HETTANGIAN AND SINEMURIAN MAGNETOSTRATIGRAPHY FROM PARIS BASIN

A long core (1110 m) drilled at Montcornet (northeastern Paris Basin) provides early Jurassic magnetostratigraphic data coupled with biochro nological control. About 600 paleomagnetic samples were obtained from a 148-m-thick series of Hettangian and Sinemurian rocks. A composite d emagnetization using thermal (up to 300 degrees C) followed by alterna ting field technique (up to 100 mT) is used to separate the magnetic c omponents. A low unblocking temperature component (<250 degrees C) wit h an inclination of about 65 degrees is interpreted as a present-day f ield overprint. The characteristic remanent component with both normal and reversed antipodal directions is then isolated between 5 and 70 m T. Eighty-one polarity intervals are recognized in this study. The hig her reversal frequency of the late Hettangian/early Sinemurian time in terval contrasts with a lower reversal rate observed in the rest of th e early Liassic. A rough mean estimate of about 5 reversals/m.y. can b e proposed for the earliest Jurassic. These results represent a signif icant contribution to the magnetic polarity reversal timescale for a t ime interval hitherto poorly known and add to the magnetic reversal fr equency curve of the last 350 m.y.