3. CAMPANIAN THROUGH EOCENE MAGNETOSTRATIGRAPHY OF SITES 1257-1261, ODP LEG 207, DEMERARA RISE (WESTERN EQUATORIAL ATLANTIC)

Ocean Drilling Program (ODP) Sites 1257–1261 recovered thick sections of Upper Cretaceous–Eocene oceanic sediments on Demerara Rise off the east coast of Surinam and French Guiana, South America. Paleomagnetic and rock magnetic measurements of ~800 minicores established a high-resolution composite magnetostratigraphy spanning most of the Maastrichtian–Eocene. Magnetic behavior during demagnetization varied among lithologies, but thermal demagnetization steps >200°C were generally successful in removing present-day normal polarity overprints and a downward overprint induced during the ODP coring process. Characteristic remanent magnetizations and associated polarity interpretations were generally assigned to directions observed at 200°–400°C, and the associated polarity interpretations were partially based on whether the characteristic direction was aligned or apparently opposite to the low-temperature “north-directed” overprint. Biostratigraphy and polarity patterns constrained assignment of polarity chrons. The composite sections have a complete polarity record of Chrons C18n (middle Eocene)–C34n (Late Cretaceous). 1Suganuma, Y., and Ogg, J.G., 2006. Campanian through Eocene magnetostratigraphy of Sites 1257– 1261, ODP Leg 207, Demerara Rise (western equatorial Atlantic). In Mosher, D.C., Erbacher, J., and Malone, M.J. (Eds.), Proc. ODP, Sci. Results, 207, 1–48 [Online]. Available from World Wide Web: . 2Paleogeodynamics Research Group, Geological Survey of Japan, AIST, 1-1-1 Central 7, Higashi, Tsukuba, Ibaraki, 305-8567, Japan. y.suganuma@aist.go.jp 3Department of Earth and Atmospheric Sciences, Civil Engineering Building, Purdue University, 550 Stadium Mall Drive, West Lafayette IN 47907-2051, USA. Initial receipt: 26 May 2004 Acceptance: 23 May 2005 Web publication: 14 March 2006 Ms 207SR-102 Y. SUGANUMA AND J.G. OGG CAMPANIAN–EOCENE MAGNETOSTRATIGRAPHY, SITES 1257–1261 2 LOCATION, GENERAL STRATIGRAPHY AND PALEOMAGNETIC GOALS Demerara Rise is a projection of the continental margin off the northeast coast of Surinam and French Guiana, South America, in depths deeper than 700 m. Ocean Drilling Program (ODP) Leg 207 employed coring in multiple holes at each site to obtain continuous records of the expanded sections of Albian–Oligocene strata (Erbacher, Mosher, Malone, et al., 2004) (Fig. F1). Sites 1257–1261 recovered nearly 5000 m of sediment along a depth transect at 1900–3100 meters below seafloor (mbsf). In addition to high-resolution records of the Late Cretaceous Ocean Anoxic Events, Paleocene/Eocene Thermal Maximum, and Eocene/Oligocene boundary, several stratigraphic intervals were characterized by cyclic sedimentation that appear to be responses to Milankovitch orbital cycles. These cyclic intervals were the focus of our detailed magnetostratigraphic studies. The generalized stratigraphy across the Demerara Rise consists of four main units: 1. Albian clay to sandstone overlain by Cenomanian–Santonian organic-rich “black shale;” 2. Campanian–Paleocene chalk with subtle oscillations in color and physical properties; 3. Eocene–lower Oligocene chalk to calcareous ooze with cyclic characteristics. Most sites have a major hiatus within the middle Eocene; and 4. Post-Oligocene deposition of primarily nannofossil ooze. Magnetostratigraphy with constraints from biostratigraphy provides a chronostratigraphic framework in deep-sea sediments for paleoceanographic studies, including stable isotope trends and developing a highresolution cyclostratigraphy tuned to orbital cycles. The primary goal of our paleomagnetic study was to establish a high-resolution magnetostratigraphy spanning the Late Cretaceous–Eocene at all Leg 207 sites with particular emphasis on continuous splices of cyclic sediments. Note: for clarity in discussing calibrated chronostratigraphy vs. relative stratigraphic positions, we capitalize the official international subdivisions of epochs (e.g., Middle Eocene subepoch spans the Bartonian and Lutetian stages; Early Eocene subepoch is the Ypresian stage, etc.) The term “Lower” Eocene indicates the associated rock record that corresponds to the Early Eocene. Terms in lower case (“upper Eocene,” “lower Lower Eocene,” etc.). indicate only a relative position within the recovered strata of that international epoch or subepoch. METHODS Sampling Shipboard magnetometer measurements were generally compromised by weak magnetizations, slurry intervals separating coherent blocks, and the inability to perform progressive thermal demagnetization. Therefore, we relied on extensive shore-based paleomagnetic and rock magnetic measurements of discrete samples. 3000