PALEOMAGNETISM

Shipboard and shore-based paleomagnetic measurements from Holes 1258A, 1258B, and 1258C potentially resolved Chrons C33r–C29r of Campanian–Maastrichtian age and Chrons C26r–C20r of late Paleo-cene–middle Eocene age.

Shipboard and Shore-Based Procedures and Data Filters

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 this shipboard 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). As at the other sites drilled during Leg 207, 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 from all Campanian–middle Eocene sediments from Hole 1258B for combined progressive AF and thermal demagnetization at the magnetic-shielded room facility at the University of Munich, Germany. Additional postcruise sampling of sediments from Holes 1258A and 1258B enabled 1-m resolution of most polarity zone boundaries and replicated the majority of the polarity succession in the adjacent hole. 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.

One or two faults cause a relative displacement of strata as much as 20 m among the holes of Site 1258; therefore, adjustments of mbsf depth to meters composite depth (mcd) is important to compare stratigraphy and paleomagnetic results (left columns of Figs. F11, F12). The biostratigraphy for this composite stratigraphy is an average of the zonations for Holes 1258A and the combined zonations Holes 1258B and 1258C, but the relative placements of zonal boundaries were not always consistent on this composite depth scale. Another problem for exact biostratigraphic calibration of the polarity zones was that the foraminifer and calcareous nannofossil zones in the holes did not always correspond to their proposed correspondence (and associated calibration to the magnetic polarity timescale) on the reference timescale used during Leg 207 (Fig. F5 in the "Explanatory Notes" chapter). These uncertainties in biostratigraphic constraints imply that some assignments of polarity chrons to the observed polarity zones might undergo future modification.

Paleomagnetic Behavior and Interpretations of Magnetostratigraphy

Lower and Middle Eocene

A reddish brown carbonate chalk predominates from ~40 to 100 mcd in the composite stratigraphy of Site 1258. Compared to other Paleocene–Eocene intervals, this reddish zone displays a relatively high magnetic intensity and susceptibility (Fig. F11). Applying 5- to 10-mT AF demagnetization removed a steep positive inclination that probably represents a drilling-induced overprint. After this initial change, the magnetization of the reddish sediments displayed no significant difference in the intensity and inclination upon 15- or 20-mT AF demagnetization steps during shipboard analyses (Fig. F11). However, shore-based thermal demagnetization of the extensive minicore suite above 150C was very effective in removing a normal polarity overprint, and samples with reversed polarity displayed an increasing magnetic intensity through 200C followed by a univectorial decay. Characteristic directions were computed for each sample from the 250–400C or 450C thermal demagnetization steps, and the polarity pattern was duplicated in detail in Holes 1258A and 1258B (Fig. F12).

Other than this zone of reddish brown chalk spanning the lower (Ypresian)–middle (Lutetian) Eocene transition, the Eocene is primarily greenish white foraminifer nannofossil chalk. Magnetic intensity of these sediments after 15-mT AF demagnetization was generally in the range of 10–5–10–3 A/m, but a large number of shipboard measurements were filtered out by the lower limit of significant background noise at 3 x 10–5 A/m (Fig. F11). These chalks yielded a relatively well defined suite of polarity zones during shore-based thermal demagnetization of minicores (Fig. F12).

Polarity chron assignments to each polarity zone are based on average paleontological ages. The uppermost zone of reversed polarity in nannofossil Zone NP15 is constrained to be Chron C21r, and the reversed polarity zone at the Paleocene/Eocene boundary is constrained to be Chron C24r. Between these two zones, the polarity pattern in each hole closely matches Chrons C24r–21n, including resolution of the brief reversed polarity subchrons within Chrons C24n and C23n. The only discrepancy is the apparent width of the reversed polarity zone that is interpreted to correspond to Chron C23r is significantly thinner in Hole 1258A relative to Hole 1258B, whereas C24n is thicker. The same offset is observed in the shipboard assignment of the boundary between foraminifer Zones P8 and P7, which suggests the possibility of another fault in Hole 1258A at this level that may have displaced a significant portion of the Chron C23r–C24n interval.

The extensive minicore suite indicates that Site 1258 was slightly south of the paleoequator throughout the early Eocene. Zones of normal polarity are characterized by a predominance of low-angle negative inclinations, and reversed polarity zones are associated with positive characteristic inclinations. This southern paleolatitude during early Eocene was also documented at other Leg 207 sites.

Paleocene

The greenish white foraminifer nannofossil chalk of the Paleocene is generally characterized by relatively weak intensity and susceptibility. Magnetic intensity of the greenish white chalk after 15-mT AF demagnetization was generally in the range of 10–5–10–3 A/m, with a large number of measurements below the noise limit of the long-core cryogenic magnetometer (Fig. F11). The composite polarity zone pattern obtained from the analyses of postcruise minicores is consistent with Chrons C26r–C25n, although a reliable assignment is inhibited by the discrepancy with foraminifer and nannofossil zonal ages in the lower portion (Fig. F12).

Campanian–Maastrichtian

The uppermost Maastrichtian contains reddish brown chalk layers that are similar to the Eocene reddish chalk interval in their high intensity and susceptibility and in their persistent normal overprints during AF demagnetization. Thermal demagnetization of minicores yielded reversed polarity in the uppermost Maastrichtian in both Holes 1258A and 1258B, which is constrained by nannofossil Zone CC26 to be Chron C29r (Fig. F12).

The underlying chalk to calcareous nannofossil clay of the Maastrichtian–Campanian is characterized by weak magnetic intensity and susceptibility (Fig. F11). The magnetic intensity of many minicores approached the noise level of the cryogenic magnetometer at the Munich laboratory upon heating beyond 200C. The weak magnetizations added uncertainty to several polarity interpretations but clustering of normal and reversed polarity samples in both holes allowed a generalized identification of the main polarity zones (Fig. F12). However, a reliable assignment of polarity chrons to these zones is inhibited by lack of detailed biostratigraphic constraints. The general pattern and age assignments are consistent with the top of Chron C34n to C30n, but these tentative interpretations may be modified after further paleontological studies.

In summary, Site 1258 yielded preliminary magnetostratigraphic patterns that, when combined with the shipboard paleontological constraints, can be unambiguously correlated to the biomagnetic polarity timescale for the majority of the late Paleocene–earliest middle Eocene. The polarity chron assignments in the Upper Cretaceous sediments are more ambiguous. The high-resolution magnetostratigraphy of the relatively expanded lower Eocene and its continuous extension into the lowermost middle Eocene section provides an important reference section for future cycle and isotope stratigraphy studies.

NEXT