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

Abstract

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.