High-resolution seismic stratigraphy of a narrow, bedrock-controlled estuary: The Guadiana estuarine system, SW Iberia

The late Quaternary sedimentary architecture of the Guadiana estuary (southwestern Iberian Peninsula), a narrow, bedrock-controlled estuary with moderate sediment supply, was studied by applying concepts of high-resolution seismic stratigraphy. The estuarine sedimentary infill consists of a discontinuous basal interval overlain by five seismic units bounded by laterally continuous seismic horizons. The correlation between seismic facies and a stratigraphic section of the Guadiana valley enables the proposal of a detailed sequence stratigraphic interpretation of the estuarine infill. During the last glacial lowstand, the Guadiana was a subaerial valley with no significant accumulation of fluvial deposits because of increased sediment bypass towards the present-day middle and outer shelf. Towards the end of the postglacial transgression, the subaerial valley was transformed into an estuary, and sediments began preferentially accumulating in structural depressions. Furthermore, flood tidal currents constrained by basement highs enhanced sand deposition in the upper part of the estuary. Wave influence was reduced and confined to the lower estuarine system. Here, the narrow morphology of the valley led to an increased sediment export to the shelf during the Holocene highstand period. The lower part of the estuarine infill consists of four fifth-order depositional sequences, composed of regressive deposits (HST). The last glacial maximum is recorded by a distinct stratigraphic surface, representing simultaneously the sequence boundary and the transgressive surface. A tidal ravinement surface is characterized by strong erosion and channel formation in the outer estuarine zones. The maximum flooding surface is identified by change of stratal patterns between landward-prograding transgressive deposits and downlapping highstand deposits. Both transgressive (TST) and regressive tracts (HST) were deposited during the final part of the postglacial transgression and subsequent highstand.