7. Site 1211¹

Shipboard Scientific Party²

PRINCIPAL RESULTS

Site 1211 is located in lower bathyal (2907 m) water depth on the southern flank of the Southern High of Shatsky Rise. The site is on seismic line TN037-17A (Fig. F1) at the location of Deep Sea Drilling Program (DSDP) Site 305, which was rotary core barrel (RCB) cored. The drilled section at Site 305 contains a relatively thick sequence of Lower Cretaceous chalk and chert, Upper Cretaceous ooze and chert, and Cenozoic ooze (Larson, Moberly, et al., 1975). The unindurated part of the sequence at Site 305 was highly disturbed by rotary drilling and the record of the Eocene-Oligocene transition, the Paleocene-Eocene Thermal Maximum (PETM), and the Cretaceous/Tertiary (K/T) boundary has been lost. Thus, the triple advanced piston corer (APC) coring strategy for Site 1211 was designed to recover a complete and undisturbed record of the Site 305 sequence.

Site 1211 is the second deepest site in the Upper Cretaceous-Paleogene part of the Shatsky Rise depth transect. The site is more than 200 m deeper than Site 1212, the next shallowest site, and more than 500 m deeper than Site 1209, the shallowest site. Thus, drilling results from Site 1211 will contribute to broad, leg-based objectives that are aimed at understanding changes in Late Cretaceous and Paleogene ocean circulation at a time of global warmth.

Holes 1211A, 1211B, and 1211C were cored with the APC. Hole 1211A terminated at 158.9 meters below seafloor (mbsf) at the highest chert layer in the upper Maastrichtian. In Hole 1211B, this chert layer was penetrated with extended core barrel (XCB) center bit drilling and a total of 169.9 m was cored reaching the lower Maastrichtian. Because there is some recovery of coherent Maastrichtian and Campanian ooze from Site 305, we decided not to penetrate multiple chert layers to core the lowermost Maastrichtian and Campanian sequence with the APC. Hole 1211C was cored to immediately below the K/T boundary at 138.3 mbsf to fill stratigraphic gaps in the composite sequence constructed from Holes 1211A and 1211B. The lack of overlap between the first two holes resulted from double coring an interval in the upper section of Hole 1211B (see "Composite Depths").

Coring at Site 1211 recovered three lithologic units that have been separated based on sediment composition (Fig. F2). Lithologic Unit I ranges from Holocene to lower Miocene (0 to ~17.3 Ma; 0-53.4 mbsf) and consists of nannofossil ooze, clayey nannofossil ooze, and nannofossil clay that are shades of gray, green, yellow, orange, and brown in color. This unit has a higher clay content than underlying units. Two subunits are distinguished. Subunit IA (Holocene to lower Pliocene; 0 to ~4.8 Ma; 0-41.4 mbsf) is olive-gray and yellowish gray in color, contains siliceous microfossils and rare ash layers, and often shows a marked decimeter-scale cyclicity. Subunit IB (lower Pliocene to lower Miocene; ~4.8 to ~17.3 Ma; 41.4-53.4 mbsf) is yellowish orange and grayish orange in color and contains centimeter to decimeter-scale cycles. An unconformity from lower Miocene to upper Oligocene separates lithologic Unit I from Unit II (see "Biostratigraphy" in "Specialty Syntheses" in the "Leg 198 Summary" chapter). Lithologic Unit II ranges from the upper Oligocene to lowermost Paleocene (~27 to 65 Ma; 53.38-133.80 mbsf) and consists of yellowish brown and dark yellowish brown nannofossil ooze with minor amounts of nannofossil ooze with clay, clayey nannofossil ooze, and clay with nannofossils. A number of minor diastems occur in this interval as indicated by thin, darker horizons. More significant unconformities are found in the middle Eocene (~40-45 Ma) and upper Paleocene (~56-59 Ma). The unit has a generally higher carbonate content than Unit I. Color reflectance data show decimeter-scale carbonate cyclicity. Lithologic Unit III ranges from the uppermost to lower Maastrichtian (65 to ~70 Ma; 133.8-169.9 mbsf) and consists of pale orange nannofossil ooze and chert. The unit has extremely high carbonate contents. One chert layer was penetrated in lithologic Unit III.

The Site 1211 stratigraphic section shows broad similarity to the sections recovered at Sites 1209 and 1210 suggesting, in general, common sedimentation histories. In particular, the critical boundaries, for example the Paleocene/Eocene (P/E) and K/T boundaries, show a similar sequence of lithologies. However, sedimentation rates throughout the section are generally significantly lower at Site 1211 than at the previous two sites (Fig. F3), and there are additional unconformities and condensed intervals. All of these differences appear to reflect changes in carbonate preservation coincident with the greater water depth.

