Stacked fluvial and tide‐dominated estuarine deposits in high‐frequency (fourth‐order) sequences of the Eocene Central Basin, Spitsbergen

Abstract

AbstractEighteen coastal‐plain depositional sequences that can be correlated to shallow‐ to deep‐water clinoforms in the Eocene Central Basin of Spitsbergen were studied in 1 × 15 km scale mountainside exposures. The overall mud‐prone (>300 m thick) coastal‐plain succession is divided by prominent fluvial erosion surfaces into vertically stacked depositional sequences, 7–44 m thick. The erosion surfaces are overlain by fluvial conglomerates and coarse‐grained sandstones. The fluvial deposits show tidal influence at their seaward ends. The fluvial deposits pass upwards into macrotidal tide‐dominated estuarine deposits, with coarse‐grained river‐dominated facies followed further seawards by high‐ and low‐sinuosity tidal channels, upper‐flow‐regime tidal flats, and tidal sand bar facies associations. Laterally, marginal sandy to muddy tidal flat and marsh deposits occur. The fluvial/estuarine sequences are interpreted as having accumulated as a series of incised valley fills because: (i) the basal fluvial erosion surfaces, with at least 16 m of local erosional relief, are regional incisions; (ii) the basal fluvial deposits exhibit a significant basinward facies shift; (iii) the regional erosion surfaces can be correlated with rooted horizons in the interfluve areas; and (iv) the estuarine deposits onlap the valley walls in a landward direction. The coastal‐plain deposits represent the topset to clinoforms that formed during progradational infilling of the Eocene Central Basin. Despite large‐scale progradation, the sequences are volumetrically dominated by lowstand fluvial deposits and especially by transgressive estuarine deposits. The transgressive deposits are overlain by highstand units in only about 30% of the sequences. The depositional system remained an estuary even during highstand conditions, as evidenced by the continued bedload convergence in the inner‐estuarine tidal channels.