Site 1049 penetrated mid-Cretaceous alternations of calcareous clay and chalk, overlain by a relatively compact succession of Maastrichtian through middle Eocene nannofossil chalk. Paleomagnetic minicores were obtained from Hole 1049A, except the Cretaceous/Tertiary (K/T) boundary interval was sampled in Hole 1049C.
Albian Subchron M''-2r'' is resolved in two holes, and the condensed uppermost Cretaceous and Paleogene strata yielded an incomplete record of polarity Chrons C33n through C19n.
The Aptian through lower Albian sediments are dominated by the normal polarity of Chron C34n (Cretaceous Long Normal Polarity Chron). Shipboard indications of two thin reversed-polarity zones within this interval were verified by the minicores (Fig. F3; Table T1). A reversed-polarity interval in the basal Albian within Sections 171B-1049A-20X-1 and 20X-2, 171B-1049B-11X-2, and 171B-1049C-12X-2 is immediately above a 0.5-m-thick interval of distinctive black shale. This organic-rich level has been correlated to Atlantic-Tethys oceanic anoxic event 1b (OAE 1b) and the equivalent Jacobi event in southeastern France, which is a candidate for defining the Aptian/Albian stage boundary (J. Erbacher, pers. comm., 1998). The overlying reversed-polarity zone coincides with an anomalous yellowish and greenish staining on the original sediments. This suggests that the apparent reversed-polarity magnetization is a diagenetic artifact of post-Albian iron mobilization induced by redox contrasts near this organic-rich interval.
The second reversed-polarity zone occurs 8.5 m above the base of the OAE 1b organic-rich shale (paleomagnetic minicore at Section 171B-1049A-19X-2, 103 cm, and shipboard magnetometer data at interval 171B-1049C-11X-3, 10-35 cm). This thin reversed-polarity band within the upper Ticinella primula planktonic foraminifer zone of the lower Albian appears to be coeval with brief reversed-polarity episodes reported from near the boundary of the T. primula and Biticinella breggiensis planktonic foraminifer zones at other DSDP sites (e.g., Leg 27 by Jarrard, 1974; Leg 40 by Keating and Helsley, 1978) and in Italian sections (VandenBerg and Wonders, 1980; Tarduno et al., 1992). Polarity Chron M0r is at the base of the Aptian, and the DSDP Leg 40 Scientific Party (Ryan et al., 1978) proposed a M''-1r'' through M''-3r'' nomenclature for the series of brief reversed-polarity subchrons within the Aptian-Albian. Further documentation is required to verify whether the reported M''-2r'' event or events represent true reversed-polarity episodes (J.E.T. Channell, pers. comm., 1999). Based on the Leg 40 ages for these episodes, we assign the thin (25 cm) reversed-polarity subzone of Site 1049 as the earliest subchron of the M''-2r'' suite. Albian sedimentation at Site 1049 was truncated shortly above this M''-2r'' polarity subzone.
The upper Aptian through lower Albian sediments display oscillation in geochemistry and reddish to greenish white color. Spectra of these variations yield ratios consistent with the suite of Milankovitch orbital-climate cycles of 413 and 100 k.y. (eccentricity), 54 and 40 k.y. (obliquity), and 23 and 19 k.y. (precession) (Ogg et al., 1999). The implied sedimentation rate for the lower Albian derived from these Milankovitch cycles is ~100 k.y./m. This rate implies that reversed-polarity Subchron M''-2r'' commenced ~0.75 m.y. after the initiation of OAE 1b. Polarity subzone M''-2r'' spans only ~25 cm in Hole 1049C; therefore, this polarity subchron had a duration of ~25 k.y.
All three holes at Site 1049 recovered a spectacular record of the end-Cretaceous impact ejecta. Shipboard pass-through cryogenic measurements indicate that the greenish layer of spherules at the K/T boundary is reversed polarity, consistent with the placement of this event within polarity Chron C29r. This boundary layer is the only interval with an unambiguous polarity chron assignment.
