Shore-based paleomagnetic analyses of minicores from the combined holes at Site 1260 resolved a high-resolution record of Chrons C18r–C21r from the middle Eocene. A lower-resolution identification was possible for Chrons C31r–C29 from the Campanian to Maastrichtian and Chrons C26n–C23n from the late Paleocene to early Eocene.
Details are given in "Paleomagnetism" in the "Explanatory Notes" chapter of the standard shipboard analysis using the pass-through cryogenic magnetometer, of the filtering and polarity interpretation procedures of this shipboard data, and of the shore-based progressive demagnetization of discrete minicores. Measurements were made at 5-cm intervals using the shipboard pass-through cryogenic magnetometer on archive halves of cores longer than 15 cm. Sections were measured at natural remanent magnetization and at 10- and 15-mT alternating-field (AF) demagnetization steps (Table T9). The 10-mT step appeared to be effective in removing extraneous overprints induced by the drilling process. The Cenomanian–Coniacian black shale intervals displayed magnetizations near the background noise level of the shipboard cryogenic magnetometer. These shales are known to have been deposited during the Long Normal Polarity Cretaceous Superchron C34n. As at the other Leg 207 sites, we decided to leave the majority of the black shale cores intact rather than partially demagnetize the sediments without prospects of obtaining useful shipboard information. However, the basal facies of clayey limestone of early Albian age underwent the routine analysis procedure.
The generalized stratigraphy of sediment facies, biostratigraphic ages, and magnetization characteristics of Site 1260 from the shipboard pass-through cryogenic magnetometer are summarized in Figure F11. Shipboard identification of polarity zones through nearly half of the succession was not possible because of weak magnetization near the noise limit of the cryogenic magnetometer or because of secondary overprints associated with reddish coloration that could not be removed by shipboard AF demagnetization. Less than 20% of the shipboard measurements at 15 mT for Hole 1260A remained after the filtering procedures. Therefore, we drill pressed a suite of oriented paleomagnetic cylinders from every second section (3-m spacing) of Hole 1260B spanning the Campanian–middle Eocene and the lower Albian. Additional minicores from Hole 1260A covered the lower portion of the middle Eocene stratigraphic interval that was not recovered in Hole 1260B. These minicores underwent combined progressive AF (5 mT) and thermal demagnetization at the magnetically shielded facility at the University of Munich, Germany. The magnetic polarity of each minicore was interpreted from an examination of the movement of its magnetic vector during progressive demagnetization (see "Paleomagnetism" in the "Explanatory Notes" chapter) (Table T10). These shore-based measurements enabled resolution of removed and characteristic components of magnetization and significantly modified the tentative shipboard polarity interpretations from all facies (Fig. F12).
A pronounced downward decrease in both magnetic intensity and susceptibility of the foraminifer nannofossil chalk was observed from the yellowish colored chalk in the uppermost meters (~10–2 A/m after 15-mT demagnetization) to the greenish white chalk at 25 mbsf (~10–5 A/m) (Fig. F11). This phenomenon is postulated to be a rock magnetic response to a progressive dissolution of magnetic oxides associated with the observed redox diagenetic trend (e.g., oxidized in the upper meters but reduced iron minerals below). However, we believe that those magnetic minerals responsible for the paleomagnetic polarity record are not significantly affected by this dissolution. We will analyze the rock magnetism from these facies of varying coloration to determine the magnetic minerals responsible for this decrease.
A series of slumps at 20–45 mbsf has redeposited upper Eocene–lower Oligocene greenish white foraminifer nannofossil chalk. These sediments display "polarity zones" of clusters of negative and positive inclination samples. It is unknown whether this apparent polarity record was acquired before (i.e., fixed in the sediment mass prior to downslope transport) or during the slumping episodes. In theory, one could distinguish between these alternatives by performing a paleomagnetic "fold test" in the future, in which one compares the characteristic magnetic directions from discrete samples taken from strata with different tilt magnitudes and orientations from within the slump.
The upper portion of the middle Eocene (~40–185 mbsf) is primarily greenish white foraminifer nannofossil chalk. Magnetic intensity of these sediments after 15-mT AF demagnetization were very low, generally in the range of 10–5–10–4 A/m, and a large number of measurements were below the background noise level of 3 x 10–5 A/m (Fig. F11). Progressive thermal demagnetization of minicores from the two holes yielded a well-defined polarity pattern identical to Chrons C21n–C18r, as expected from the paleontological ages.
