Shipboard paleomagnetic measurements from the two holes at Site 1261 were disappointing, but shore-based thermal demagnetization of minicores revealed the main successions of polarity chrons bound by hiatuses. Maastrichtian Chrons C31r–C29r, early Paleocene–early Eocene Chrons C27n–C24r, and middle Eocene Chrons C21r–C19r were delimited.
Details are given in "Paleomagnetism" in the "Explanatory Notes" chapter of the standard shipboard analysis procedures 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 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 207-1261A-7R through 40R and from Cores 207-1261B-2R through 5R, with the exceptions of most cores in the Miocene debris flow. 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 magnetic intensities near the background noise level of the shipboard cryogenic magnetometer and were deposited during the Cretaceous Long 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, we took a set of oriented paleomagnetic cylinders from every second section (3-m spacing) of Eocene–Campanian sediments (Cores 207-1261A-20R through 40R, and 207-1261B-2R through 4R) for combined progressive AF and thermal demagnetization at the magnetic-shielded 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 (Table T8).
The generalized stratigraphy of sediment facies, biostratigraphic ages, and magnetization characteristics of Site 1261 from the shipboard pass-through cryogenic magnetometer are summarized in Figure F9. Intervals of greenish white to 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. In contrast, the normal polarity overprint dominating the strongly magnetized Paleocene and Miocene clay-rich facies did not respond to AF demagnetization. The polarity zones of all facies were resolved by applying thermal demagnetization to minicores, followed by interpreting the polarity of each minicore from an examination of the movement of its magnetic vector during progressive demagnetization (see "Paleomagnetism" in the "Explanatory Notes" chapter).
Cores 207-1261A-7R through upper 14R consist of a gray silty claystone of late Miocene (M13, NN11) to earliest Pliocene (PL1) age. This facies had a uniformly strong magnetization that averaged 5 x 10–5 A/m after 10-mT demagnetization. The 10-mT step was effective in removing a steep downward drilling-induced magnetic overprint, but application of an additional 15 mT did not significantly change the intensity or directions (Fig. F9). This resistance to AF demagnetization, coupled with the near-continuous normal polarity inclinations during an interval of time that should be characterized by frequent magnetic reversals, implies that the gray claystone probably has a persistent overprint that is perhaps carried by goethite-type minerals. Thermal demagnetization >200°C is generally required to remove overprints carried by goethite, but this interval was not a priority for our shore-based program of measurements.
Debris flows and redeposited material into upper Miocene (nannofossil Zone NN10) host sediments compose the majority of Cores 207-1261A-15R through 20R. We measured only a few sections in this interval, primarily the large 45° tilted block of light green chalk of the middle Miocene (nannofossil Zone NN6) spanning all of Core 207-1261A-16R and uppermost Core 17R. Frequent magnetic reversals are characteristic of the original depositional age of this block (Serravallian stage). Intact pieces in this 10-m-thick chalk display well-defined polarity reversals with simultaneous 180° changes of declination and inclination. It is rather ironic that the best polarity data obtained from shipboard measurements taken during Leg 207 are from a displaced olistrostrome. In all other units, shore-based progressive thermal demagnetization of minicores was required to adequately resolve magnetic polarity zones.
The light greenish gray to medium gray lower–middle Eocene chalk was very weakly magnetized, with an average magnetic intensity of ~3 x 10–5 A/m after the 15-mT demagnetization step. Therefore, only a few intervals were significantly above the effective background noise of the shipboard magnetometer (3 x 10–5 A/m) (Fig. F9). The shore-based cryogenic magnetometer enabled an additional order of magnitude in sensitivity, and we were able to adequately resolve the polarity patterns from this facies (Fig. F10).
The uppermost meters of the Eocene yielded a dominance of normal polarity, which is tentatively assigned to Chron C18n from its coincidence with foraminifer Zones P13 and P14. Below a gap in paleomagnetic sampling, we resolved the complete succession of Chrons C19r through probably C20r and perhaps the underlying Chrons C21n and C21r. A hiatus at the early/middle Eocene boundary is indicated by biostratigraphy, which probably encompasses Chrons C22n and C22r. A sediment piece recovered from the interval spanned by Core 207-1261A-31R has normal polarity, which is suggested by its biostratigraphic age to correspond to a portion of Chron C23n. Another hiatus in the middle of the early Eocene probably spanned much of Chrons C23r and C24n, and the lower lower Eocene section at Site 1261 is entirely within Chron C24r.
The Paleocene medium–light gray clayey chalk had a slightly stronger magnetization (averaging ~5 x 10–5 A/m), but only positive inclinations were resolved by AF demagnetization (Fig. F9). Thermal demagnetization successfully removed this normal polarity overprint. The upper Paleocene section contains the record of lower Chron C24r and Chrons C25r–C27n (Fig. F10).
The greenish gray clayey chalk with light–dark cycles of Maastrichtian and Campanian age is characterized by weak intensity and susceptibility (Fig. F9). The magnetic signals from many of the minicores from both holes were commonly reduced to near the noise level of the Munich cryogenic magnetometer upon heating >200°C, and polarity interpretations were commonly uncertain. When the data from the two holes are merged using the composite depth (mcd), then general magnetic characteristics and biostratigraphic constraints suggest Chrons C30n–C31n and the underlying Chron C31r (Fig. F10).