The major highlight of coring at Site 1211 is similar to the highlights at Sites 1209 and 1210, namely the recovery of all of the critical intervals, most of which were recovered in all three holes. These include the Eocene/Oligocene (E/O) boundary, the PETM, a biotic event in the mid-Paleocene, the K/T boundary, and probably the mid-Maastrichtian deepwater event (MME). In the first part of this section we describe the general stratigraphy of these intervals. In the absence of detailed data and analysis, however, a discussion of the significance of these events would be broadly similar to Sites 1209 and 1210. More detail on the critical events is presented in "Principal Results" in the "Site 1209" chapter.

The stratigraphic record at Site 1211 reveals differences from that at Sites 1209 and 1210 that appear to be related to increased water depth. In the second part of this section, we integrate results from the three sites and explore the significance of these differences in the framework of the Shatsky Rise depth transect.

Recovery of Critical Events

Sediment recovered at Site 1211 contains the record of the Eocene-Oligocene transition, the PETM, an event in the mid-Paleocene, the K/T boundary, and possibly the MME. The lithologic record of each of these intervals at Site 1211 appears to be similar to those of Sites 1209 and 1210, but it also shows significant differences.

The Eocene-Oligocene transition has been recovered in Holes 1211A, 1211B, and 1211C. Cores and reflectance data from the boundary transition show a subtle gradual upward lightening in color, likely as a result of an increase in carbonate content (Fig. F4). This change is evidence for a deepening of the lysocline and calcite compensation depth (CCD). Prominent color cycles in the transition interval suggest an orbital control on dissolution. The amplitude of the reflectance variations is higher than at previously drilled Southern High sites, suggesting greater variation in dissolution intensity at the deeper Site 1211.

The PETM was recovered in Holes 1211A, 1211B, and 1211C; however, the lithologic record in the three holes is different (Fig. F5), and magnetic susceptibility records are hard to correlate (see "Composite Depths"). Part of this problem arises from the fact that the event in Holes 1211A and 1211C was cut by section breaks. In these two holes, the PETM was recovered in a yellowish brown clayey nannofossil ooze. The contact of this layer with the underlying grayish orange nannofossil ooze is affected by bioturbation (Fig. F5). The lower part of the clayey nannofossil ooze in Hole 1211A also appears to have been cut by a burrow that contains grayish orange ooze. The clayey nannofossil ooze layer is ~11 cm thick in Hole 1211A and 8 cm thick in Hole 1211C and is overlain abruptly by grayish orange nannofossil ooze. In Hole 1211B, the base of the PETM corresponds to a sharp change from a pale orange nannofossil ooze to a yellowish brown clayey nannofossil ooze. This color is uniform for 16 cm then grades to a grayish orange over the next ~30 cm. All sediment in the section above the base of the PETM is darker than it is below the PETM base.

Based on lithology and color, it is impossible to correlate the PETM in Holes 1211A and 1211C with the event in Hole 1211B. However, biostratigraphy suggests that there is a diastem right above the base of the event in Hole 1211B. Samples from the lower 6-7 cm of the clay-rich unit in Holes 1211A and 1211C contain the nannolith Fasciculithus in similar abundance to the lower part of the PETM at Sites 1209 and 1210. Nannofossils in the lower part of the event are also poorly preserved, and abundant calcite blades are seen in samples as at the previous sites. The planktonic foraminifer Morozovella velascoensis, the last occurrence (LO) of which defines the top of Zone P5, occurs 33 cm above the base of the event in Hole 1211A and ~24 cm above the base of the event in Hole 1211C. In Hole 1211B, however, Fasciculithus disappears 1-2 cm above the base of the event and M. velascoensis is absent in a sample 6 cm above the base. The nannolith Discoaster diastypus, the first occurrence (FO) of which defines the base of nannofossil Zone CP9, is found 2-3 cm above the base of the event in Hole 1211B, 34 cm above the base of the event in Hole 1211C, and 10 cm above the base in Hole 1211A, which may have a slight gap at this level (this event is found 2 to 4 m above the base of the event at Sites 1209 and 1210). These preliminary data show that the PETM interval in Holes 1211A and 1211C is correlative with the lowermost 1 cm of the event in Hole 1211B. All three sections are highly condensed compared to records from Sites 1209, 1210, and 1212. The condensed nature of the PETM at Site 1211 suggests that it was in a depth range that was sensitive to carbonate solubility changes across the PETM.

The mid-Paleocene biological event was recognized in Holes 1211A, 1211B, and 1211C by a 5- to 10-cm-thick nannofossil ooze that has a slightly darker color than surrounding sediment. The base of the unit is gradational with the underlying sediment, except in Hole 1211C, where a void lies directly under the event. The interval shows a sharp magnetic susceptibility peak and a density decrease. Preliminary nannofossil biostratigraphy suggests significant differences between Sites 1209 and 1210 and Site 1211. At the former sites, this event corresponds to the evolutionary FO of the nannolith Heliolithus kleinpellii, an important component of late Paleocene assemblages and a marker for the base of Zone CP5 (58.4 Ma in the timescale of Berggren et al., 1995). Assemblages in the lower part of this interval are highly dissolved. At Site 1211, a sharp increase in H. kleinpellii occurs within the darker sediment interval; however, this coincides with the FO of younger taxa Discoasteroides bramlettei and Discoaster mohleri. In continuous sections, the FO of D. bramlettei lies in the upper part of Zone CP5 (e.g., Perch-Nielsen, 1985), whereas the FO of D. mohleri at 57.5 Ma defines the base of Zone CP6. The co-occurrence of these FOs suggests that the dark claystone represents a condensed interval of close to 1 m.y.

