The standard geomagnetic timescale for the Late Cretaceous–Cenozoic is the "C-sequence" of marine magnetic anomalies and their calibration in other ODP-DSDP sites to foraminifer and nannofossil datums (e.g., Cande and Kent, 1992, 1995; Berggren et al., 1995). Characteristic portions of this pattern, when coupled with biostratigraphic constraints, generally enabled unambiguous correlations to the polarity zones recorded at Sites 1257, 1258, 1259, 1260, and 1261. For example, the relatively long reversed-polarity Chron C24r that spans the Paleocene/Eocene boundary could be confidently assigned to the reversed-polarity zone spanning this boundary at all sites. In sedimentary intervals deposited by fluctuating deposition rates or spanning rare interruptions by major hiatuses, the available biostratigraphic control was essential for proposing polarity chron assignments to the distorted pattern of polarity zones. Of course, it is inevitable that a few intervals among the array of sites had unreliable polarity interpretations or lacked biostratigraphic constraints which precluded unambiguous chron assignments. We have flagged uncertain chron assignments, but later revisions in biostratigraphy and/or methods of polarity determination may alter a few of the other assignments.
The following site-by-site summary of the Campanian–Eocene magnetostratigraphy is updated from our paleomagnetic sections in the site chapters within the Leg 207 Initial Reports volume (Erbacher, Mosher, Malone, et al., 2004). Paleomagnetism of the underlying Albian–Santonian at each site is merged into a concluding synthesis discussion.
Paleomagnetic analysis of minicores from combined Holes 1257A, 1257B, and 1257C revealed upper Chron C33n–C31r within a thick upper Campanian–lower Maastrichtian section, Chron C26n–C24r in an expanded upper Paleocene section, and portions of Chron C20r–C17n within a relatively condensed Middle Eocene interval (Shipboard Scientific Party, 2004b) (Fig. F5A).
When Campanian–Maastrichtian magnetostratigraphies from the three independent holes of Site 1257 are merged using the composite depth offsets, a consistent and simple polarity pattern emerges. The upper Campanian interval is constrained by foraminifer biostratigraphy to be uppermost Chron C33n–C32n (Fig. F5A). The early Maastrichtian age of the reversed polarity zone in the upper half of the succession implies an assignment to Chron C31r.
The thick upper Paleocene yielded two major pairs of normal–reversed polarity zones, and their placement within foraminifer Zone P4 indicates an assignment to Chrons C26n-C25r-C25n-C24r. The base of this Paleocene section is a massive slump deposit, and its magnetization appears to have been acquired during redeposition in a normal polarity field, which was probably Chron C26n.
A compact, hiatus-delimited slice of Lower Eocene yielded primarily normal polarity, which may be Chron C24n. The apparent normal polarity of the uppermost Paleocene below the basal Eocene hiatus has no equivalent chron in the standard reference set (e.g., Cande and Kent, 1992), therefore may be an early Eocene remagnetization associated with the nondeposition interval or other artifact.
A hiatus spanning a minimum of foraminifer Zones P7–P10 separates lower Eocene strata from the Middle Eocene unit. The relatively compact Middle Eocene greenish white foraminifer nannofossil chalk yields a relatively well defined suite of polarity zones. However, assignments of these zones to polarity chrons is inhibited by the low biostratigraphic resolution and apparent inconsistencies between currently available zonal identifications from the foraminifer and calcareous nannofossil assemblages and by gaps in core recovery. The limited stratigraphic control suggests that the basal polarity zones of this Middle Eocene segment might correspond to Chrons C20r–C20n and that the upper portion records Chrons C18r-C18n-C17r and the base of Chron C17n.
A thin Upper Eocene unit (foraminifer Zone P16) of normal polarity is bounded by disconformities between the underlying Middle Eocene (Zone P14) and overlying lower Oligocene (Zone P18) units. The only normal polarity chron within foraminifer Zone P16 is Chron C15n.
