Abstract 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.

Abstract Sharp-based marine shoreface sandstones interpreted as forced regressive deposits are a characteristic feature of the Gassum Formation in the intracratonic Danish Basin. Detailed process-based sedimentological and a high-resolution, sequence-stratigraphic interpretation of cores from closely-spaced wells has led to improved understanding of the erosional and depositional processes active during the formation of the sharp-based sandstones. Each sandstone shows an internal stacking of forced regressive shoreface units separated by thin muddy offshore facies. This stacked pattern records low-amplitude but widespread changes in relative sea-level during the overall progradation due to low depositional gradiednts. Laterally, the stacked forced regressive shoreface deposits show a seaward-dipping, shingled geometry indicating seaward displacement of the shoreline through stepwise, forced regressions during overall fourth-order relative sea-level fall. Thereby each sharp-based shoreface sandstone records deposition resulting from interaction of from two scales of superimposed relative sea-level fluctuations: a lower fourth-order fall responsible for the overall seaward shoreface displacement, and a higher fifth-order oscillation that resulted in repeated forced regression within the lower-order sequences. Although these stepwise, forced regressive deposits dynamically resemble ‘stranded’ parasequences, they differ from the conceptualized picture of ‘stranded’ parasequences as simple downstepping of forced regressive deposits, because of their gently dipping shingled geometry and distinctive deposition component resulting from intervening, high-order drowning. For both the fifth-order forced regressive units and the lower-order forced regressive sharp-based sandstones it is possible to differentiate between: (1) deposits formed during falling sea level as part of the forced regressive systems tract and (2) the last, forced regressive to progradational part formed at sea-level lowstand representing the lowstand systems tract. Accordingly, the sequence boundary, whether of high- or low-order, is placed below the last, forced regressive deposits and associated lowstand progradational deposits, but above the deposits formed during falling relative sea-level. Thus the sequence boundary is placed at the surface of subaerial exposure passing seaward into a marine regressive surface of erosion reflecting maximum regression. The basal, regressive surface of erosion below the fourth-order forced regressive systems tract is demonstrated to consist of coalesced fifth-order forced regressive surfaces. Therefore, the fourth-order regressive surface is a composite surface reflecting a series of forced regressions and intervening drowning and as such is diachronous. The basinwide dominance of sharp-based, forced regressive shoreface deposits in Upper Triassic of the Danish Basin is interpreted to reflect the interaction between low gradient and shallow palaeobathymetry, sediment supply and low-amplitude relative sea-level changes.