Abstract Determining how autogenic and allogenic processes and responses in deltas scale up from meter-scale laboratory experiments to actual field examples remains a challenge. This study was devised to bridge that scale gap using field data from small, hundreds of meter-scale natural deltas. Ground-penetrating radar and core data were collected from four different river-dominated delta morphotypes developing at the margins of freshwater coastal lagoons in southern Brazil. Since the sediment supplying these deltas is sourced from a nearby dune field and is similar between the deltas, it is hypothesized that major morphological differences in the four deltas are primarily the result of differences in sediment discharge rates (sediment-water ratio). As observed in published tank experiments, channel cross-section and distributary channel patterns in the deltas vary as a function of sediment discharge, from shallow sheet-like flow at high discharge to well-established, stable distributary channels (i.e., birdsfoot pattern). A contributing factor may be the development of vegetation on the slower growing deltas influencing sediment cohesion, a key control in laboratory-scale deltas. As in many tank experiments, these lagoon deltas are steep and sandy, with the Froude number modulated to just below Froude critical flow (i.e., they are Froude-scaled). Ground-penetrating radar sections were processed, interpreted, and integrated with cores, allowing the definition of radar units. Analysis of the radar units demonstrates the presence of both allogenic and autogenic signals. Allogenic control is identified in the stacking of clinoforms and is perceptible in both sides of a single delta (delta 4), as well as in two other deltas (deltas 1 and 2). An autogenic signal varies according to delta planform shape and was identified by the stacking of lobe elements, both in dip and strike. Base-level change (lake level) and autogenic avulsion cycles occur on similar timescales, and therefore it is a significant challenge to separate these different processes in the stratigraphy. The potential uses of these types of data include understanding the link between delta dynamics, channel patterns, and stratigraphy to develop improved genetic models of steep sandy deltas common in the stratigraphic record.

Abstract The Holocene barrier along the southern Brazilian coast is most likely the most continuous barrier structure in the world. This coast is characterized by a Holocene barrier extending for 760 km, interrupted by only six discontinuities, which are associated with narrow lagoonal inlets and river mouths. This coast is marked by a huge availability of sand, a gently sloping continental shelf, and incident waves of moderate to high energy. Together, these factors constitute ideal conditions for the formation of coastal barriers. During periods of low sea-levels, the continental shelf in front of the La Plata estuary was fed by immature fluvial sands delivered by the Paraná River, which were transported to the southern Brazilian coast by a predominantly northward littoral drift. Based on forecasts of sea-level rise, it will take decades or centuries for change to occur in the progradation of the regressive barriers along the coastal embayments. During the last centuries and millennia (Middle to Late Holocene), the evolution of this coast has been controlled by the variations in wave energy along the coast.