The assignment of polarity chrons within the underlying uppermost Cretaceous is compromised by discontinuous strata, hiatuses, poor paleomagnetic characteristics, and synsedimentary slumping and/or drilling-induced disruption. The Maastrichtian and upper Campanian chalk recovered from the three holes at Site 1049, even though these holes are spaced only 10 m apart, yielded different biostratigraphic successions and relative thicknesses of biostratigraphic zones (Shipboard Scientific Party, 1998b; B. Huber and J. Self-Trail, pers. comm., 1998). We obtained independent Maastrichtian magnetostratigraphies from minicore sampling in Holes 1049A and 1049C. A reversed-polarity zone in Core 171B-1049A-18X is not present in the equivalent depth interval (relative to the K/T spherule horizon) within Hole 1049C. Biostratigraphic constraints suggest that this discontinuous reversed-polarity zone is polarity Chron C30r. There is no indication that mid-Maastrichtian polarity Chrons C31n-C31r are present, and the biostratigraphy associated with the underlying normal-polarity zone indicates a probable juxtaposition of C33n of the late Campanian and C32n of the early Maastrichtian.
Paleocene polarity Chrons C29n through C26n appear to be represented by alternating normal- and reversed-polarity zones in Hole 1049A, although resolution of the thicknesses and relative completeness of these zones is precluded by the patchy recovery of the interbedded chalk-chert-ooze facies. As a result, the assignment of the polarity chrons is based entirely on the biostratigraphic ages. Latest Danian polarity Chron C27n was not resolved, and the Paleocene/Eocene boundary interval spanning polarity Chrons C25r-C25n-C24r is either condensed or absent at this site.
Biostratigraphy indicates that the apparent pair of normal- and reversed-polarity intervals spanning the lower Eocene (Cores 171-1049A-12X through 5H) is the concatenation of polarity Chrons C24n-C23n-C22n and C21r-C20r, respectively. The record of polarity Chron C23r may be absent because of a recovery gap. Measurements with the shipboard pass-through magnetometer suggest that a sliver of polarity Zone C21r of basal Lutetian (lowermost middle Eocene) may be present below the lower Lutetian hiatus spanning the majority of Chron C21r and all of Chron C21n.
The upper Lutetian portion of the middle Eocene is represented by polarity Chrons C20r, C20n, C19r, and C19n. In contrast to the underlying Paleocene and lower Eocene, this interval is relatively expanded and appears to be continuous sedimentation.
Site 1050 penetrated upper Albian claystone and lower Cenomanian chalk, overlain by a highly condensed Turonian through Coniacian and a nearly continual succession of upper Campanian through middle Eocene clayey to siliceous chalk and ooze. Paleomagnetic minicores were obtained from the Albian through Danian (lower Paleocene) in Hole 1050C and from the upper Paleocene through middle Eocene in Hole 1050A.
The magnetostratigraphy resolved a nearly complete succession of polarity Chrons C33n (late Campanian) through C19n (mid-middle Eocene).
The suite of cores spanning the upper Albian claystone, lower Cenomanian chalk, and condensed Turonian-Coniacian yielded normal polarity, consistent with their deposition during the Cretaceous Long Normal Polarity Chron C34n (Fig. F4; Table T2).
A series of polarity zones is present within the upper Campanian through Maastrichtian. However, the assignment of polarity chrons is slightly ambiguous because of the presence of two major coring gaps (nonrecovery in Cores 171B-1050C-14R and 12R), a synsedimentary slumped interval in the uppermost Campanian that has probably reset the magnetization, possible magnetic overprints, and the lack of a well-calibrated biomagnetic reference time scale for this time period. The placement of the Campanian/Maastrichtian boundary indicates that Cores 171B-1050C-16R through 18R are probably polarity Chron C32n, including its reversed-polarity Subchron C32n.1r. The Maastrichtian biostratigraphy and magnetic polarity pattern above Chron C32n can be interpreted in at least two ways; therefore, dual alternatives spanning Chrons C31r through C30n are displayed in the polarity column (Fig. F4).
