Ocean Drilling Program (ODP) Leg 171B obtained expanded sections of Maastrichtian through Eocene strata and portions of the Albian-Cenomanian along a transect of the Blake Spur portion of the Blake Plateau off Florida. The sediments were dominated by a siliceous chalk facies that generally had excellent preservation of calcareous nannofossils, planktonic and benthic foraminifers, diatoms, and radiolarians. Several intervals of the stratigraphy at different sites display color and compositional oscillations that appear to reflect a spectrum of Milankovitch orbital 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 establish 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 consistent with our suites of results from Leg 171. At this stage, the main constraint appears to be the lower resolution of the paleontological investigations relative to the sampling density of the magnetostratigraphy. The uppermost Cretaceous (Campanian-Maastrichtian) lacks a well-calibrated reference scale, and combined biostratigraphic and magnetostratigraphic results from Leg 171 are an important step in this direction. 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 reversed-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-magnetostratigraphic analyses should be feasible through the Paleogene succession of Leg 171.