Morphodynamics and seismostratigraphy of a deep hole at tidal channel confluence

Abstract

The sedimentary circulation pattern at a deep hole at a tidal channel junction in an estuary environment is examined using morphosedimentological and seismostratigraphic data. The bottom and sub-bottom acoustic information (side scan sonar and 3.5 kHz pro fi ler) obtained at a tidal channel con fl uence in Bahía Blanca estuary (Argentina) evidences the depositional and erosive features, bed load sediment transport pathways and morphological evolution of the depression. This important depression in the con fl uence zone of the channels is characterised by the presence of steep scour faces (14°) at the mouth of both channels and gentle slopes (1.5°) opposite to them. Bed load sediment transport analysis reveals a differential circulation pattern of sand on both sides of the hole, which is associated with the different sediment availability in each channel. The side scan sonar information therefore indicates that there is no available sand in one of the channels whereas the other channel is characterised by the presence of a large amount of sediment coming from its inner area. Deposition therefore occurs on the side where sediment is mobilised as bed load by the ebb current dominance and on the face developed downstream. In the latter, a part from the sediment is de fl ected inward by the tidal fl ood currents which produce the deposition of sediment on the gentle side. The rest of the sediment is exported from the system by ebb currents. In contrast, erosion occurs on the opposite side of the hole and at the mouth of each channel and it is manifested by the outcrop of old sedimentary strata. The seismostratigraphic information collected in this study indicates that the depression was originally a fully erosive morphological feature. A descriptive model of the morphological evolution at a channel con fl uence is presented in this study.