A complete polarity succession of Chrons C29r through C24r spans a 200-m interval in upper Hole 1050C and lower Hole 1050A (Fig. F4). The relative widths of these polarity zones are nearly identical to the relative duration of chrons in the marine magnetic anomaly model (Cande and Kent, 1995), implying a fairly constant sedimentation rate. The only exception is that polarity Chron C27n is slightly expanded relative to the oceanic model (Fig. F2), a pattern also noted in nearby Hole 1051A and in Hole 1001A in the Caribbean Sea (V. Louvel and B. Galbrun, unpubl. data), suggesting that the relative durations in the oceanic model require slight modification.
Sediments deposited during polarity Chrons C27r and C27n display oscillations of color and relative carbonate content. This combined polarity Chron C27r-C27n spans 35-36 obliquity cycles in both the high-resolution geochemical stratigraphy and the natural gamma downhole logs (Röhl et al., in press). This yields a cycle-tuned duration of 1.45 m.y. for polarity Chron C27r-C27n. Similar cycle-duration analyses are underway for polarity Chrons C26r through C25n (U. Röhl and J. Ogg, unpubl. data). The cyclostratigraphy of the Paleocene/Eocene boundary interval within polarity Chron C24r implies an anomalous release of carbon at the associated isotopic excursion (Norris and Röhl, 1999).
Early Eocene polarity Chrons C24r through earliest C22n are assigned from the pattern of polarity zones in association with the biostratigraphy (Fig. F4). Within the interval corresponding to reversed-polarity Chrons C22r and upper C24r are narrow subzones of normal polarity that may represent either pervasive normal-polarity overprints at particular horizons or normal-polarity subchrons not present in the reference magnetic anomaly scale (Fig. F2). It is interesting that similar levels of normal polarity or uncertain magnetization occur within these two chrons at Site 1051 (Fig. F5).
The lower/middle Eocene boundary (Ypresian/Lutetian stage boundary) is a hardground at 153 mbsf (Section 171B-1050A-16X-7, 42 cm) that coincides with the omission of calcareous nannofossil Zone CP12a (e.g., fig. 8 in Shipboard Scientific Party, 1998c). This hardground has juxtaposed sediments deposited during the earliest part of polarity Chron C22n and the later portion of Chron C21n, implying a hiatus of ~1.5 m.y. at this site.
Middle Eocene siliceous chalks at most sites were not as generous in yielding their paleomagnetic secrets as lower Eocene and older sediments. A rapid loss of magnetic intensity upon thermal demagnetization, coupled with inadequate removal of secondary overprints prior to loss of signal, resulted in the majority of paleomagnetic samples being unsuitable for use in determining paleolatitudes. Prolonged contact with oxidized bottom waters after cessation of sedimentation in the late Eocene caused a secondary yellowish discoloration and associated magnetic overprinting of the upper 40 m in each hole (lithologic Unit IIB). Therefore, we are confident in assigning polarity Chrons C20r and C20n, but resolution of Chrons C19r and C19n is difficult (Fig. F4).
Site 1051 penetrated a thick succession of lower Paleocene through middle Eocene siliceous chalk to ooze. The magnetic properties and stratigraphy are similar to the coeval facies at Site 1050. The magnetostratigraphy from minicores in Hole 1051A, supplemented by shipboard data from Hole 1051B, resolved the complete succession of polarity Chrons C27n (Danian stage of the early Paleocene) through C16r (base of late Eocene).
In contrast to Site 1050, the gray chalks in the lower portion of the Paleocene (Danian and Selandian stages) at Site 1051 displayed relatively poor magnetic properties. The characteristic magnetization of the lower 80 m of Hole 1051A is dominated by poor-quality results of apparent normal polarity, and therefore are rated "NPP" (Fig. F5; Table T3). However, the biostratigraphy indicates that this upper Danian-lower Selandian interval should be dominated by reversed polarity of Chrons C27r and 26r, with only a relatively narrow normal-polarity interval associated with Chron C26n (Fig. F2). Therefore, we tentatively consider the "NPP"-dominated intervals to represent incomplete removal of a normal-polarity overprint from a primary reversed-polarity original magnetization. Chron C27n is assigned only to the relatively high-rated normal-polarity ("N" and "NP") characteristic directions from ~620 to 630 mbsf within planktonic foraminifer Zone P2, and the adjacent sediments have indeterminate polarity.