The K/T boundary stratigraphy at Site 1211 is very similar to that at Sites 1209, 1210, and 1212. The boundary succession includes pale orange nannofossil ooze of latest Maastrichtian age (nannofossil Zone CC26) overlain by lowermost Paleocene (foraminiferal Zone P) yellowish orange foraminiferal ooze. This lithology grades upward into a white foraminiferal nannofossil ooze then back into a grayish orange nannofossil ooze. The boundary between the uppermost Maastrichtian and the lowermost Paleocene is clearly bioturbated, and careful sampling of burrows yields planktonic foraminifers dominated by Guembelitria with rare Hedbergella holmdelensis that suggest a possible Zone P0 age. Light brown to amber, spherical particles ~100 µm in diameter found in a sample from these burrows may be altered tektites.

The MME is likely to have been recovered at Site 1211. Although Inoceramus prisms are not visible in the sediments, they are present in washed residues from Sample 198-1211B-17H-CC.

Carbonate Record at Site 1211

The Shatsky Rise depth transect was designed to reconstruct the effect of short-term and long-term variations in the lysocline and CCD on the Upper Cretaceous and Paleogene sedimentary record. The total depth range of sites included in this transect is almost 1000 m, from Site 1209 (2387 m) to Site 1208 (3346 m). Site 1208 was clearly below the CCD for a significant part of the early Paleogene and the Maastrichtian, as much of this stratigraphic interval corresponds to an unconformity (see "Principal Results" in the "Site 1208" chapter). Site 1211 (2907 m) is the deepest site in the Shatsky Rise transect with a nearly complete Maastrichtian to Eocene section.

The Site 1211 section shows broad similarities to its shallower neighbors, Sites 1209 and 1210 (2573 m) but is considerably more condensed (Figs. F3). Lower sedimentation rates in the major, continuous parts of the Site 1211 section as well as the longer duration of several hiatuses have reduced the thickness of the stratigraphic column. The major Miocene-Oligocene hiatus is similar in duration at all of the sites, however. This hiatus is thought to represent regionally increased erosion coincident with intensified deepwater flow during the latest Oligocene and early Miocene (see "Site 1207" and "Site 1208" in "Principal Results" in the "Leg 198 Summary" chapter). Preliminary biostratigraphy suggests unconformities in the Site 1211 section in the middle Eocene (~40-45 Ma) and upper Paleocene (~56-59 Ma) that are unique among the Southern High sites or at least much longer in duration than gaps in these intervals at the other sites.

Throughout the Paleogene section at Site 1211 are a number of 5- to 20-cm-thick yellow-brown clay-rich nannofossil ooze levels that have abundant phillipsite-, pyrite-, and manganese-coated foraminifers. Carbonate content of these layers are as low as ~50 wt%. There are ~26 such intervals >5 cm in thickness. These levels are thought to result from lengthy seafloor exposure during intervals of intense dissolution when the site was located close to the lysocline/CCD. The late Paleocene to middle Eocene is an interval with a steadily falling but generally shallow CCD in the North Pacific (Rea et al., 1995). The paleodepth of Site 1211 is thought to be comparable to the present day (see "Biostratigraphy" in "Specialty Syntheses" in the "Leg 198 Summary" chapter); thus, the unconformities at Site 1211 may be intervals when the CCD shoaled to depths around 2900 m. A few clay-rich intervals in the upper Maastrichtian may also represent short-term lysocline/CCD shoaling events.

Superimposed on this long-term record for CCD variation are short-term (<1 m.y.) events that led to abrupt shoaling of the lysocline/CCD. The two most prominent events are in the mid-Paleocene at the time of a prominent biological event (see "Principal Results" in the "Site 1209" chapter). The second is at the PETM. Both of these events are highly condensed at Site 1211 compared to Sites 1209, 1210, and 1212 as a result of dissolution. For the mid-Paleocene event, dissolution lasted up to 1 m.y. beyond the end of the event.

Color reflectance data from Site 1211 show low-amplitude cyclic variation throughout the Paleogene (see "Lithostratigraphy"). The percent total reflectance is usually closely correlated with carbonate content. For much of this interval, carbonate likely reflects the amount of dissolution. This suggests that the intensity of dissolution was controlled by a mechanism that varied in intensity on orbital timescales. Two possible mechanisms are changing deepwater circulation and surface water productivity. The cycle record at Site 1211 should allow development of an orbital chronology for the Paleogene section.

¹Examples of how to reference the whole or part of this volume can be found under "Citations" in the preliminary pages of the volume.
²Shipboard Scientific Party addresses can be found under "Shipboard Scientific Party" in the preliminary pages of the volume.

Ms 198IR-107