Miocene lithological, foraminiferal and palynological data from the Belverde borehole (Portugal)

Abstract

Preliminary data on lithostratigraphy and biostratigraphy of the Belverde borehole (http://www.dct.fct.unl.pt/Sondagem/Sondagem/coluna/index_col.html) is presented. The borehole attained 619,77 m of depth, crossing 130 m of Plio-Pleistocene and 460 m of Miocene deposits. Main lithologies are fine sands, marls and biocalcarenites. A coral reef was recognized in the lower part of the borehole (530-585 m). Gamma ray, neutrons and sonic diagraphies were obtained. Eight 87Sr/86Sr isotopic ages (H. Elderfield, Cambridge University) and the presence of some of the main planktonic foraminifera markers were used to establish the chronostratigraphic framework. Planktonic spikes are related to the major transgressions of the Lower Tagus Basin (LTB): M. Burdigalian (sample Belv 459; 446 m), L. Serravallian (Belv 76; 244 m) and L. Tortonian (Belv 51, 202 m). Benthic assemblages are dominated by Ammonia, Elphidium and Nonion. Spores suggest a mild and humid climate during the uppermost Burdigalian to the Tortonian. Pollen is scarce. Thirty dinoflagellates taxa (26 Gonyaulacoids and 4 Peridinioids) have been identified, indicating coastal to shallow marine environments. In the uppermost Burdigalian to the Tortonian the presence of peridinioids and scarce acritarchs suggest influence of inshore to brackish environments. Several 87Sr /86Sr ages obtained by H. Elderfield (Cambridge University) fit well with the biostratigraphy for the Lower and Middle Miocene, but values given by Howarth and Mcarthur (1997) are closer to the Upper Miocene biostratigraphic data.