Paleomagnetic analysis of combined Holes 1258A, 1258B, and 1258C reveal Chrons C26r–C20r of the late Paleocene through the early Middle Eocene (Shipboard Scientific Party, 2004c) (Fig. F5B). Polarity chron assignments within the Campanian–Maastrichtian sediments are more ambiguous due to weak magnetizations but seem consistent with Chrons C33r–C29r of age. The high-resolution magnetostratigraphy of the relatively expanded Lower Eocene–lowermost Middle Eocene provides an important reference section for future cycle and isotope stratigraphy studies.
The calcareous nannofossil clay to chalk of Campanian–middle Maastrichtian is characterized by very weak magnetic intensity. Many minicores were demagnetized to near the noise level of the cryogenic magnetometer upon heating >200°C, thereby introducing significant uncertainty in polarity interpretations. Clustering of normal and reversed polarity samples in both holes allowed identification of the main polarity zones (Fig. F5B), but assignment of polarity chrons is inhibited by the lack of detailed biostratigraphic constraints. The general pattern and age are consistent with the uppermost Chron C34n–C30n, but these tentative interpretations may be modified after further paleontological studies.
The uppermost Maastrichtian contains reddish brown chalk layers that display high magnetic intensities with NRMs dominated by normal overprints. Thermal demagnetization yielded reversed polarity in both Holes 1258A and 1258B, which is constrained by nannofossil Zone CC26 to be Chron C29r (Fig. F5B).
The greenish white foraminifer nannofossil chalk of the Paleocene is generally characterized by relatively weak intensity. The composite polarity zone pattern and foraminifer zonation of the merged holes is consistent with Chrons C26r-C26n-C25r-C25n-C24r (Fig. F5B), although the lowermost portion currently has a significant discrepancy between the foraminifer and the nannofossil age assignments.
The greenish white foraminifer nannofossil chalk that dominates the Eocene succession yields a relatively well defined suite of polarity zones (Fig. F5B). A reddish brown chalk that spans the Lower Eocene (Ypresian)–Middle Eocene (Lutetian) at ~40–100 meters composite depth (mcd) has strong magnetic intensity with a relatively steep downward-directed magnetic inclination. Thermal demagnetization >150°C of this reddish brown chalk was very effective in removing this downward overprint, and univectorial characteristic directions are from 250° through 400° or 450°C. Polarity chron assignments are based on average paleontological ages. The reversed polarity zone at the Paleocene/Eocene boundary must be Chron C24r; therefore, the overlying normal polarity zone is Chron C24n. The assignment of Chron C23n to the normal polarity zone at ~100 mcd is dictated by its position in the upper portion of foraminifer Zone P7. This implies that the narrow reversed polarity zone underlying Chron C23n in both holes must be a relatively condensed record of Chron C23r. The uppermost zone of reversed polarity within nannofossil Zone NP15 is constrained to be Chron C21r, and the underlying polarity pattern and biostratigraphic constraints are consistent with Chrons C22n and upper C22r.
However, there is an inconsistency in the interpreted polarity patterns between Holes 1258A and 1258B after adjusting to a composite meter depth scale in the interval between lower Chron C23n (~100 mcd) and Chron C22n (~50 mcd) (Fig. F5B). First, the top of polarity Chron C23n is offset between the two holes. Fault displacements that caused apparent relative removal of ~20 m of stratigraphic section between holes (shown by yellow squares in Fig. F5B) were assigned from shipboard comparisons of sedimentary features, biostratigraphic datums vs. depth, and other inconsistencies in the composite stratigraphy of the Paleocene–Early Eocene. An offset in the shipboard assignment of the foraminifer Zone P8/P7 boundary near the top of this distorted Chron C23n suggests the possibility of another unrecognized fault in Hole 1258A, which may have truncated uppermost Chron C23n in Hole 1258B and/or truncated the overlying Chron C22r in Hole 1258A. More worrisome is the interval of apparent normal polarity at ~80 mcd at each site, which, being between our assignments of Chron C22n and C23n, would seem to be in the middle of reversed polarity Chron C22r. There are at least two possible explanations for this "anomalous" normal polarity zone: (1) a stratigraphic interval within the reddish brown chalk characterized by a pervasive normal polarity overprint that was not effectively removed by our thermal demagnetization procedure, or (2) a duplication of a portion of the polarity pattern by another fault that could not be resolved from biostratigraphic constraints. Other than this annoying inconsistency within upper Chron C23n and lower Chron C22r, the Eocene polarity pattern is well resolved.
