The sedimentary record of Site 1257 is in packets separated by major hiatuses; therefore, the assignments of polarity chrons to interpret magnetostratigraphic zones rely on biostratigraphic constraints. The magnetostratigraphy of the combined holes is interpreted to resolve portions of Chrons upper C33n–C31r spanning a thick Campanian/Maastrichtian boundary interval, portions of Chrons C20r–C17n in a relatively condensed middle Eocene interval, and Chrons C26n–C24r in an expanded upper Paleocene section.
Details are given in "Paleomagnetism" in the "Explanatory Notes" chapter of the standard shipboard analysis using the pass-through cryogenic magnetometer, the filtering and polarity interpretation procedures of these data, and the shore-based progressive demagnetization of discrete minicores. Shipboard measurements of each section were at natural remanent magnetization (NRM) 10- and 15-mT alternating-field (AF) demagnetization steps, with an additional 20-mT step applied if core flow permitted (Table T9). The 10-mT step appeared to be effective in removing extraneous overprints induced during the drilling process. In general, the additional 20-mT demagnetization step did not significantly alter the magnetic directions obtained at the prior 15-mT step for the majority of the sediment types. The black shale intervals of the Cenomanian–Santonian displayed magnetizations near the background noise level of the shipboard cryogenic magnetometer, are commonly highly fractured or biscuited, and are within the Long Cretaceous Normal Polarity Superchron C34n. We decided to leave the majority of the black shale cores intact rather than partially demagnetize the sediments without the prospect of obtaining useful shipboard information.
Oriented paleomagnetic cylinders were drill-pressed at one-per-section spacing in the entire Campanian–middle Eocene in Hole 1257B for combined progressive AF and thermal demagnetization at the magnetic-shielded room facility at the University of Munich, Germany. These shore-based measurements enabled resolution of removed and characteristic components of magnetization and significantly modified the tentative shipboard polarity interpretations from all facies. 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). In addition, revisions to the initial shipboard biostratigraphy necessitated revisions of assignments of polarity chrons in portions of this succession that are interrupted by several hiatuses.
The uppermost 40 m of Hole 1257A was APC cored. The lower three of these five cores were oriented with the Tensor tool. The initial 10-mT step of AF demagnetization of the Oligocene foraminifer nannofossil oozes removed a component with positive inclination. The progressive 15- and 20-mT steps generally shifted the inclinations to low positive or negative values but with a high degree of scatter. No significant intervals of sustained positive inclination were observed. Magnetic intensity after 20 mT was generally in the range of 10–4 to 10–2 A/m and displayed an artifact of higher intensity in the top section of each core (right-hand column in Fig. F8). The removed component is probably a composite overprint from drilling-induced magnetization and from shear remagnetization near the margins of the piston core liner (e.g., review by Acton et al., 2002) and the present-day normal polarity field. NRM declinations and inclinations were generally random, and although the progressive AF demagnetization resulted in a relatively stable direction for each independent sample, this scatter in sample groups was not improved (inclination column for Hole 1257A in Fig. F8).
Our preferred explanation for this abundance of negative inclinations and scattered declinations is that these Oligocene oozes have variable and semipersistent overprints of present-day normal polarity superimposed on a primary reversed polarity component. This interpretation is supported by the earliest Oligocene paleontological age (Zones NP21–NP23 and P18–P19); therefore, we assign the entire set of lower Oligocene sediments to Chron C12r. No discrete minicores were analyzed from the Oligocene facies at any site; therefore, the interpretation of Chron C12r assumes that Site 1257 was located significantly north of the Oligocene paleoequator. The recovered Oligocene interval of the reversed polarity Chron C12r zone apparently did not include either of the bounding normal polarity zones (base of Chron C12n or top of Chron C13n).
The uppermost 1.5 m of Hole 1257A (Section 207-1257A-1H-1 to Sample 1H-2, 10 cm) is a Miocene reddish brown to greenish brown clayey carbonate ooze. A sharp polarity transition at Sample 207-1257A-1H-1, 100 cm, indicated by both declination and inclination shifts, separates an upper interval dominated by normal polarity and an underlying reversed polarity zone. It was not possible to unambiguously make a polarity chron assignment from the broad shipboard paleontology of this capping sediment unit.
