High-resolution chronostratigraphy from downhole susceptibility logging tuned by palaeoclimatic orbital frequencies

High-resolution magnetic records have been obtained by downhole measurement with the Geological High Resolution Magnetic Logging Tool (GHMT) in a well drilled in the North Sea for hydrocarbon exploration. This tool enables a continuous measurement of the polarity of the magnetic remanence and of the magnetic susceptibility throughout the drilled formations. A high-resoluti on magnetic chronostratigraphy between 2.6 and 1.6 Ma is deduced from the t ie points given by the downhole magnetostratigraphy and from a detailed spe ctral analysis of climatically influenced variations of susceptibility. The orbital frequencies identified by spectral analysis of the magnetic suscep tibility in the depth domain allow a refined estimation of the sedimentatio n rate and determination of a high-resolution chronology of the sequence. C haracteristic features of the onset of Northern Hemisphere glaciation and i ts fluctuations during the Late Pliocene are identified through this analys is of the susceptibility record. Our analysis indicates that massive discha rge events of non pelagic deposits - similar, but with a higher amplitude, to Heinrich events that are observed during the last glaciation occurred be tween 2.5 and 2.0 Ma and were probably related to instabilities of the Laur entide ice sheet. The relative power of the orbital forcing factors is comp ared with that documented at other North Atlantic Ocean sites. Comparison w ith an insolation model does not indicate direct correlation but suggests a strong influence of the Northern Hemisphere ice cover and its local variat ions. (C) 1999 Published by Elsevier Science B.V. All rights reserved.