Spatial variability of prodeltaic undulations on the Guadalfeo River prodelta: support to the genetic interpretation as hyperpycnal flow deposits

Two fields of prodeltaic undulations located off the Guadalfeo River were studied by integrating surficial (multibeam bathymetry, backscatter, sediment samples) and sub-surface (seismic profiles, sediment cores) data. Our main motivation was to analyze the along- and across-shelf variability of the seafloor undulations, in order to obtain useful insights into genetic mechanisms. A geostatistical analysis was performed, based on the determination of characteristic parameters and derived relationships. The undulations occur over a concave-upward surface which shows a seaward-decreasing slope. Most of the undulations are symmetrical to asymmetrically-oriented toward the coast. Two main fields are correlated with the present and previous river mouths. The western field, associated with the modern river mouth, is highly symmetrical, with the higher undulations in an axial position and diminishing the width/height relationship both laterally and downslope. In contrast, the eastern field, associated with an historic river mouth, shows lower-amplitude undulations, the width/height changes are less pronounced, and the undulations are more elongated. The two undulation fields exhibit subseafloor reflections that are subparallel to the seafloor, with peaks that migrate upslope upward in the stratigraphic column and which appear to correlate with coarse-grained layers. We support the contention that prodeltaic undulations off the Guadalfeo River should be regarded as sediment waves. Assuming a sediment-wave process, a strong normal-to-contour sediment flows with a riverine origin (e.g., hyperpycnal flows) may have been active during undulation generation. Both morphometric parameters of the river basin and estimations of sediment concentration during exceptional flood events are in agreement with an episodic activity of high freshwater discharges. Most of the geomorphic parameters and stratigraphic observations indicate a change of sediment supply conditions related to the shift in river mouth position, attributed to a temporal change in the activity of hyperpycnal flows.