Shipboard paleomagnetic measurements from the three holes at Site 1259 resolved numerous Campanian–lower Miocene magnetic polarity zones. Chrons C31r–C29r are tentatively assigned to the Maastrichtian succession, and Chrons C24r–18r of uppermost Paleocene to middle Eocene are well defined.
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. Shipboard measurements of natural remanent magnetization (NRM) and after 10- and 15-mT alternating-field (AF) demagnetization steps were made at 5-cm intervals on all archive halves longer than 15 cm from Cores 207-1259A-1R through 52R, 207-1259B-2R through 18R, and 207-1259C-1R through 17R. In general, the 10-mT step was effective in removing steep downward overprints induced by the drilling process, and the 15-mT step was usually needed to resolve trends toward negative inclination directions but also reduced the magnetic intensity of the majority of the sediment intervals to the background noise level of the cryogenic magnetometer. As at the other Leg 207 sites, we did not analyze the black shale cores rather than partially removing the magnetization of these sediments without obtaining any useful shipboard information. These black shale intervals typically display magnetizations near the background noise level of the shipboard cryogenic magnetometer and were deposited during the Long Cretaceous Normal Polarity Superchron C34n.
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, secondary overprints associated with reddish coloration that could not be removed by shipboard AF demagnetization, and several intervals characterized by reduced core recovery. Therefore, oriented paleomagnetic cylinders were drill pressed at 3-m spacing from all Campanian–middle Eocene sediments from Hole 1259B and Campanian–Maastrichtian sediments from Hole 1259A for combined progressive AF and thermal demagnetization at the magnetic-shielded facility at the University of Munich, Germany. Additional postcruise sampling of sediments from Hole 1259A filled some of the gaps in recovery in Hole 1259B. 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.
The generalized stratigraphy of sediment facies, biostratigraphic ages, and magnetization characteristics at Site 1259 from the shipboard pass-through cryogenic magnetometer are summarized in Figure F12. Intervals of greenish white to light gray chalk are characterized by very weak magnetization, and nearly half of the measurements after the 15-mT demagnetization step for such intervals were below the 3 x 10–5 A/m background noise level of the magnetometer system. There was a general association of "expanded stratigraphic sections" (e.g., the Maastrichtian, middle Eocene, and upper Oligocene) with these low intensities of magnetization, which suggests a dilution of the magnetic minerals by enhanced biogenic input of carbonate and silica tests. Relatively compact intervals, such as upper Eocene and Campanian–Maastrichtian greenish gray chalks, had a much stronger magnetic intensity.
The 85-m-thick unit of yellowish to greenish foraminifer–sand facies of early Miocene age (Cores 207-1259A-1R to upper 10R) has moderately strong magnetic intensities, averaging ~1 x 10–4 A/m, with numerous normal and reversed polarity zones (Fig. F12). The green–yellow color transition at ~40 mbsf does not appear to affect the magnetic characteristics. The early Miocene, which spans ~5 m.y., encompasses ~12 pairs of magnetic reversals (see Fig. F5 in the "Explanatory Notes" chapter). The only "fingerprint" in this polarity pattern is the relatively long normal polarity Chron C6n in the upper M2 foraminifer zone and the upper NN2/lower NN3 nannofossil zones, which we tentatively correlate to the normal polarity zone in Core 207-1259A-5R at this same biostratigraphic level. An assumption of relatively constant sedimentation rates implies that the two overlying polarity pairs in the resedimented foraminifer sand dominated by Oligocene microfossils could be Chrons C5E and C5D, but shipboard resolution of this interval was very poor. Chron C6C of predominant reversed polarity spans the Oligocene/Miocene boundary; therefore, the reversed polarity zone at this biostratigraphic position in Hole 1259A is tentatively assigned to this chron. Limited core recovery precludes establishing the lower Miocene polarity zone pattern, so other polarity chron assignments are not reliable.
The 40-m-thick Oligocene unit of greenish white chalk has an average magnetization after 15-mT demagnetization of only 3 x 10–5 A/m, which is the background noise level of the magnetometer. Less than 20% of the measurements provide useful polarity information, but the broad biostratigraphic constraints and limited intervals with adequate magnetization are consistent with a possible assignment of Chron C12–Chron C10r. These tentative polarity zone identifications would require future verification from paleomagnetic studies of discrete minicores. We did not attempt to resolve this magnetostratigraphy in our postcruise program.
