Donatella Mellere

Turbidite Variability and Architecture of Sand-Prone, Deep-Water Slopes: Eocene Clinoforms in the Central Basin, Spitsbergen

ABSTRACT The architecture and turbidite variability within six wedge-shaped (downslope-thinning), sand-prone slope accumulations are documented from Eocene shelf margins on Spitsbergen. The Central Basin formed as a small foreland or piggy-back basin, and the studied turbidites accumulated mainly on the slope portion of sand-prone clinoforms that developed during depocenter migration and infilling of the basin. The shelf-margin clinoforms have amplitudes (minimum water depths) from 100 to 350 meters, and their shelf, slope, and basin-floor segments are well imaged, and can be walked out along many of the mountainsides. Only a small percentage of the clinoforms are sand prone, and these developed when sea level occasionally fell to or below the shelf edge. Of the sand-prone clinoforms, some had their sand budget partitioned mainly out onto the basin floor (basin-floor fans), but most trapped the sand on the slope only. The latter are now visible as downslope-thinning wedges, some 2.5-3.5 km in downdip extent. The turbidites within this type of clinoform have been examined and classified. The lower-slope to base-of-slope segment of the studied clinoform complexes are dominated by lobes consisting of broad, shallow channels and sheet-like turbidites, becoming heterolithic and muddy out on the basin floor. Beds on the lower slope vary from thick (up to 4.5 meters), ungraded or laminated sandstones, to thinner ungraded sandstones with coarse cappings. The middle-slope segment of clinoform complexes is dominated by narrow channels (chutes) that feed downslope to progradational chute-mouth lobes. Chutes contain ungraded and laminated sandstone beds up to 3 m thick, whereas the chute-mouth lobes show alternations of thinner, ungraded to laminated or rippled sandstones. These lobes become more heterolithic and muddy downslope. The shelf-edge to upper-slope segment of clinoform complexes is dominated by upward-coarsening and -thickening sheetsands of steep-fronted shelf-edge deltas. The sandsheets of the delta front can be traced updip into mouth-bar and distributary-channel sandstones. It is argued that shelf-margin accretion, represented by the sand-prone slope wedges, was achieved mainly by sand-laden currents that flooded from the shelf edge as hyperpycnal flows. This hypothesis is supported by: (1) the direct connection between channel and mouth bar systems at the shelf edge, and the turbidites of the slope lobes, (2) the systematic progradational character of the slope lobes, (3) the absence of large-scale slump scars, gullies, or canyons on the slope, and (4) the dominance of a type of turbidite that implies deposition from sustained flow. Detailed examination of the architecture of one of the slope wedges shows that there are unconformities developed within the stratigraphy below the shelf edge and that these erosional terraces beheaded the deltas perched on the uppermost slope. The erosion surfaces indicate fall of sea level to this position. Despite the magnitude of this fall (up to 80 meters), the lack of canyons on the slope prevented the construction of basin-floor fans. Such falls of sea level, on non-canyoned slopes, simply promote sand-prone accretion of the shelf margin.

Style contrast between forced regressive and lowstand/transgressive wedges in the Campanian of south-central Wyoming (Hatfield Member of the Haystack Mountains Formation)

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 (H1 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 that 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.