ANATOMY OF SHELF DELTAS AT THE EDGE OF A PROGRADING EOCENE SHELF MARGIN, SPITSBERGEN

Abstract

Although shelf-edge deltas are well-imaged seismic features of Holocene and Pleistocene shelf margins, documented outcrop analogues of these important sand-prone reservoirs are rare. The facies and stratigraphic architecture of an outcropping shelf-edge delta system in the Eocene Battfjellet Formation, Spitsbergen, is presented here, as well as the implications of this delta system for the generation of sand-prone, shelf-margin clinoforms. The shelf-edge deltas of the Battfjellet Formation on Litledalsfjellet and Hogsnyta produced a 3-5 × 15 km, shelf edge-attached, slope apron (70 m of sandstones proximally, tapering to zero on the lower slope). The slope apron consists of distributary channel and mouth-bar deposits in its shelf-edge reaches, passing downslope to slope channels/chutes that fed turbiditic lobes and spillover sheets. In the transgressive phase of the slope apron, estuaries developed at the shelf edge, and these also produced minor lobes on the slope. The short-headed mountainous rivers that drained the adjacent orogenic belt and fed the narrow shelf, and the shelf-edge position of the discharging deltas, made an appropriate setting for the generation of hyperpycnal turbidity currents on the slope of the shelf margin. The abundance of organic matter and of coal fragments in the slope turbidites is consistent with this notion. Evidence that many of the slope turbidites were generated by sustained turbidity currents that waxed then waned includes the presence of scour surfaces and thick intervals of plane-parallel laminae within turbidite beds in the slope channels, and thick spillover lobes with repetitive alternations of massive and flat-laminated intervals. The examined shelf-edge to slope system, now preserved mainly below the shelf break and dominated by sediment gravity-flow deposits, has a threefold stratigraphic architecture: a lower, progradational part, in which the clinoforms have a slight downward-directed trajectory; a thin aggradational zone; and an upper part in which clinoforms backstep up onto the shelf edge. A greatly increased density of erosional channels and chutes marks the regressive-to-transgressive turnaround within the slope apron, and this zone becomes an angular unconformity up near the shelf edge. This unconformity, with both subaerial and subaqueous components, is interpreted as a sequence boundary and developed by vigorous sand delivery and bypass across the shelf edge during the time interval of falling relative sea level. The studied shelf-margin clinoforms accreted mostly during falling stage (sea level below the shelf edge), but the outer shelf later became estuarine as sea level became re-established above the shelf edge.