Miocene and Pliocene palynology of the Belverde borehole, lower Tagus Basin, Portugal

A palynological analysis ofthe Neogene ofthe Belverde borehole (38'35'54"N; 9"8'24W),drilled in the distal sector of the lower Tagus basin, is presented. It crossed 130m of plio-pleistocene continental deposits and 460m of marine Miocene levels. planktonic foraminifera and 87Sr/86Sr isotopic ages were used to establish a chronostratigraphic framework. In some of the Miocene samples, the spores/pollen are commonest than dinofl agellates; in others, dinoflagellates are particularly frequent. Acritarchs (Cyclopsiella granosa, Quadrina sp.) and peridinioi ds (Selenopemphix nephroides, S. brevispinosa, Lejeunecysta sp.) are represented. During the Burdigalian, Polysphaeridium zoharyi, CIistosphaeridium pIacacanthum and Cribroperidinium tenuitabulatum occur. In the Langhian, C. tenuitabulatum is abundant; P. zoharyi and Operculodinium israelianum are common. During the Serravallian, Spiniferites sp ., Spiniferites I Achomosphaera, Hystrichosphaeropsis obscura, L. machaerophorum and O. i sraelianum are frequent. Spiniferites I Achomosphaera, S. pseudofurcatus, L. machaerophorum and Homotryblium spp. are common during the Tortonian. In the Miocene, dinoflagellates indicate littoral environments during the Burdigalian and the Tortonian. The peridinioids and acritarchs from the Tortonian suggest a shallow marine brackish environment. During the Pliocene, the pollen spectra show the dominance of Pinus, Myrica and Quercus, Ericaceae and Oleaceae occur. The herbs are dominated by Amaranthaceae-Chenopodiaceae, poaceae; Asteraceae are scarce. The presence of mesothermic forms (Quercus, Alnus, Liquidambar), mega-mesothermic elements (Myrica) and scarce Engelhardia suggest a mixed forest developed under relative humid conditions. The increase of Cistaceae, Amaranthaceae-Chenopodiaceae and Poaceaea and the decrease of Myrica and Engelhardia to the top of the borehole, points out to a transition to a prairie habitat.