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9. Aptian Through Eocene Magnetostratigraphic Correlation of the Blake Nose Transect (Leg 171B), Florida Continental Margin1

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9. Aptian Through Eocene Magnetostratigraphic Correlation of the Blake Nose Transect (Leg 171B), Florida Continental Margin1 Kroon, D., Norris, R.D., and Klaus, A. (Eds.) Proceedings of the Ocean Drilling Program, Scientific Results Volume 171B 9. APTIAN THROUGH EOCENE MAGNETOSTRATIGRAPHIC CORRELATION OF THE BLAKE NOSE TRANSECT (LEG 171B), FLORIDA CONTINENTAL MARGIN1 James G. Ogg2 and Leon Bardot3 ABSTRACT The full suite of magnetic polarity chrons from Subchron M″-2r″ (early Albian) through Chron C13r (latest Eocene) were resolved at one or more Ocean Drilling Program sites on the Blake Nose salient of the Florida continental margin. These sediments preserve diverse assem- blages of calcareous and siliceous microfossils; therefore, the composite 1Ogg, J.G., and Bardot, L., 2001. suite provides a reference section for high-resolution correlation of bio- Aptian through Eocene magnetostratigraphic correlation of stratigraphic datums to magnetic polarity chrons of the Late Cretaceous the Blake Nose Transect (Leg 171B), and Paleogene. Relative condensation or absence of polarity zones at Florida continental margin. In Kroon, different sites along the transect enhance the recognition and dating of D., Norris, R.D., and Klaus, A. (Eds.), depositional sequences and unconformities within the margin succes- Proc. ODP, Sci. Results, 171B, 1–58 sion. A stable paleolatitude of ~25°N was maintained from the late Ap- [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/ tian through Eocene. publications/171B_SR/VOLUME/ CHAPTERS/SR171B09.PDF>. [Cited YYYY-MM-DD] INTRODUCTION 2Department of Earth and Atmospheric Sciences, Purdue Ocean Drilling Program (ODP) Leg 171B obtained expanded sections University, West Lafayette IN 47907- 1397, USA. [email protected] of Maastrichtian through Eocene strata and portions of the Albian–Cen- 3Department of Earth Sciences, omanian along a transect of the Blake Spur portion of the Blake Plateau University of Oxford, Parks Road, off Florida. The sediments were dominated by a siliceous chalk facies Oxford, OX1 3PR, United Kingdom. that generally had excellent preservation of calcareous nannofossils, planktonic and benthic foraminifers, diatoms, and radiolarians. Several Initial receipt: 14 June 1999 Acceptance: 8 September 2000 intervals of the stratigraphy at different sites display color and composi- Web publication: 3 April 2001 Ms 171BSR-104 J.G. OGG AND L. BARDOT MAGNETOSTRATIGRAPHIC CORRELATION OF THE BLAKE NOSE TRANSECT 2 tional oscillations that appear to reflect a spectrum of Milankovitch or- bital climate cycles. The paleomagnetic component is critical to three main objectives of Leg 171: (1) to refine the Paleogene-Cretaceous biochronology and magnetochronology of the Paleogene-Cretaceous period, (2) to develop a cyclostratigraphy tuned to orbital cycles that could be used to estab- lish an extremely accurate chronology for biozones and magnetochrons in the Cretaceous and Paleogene, and (3) to constrain Late Cretaceous through Eocene paleomagnetic poles for the North American plate. This report summarizes the detailed magnetostratigraphy of the various sites. A preliminary magnetostratigraphy at each site was presented in the Leg 171B Initial Reports volume (Norris, Kroon, Klaus, et al., 1998), but we have now analyzed many more discrete minicores to enhance resolution of polarity zone boundaries and we also have reinterpreted several of the polarity chron assignments. Paleolatitudes are derived from the same paleomagnetic data set, and an associated compilation of the Cretaceous–Paleogene polar wander path for North America. In general, the reference scale of oceanic biomagnetochronology compiled by Berggren et al. (1995) for the Paleogene interval is consis- tent with our suites of results from Leg 171. At this stage, the main con- straint appears to be the lower resolution of the paleontological investigations relative to the sampling density of the magnetostratigra- phy. The uppermost Cretaceous (Campanian–Maastrichtian) lacks a well-calibrated reference scale, and combined biostratigraphic and mag- netostratigraphic results from Leg 171 are an important step in this di- rection. This Campanian–Maastrichtian interval is the focus of a separate paper (L. Bardot and J. Self-Trail, unpubl. data). In this paper, polarity chron (time) and polarity zone (stratigraphy) nomenclature is taken from the system of Cande and Kent (1992), with the suffix n denoting normal polarity or r denoting the preceding re- versed-polarity interval. The relative timing (position) of an event (level) within a polarity chron (zone) is “defined as the relative position in time or distance between the younger and older chronal boundaries” (system of Hallam et al., 1985, p. 