Morphodynamic behavior and seismostratigraphy of a sandbank: Bahía Blanca estuary, Argentina

Abstract

Sediment dynamics in Bahía Blanca estuary (Argentina) permit the formation of longitudinal banks, one of which is La Lista Bank, located at the mouth of the La Lista tidal channel. The purposes of the present research were to study the processes and mechanisms involved in the formation and maintenance of this bank, and to identify seismic sequences which could be indicative of its origin and current morpho-evolutionary trend. To this end, different methods and techniques, such as side-scan sonar, 3.5 kHz profiler and bottom sediment sampling were followed. The analysis of seismic-acoustic records revealed bottom features (sand dunes, sand ribbons, current marks, escarpment) which, based on our analysis of current measurements and our estimations of transport as bedload, have contributed to tracing a sediment circulation pattern in the study area. Taken together, these findings allowed us to identify the areas dominated by erosion, sediment transport or depositional processes. Bedload material was found to be exported to the outer sector and held on the flanks of the bank, particularly in its southern part. Inference on the morpho-evolutionary trend in the study area was based on evidence related to migration patterns and changes observed on the flanks and inner structure of the bank. The morphological evolution of La Lista Bank is associated to a residual sediment transport pattern with trajectories opposite on its two flanks, which results from the action of ebb tides in the northern flank and flood tides on the southern flank. This process induces a gradual increase in the bank height and longitudinal growth. The La Lista Bank evolution agrees with the motion sequence and development pattern proposed by Caston (1972) for a linear bank, except that this model is repeated as the bank grows. The seismostratigraphic data presented here indicates that the origin of this bank is associated with the Holocene marine regression process.