Eocene Clinoform Growth in Front of a Storm-Wave-Dominated Shelf, Central Basin, Spitsbergen: No Significant Sand Delivery to Deepwater Areas

Abstract

ABSTRACT Eocene shelf-slope clinoforms on Spitsbergen reflect asymmetric basin infill across the coastal plain, shelf, slope, and deepwater segments of a foreland basin. Although four main types of clinoforms are recognized in the basin, this study focuses on clinoforms generated by a storm-wave regime on the shelf margin. The key aspect of this type of clinoform (Type 3) is that only modest volumes of sand were delivered across the shelf edge onto the slope, and little or no sand onto the basin floor, despite the sand-prone nature of the sediment delivery system, right to the shelf edge. The sedimentology and geometry of a series of such clinoforms are documented along a continuously exposed outcrop transect, some 800 m high and 4 km long. Sandstone units of individual clinoforms are 2-12 m thick, and are separated by thicker shale. Clinothems have subhorizontal topset and bottomset segments and steeper mid-portions, and are up to 150 m high, nearly an order of magnitude higher than the deltas and shorefaces that constructed them. This clinoformal configuration is thus interpreted to reflect shelf, shelf-edge, slope, and base-of-slope depositional environments, and water depths at least as great as the decompacted clinoform amplitudes. Exposed shelf segments dip up to 0.4 degrees whereas slope segments (some 2150 m wide) dip up to 4 degrees. The shelf and upper-slope reaches of clinothems consist of hummocky and swaly cross-stratified sandstone units without any significant interbedded shale. On the middle slope and below, these sandstone units become finer grained, are ripple- and plane-parallel laminated, and separated with interbedded shale. Individual topsets of clinothems are interpreted to have formed by strandplain or wave-dominated delta progradation across the morphological shelf platform during short periods of sea-level stillstand or slight fall. The shale units immediately above sandy clinothems developed during longer intervals of relative sea-level rise, when the wave-dominated shoreline retreated back across the shelf platform. The long-term trajectory of shelf-margin growth shows a significant aggradational component during overall progradation. Compared to other clinoforms in this same basin where the sediment-delivery system was fluvially driven and brought great volumes of sand onto the basin floor and slope, respectively, storm-wave-generated clinoforms brought only limited sand to the upper slope and little or no sand to the basin floor.