Análisis preliminar de los ostrácodos del Mioceno Inferior de las Bardenas Reales de Navarra (Cuenca del Ebro)

Abstract

The outcrops of the Tudela Formation, Lower Miocene (Ramblian), in the Bardenas Reales of Navarra (western area of the Ebro Basin) are very rich on vertebrate remains (fishes, amphibians, reptiles, birds and mammals), gastropods and microfauna of ostracodes and charas. Lithologies are composed by silts, marls and limestones, with sporadic levels of sandstones, deposited in a distal alluvial and lacustrine environment. Ostracode assemblages that have been studied from nine sections of this formation, show a dominance of the genera Nannocandona, Heterocypris, llyocypris and limnocytherids, which are characteristics of a shallow lacustrine environment, with fresh to oligohalyne and vegetated waters. The analysis of the assemblages, salinity and other ecologic ranges of these ostracode faunas allow to differentiate two phases in the lake evolution: the first phase (biozone «Z», Daams and Freudenthal, 1 989) with a fresh water, carbonated and warm climate lake, and a second phase (biozone «A», Daams and Freudenthal, 1 989) with bicarbonated, alcaline waters in a climate context where seasonality was more evident. The transition between both phases was marked by several euryhaline intervals, indicators of the environmental instability that affected those microfaunas during the water chemistry transition