Paleomagnetic analysis of minicores from combined Holes 1259A and 1259B resolved numerous magnetic polarity zones within the Campanian–Eocene section (Shipboard Scientific Party, 2004d) (Fig. F5C). Chrons C31r–C29r are assigned to the Maastrichtian succession, and a complete record of Chrons C23n–C18r is resolved in the strata spanning late Early Eocene–Middle Eocene. This is one of the rare sites with a continuous magnetostratigraphy spanning the boundary interval between Early Eocene and Middle Eocene—a time span that is typically represented as a hiatus in most marine sections.
The greenish gray chalk of upper Campanian–Maastrichtian is characterized by light–dark cycles. This interval was extensively sampled for postcruise paleomagnetic analyses for applying cyclostratigraphy to constrain the duration of the polarity chrons. The merged Maastrichtian polarity pattern from Holes 1259A and 1259B is consistent with Chrons C31r–C31n and C30n–C29r.
A ~25-m interval of negligible recovery within the upper Paleocene gray chalk and the extreme condensation of the lower Paleocene reddish brown chalk precludes reliable assignments of chrons to the observed polarity zones. If one assumes that the recovery gap had a similar sedimentation rate as the Lower Eocene, then it represents nonrecovery of Chron C25n and the underlying polarity pattern of the lower Upper Paleocene must be Chrons C26r-C26n-C25r.
The uppermost Paleocene and lower Lower Eocene is weakly magnetized white chalk of predominantly reversed polarity, which is consistent with the expected Chron C24r at the Eocene/Paleocene boundary (Fig. F5C). Within Chron C24r, a thin normal polarity band is recorded in Core 207-1259A-37R—this and similar intra-C24r features at other sites are examined in the summary discussion.
An apparent hiatus in sedimentation during the middle Lower Eocene spans the time interval of foraminifer Zone P7, nannofossil Zone NP11 and portions of adjacent microfossil zones, and the associated Chrons C24n and C23r.
The upper Lower Eocene strata (foraminifer Zones P8–P9) display distinctive cycles of reddish brown–gray or dark–light green. Similarly to the coeval cyclic unit at Site 1258, hematite seems to be both the source of the reddish coloration and the carrier of a normal polarity overprint. Progressive thermal demagnetization was effective in resolving polarity zones, which are constrained by the biostratigraphy to be Chrons C23n-C22r-C22n.
The greenish white chalk facies of the lower Middle Eocene (foraminifer Zones P10 and P11) and upper Middle Eocene (foraminifer Zones P12–P14) have, respectively, very weak and moderate magnetic intensities (Fig. F5C). The pattern of well-defined polarity zones is constrained by biostratigraphy to be the complete succession of Chrons C21r–C18r and, possibly Chron C17r above a gap in recovery.
A narrow 10-m-thick zone of Late Eocene greenish gray chalk has relatively strong magnetization, and biostratigraphy (foraminifer Zone P16) constrains the dominance of normal polarity to be Chron C15n.
Closely spaced paleomagnetic sampling of combined Holes 1260A and 1260B produced a detailed record of Middle Eocene Chrons C21r–C18r (Shipboard Scientific Party, 2004e) (Fig. F5D). Lower-resolution sampling was adequate to identify Chrons C26n–C23n of the Late Paleocene and Early Eocene.
