Abstract

This paper examines the differences between tide-dominated and tide-influenced deltas, as well as tide-dominated deltas and tide-dominated estuaries. The deltaic deposits of the Middle Devonian Kernave and Aruküla formations were documented in cores and outcrops in the Baltic Basin and interpreted as tide-dominated delta deposits. These tide-dominated deposits consist of three vertically stacked progradational to aggradational packages, 20–40 m thick. Each package consists of two stratigraphic intervals. The lower upward-coarsening interval contains seaward-accreting prodelta to distal tidal-bar and proximal tidal-bar deposits. The upper upward-fining interval consists of tidal-flat deposits and minor tidal gully, distributary-channel, supratidal muds, and paleosol deposits. The overall character of these delta deposits indicates a subaqueous delta with no river-dominated delta-plain. Comparison of these successions with modern and ancient tide-dominated and tide-influenced deltas suggests that tide-dominated deltaic deposits tend to form in conditions of relative sea-level rise succeeding transgressions, when tidal currents are strong enough to redeposit most river-derived sediments. Tide-dominated deltas form subaqueous deltas, where the bulk of the deposits are tidally reworked. In contrast, tide-influenced deltas contain tidal indicators in delta-front and lower-delta-plain deposits, whereas the upper delta plain is river-dominated. Our data suggest that tide-dominated deltas may change into tide-influenced deltas during delta evolution when they prograde to the mouth of the restricted or funnel-shaped bay, given the rate of fluvial sediment supply exceeds the rate of accommodation increase.

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