Abstract
Upper-flow-regime bedforms and their role in the evolution of marine and lacustrine deltas are not well understood. Wave-like undulations on delta foresets are by far the most commonly reported bedforms on deltas and it will take time before many of these features get identified as upper-flow-regime bedforms. This study aims at: (1) Providing a summary of our knowledge to date on deltaic bedforms emplaced by sediment gravity flows; (2) illustrating that these features are most likely transitional upper-flow-regime bedforms; and (3) using field case studies of two markedly different deltas in order to examine their role in the evolution of deltas. The study combines numerical analysis with digital elevation models, outcrop, borehole, and high-resolution seismic data. The Mazzarrà river delta in the Gulf of Patti, Italy, is selected to show that upper-flow-regime bedforms in gullies can be linked to the onset, growth, and evolution of marine deltas via processes of gully initiation, filling, and maintenance. Ice-marginal lacustrine deltas in Germany are selected as they illustrate the importance of unconfined upper-flow-regime bedforms in the onset and evolution of distinct delta morphologies under different lake-level trends.
Highlights
Deltas can be classified based on a variety of criteria
The results suggest that it is unlikely for submarine antidunes and cyclic steps of the same magnitude to form in a single train of sediment waves
Upper-flow-regime bedforms emplaced by supercritical sediment gravity flows have been recognized as crucial building blocks of deep-sea fans and canyon-channel systems [23]
Summary
Deltas can be classified based on a variety of criteria. Other classifications of deltas emphasize foreset/topset geometry [4,5], sediment grain size [2], or delivery system type [6,7,8]. More recent studies demonstrate that external geometry and internal characteristics of deltas greatly vary in response to the falling and rising of sea level [9,10,11,12,13]. This paper focuses on upper-flow-regime bedforms observed on diverse deltas in marine and lacustrine settings at water depths of up to about 150–200 m. Parts of deltas that extend beyond water depths of 200 m are not investigated
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