A highstand shelf-margin delta system from the Eocene of West Spitsbergen, Norway

摘要

Demonstration of shelf-margin accretion by shelf-edge deltas during rising and highstand of relative sea level has important consequences for deepwater sand depositional models. Although highstand shelf-edge deltas are conceptually feasible and have been recently argued from subsurface data, we describe here the first outcrop example, thus providing facies and architectural data on this important category of delta. Deltas are able to reach the shelf-edge during rising sea level, if one or more of the key conditions of sediment supply, shelf width/gradient, or basinal processes are such as to allow complete cross-shelf progradation before the onset of delta auto-retreat. Such highstand deltas promote the retention of high volumes of sand on the aggrading shelf and coastal plain, and thus potentially have a reduced sand budget available for delivery to the deeper water areas. Clinoform 17, one of a series of eastward-prograding, shelf-margin clinoforms from the Eocene Battfjellet Formation on West Spitsbergen, contains a sand-rich delta complex sited near the clinoform shelf-slope rollover, and is argued to be a highstand (rising relative sea level) shelf-margin delta based on: (1) its highly aggradational architecture shown by an unusual (compared to other clinoforms) regressive unit thickness and its marked stacking of parasequences, (2) coeval accumulation of delta-plain and lagoonal deposits that are well-preserved in the landward reaches of the same clinoform, and (3) its context within a mappable, longer-term rising shelf-edge trajectory (through 5 clinoforms). It is likely that the delta reached its shelf-edge location because the shelf was narrow (less than 20 km), and not because of high sediment supply or relative sea-level fall. The delta system was markedly wave-dominated as might be predicted at a shelf-edge site. The sand-rich, shelf-edge portion of Clinoform 17 consists of (1) a 30-35 m thick regressive deltaic unit with offshore mudstones and thin tempestite layers, wave-dominated delta-front sandstones, and tidal-fluvial-distributary channels on the delta topsets, (2) an overlying 15-23 m thick, aggrading-to-transgressive shoreface/barrier unit with associated tidal-inlet/estuarine channel-fill deposits, and (3) an uppermost, <20 m thick regressive deltaic unit similar to (1). The slope successions of the units described in (1) and (3), beyond and below the shelf-edge, contain thin upper-slope tempestite sheet sandstones, within an otherwise shale-dominated environment. Neither sandy slope channels nor basin-floor fans are observed within the otherwise shale-prone deepwater segments of the clinoform.