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Bayhead Deltas and Shorelines: Insights From Modern and Ancient Examples


Bayhead deltas are important stratigraphic markers in the ancient record, house populous ports and communities along modern coasts, and are host to hydrocarbon reservoirs within many subsurface incised-valley successions. They are a critical component of wave-dominated estuarine systems and are sensitive to both allogenic and autogenic forces. However, little is known about how they differ from their open-ocean equivalents or their natural variability. We review their occurrence, deposits, and past responses to allogenic and autogenic processes. Based on this review and new observations from modern and ancient examples, we suggest a general classification scheme for bayhead deltas and similar estuarine depositional systems. We distinguish transgressive bayhead shorelines and bayhead deltas, which are both located at the upper reaches of estuaries and bays at the confluence of the fluvial and estuarine realm. Bayhead deltas are actively building depositional systems and leave a clear record of sedimentary deposits, while transgressive shorelines have a “destructive” morphology and leave only thin deposits behind within the sedimentary record. We subdivide bayhead deltas further into confined and unconfined systems. Confined bayhead deltas are highly influenced by their antecedent topography – usually an incised valley, structurally-controlled basin, or fjord. Unconfined bayhead deltas are located in backbarrier environments or as part of larger deltaic systems. Confined bayhead deltas are subject to autogenic processes created by interactions with the confining valley or structurally controlled walls. These processes include the flooding of relict topography and the stabilization or auto-acceleration at tributary junctions during transgression or regression, respectively. Both unconfined and confined bayhead deltas are sensitive to climate, sea-level, and tectonic forcings and thus provide excellent records of changes in these past influences. In addition to providing a classification scheme for bayhead deltas and transgressing shorelines, we review their deposits and occurrence both in time and space. In general, they are thinner than their more open-ocean equivalents, except for those within fjords. Confined bayhead deltas are most commonly formed during the transgression, while unconfined bayhead deltas are also common during regression.