Formation of Danube delta beach ridge plains and signatures in morphology

Danube delta, as most of the large deltas, favours complex interactions between river sediment supply and marine dispersing forces which create complex configurations with concurrent morphologies (i.e. river-versus wave-influenced) or processes (accretion versus erosion) and even distinct landscapes for the same accretionary features: strandplains with low monotonous beach ridges or covered by transgressive dunefields. Beach ridge plains (BRPs) are a common feature of the wave-influenced deltaic lobes, that are created by juxtaposition of succesive berms on the prograding sandy coasts with different shapes and sizes depending on the accretionary mechanisms and the accomodation space. This study reports the formation and dynamics of Danube delta BRPs based on numerical age determination (OSL and AMS) of the paleoshorelines, topographic and geophysical (GPR) surveys and stratigraphic records by cores. A chronological framework is established for all deltaic BRPs which points to the importance of formation timespan and growth rate (as a consequence of sediment supply) on the resultant morphology varying inter-site from small BRP (formation time <300 years) to large quasi-equilateral triangle-shaped BRP (1400-3000 years) and intra-site from low ridgesets with subparallel beach ridges (progradation rates of 3.5-12 m/yr) to high ridgesets where the original configuration was replaced by massive parabolic dunes or transgressive dunefields (progradation rates <= 3.5 m/yr). A six-type morphogenetic classification of BRPs is discussed in respect with the various spatial patterns of progradation - from the large strandplains developed on the updrift side of asymmetric lobes to the small ones accommodating the prograding sectors of the river mouths spits - and with the progradation types encountered: i) beach face accretion during fair-weather, ii) nearshore bar welding, and iii) berm crest building during storms. Although the last is building ca. 1.5% from deltaic BRPs, this is the only case when the visible beach ridges are made by wave processes. For the other two, the aspect of beach ridges is the result of aeolian processes that create either unitary foredune or thin eolian sheets (incipient foredune) tens of centimeters higher than adjacent berms. (C) 2016 Elsevier Ltd and INQUA. All rights reserved.