Sandy shelf-margin clinoforms in the Eocene strata of the Central Tertiary Basin of Spitsbergen are usually generated by river-dominated shelf deltas, or by wave-dominated shorelines, though these two regimes can also be strike-equivalent to each other. Clinoforms occur in series or sets that show both sub-horizontal and rising trajectories of shelf-edge accretion. Clinothems involved in the former style of margin growth, however, tend to be dominated by delta deposits. Shelf-edge deltas of such clinoforms are commonly severely eroded by their own distributary channels, and this is especially noticeable at (though not restricted to) shelf-edge locations. Fluvially incised shelf edges are commonly linked directly across the shelf break, to turbidite-filled channels, gullies and small canyons on the slope. Examples of this type of shelf-edge situation are present on Brogniartfjellet in Van Keulenfjorden, where the outer-shelf segment of the clinothem contains shelf-edge deltaic units that are 20-30m thick deposited during falling base level and lowstands. The deltas have been cut by deep erosive channels (up to 12 m) paved by shale rip-up conglomerates. The channel infill is dominated by up to 3 m-thick, flat and low-angle laminated, medium-grained sandstone bedsets deposited from upper-flow-regime conditions in riverine and shallow sand flats. Multiple phases of erosion can be demonstrated, separated by phases of minor re-establishment of delta-front facies. At peak regression of the delta system, still during falling relative sea level, the channels have reached the shelf break and allowed the river system to feed sediment directly into slope channels that were turbidity-current conduits to the basin floor. These are incised more than 25m deep on the upper slope, appear to have originated from fluvial input and retrogressive slumping on the slope, and link back up to the shelf-edge incisions. The infill of the slope conduits strongly suggests repeated phases of erosion/bypass that alternated with phases of low-efficiency, hyperpycnal-flow deposition. The apparent off-lapping architecture within the slope conduits strongly suggests oblique or downslope accretion of infill during continued relative fall (forced regressive and lowstand conditions) of sea level, and probably during basin-floor growth of the fan. In the latest stage of the lowstand, the shelf-edge deltas have re-established themselves onto the shelf, aggrading and prograding onto the underlying canyonized succession, thus forming a lowstand prograding wedge. Minor fluvial incision occurs, but overall the system is less sand prone. During the subsequent transgression of the shelf, when sea rose back up to and above the shelf edge, the slope is blanketed by mud, there is tidal re-working and infilling of the older shelf-edge channels and a transgressive barrier/lagoon or estuary system migrated landwards.

Abstract: The Campanian Hatfield Member of the Haystack Mountains Formation is composed of two well-exposed marine sandstone tongues that extend up to 35 km basinward from their earliest shoreline position into the Western Interior Seaway. Each tongue (HI and H2) is comprised of two parts that have characteristic architecture, external geometry and facies assemblages. Together, the tongues form a stratigraphic sequence that is formed of four systems tracts and bounded by erosional unconformities. The sequence is interpreted to have been generated over an interval of less than 1 Ma during a fall-to-rise cycle of relative sea level. The earliest and latest systems tracts of the sequence, interpreted as lowstand prograding deltaic wedge and forced regressive shoreface respectively, are distinguished on the basis of their position with respect to the sequence-bounding unconformities, reconstructed shoreline trajectories, and by their component facies that indicate the dominant depositional regime. The mapped basinward shift of the Hatfield 1 lowstand prograding wedge from the previous shoreline deposits and erosional relief on the sequence boundary, indicates a relative sea-level fall prior to its deposition. The lowstand prograding wedge consists of parasequences that are dominated by tidally influenced cross-stratified sandstones and step for more than 30 km basinward, and are readily distinguished from the underlying highstand shoreface facies. Distal aggradational stacking of the lowstand produced a slightly rising shoreline trajectory that in combination with proximal onlap against the underlying erosional unconformity indicates accumulation under conditions of rising relative sea-level with abundant sediment supply. The domination of tidally influenced facies and an estimated relief of at least 20 m in proximal reaches of the underlying sequence boundary suggests that the lowstand wedge was a tidally dominated deltaic system localized and fed through an incised valley. This systems tract resembles other cross-stratified Mancos-type sandstone bodies of the Western Interior Seaway which have been under debate. However, unlike most of these, the Hatfield 1 has great outcrop extent and the updip relationship of the lowstand wedge with the older shoreline deposits can be traced. The overlying retrogradational Hatfield 1 transgressive systems tract has comparable facies to the lowstand wedge and also shows proximal onlap of the sequence boundary, suggesting that it developed within a tidally influenced estuary. As such, the lowstand and transgressive systems tracts form a distinctive cross-bedded tidally influenced lithosome that is readily distinguished from the wave-dominated lithosomes of the preceding Hatfield 1 highstand systems tract and the overlying Hatfield 2 highstand and forced regressive systems tracts. The Hatfield 2 forced regressive systems tract is a wave-dominated shoreface that like the preceding Hatfield 2 highstand shoreface is strongly progradational. However, in contrast to the highstand shoreface from which it builds, the forced regressive shoreface is relatively thin, lacks shaley offshore transitional facies at its base, and displays a downstepping trajectory relative to the underlying MFS. The basal surface of the forced regressive shoreline also has an enrichment of coarse glauconitic grains derived from erosion of the underlying condensed section whereas the upper bounding surface of the systems tract is an erosional unconformity, documenting the maximum fall in relative sea level. There is a clear sedimentological distinction of the lowstand and forced regressive systems tracts because whereas the former has a tidally influenced facies association, forced regressive facies tend to be wave-dominated. Such facies partitioning and style contrast are thought to reflect the less-confined nature of the highstand and forced regressive shorelines in comparison to the incised or embayed nature of the lowstand and transgressive shorelines.