40Ar/39Ar dating of a tephra layer in the Pliocene Seneze maar lacustrine sequence (French Massif Central):: constraint on the age of the Reunion-Matuyama transition and implications on paleoenvironmental archives

The sedimentary sequence cored in the Seneze maar (Velay, France) is very favourable for magnetostratigraphy of the Late Pliocene and for studying the paleoenvironmental changes that occurred during that period. The magnetostratigraphic studies revealed that the sediments from this lacustrine sequence were deposited during the Matuyama reverse period, except for the upper part where a short normal polarity episode was recorded. A tephra layer interbedded in the sediments contemporaneous with this normal polarity event is dated at 2.10 +/- 0.01 Ma by the 40Ar/39Ar method. Using this chronological marker, it can be inferred that the normal polarity episode recorded in the Seneze sequence corresponds to the Reunion event and that the age of the normal-reverse transition closing this event is ca. 2.09 Ma. These chronological constraints also permit to (1) demonstrate that the 'Villafranchian' mammal fauna found in the Seneze maar is younger than 2.09 Ma and (2) correlate the climatic events recorded in the Seneze sequence to the marine delta O-18 records. This suggests that the Seneze pollen sequence (5-120 m depth) ranges from isotopic stage 85 to 76. (C) 2000 Elsevier Science B.V. All rights reserved.