The polarity pattern assigned to Chrons C20n and C20r has one caveat in that it may contain artifacts introduced during the minicore drill-pressing process in this weakly magnetized soft–pliable chalk facies. We assigned polarity to each individual minicore on the basis of observed magnetic declination rotations and intensity changes during the progressive demagnetization. The majority of the characteristic declinations (after initial AF and thermal demagnetization steps) from Cores 207-1260B-9R and 10R and a significant number of those from Cores 207-1260A-18R and 19R are within 30° of a 180° direction and display horizontal to very low angle inclinations. In Ocean Drilling Program (ODP) coordinates, this corresponds to a magnetic vector directed along the axis of each minicore. It is common for ODP cores to have a downward magnetic overprint introduced during the main drilling process, and we are suspicious that this clustering of characteristic declinations near 180° direction might be an analogous artifact produced during the minicoring for this particularly soft chalk. However, an alternative explanation is a statistical coincidence in alignment of the rotated cores, and perhaps the sediment in each core did not experience any significant relative rotation among identified blocks. The apparent uniformity of the assigned polarity for the succession of minicores, as individually interpreted from declination and intensity behaviors during demagnetization, and the excellent apparent correlation of the polarity pattern with the expected biostratigraphic ages and relative widths of Chrons C21r–C18r support this alternative explanation, thereby implying that the polarity pattern is reliable.
The lower portion of the middle Eocene to the uppermost lower Eocene (~200–245 mbsf) is a reddish brown carbonate chalk with a relatively high magnetic intensity and susceptibility (Fig. F11). Applying a 10-mT demagnetization step reduced the magnetic intensity to less than half and removed a steep positive inclination that probably represents drilling-induced overprints. After this initial change, the magnetization of the reddish colored sediments displayed no significant difference in the intensity and inclination upon applying a 15-mT AF demagnetization and displayed a consistent positive inclination averaging 20° (Fig. F11). Thermal demagnetization of minicores was very effective in removing this AF-resistant overprint. The resulting polarity pattern is assigned to the lower portion of Chron C21n through the uppermost portion of Chron C22n (Fig. F12). Biostratigraphy indicates a hiatus spanning foraminifer Zone P9 and nannofossil Zone NP13 in the uppermost part of Section 207-1260A-25R-1, thereby causing an apparent juxtaposition of the normal polarity zones from uppermost Chron C22n (top of this section) and that of lower Chron C23n (lower sections).
The underlying lower Eocene clayey chalk is generally characterized by relatively weak intensity (~10–5–10–4 A/m) and low susceptibility (Fig. F11). The paleomagnetism of the minicores yielded a pattern of polarity zones that matches Chron C24r through Chron C23n (lower portion).
The upper Paleocene clayey nannofossil chalk is similar in its weak magnetic characteristics to that from the lowermost Eocene. The magnetostratigraphy was resolved from minicores. The polarity pattern and biostratigraphic constraints from the upper Paleocene match the lower portion of Chrons C26n–C24r. Apparent rapid polarity changes, coupled with apparent biostratigraphic gaps among foraminifer and nannofossil datums, preclude an unambiguous chron assignment to the minicore results from the lower Paleocene.
The uppermost Maastrichtian reddish brown chalk displayed a normal polarity overprint that resisted shipboard AF demagnetization. Thermal demagnetization of minicores from this facies was effective in revealing the reversed polarity of Chron C29r.
The underlying Campanian–Maastrichtian white to greenish white chalk had weak magnetizations that generally approached the background noise level of the Munich cryogenic magnetometer upon heating >200°C. Very few minicores yielded reliable characteristic directions, and the polarity assignments of some intervals are uncertain (Fig. F12). The broad biostratigraphic constraints are inadequate to make assignments of polarity chrons to the strata older than Chron C31n.
The Albian quartz siltstone is characterized by relatively high intensity (~10–4–10–3 A/m after 15-mT AF demagnetization) and high susceptibility. Application of 10-mT AF demagnetization removes a secondary component that was probably acquired during the drilling process. Applying an additional 15-mT AF step does not affect the remanent directions of the 10-mT step, which averaged +30° in inclination. Progressive thermal demagnetization of several minicores yielded normal polarity behaviors with well defined characteristic directions, but these inclinations are uniformly positive. The implied mean paleolatitude is ~15–20°N, which is slightly north of the present 9°–10° latitude of this site, is in conflict with paleogeographic reconstructions that project Site 1260 to have been near or south of the paleoequator during the mid-Cretaceous (C. Scotese, pers. comm., 2002). We suggest that the mid-Cretaceous reconstructions of the position of this portion of the South American margin requires further refinement in the paleolatitude constraints. In summary, Site 1260 yielded preliminary magnetostratigraphic patterns which, when combined with the shipboard paleontological constraints, can be unambiguously correlated to the biomagnetic polarity timescale for the late Paleocene–middle Eocene.