Palaeoenvironmental interpretation of dinosaur- and mammal-bearing continental Maastrichtian deposits, Haţeg basin, Romania

Abstract

The Haţeg basin, South Carpathians, Romania, contains a thick sequence of Maastrichtian continental deposits from which a rich dinosaur and mammal fauna is known. Field data as well as mineralogical and stable isotope analyses from three representative profiles were integrated in order to reconstruct environmental conditions during Maastrichtian time. Tuştea quarry is characterized by the presence of well drained calcisols, with smectite (montmorillonite) as the main clay component. Along the profile, the d18 O and d13 C isotopic compositions of calcretes show a small variation, of up to 0.9. The profile along the Bărbat Valley shows preponderantly calcisols, the main clay mineral being smectite, with subordinate illite and chlorite. The oxygen isotopic compositions of calcretes are ~0.5 lighter than those from Tuştea. The soils are interpreted as having formed under more humid conditions and they are similar to those situated at the bottom of the sequence developed along Sibişel Valley. The abundant smectite from the Tuştea and Bărbat Valley deposits, as well the presence of good developed soils, reflects palaeoenvironmental conditions predominantly controlled by climate. Preliminary magnetostratigraphic data along the Sibişel Valley section indicate that sedimentation started at the end of chron C32n. All other palaeomagnetic sites distributed upstream, as far as the upper limit of this formation, have only reversed polarity and the corresponding time interval is probably chron C31r. Along this valley, the sequence shows a general coarsening upward trend. The palaeosol type changes from calcisol- to vertisol-dominated sequences. The soils are moderate to weakly developed. The mineralogical composition of the clay fraction also changes, from smectite- to illite and chlorite-dominated. These features points towards unstable tectonic conditions and higher uplift rates of the surrounding area within chron C31r. Towards the top of the sequence, the oxygen and carbon isotopic composition of calcretes become 1 and ~2 lighter, respectively. These changes indicate a transition from generally semi-arid towards more humid and possible cooler conditions and correlate with the worldwide trend for chron 31r.