The sedimentary record of the post-glacial transgression on the Gulf of Cadiz continental shelf (Southwest Spain)

Four seismic units (T A to T D) have been identified from the analysis and interpretation of high-resolution seismic profiles in a sector of the Gulf of Cádiz shelf. They constitute a composite seismic unit attributed to the transgressive systems tract (TST) of the last depositional sequence. The earliest transgressive unit was deposited on the outer shelf, and represents the distal facies of a coastal deposit. A process of shelf partitioning seems to have occurred during the formation of the three later transgressive parasequences. The shelf sector offshore from the Guadiana River mouth was a high-energy environment dominated by storm events. By contrast, the shelf sector offshore from the Doñana National Park was a low-gradient shelf, where large barrier island and lagoon systems were formed. The sparse occurrence of marine deposits within the TST is a consequence of the episodic nature of the sea-level rise. The generation and preservation of these transgressive deposits results from the interaction of the following controlling processes: (1) Relative sea-level rise: The formation of coastal transgressive deposits is related to intervals of reduced sea-level rise or stillstands within a period of continuous sea-level rise. Those sea-level changes were probably driven by short-term periods of colder climate during the Late Pleistocene–Holocene deglaciation. The Younger Dryas interval is the most widely recognised climatic event of this type, but probably other events of similar characteristics had the same effect on the glacio-eustatic sea-level rise. (2) Sediment supply changes: Short-lived climatic events probably also involved changes in the sediment supply to the continental shelf due to changes in erosion rates and river regimes of the river basin hinterlands. (3) Effect of the paleophysiography/paleogeography: The sedimentary environments' differentiation processes are associated to the formation of an exposed coastal promontory offshore from the Guadiana River mouth and a semi-protected embayment offshore from Doñana National Park. This coastal configuration was controlled by previous tectonic evolution and influenced the non-uniform landward migration of the coastline.