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 Cadiz shelf. They constitute a composite seismic unit attributed to the transgressive systems tract (TST) of the last depositional sequence. The ea rliest transgressive unit was deposited on the outer shelf, and represents the distal facies of a coastal deposit. A process of shelf partitioning see ms to have occurred during the formation of the three later transgressive p arasequences. 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 Donana National Park was a low-gradient shelf, whe re large barrier island and lagoon systems were formed. The sparse occurren ce of marine deposits within the TST is a consequence of the episodic natur e of the sea-level rise. The generation and preservation of these transgressive deposits results fro m the interaction of the following controlling processes: (1) Relative sea- level rise: The formation of coastal transgressive deposits is related to i ntervals of reduced sea-level rise or stillstands within a period of contin uous sea-level rise. Those sea-level changes were probably driven by short- term periods of colder climate during the Late Pleistocene-Holocene deglaci ation. The Younger Dryas interval is the most widely recognised climatic ev ent of this type, but probably other events of similar characteristics had the same effect on the glacioeustatic sea-level rise. (2) Sediment supply c hanges: Short-lived climatic events probably also involved changes in the s ediment supply to the continental shelf due to changes in erosion rates and river regimes of the river basin hinterlands. (3) Effect of the paleophysi ography/paleogeography: The sedimentary environments' differentiation proce sses are associated to the formation of an exposed coastal promontory offsh ore from the Guadiana River mouth and a semi-protected embayment offshore f rom Donana National Park. This coastal configuration was controlled by prev ious tectonic evolution and influenced the non-uniform landward migration o f the coastline. (C) 2001 Elsevier Science B.V. All rights reserved.