Mineralogical assemblages, geochemistry and fossil associations of Pleistocene-Holocene complex siliciclastic deposits from the Southwestern Donana National Park (SW Spain): A palaeoenvironmental approach

摘要

On the basis of mineralogy, chemical and palaeontological data collected in a long core (93 m) three associations of facies are distinguished in the Late Pleistocene-Late Holocene evolution of the sedimentary infilling in nowadays Donana National Park. From the lower, middle and upper associations of facies three sedimentological phases have been established. In the oldest phase (> 44 kyr BP), this area was occupied by supratidal, freshwater marshes located close to a brackish lagoon and inundated suddenly by brackish-marine inputs. The following phase (44-7 kyr BP) is characterized by the transition from supratidal to intertidal conditions, with the alternation of freshwater or brackish marshes situated very close or in the margin of the former lagoon and submitted to episodes of desiccation or tidal inundation. The last phase (7-3.0 cal kyr BP) comprised: a) a marine inundation during the maximum of the Flandrian transgression (～7-6.5 cal kyr BP); and b) the progressive infilling of a brackish lagoon with marine connection and permanent fresh water inputs (6.5-3.5 cal kyr BP), partially enclosed by the Donana spit. The phyllosilicate-rich, clayey-silty sequence of this latter infilling is interrupted by tsunami-like deposits at 4.2-3.6 cal kyr BP and 3.6-3.0 cal kyr BP. These high-energy events are characterized by textural (bimodal grain-size distribution, sorting decrease), mineralogical (crystallochemical changes in illites, increasing dolomite content) and geochemical changes (Ti anomaly), besides a sudden introduction of marine bivalves and microfauna and a strong decrease or the almost disappearance of the brackish ostracode assemblage. Ages of these deposits coincide with those indicated for other tsunamigenic layers detected in the southwestern Spanish coasts and some deep cores collected near the African-European plate boundary.