The upper Paleocene (Thanetian stage) yielded a good-quality magnetostratigraphy pattern that is assigned to polarity Chrons C26n, C25r, C25n, and early C24r (Fig. F5).
The early Eocene (Ypresian) pattern of polarity Chrons C24r, C24n, C23r, C23n, C22r, and early C22n is reflected without significant distortion in the magnetostratigraphy. The only exceptions are an interval of anomalous normal-polarity overprint or subchron in the upper portion of the polarity zone assigned to Chron C24r (~460-475 mbsf) and a band of indeterminate polarity within the zone assigned to Chron C23r. These levels seem to be coeval with similar features in the magnetostratigraphy of Site 1050.
The Ypresian/Lutetian stage boundary interval (contact between the lower Eocene and middle Eocene) is a biostratigraphic hiatus at all sites of Leg 171B. As in Site 1050, the majority of polarity Chron C22n of the latest Ypresian and the early portion of Chron C21r of earliest Lutetian are absent but the sediments overlying this hiatus at both holes of Site 1051 record the latest portion of C21r as a narrow band of reversed polarity (Fig. F5) (Shipboard Scientific Party, 1998d).
The middle Eocene Lutetian and Bartonian stages are relatively expanded, and the magnetostratigraphy pattern displays an excellent record of polarity Chrons C21n through C17n, including indications of the brief Subchrons C17n.1r and C17n.2r within polarity Chron C17n (Fig. F5). Within this succession, the apparent thickness of the polarity zone assigned to mid-Bartonian Chron C18n is shortened relative to the reference magnetic polarity time scale of Cande and Kent (1995). Shipboard measurements of cores from the upper portion of Hole 1051B in the pass-through cryogenic magnetometer suggest a reversed-polarity zone at the base of the upper Eocene, and the associated calcareous nannofossil Zone CP15 indicates a possible assignment to polarity Chron C16r.
Site 1052 penetrated upper Albian and lower Cenomanian silty claystone overlain by Maastrichtian through lower Paleocene clayey chalk to calcareous claystone, followed by a hiatus to middle and upper Eocene siliceous chalk and ooze. Paleomagnetic minicores were obtained from Albian through Danian (lower Paleocene) in Hole 1052E and uppermost lower Paleocene through upper Eocene in Hole 1052A. The suites from these two holes duplicate the uppermost portion of the Paleocene.
Above the Albian-Cenomanian normal polarity of Chron C34n, the magnetostratigraphy resolved a complete succession of polarity chrons C31r (early Maastrichtian) through C26r (basal Selandian of late Paleocene) followed by the series from C19r (latest Lutetian of middle Eocene) through C16n (early Priabonian of late Eocene).
The upper Albian and lower Cenomanian consists entirely of normal polarity, consistent with deposition during the Cretaceous Long Normal Polarity Chron C34n (Fig. F6; Table T4). Cenomanian sedimentation is truncated, and deposition resumed with Maastrichtian clayey chalk of reversed polarity.
Assignment of polarity Chrons C31r through C29r to the Maastrichtian magnetostratigraphy is based upon the distinctive polarity pattern and the biostratigraphy (Fig. F6). The upper limit of the polarity zone assigned to Chron C31n is at a synsedimentary slump feature, and a slump within the uppermost portion of the zone assigned to Chron C31r coincides with a nannofossil subzonal boundary. However, the relatively expanded biomagnetic stratigraphy of the Maastrichtian interval provides a useful reference site for calibration of biochronology to polarity chrons.
The K/T boundary interval displays reversed polarity in shipboard pass-through magnetometer measurements, consistent with assignment to polarity Chron C29r (Shipboard Scientific Party, 1998e). An extended interval of normal polarity extends for ~55 m above the K/T boundary. Biostratigraphic constraints indicate that this normal-polarity zone represents the concatenation of polarity Chron C29n with Chron C28n. Reversed-polarity Chron C28r, which coincides approximately with the boundary between foraminifer Zone P1c and P2, is absent.