126). In this proportional stratigraphic convention, the location of the Cretaceous/Tertiary at Gubbio (Alvarez et al., 1977) occurs at C29r.75, indicating that 75% of reversed-polarity Zone C29r is below the event. (Cande and Kent [1992] used an inverted stratigraphic placement relative to present; therefore, C29r.3 in their notation indicates that 30% of reversed-polarity Chron C29r followed the event. This system mirrors the convention of measuring geological time and the numbering of magnetic anomalies backward from the present.) The number of orbital cycles within each polarity zone provides a means to assign absolute durations to the associated polarity chrons and spreading rates to the corresponding marine magnetic anomalies. For example, the cyclostratigraphy of polarity Chron C27 (late Danian) at Site 1050 indicates a cycle-tuned duration of 1.45 m.y. (Röhl et al., 2001), which is about 10% less than in the magnetic polarity time-scale model derived by Cande and Kent (1995). Similar cycle-magnetostrati- graphic analyses should be feasible through the Paleogene succession of Leg 171. J.G. OGG AND L. BARDOT MAGNETOSTRATIGRAPHIC CORRELATION OF THE BLAKE NOSE TRANSECT 3 PROCEDURES AND GENERAL MAGNETIC PROPERTIES Sampling and Demagnetization Approximately 1100 discrete minicores were extracted from the “A” holes at the five sites, the “C” hole extension of Site 1050, and the Cre- taceous/Paleogene boundary interval in the “C” hole of Site 1049. Sam- pling density averaged about two minicores per core section. Soft sediment samples were taken using oriented plastic cylinders (~10 cm3) followed by extrusion, trimming, and air drying. Indurated sediment minicores were drilled into the cut face of the working half of the core with a water-cooled nonmagnetic drill bit attached to a standard drill press. Minicore orientations are perpendicular to the axis of the core, and therefore horizontal relative to bedding strata. Some of the ad- vanced hydraulic piston cores (denoted by the suffix “H” in the core name) had attempts at downhole orientation relative to magnetic north using a tensor tool mounted on the core barrel, but these at- tempts were generally unreliable. Therefore, declinations of magnetiza- tion of the minicores are random, although stability or progressive rotation of the magnetic declinations during demagnetization can be used to identify characteristic magnetic polarity. Shipboard pass-through cryogenic magnetometer measurements were made at ~5-cm intervals on all cores from all holes using alternat- ing-field demagnetization at 10–20 mT. Even though these shipboard measurements were commonly distorted by coring recovery artifacts, such as biscuiting slurry, and were generally inadequate to remove mag- netic overprints, the main polarity patterns were often remarkably in- dicative of the detailed demagnetization results from the suites of minicores. Paleomagnetic analyses of the suites of minicores were performed at the University of Oxford and at the University of Michigan. Progressive thermal demagnetization was tailored to each facies based on pilot studies of each lithology from each site, but it generally consisted of 30°C increments from ~100°–360°C, with continuation to higher ther- mal steps for the more stable samples. Bulk susceptibility was generally monitored after every second thermal demagnetization step <300°C, and after every higher temperature step to monitor changes in mag- netic mineralogy and onset of viscous magnetization associated with formation of new magnetic phases. Magnetic directions were measured on a cryogenic magnetometer isolated from the Earth’s magnetic field in a shielded room (Michigan) or with Helmholtz coils (Oxford). The practical limit on resolution of the natural remanence of the minicores was ~5 × 10–6 A/m, and duplicate measurements were taken whenever the intensity was <2 × 10–5 A/m. Common Magnetic Overprints and Demagnetization Behavior Even though magnetic characteristics varied greatly among the vari- ous lithologies, it is possible to summarize the typical behavior. Initial natural remanent magnetization (NRM) directions are dominated by a downward inclination, which is presumed to be the combined second- ary overprints of normal polarity oriented parallel with the present di- pole field and a drilling-induced remanence. The drilling-induced J.G. OGG AND L. BARDOT MAGNETOSTRATIGRAPHIC CORRELATION OF THE BLAKE NOSE TRANSECT 4 overprint has always plagued Deep Sea Drilling Project (DSDP) and ODP paleomagnetic studies and is characterized by a downward (high posi- tive inclination) and radially inward orientation (e.g., the series of in- vestigative studies made by the paleomagnetists during Leg 154, as summarized in successive site chapters in Curry, Shackleton, Richter, et al., 1995). This radial-inward overprint is exhibited as an apparent pref- erential declination toward 0° (“north”); therefore,
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