A narrow 10-m-thick zone of greenish gray chalk from the upper Eocene (Priabonian stage) has a relatively strong magnetization (average = 2 x 10–4 A/m after 15-mT demagnetization) and is characterized by normal polarity with a possible thin reversed polarity band near the base (Fig. F12). Progressive demagnetization of minicores verified the shipboard interpretation (Fig. F13). This polarity and the biostratigraphic constraints (foraminifer Zone P16) may indicate Chrons C15n and C15r.
The upper portion of the middle Eocene greenish white chalk (foraminifer Zones P12–P14) has a moderate magnetic intensity that averaged 1 x 10–4 A/m after 15 mT. The lower portion (foraminifer Zones P10 and P11) is very weakly magnetized (averaging <5 x 10–4 A/m) (Fig. F12). Progressive thermal demagnetization of minicores yielded well-defined polarity zones, which are constrained by the biostratigraphy and characteristic pattern to be Chrons C21n–C18r. Low core recovery and wide spacing of minicores in the uppermost portion (Cores 207-1259A-16R to 18R) precluded a reliable chron assignment.
The upper half of the lower Eocene strata (foraminifer Zones P6–P9) displays distinctive cycles of reddish brown to gray or dark to light green. In the cyclic unit of similar age at Site 1258, it appeared that hematite was both the source of the reddish coloration and the carrier of a normal polarity overprint that persisted during AF demagnetization. Progressive thermal demagnetization of minicores was effective in resolving polarity zones, and these are constrained by the biostratigraphy to be Chrons C23n–C22n.
There is a possible hiatus in sedimentation during the middle of the early Eocene that appears to encompass foraminifer Zone P7, nannofossil Zone NP11, and portions of adjacent zones. The polarity zone pattern is consistent with the associated absence of Chrons C24n and C23r.
The lower half of the lower Eocene succession is a weakly magnetized white chalk. This interval and the underlying uppermost Paleocene displays reversed polarity, which is consistent with Chron C24r at the Paleocene/Eocene boundary interval. This thick reversed polarity interval contains a thin band of normal polarity in Core 207-1259A-37R (Fig. F13). The biostratigraphic position of this normal polarity subzone in the lower part of foraminifer Zone P6 is similar to the age of thin normal polarity bands observed in the paleomagnetic results of Site 1258 (Fig. F13) and of possible normal polarity subzones in Chron C24r at Site 1051A (Ogg and Bardot, 2001). However, without declination control on the minicore paleomagnetic behaviors during demagnetization, the validity of any thin polarity band in ODP cores is uncertain. In summary, the lower–middle Eocene succession contains a nearly complete record of Chrons C24r–C18r.
It was not possible to make unambiguous identification of polarity zones in the upper Paleocene gray chalk or lower Paleocene reddish brown chalk. If one assumes that the ~25-m interval of negligible recovery from Cores 207-1259A-40R to 42R, which were not cored in Holes 1259B or 1259C, contained the record of Chron C25n, then the underlying polarity pattern resolved from the minicores could be Chrons C26r–C25r. The relatively condensed reddish brown facies of the basal Paleocene (upper portion of Core 207-1259A-47R) carried a persistent normal polarity overprint that did not respond to AF demagnetization but was not sampled for postcruise analyses due to proximity to the K/T boundary.
The greenish gray chalk of Campanian–Maastrichtian age is characterized by light–dark cycles and was extensively sampled for postcruise paleomagnetic analyses with the goal of placing cyclostratigraphic constraints on the durations of the polarity chrons. This facies displayed weak intensity and susceptibility during shipboard measurements, and few pieces yielded magnetic intensities above the background noise of the magnetometer upon AF demagnetization (Fig. F12). Progressive thermal demagnetization of the suites of minicores from the duplicate intervals in Holes 1259A and 1259B yielded similar patterns of potential polarity interpretations (Fig. F13). Our tentative interpretation of the combined polarity pattern is a relatively expanded Chron C29r above a more compact record of Chrons C31r–C30n of the lower–middle Maastrichtian.