Mixed-energy shallow-marine systems with emphasis on tidal influence
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This research investigates mixed-energy shallow marine depositional systems (i.e., subject to the influence of river, wave and tidal currents), with particular emphasis on the role of tidal currents in controlling the final stratigraphic product. In modern coastal areas and in the rock record, many sedimentary systems bear the signature of changing and overlapping coastal processes. Understanding the evolution of the mixed tidal systems and their stratigraphic expression is fundamental both to the science of dynamic stratigraphy and for a proper exploitation of the stored natural resources. The research was carried out using four datasets: an outcrop dataset of measured sedimentological sections from the Jurassic Lajas Formation, Argentina; a dataset of previously published literature of process variability and sedimentary structures; a dataset of numerical simulations produced with Delft3D software; and an outcrop dataset of measured sedimentological sections from the Pleistocene Siderno Strait, Italy. The data here presented highlight the great degree of process variability in the rock record, and the importance of tidal currents in controlling deltaic morphology and stratigraphic architecture. The strata of the Lajas Formation show a clear process partitioning in different reaches of the deltaic system (proximal vs. distal and regressive vs. transgressive). In particular, tidal currents strongly reworked the delta front at times, creating sand-rich and amalgamated sandbodies. This project demonstrates that disentangling the signals of river, wave and tidal currents in the stratigraphy leads to a better interpretation of ancient mixed-energy systems. The study of the literature database shows that some sedimentary structures can be considered reliable indicators of a particular process (river, waves or tides), whereas other structures cannot be tied with confidence to any particular process. A process probability value is calculated for each sedimentary structure, and thus quantifies process variability and its uncertainty. This work encourages a new, more detailed and more quantified field methodology for facies sedimentology. The numerical modeling of tide-influenced deltas using Delft3D shows how different degrees of tidal influence in river-dominated deltas affect delta morphology and stratigraphy, when tidal currents are flowing perpendicularly to the shoreline. Increasing tidal influence induces deeper and more stable distributary channels that act as efficient conduits for sediment transport basinward. The delta-front geometry is also affected by tidal current reworking, evolving into a compound clinoform geometry. The research on the Siderno Strait, in contrast, highlights tidal influence on deltaic stratigraphic evolution when tidal currents flow parallel to the coastline. River-dominated deltas entering the tide-dominated strait tend to show a deflection of the delta-front sands in a direction parallel to the dominant tidal current. The delta-front sands became reworked by tidal currents into large dune fields within the strait.