95 = 5°, k = 25) (Suganuma and Ogg, unpubl. data).
Magnetostratigraphic correlations of all five sites are shown in Figure F6. In general, the magnetostratigraphy patterns, when coupled with biostratigraphic constraints, indicate that most sites of Leg 207 contain semicontinuous records of Chrons C18n–C27r of Middle Eocene to Late Paleocene age and Chrons C29r through potentially C33r of Maastrichtian–Campanian.
Cretaceous strata display progressive expansion toward the deeper sites. Although the pre-Campanian strata were not useful for magnetostratigraphy, owing to their deposition during the long normal polarity Superchron C34n, the paleomagnetism is important for determination of paleolatitudes.
Thermal demagnetization of clayey carbonate siltstone of Albian age at Site 1257 displays a relatively weak magnetization of normal polarity with positive inclination. Albian quartz siltstone of Site 1260 is characterized by relatively high intensity (~10–4–10–3 A/m after 15-mT AF demagnetization) and high susceptibility. Progressive AF demagnetization of shipboard cores of Site 1260 siltstone indicated normal polarity with a mean inclination of +30°. Progressive thermal demagnetization of several minicores of this facies also yielded normal polarity with inclinations being uniformly positive.
The overlying black shale of the Cenomanian–Santonian at Site 1257 displayed the highest intensity magnetization (10–2–10–1 A/m after 20-mT AF demagnetization using shipboard pass-through cryogenic magnetometer) of any facies at Site 1257. In contrast, this black shale unit at Site 1258 has magnetic intensities near the background noise level of the shipboard magnetometer. The positive inclinations that dominate the remanent magnetization of this black shale also indicate a paleolatitude north of the paleoequator. Of course, with any unit of entirely normal polarity, it possible that there is a contribution from a persistent overprint of present-day normal polarity (20° dipole inclination for Leg 207 sites). However, the fact that the mean inclinations of characteristic magnetization of Albian and Cenomanian paleomagnetic minicores are steeper than the present-day field is an indication that the Albian–Cenomanian paleolatitude was slightly northward of the present location of these sites. The Albian samples yield a paleolatitude of 15°N latitude (95 = 5°, k = 25) (Suganuma and Ogg, unpubl. data).
Magnetostratigraphies of Campanian and Maastrichtian chalk from Sites 1257, 1258, 1259, and 1261 are consistent with assignments and correlations of Chrons C29r–C32n (Site 1260 had inadequate sampling and weak magnetizations that precluded useful polarity zone interpretations). However, until the array of sites have enhanced biostratigraphic studies to delimit microfossil datums, we caution that it is possible that portions of the current biostratigraphy polarity patterns at each site might have alternate assignments to polarity chrons. The expanded succession with duplicate magnetostratigraphic records from two holes at Site 1258 appears to have resolved the complete sequence of magnetic Chrons C29r–C34n. The Cretaceous/Paleogene (K/P) boundary impact event layer at Sites 1258, 1259, and 1260 is within polarity Chron C29r, as is expected from the reference magnetic polarity timescale.
At all sites, the Danian stage (Chrons C29r–C27n) is condensed and/or includes hiatuses. In contrast, the Upper Paleocene (Selandian and Thanetian stages) is present at all sites, and most holes yielded records of Chrons C26r–C24r. The distinctive Chron C26n provides a useful correlation marker among all sites. Magnetostratigraphic correlations and comparison to the reference magnetic polarity timescale suggest that relative sedimentation rates were variable within and among the sites, and minor hiatuses punctuate the record at some locations. For example, the relative widths of polarity zones associated with Chron C26n and overlying Chron C25r indicate relatively rapid deposition at Site 1257, where the early Paleocene is absent above the K/P boundary, whereas the coeval accumulation rates are significantly slower at sites where Paleocene sedimentation was initiated.
Portions of the Eocene are significantly expanded at some sites. Site 1258 has a very thickened Early Eocene (Chrons C24r–C22n), and Sites 1259 and 1260 have a very thick Middle Eocene (Chrons C21r–C18r). At each site, the chron assignments are constrained by foraminifer and nannofossil biostratigraphy, but many portions also have a distinctive "fingerprint" match to the reference magnetic polarity timescale. A hiatus of variable duration occurs at Sites 1257, 1260, and 1261 between the Lower and Middle Eocene that removes the uppermost Lower Eocene (Chrons C22n and C22r; and an even longer time span at Site 1257). In contrast, the interpreted presence of Chron C22n and overlying Chron C21r at Sites 1258 and 1259 suggests a continuous record across this subepoch boundary.
A curious feature within the 2-m.y.-duration Chron C24r of latest Paleocene into Early Eocene are apparent thin normal polarity bands (e.g., at Site 1257 and Site 1258 within lower foraminifer Zone P6). Similar intra-C24r normal polarity subzones were interpreted in ODP Hole 1051A (Ogg and Bardot, 2001). Even though this apparent occurrence at multiple sites suggests that Chron C24r may include one or more short-duration normal polarity events that are not in the standard C-sequence model (Cande and Kent, 1992), we are hesitant to propose subchrons within Chron C24r without confirmation in a land-based section with fully oriented paleomagnetic data.
One major objective of our study is to provide a high-resolution magnetostratigraphic framework for a detailed cycle-stratigraphy analysis of the subtle facies oscillations and of the duration of polarity chrons and thereby estimation of the spreading rates for the corresponding marine magnetic anomalies. We successfully obtained expanded magnetostratigraphic record of Chrons C24r-C24n-C23r-C23n at Sites 1258 and 1260 and of Chrons C21r-C21n-C20r-C20n-C19r at Sites 1259, 1260, and 1261. This enables multiple sites to be used to verify the cycle-stratigraphy interpretations.
Although the Bartonian stage of the upper Middle Eocene (Chrons C18n–C17n) was recovered at four sites, assignment of the polarity zones to chrons was generally uncertain. The recovered strata of the Priabonian stage at Sites 1257 and 1259 on the northeast margin of Demerara Rise record only a brief portion of the Late Eocene Chrons C17n–C15n.