Compared to other sites, the magnetostratigraphy of the Campanian–Maastrichtian was less successful. This white to greenish white chalk at Site 1260 displayed very weak magnetizations that generally approach the background noise level of the Munich cryogenic magnetometer upon heating >200°C. Very few minicores yielded reliable characteristic directions; and, especially in the Campanian, polarity assignments were also difficult (Fig. F5D). In addition, the biostratigraphic constraints are too broad to make assignments of polarity chrons to strata older than Chron C31n.
In contrast, the thermal demagnetization of the relatively strongly magnetized reddish brown chalk of the uppermost Maastrichtian effectively resolved reversed polarity Chron C29r.
Paleocene clayey nannofossil chalk is also characterized by weak magnetization, but it is significantly better than the underlying Campanian–Maastrichtian. No chron assignments were attempted in the relatively condensed lower Paleocene where biostratigraphic ages are inconsistent between foraminifer and nannofossil assignments. The upper Paleogene polarity pattern resolved by minicore analyses is constrained by biostratigraphy to represent Chron C26n through the lower portion of Chron C24r.
The clayey chalk of the earliest Eocene and reddish brown chalk of middle Early Eocene yielded a pattern of polarity zones that matches upper Chron C24r–lower C23n.
The lower Middle Eocene is also reddish brown chalk. Its polarity pattern fits uppermost Chron C22n–lower C21n (Fig. F5D). A hiatus spanning foraminifer Zone P9 and nannofossil Zone NP13 in the uppermost part of Core 207-1260A-1R apparently caused the juxtaposition of two normal polarity zones—uppermost Chron C22n directly overlies lower Chron C23n.
The upper Middle Eocene (~40–185 mbsf) mainly consists of greenish white foraminifer nannofossil chalk. The combined magnetostratigraphy of the two holes yielded a detailed polarity pattern identical to Chrons C21n–C18r, which is consistent with the paleontological ages.
Compared to other sites, Site 1261 is relatively condensed and the main successions of polarity chrons are bound by hiatuses (Shipboard Scientific Party, 2004f) (Fig. F5E). Chrons C31r–C29r (Maastrichtian), Chrons C27n–C24r (Paleocene–Early Eocene), and Chrons C21r–C19r (Middle Eocene) were resolved.
As in Site 1260, the weakly magnetized Maastrichtian–Campanian greenish gray clayey chalk with light–dark cycles was commonly near the noise level of the Munich cryogenic magnetometer upon heating >200°C. Even though many of the individual-sample polarity interpretations are uncertain, when the data from the two holes are merged using composite depths, the generalized polarity zones and biostratigraphic constraints suggest Chrons C31r, C30n/C31n, and C29r (Fig. F5E).
Medium–light gray clayey chalk of the Paleocene has slightly stronger magnetizations than Late Cretaceous sediments. The upper Paleocene section contains the record of Chrons C27n–lower Chron C24r (Fig. F5E).
Light greenish gray to medium gray chalk of Early–Middle Eocene age is very weakly magnetized, but the polarity of most samples was evident. The lowermost Eocene section at Site 1261 is entirely within Chron C24r, but is truncated by a hiatus that probably spanned Chrons C23r and C24n of middle Early Eocene. A second hiatus at the Early (Ypresian)/Middle (Lutetian) Eocene stage boundary spanned Chrons C22n and C22r. A sediment piece recovered from between these two hiatuses has normal polarity, and its biostratigraphic age suggests an assignment to Chron C23n.
Within the Middle Eocene, we resolved the complete succession of Chrons C20r-C20n-C19r and possibly portions of the underlying Chrons C21n and C21r. Above a sampling gap, the uppermost meters of the Middle Eocene yielded a dominance of normal polarity, and its age within with foraminifer Zones P13 and P14 indicates an assignment to Chron C18n.