Morphosedimentological features of the sea floor in the outer sector of the Bahia Blanca estuary, Argentina

The purpose of this paper is to analyze the morphology of the Quaternary relict features and bedforms that characterize the outer zone of an extensive mesomareal system, which is part of the Bahía Blanca estuary (Argentina), and to estimate the prevailing hydro-sedimentological conditions on the continental shelf adjacent to the estuary. Surveys with a multibeam bathymetric system, side-scan sonar and a high-resolution seismic system (3.5 kHz) were carried out and bottom sediment samples were collected. In general, the seafloor presents a varied configuration, marked by changes in its physiography. The Quaternary relict features are composed of two terraces (12–14 m and 18–22 m deep), with steeply sloping escarpments, that are oriented approximately parallel to the current coastline. Associated with these terraces, wide abrasion platforms develop with irregular and plateau-like structures up to 4 m high, together with large rocky blocks up to 6 m in diameter. Here, the predominant sediment corresponds to a gravel-sized material composed of quartzite boulders, siltstone fragments of up to 7 cm, and biogenic remains with different degrees of fragmentation. These sedimentological data, together with the bathymetric and seismostratigraphic information, indicate that the terraces and abrasion platforms are ancient shorelines that originated in an environment with high wave energy, corresponding to periods of stabilization of sea level rise during the final stage of the last postglacial transgression (late Pleistocene-Early Holocene). On the other hand, asymmetric dunes of different sizes (0.5 m < height < 5 m, 5 m < wavelength < 200 m), sand ribbons (length > 300 m) and comet marks originated in equilibrium with the present-day hydro-sedimentological conditions. The availability of sandy material mobilized as bedload is the determining factor for the generation of the varied bedforms. Thus, sand ribbons and comet marks are generated especially in the northern sector of the area, with mobile sediment thicknesses ≤ 0.5 m. With larger thicknesses the formation of dunes occurs. Their size and asymmetry show an intense transport of sand as bedload towards the continental shelf, the southern sector of the area being the main route of the sediment that leaves the estuary. This sediment is the supply source of the sandy shoals that are part of the great ebb delta developed on the continental shelf in the external area of the Bahía Blanca estuary.