Abstract

The Neogene succession of the Lepe-Ayamonte sector (western Guadalquivir Basin) shows a substantial difference with the classic lithostratigraphic succession described towards the east of the Odiel River. The older unit in the first is constituted by white silts interpreted as marginal shallow marine environments of Tortonian age. Overlying this unit, a sandier interval occurs in which three units can be distinguished (from bottom to top): glauconite bed, medium to coarse sandy deposits with carbonate nodules and sands with malacofauna. This interval records a condensed section that is considered a Late Tortonian regional datum although at some points of the basin its age is still troublesome. The Late Pliocene units are constituted by silty sands, sands and coarse siliciclastic sediments, with abundant ichnofossils, plants and invertebrate fossils. These facies have been dated indirectly in the Piazencian and interpreted as a coastal restricted environment. An erosional boundary has been described between the Miocene and Pliocene succession in the top of the malacofauna-rich sandy unit. This last deposit records both Miocene mollusk fossils and Late Zanclean nannoplankton assemblages which indicates a significant reworking process of the Miocene units during the Pliocene. Hence, this sector of the basin bears a hiatus including the Messinian and Zanclean that has not been observed in other areas of Huelva province. In the studied outcrop (La Antilla Section) the malacofauna-rich sandy unit is not observed and hummocky cross-stratified (HCS) sand layers are described between the Miocene and Pliocene successions. The HCS deposits show a few hundred meters of lateral continuity and are constituted by a bed set of 190 cm thick that wedge out towards the east and west. They are formed by the stacking of successive HCS beds with first-order irregular scoured base developed in the Miocene units, both white silts and glauconitic sands. Each bed is graded and composed by fine sands and muddy sands with characteristic low hummocks and swales structures. These facies were quickly covered by white clays and red sands of fluvial-estuarine origin and Late Pliocene age. The HCS is formed by large storm waves, such as hurricanes and even though, it is mainly described in shallow marine environments, it can also be formed inland when large amounts of water are pushed up onto the coast by meteorological events. Therefore, the study area was configured as an open estuary exposed to high-energy events recorded by the HCS. The age of this process is difficult to establish but the vertical sedimentary continuity and genetic relationship that HCS deposits show with the overlying deposits allow us to infer a Late Pliocene age for this event.