciclastic Shallow-Marine Stratigraphy, Gray J. Hampson, Ronald J. Steel, Peter M. Burgess, Robert W. Dalrymple
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The Cretaceous Western Interior Seaway, North America, contains numerous coastal and shelf sandstone bodies that are interesting from a commercial and academic viewpoint. Exploration and production of hydrocarbons has provided an enormous amount of subsurface data that supplement superb outcrops, and many excellent papers on detailed sedimentology and sequence stratigraphy have been produced. However, consensus regarding interpretation of some of the sandstones is still out of sight. This paper proposes consideration of yet another possible interpretation, namely large sandy spit systems.
The Western Interior Seaway was characterized by recurrent sea-level changes, long-distance shifts in coastline position, incised valleys, large influxes of sand from rivers building deltas, strong waves that caused longshore sand drift toward the south, and differential subsidence and uplift that occasionally fragmented the basin into subbasins with shoals and islands. This physiography probably favored formation of large sandy, southward-prograding spit systems; however, these have not yet been recognized, possibly due to poorly constrained paleography, partial preservation, and lack of suitable facies models.
This paper thus presents two well-constrained spit-system models based on well-preserved, uplifted Quaternary spit systems with exposures of the internal facies architecture and present-day morphology showing growth patterns. The Lyngså spit system (10 km long, 0.5-2.5 km wide, and up to 15 m thick) was formed in less than 500 years in a moderate-wave-energy, nontidal to microtidal environment, and it prograded into 10-15 m of water depth. The Skagen spit system provides a unique possibility for comparing preserved structures with processes, inasmuch as its old proximal part is uplifted 13 m and exposed in seacliffs, while its distal end is still prograding. The spit system (22 km long, 3-7 km wide, and up to 32 m thick) has been active for 5,500 years and is situated in a high-energy, wave-dominated, nontidal environment and has prograded into 20-30 m of water depth. In both systems the wave energy was transformed into strong longshore currents when the waves hit the mainland and spit coasts. Thus current-generated structures dominate both systems, whereas wave-formed structures are less common. Both systems overlie offshore mud and silt. The Lyngså system consists of coarse-grained, steep avalanche-type platform foresets, up to 10 m high, overlain by bar-trough deposits, beach sand, and gravel. The Skagen system consists of gently dipping, sandy spit-platform clinoforms composed of bioturbated storm sand beds, dune cross-beds and ripples, beach sand and pebbles, and peat beds. Both systems are comparable in size to proven hydrocarbon reservoirs, and their dimensions and structures resemble some of the sandstone ridges in the Cretaceous Seaway.