Paleomagnetic and geochronological study of a geomagnetic field reversal or excursion recorded in pliocene volcanic rocks from Georgia (Lesser Caucasus)
A 250-m thick volcanic sequence of some 63 Pliocene lava flows from Georgia (Akhalkalaki plateau) which was previously thought to record the Gauss-Matuyama reversal was re-studied in detail. About 400 oriented cores were sampled along four sub-vertical sections partially overlapping each other. At the base of the section, 16 consecutive lava flows were found to record intermediate directions. The overlying flows are all reversely magnetized. With the exception of the upper part of the sequence, four groups of three to 11 consecutive lava flows with strictly the same paleodirection can be recognized. The location of the intermediate virtual geomagnetic poles (VGPs) does not correspond to that of the VGP clusters previously proposed as indicating a long lasting near dipole configuration during reversals. The field paleointensity is close to one fifth of the present field at the bottom of the section and tends to increase uphill. Age determination ( 40Ar /39Ar ) provides an estimate of 3.6 ± 0.06 Ma, which leaves us with two possible interpretations: the transitional vectors could correspond either to an excursion within chron 2Ar or to the upper (N-R) Cochiti-Gilbert reversal.