16. Magnetostratigraphy of Paleogene and Upper Cretaceous Sediments from Broken Ridge, Eastern Indian Ocean

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 Cl 8/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. INTRODUCTION Broken Ridge is a shallow (1100 m), east-west-trending plateau in the eastern Indian Ocean with a steep southern slope and a more gently inclined northern margin (Fig. 1). Recent drilling results from Ocean Drilling Program (ODP) Legs 119, 120, and 121 (Barron, Larsen, et al., 1989; Schlich, Wise, et al., 1989; Peirce, Weissel, et al., 1989) indicate that Broken Ridge and the Kerguelen Plateau represent conjugate fragments of an oceanic platform constructed by the Kerguelen hot spot (approximately 50°S) during the middle Cretaceous and rifted apart during the middle Eocene (43-45 Ma). Subsequent seafloor spreading has resulted in approximately 20° of northward displacement of Broken Ridge (e.g., Mutter and Cande, 1983). Broken Ridge is overlain by >1500 m of Upper Cretaceous to Pleistocene sediments, which record its prerift sedimentary environment at high southern latitudes and document the rifting/drifting process. The magnetic polarity sequence preserved in these sediments, together with biostratigraphic data, provides a well-constrained time framework in which to interpret the history of Broken Ridge. The sedimentary sequence on Broken Ridge consists of two major lithologic units: (1) a flat-lying sequence of Neogene and upper Eocene nannofossil oozes that unconformably overlies (2) Upper Cretaceous and Paleogene sediments that dip gently (<2°) toward the north (Fig. 2). The latter unit comprises siliceous and ash-bearing carbonates, cherts, and a basal sequence of glauconite-bearing tuffs. The low sedimentation rate (<0.5 cm/100 yr), poor quality of the whole-core remanence data, and inadequate density of discrete samples preclude establishment of a magnetostratigraphy for the Neogene sediments. The results from lithologic Unit I are not discussed further in this paper. In contrast, the dipping and truncated sequence from Sites 752 and 754 yielded a complete polarity sequence from the lower Eocene to the Maestrichtian/Campanian boundary (Chrons C23R to C32R). 1 Weissel, J., Peirce, J., Taylor, E., Alt, J., et al., 1991. Proc. ODP, Sci. Results, 121: College Station, TX (Ocean Drilling Program). 2 Scripps Institution of Oceanography, La Jolla, CA 92093, U.S.A. 3 Bureau of Mineral Resources, Geology and Geophysics, P.O. Box 386, Canberra ACT 2601, Australia. 4 Dept. Earth and Planetary Sciences, St. Louis University, P.O. Box 8099, Laclede Station, St. Louis, MO 63156, U.S.A. In addition, the upper portion of the Cretaceous normal polarity zone (C34N) was recovered from Site 755, and the short middle Eocene section from Site 753 yielded one normal and one reversed polarity interval. The continuity and high mass-accumulation rates (Rea et al., 1990) of this sequence facilitate the comparison of midto high-latitude austral fossil assemblages to both the geomagnetic reversal time scale (GRTS) and standard low-latitude zonations (Berggren et al., 1985; Kent and Gradstein, 1985; Bolli et al., 1985). In addition, the sediments from Broken Ridge afford a unique opportunity to examine the fidelity of inclination data from marine sediments subjected to progressive compaction and diagenesis. Tectonic reconstructions for the Indian Ocean (e.g., Luyendyk and Rennick, 1977: Royer and Sandwell, 1989) together with land-based paleomagnetic studies (e.g., Klootwijk, 1971; Schmidt, 1976) predict little or no latitudinal displacement for Broken Ridge from its formation in the middle Cretaceous until rifting in the middle Eocene. These models suggest that the entire prerift sequence should record a paleolatitude of 47°-50°S, similar to the present location of the Kerguelen hot spot (50°S). Significant deviations from this expected inclination (65°-67°) may cast doubt on the fixity of the Kerguelen hot spot, tectonic models for the Indian Ocean, or the strict validity of the geocentric axial dipole hypothesis. Alternatively, such inclination discrepancies may result from incomplete separation of the primary magnetization or may reflect complications in the preservation of paleolatitudinal information in these sediments. METHODS JOIDES Resolution is equipped with a pass-through cryogenic magnetometer, made by 2G Enterprises, and alternating field demagnetization coils, automated for the measurement of split cores at spacings of 5 cm or more. Natural remanent magnetization (NRM) and the remanence after 9-mT demagnetization (the maximum field attainable with the system available during Leg 121) were routinely measured at 5or 10-cm spacing for all but the most disturbed sections. All cores from the dipping and truncated sequence on Broken Ridge were well above the practical shipboard sensitivity limit (10~ Am total moment) of the 2G magnetometer. The whole-core data have not been deconvolved. Rather, declination and inclination have been calculated by normalizing the moment of the three axes by their respective sensor