The Eocene greenish white foraminifer nannofossil chalk yielded a relatively well defined suite of polarity zones. Intensities of characteristic magnetization averaged 1 x 10–3 A/m. However, assignments of polarity chrons to these zones are inhibited by the biostratigraphic resolution and discrepancies between preliminary foraminifer and calcareous nannofossil zonal assignments compared to the reference biomagnetic polarity timescale (e.g., lower middle Eocene–lower Eocene foraminifer and nannofossil biostratigraphic zonation scales in Fig. F9 compared to those of Fig. F5, in the "Explanatory Notes" chapter).
A thin upper Eocene unit (foraminifer Zone P16) is bounded by disconformities at the overlying lower Oligocene (Zone P18) and underlying middle Eocene (Zone P16). Minicores from this upper Eocene zone yielded normal polarity, but the biostratigraphic constraints are not adequate to determine whether this normal polarity zone corresponds to Chron C15n or C16n.
The biostratigraphy of the hiatus-delimited 25-m-thick middle Eocene unit indicates a span of nearly 5 m.y. that should encompass Chrons C17–C20. Shipboard clusters of negative inclination were originally interpreted as reversed polarity zones (Fig. F8), but progressive thermal demagnetization of discrete minicores indicated that inclinations are not reliable guides to the polarities of these near-equatorial samples. Comparison between the polarity interpretations of the minicores to the generalized biostratigraphy suggests that the normal polarity interval at the top of this middle Eocene segment may correspond to the base of Chron C17n and that the reversed polarity interval at the base of this segment might correspond to Chron C20r. However, the ambiguity of these assignments and those of the intervening polarity zones merits caution (if fully resolved by our sample spacing in this compact unit, which is doubtful).
A hiatus spanning foraminifer Zones at least P7–P10 separates the lower Eocene unit from the middle Eocene unit. The behavior of magnetic vectors during progressive thermal demagnetization of minicores suggested that the lower Eocene unit is dominated by normal polarity, which we tentatively assign to Chron C24n.
In contrast to the compact Eocene, the upper Paleocene is relatively expanded. The array of minicores yielded two main pairs of normal and reversed polarity zones plus a possible thin normal polarity interval in the uppermost Paleocene.
Biostratigraphic constraints suggest that these polarity zones are Chrons C24r–C25n–C25r–C26n. The base of the Paleocene section is a massive slump deposit, and its magnetization appears to have been acquired during redeposition in a normal polarity field, which is also interpreted as Chron C26n.
The upper Campanian and lower Maastrichtian chalks have relatively weak magnetizations that were less reliable for magnetic polarity assignments (see the "Polarity Rating" column in Fig. F9). Minicores were collected across the Campanian–Maastrichtian boundary interval from all three Site 1257 holes to improve the resolution of the magnetostratigraphy. When these sets are merged using the composite depth offsets, a consistent and simple pattern of polarity zones emerges. The upper half of the succession is a single reversed polarity zone, and the early Maastrichtian age implies an assignment of Chron C31r. The upper Campanian interval is constrained by the foraminifer age span to be Chrons C32n–C32r–uppermost C33n.
The underlying black shale of the Cenomanian–Santonian displays the highest intensity magnetization (10–2 to 10–1 A/m after 20-mT AF demagnetization) of any facies at Site 1257 (Fig. F9). In contrast, the black shale unit at Site 1258 has magnetic intensities near the background noise limit of the shipboard magnetometer. Positive inclinations dominate the magnetization of this black shale, and these are interpreted as an incomplete removal of a steeper normal polarity overprint of the present day (20° dipole inclination) on the predicted Cretaceous Normal Polarity Superchron C34n.
The Albian clayey carbonate siltstone displayed a relatively weak magnetization (Fig. F9). The Albian stage is entirely within Cretaceous Superchron C34n; therefore, we expected low-angle positive polarity (if a present-day normal polarity overprint is present) or low-angle negative polarity (near-equatorial Cretaceous paleolatitude). The single minicore yielded a normal polarity with positive inclination (base of magnetostratigraphy columns of Fig. F9). This is consistent with paleomagnetic results from Albian–Cenomanian sediments at other Leg 207 sites, which suggests a mid-Cretaceous paleolatitude at ~10°–15°N.