Magnetic polarity sequences from downhole measurements in ODP holes 998B and 1001A, leg 165, Caribbean Sea

The magnetostratigraphy of sediments drilled at two sites (998 and 1001) in the Caribbean Sea during the Ocean Drilling Program (ODP) Leg 165 is estab lished directly from downhole measurements. For the first time in the ODP, the magnetostratigraphy of sediments is determined from both logging magnet ic data and paleomagnetic analysis of core material for an age interval of about 16 m.y. across the Cretaceous/Tertiary boundary. The total magnetic i nduction and the susceptibility of the surrounding sediment were recorded i n situ and continuously by the GHMT (Geological High-sensitivity Magnetic T ool). The aim of processing these data is to determine the magnetic polarit y. At Site 998 (Cayman Rise), no paleomagnetic results are available from t he recovered cores given a hard secondary overprint. Nonetheless, a magneto stratigraphic sequence from early Miocene to late Oligocene is proposed fro m the analysis of the GHMT data. The comparison with biostratigraphic resul ts shows the accuracy of the resulting magnetic polarity. At Site 1001 (Hes s escarpment), cores were recovered in two holes thus allowing the comparis on of the magnetostratigraphic results obtained from the GHMT data and from paleomagnetic analysis of core samples over a length of 200 m representing about 16 m.y. from early Eocene to late Cretaceous times. The sedimentatio n rates computed from the determined magnetochrons are very similar and cor respond to the ones obtained from shipboard biostratigraphic results. One o f the most striking result obtained from the magnetostratigraphic results, especially the GHMT data, at Site 1001, is the abrupt decrease in the sedim entation rate across the K/T boundary. This suppressed sedimentation lasted at least 4 Ma after the K/T event.