Resolution of the upper Danian (late polarity Chron C27r and early Chron C27n) is compromised by low recovery and poor magnetic behavior. The top of the Paleocene succession is a narrow band of reversed polarity that probably represents the earliest part of polarity Chron C26r of earliest Selandian. Nearly the entire late Paleocene is absent.
Sedimentation resumes above the Eocene-Paleocene unconformity with a narrow band of reversed polarity. The biostratigraphy, coupled with the polarity pattern of the succeeding middle Eocene, indicates that this band represents the latest portion of polarity Chron C19r of latest Lutetian.
A hardground at 150 mbsf (Section 171B-1052A-18X-4, 10 cm) seems to coincide with the P12/P13 foraminifer zonal boundary (approximately coinciding with the Lutetian/Bartonian stage boundary) and with the contact between polarity zones assigned to Chrons C19n and C18r. The relative widths of the polarity pattern with respect to Chrons C19n-C18r-C18n in the reference magnetic polarity time scale (Fig. F2) indicates a comparative shortening of Chron C18r. Therefore, we infer this hardground to be a temporary cessation of sedimentation during the early portion of polarity Chron C18r.
The full suite of polarity Chrons C18n through C16n, including the brief subchrons, is present within the complete sediment recovery of the upper portion of Hole 1052A. Shipboard measurements of cores on the pass-through cryogenic magnetometer suggest that the uppermost sediments were deposited during polarity Chron C15r of the mid-Priabonian stage, but this requires verification by progressive demagnetization of minicores.
Site 1053 penetrated upper Bartonian (uppermost middle Eocene) through upper Eocene siliceous chalk to ooze. The magnetostratigraphy from minicores in Hole 1053A was consistent with the shipboard cryogenic measurements of cores from Hole 1053B (Shipboard Scientific Party, 1998f). However, the sediment facies were weakly magnetized and generally did not yield reliable magnetic directions upon heating to 300°C and higher; therefore, we cannot ascertain whether the characteristic polarity at the relatively low-temperature demagnetization steps (typically 180°-270°C) represent adequate removal of later overprints. A long interval of normal polarity is bounded by 30-m-thick zones of reversed polarity in the upper Bartonian (lower 30 m of Hole 1050A) and in the uppermost Eocene (upper 30 m of each hole) (Fig. F7; Table T5). The long normal-polarity zone is interrupted by two to three narrow reversed-polarity excursions. The dominance by normal polarity is consistent with the late Bartonian through early Priabonian succession of C18n through C16n, with relatively brief reversed-polarity chrons and subchrons (Fig. F2). The biostratigraphy, especially the boundary between foraminifer Zones P14 and P15, indicates that the relatively thick reversed-polarity zone at the base of Hole 1053A is probably polarity Chron C17r.
However, assignment of polarity chrons to the overlying general pattern is hindered by the broad biostratigraphic zones (e.g., only two foraminifer zones and a single calcareous nannofossil zone). This biostratigraphy does not provide adequate constraints to evaluate either the continuity of sedimentation or whether the weakly magnetized chalk had yielded its original polarity chronology.
Therefore, two of several alternative polarity assignments are displayed for the upper half of Hole 1053A (Fig. F7). The first alternative assumes that the relatively thick reversed-polarity zone at the top of Hole 1053A is polarity Chron C13r of the latest Eocene. Under this scenario, polarity Chrons C15r and C16r are either absent, condensed, or not resolved by the demagnetization procedures. The second alternative assigns the thick uppermost reversed-polarity zone to Chron C15r of mid-Priabonian. Applying relative durations results in an assignment of Subchron C16n.1r to the narrow reversed-polarity excursion at 55 mbsf and a similar absence of polarity Chron C16r. It may be possible to improve resolution and validity of the magnetostratigraphy by detailed demagnetization of samples from Hole 1053B, but the low-resolution biostratigraphic control may preclude an unambiguous assignment of polarity chrons.