Sedimentological and paleoenvironmental characterisation of transgressive sediments on the Guadiana Shelf (Northern Gulf of Cadiz, SW Iberia)

Abstract

During the transgression following the Last Glacial Maximum, four backstepping parasequences were deposited on the northern Gulf of Cádiz shelf (SW Iberia). In contrast to other areas on the shelf, these transgressive deposits are at, or very close to, the surface to the southeast of the Guadiana Estuary mouth. This is particularly true for transgressive parasequence T C, possibly associated with the Younger Dryas event, and the youngest parasequence T D, probably linked to a slow-down in sea-level rise at around 8.2 ka BP, both of which form a sandy transgressive bulge on the upper middle shelf. The older and more distal parasequence T C is characterised on seismic records by convex low-angle sigmoidal clinoforms, and contains high amounts of quartz and bioclasts. In contrast, the younger and more proximal parasequence T D shows relatively steep concave prograding clinoforms, and contains high amounts of quartz and other terrigenous components, but comparatively low amounts of bioclasts. Furthermore, the difference in age between both parasequences is clearly marked by a difference in the content of glauconite. The results indicate that sediments found in parasequence T C are associated with deposition related to storm events, with frequent reworking of components, and the more proximal and younger sediments in T D are the result of rapid sediment accumulation related to floods in the Guadiana River basin, possibly during the transition from a dry cold period to a warmer more humid period, when vegetation cover was lowest and flood frequency increased. Additionally, accumulations of terrigenous components directly associated with the Guadiana River basin found on the outer shelf indicate that, at least during the beginning of the transgression, overflow channels active during large-scale flood events may have spilled material to an area located immediately to the south of the Guadiana Estuary mouth.