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Variation in Stacking Style of Delta-Estuary Couplets and Associated Deep-Marine Fans: An Example From the Eocene Central Basin of Spitsbergen


The Eocene of the Central Basin of Spitsbergen shows a series of eastward building clinothems deposited in a foreland basin. This basin was formed by a westerly active fold and thrust-belt which also acted as provenance area for these shallow-marine sand-wedges. Some of these shallow-marine wedges prograded onto the shelf, whereas some of them reached the shelf-edge and have associated deep-marine sand-lobes. Three of these clinothems have been studied with focus on depositional environment, lateral facies variations, internal stacking pattern and shoreline trajectory pattern. All of them show a regressive deltaic to transgressive estuary/tidal couplet. Internally, there are clear differences between the three clinothems in terms of the style of the regressive deltaic part and the transgressive estuary part. The deltaic parts range from a) fluvial and punctuated mass-flow style; b) wave reworked and delta front collapse style; and c) mixed tide and fluvial influenced delta. The transgressive parts of the clinothems show a variation of the thickness of estuary sandstones and coastal plain fines developments conditioned on the degree of aggradation. Previous studies of these Eocene clinothems have interpreted the associated deep-marine sand-lobes as due to: a) sea-level fall with shelf-incision and basinward movement of the deltaic system beyond the shelf-break; b) high sediment-supply mechanism as hyperpycnal flow within shelf-edge deltas feeding the basin-fans during sustained flow; and c) having a narrow shelf that easily gets prograded across with high sediment supply. On individual basis each of these clinothems can be interpreted with these mechanisms above. However, it is interesting to see how the shape and size of each clinothem has a direct effect on the next clinothem that occurs above. As a clinothem consist of a dominant muddy part, the mud-volume can be stored: at the shelf-edge and expand the width of the shelf, on the shelf and building up the shelf height or even be stored more landward within the lagoonal and coastal areas, starving the shelf. This study show how a volumetrically-limited clinothem enables the next clinothem above, to easily cross the shelf and feed sediments down the shelf slope from a fluvial delta. The two following clinothem faced a wider shelf that first gave a wave-dominated delta and finally a mixed tidal and fluvial delta capped by an estuary.