Turbidite variability and architecture of sand-prone, deep-water slopes: Eocene clinoforms in the Central Basin, Spitsbergen

The architecture and turbidite variability within six wedge-shaped (downslo pe-thinning), sand-prone slope accumulations are documented from Eocene she lf margins on Spitsbergen. The Central Basin formed as a small foreland or piggy-back basin, and the studied turbidites accumulated mainly on the slop e portion of sand-prone clinoforms that developed during depocenter migrati on 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 t he mountainsides. Only a small percentage of the clinoforms are sand prone I and these develo ped when sea level occasionally fell to or below the shelf edge. Of the san d-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 b een 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-lik e 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 laminate d sandstones, to thinner ungraded sandstones with coarse cappings. The midd le-slope segment of clinoform. complexes is dominated by narrow channels (c hutes) that feed downslope to progradational chute-mouth lobes. Chutes cont ain ungraded and laminated sandstone beds up to 3 m thick, whereas the chut e-mouth lobes show alternations of thinner, ungraded to laminated or ripple d sandstones. These lobes become more heterolithic and muddy downslope. The shelf-edge to upper-slope segment of clinoform complexes is dominated by u pward-coarsening and -thickening sheetsands of steep-fronted shelf-edge del tas. The sandsheets of the delta front can be traced updip into mouth-bar a nd distributary-channel sandstones. It is argued that shelf-margin accretion, represented by the sand-prone slo pe 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 ch aracter of the slope lobes, (3) the absence of large-scale slump scars, gul lies, 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 t hat there are unconformities developed within the stratigraphy below the sh elf edge and that these erosional terraces beheaded the deltas perched on t he 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 o f canyons on the slope prevented the construction of basin-floor fans. Such falls of sea level, on non-canyoned slopes, simply promote sand-prone accr etion of the shelf margin.