Magnetostratigraphy of Paleogene and upper Cretaceous sediments from Broken Ridge

Broken Ridge represents a fragment of the oceanic Kerguelen-Heard platform, constructed at high southern latitudes in the middle Cretaceous and rifted apart during the middle Eocene (43-45 Ma). The approximately 1400-m section of prerift sediments on Broken Ridge preserves a polarity sequence that spans the middle Eocene to Upper Cretaceous (Chrons C18/C20 to C34), including a 500-m continuous sequence from the lower Eocene (C23R) to the Maestrichtian/Campanian boundary (C32R). The polarity record, together with biostratigraphic data, provides a well-constrained time framework for interpreting the history of Broken Ridge. Inclinations in the lower portion of the section are generally 5°-10° shallower than the expected geocentric axial dipole inclination. Comparison of inclination and porosity changes with stratigraphic depth suggests the importance of compaction in generating these shallow inclinations. Changes in the magnetic fabric, reflected in the anisotropy of anhysteretic remanence, provide additional support for this interpretation.