PRINCIPAL RESULTS (continued)

Background

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. F29) at the location of DSDP Site 305, which was 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 PETM, and the K/T boundary has been lost. Thus, the triple 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 (Table T1). Hole 1211A terminated at 158.9 mbsf at the highest chert layer in the upper Maastrichtian. In Hole 1211B, this chert layer was penetrated with 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 spend the time to penetrate multiple chert layers to core the lowermost Maastrichtian and Campanian sequence with 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" in the "Site 1211" chapter).

Summary of Results

Coring at Site 1211 recovered three lithologic units that have been separated based on sediment composition (Fig. F30). 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, 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"). Lithologic Unit II ranges from 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 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 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. F31), 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.

Highlights

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 E/O boundary, the PETM, a biotic event in the mid-Paleocene, the K/T boundary, and probably the 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 Sites 1209 and 1210, but also to show significant differences.

The Eocene-Oligocene transition was 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. F32). This change is evidence for a deepening of the lysocline and 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. F33), and magnetic susceptibility records are hard to correlate (see "Composite Depths" in the "Site 1211" chapter). 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. F33). 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 below.

Based on lithology and color, it is impossible to correlate between 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 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 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 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 with Sites 1209 and 1210. In 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 D. 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. F31, F34). 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"). 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"); 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" in the "Site 1211" chapter). 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.

Background

Site 1212 is located in middle bathyal (2681 m) water depth on the southern flank of the Southern High of Shatsky Rise. The site is at the location of DSDP Site 47 (Fischer, Heezen, et al., 1971). Hole 47.2 was cored with RCB penetrating 129.2 m of predominantly soft sediments, which frequently were so fluid that cores could not be easily opened. The hole terminated at a hard layer interpreted as chert. The recovered sediments, mainly nannofossil ooze and chalk are Pleistocene to late early Maastrichtian in age. Site 1212 also lies ~1.3 km to the northwest of DSDP Site 577.

According to the drilling record from Hole 47.2 (Fischer, Heezen, et al., 1971), the uppermost seismic reflector correlates with a major unconformity between the upper Miocene and the lower Oligocene. The next highest reflector corresponds to the uppermost chert horizon in the Maastrichtian. The interval between the two reflectors contains a Paleogene to uppermost Cretaceous section with minor unconformities. The lower Eocene to the K/T boundary section in Hole 47.2 was apparently complete and included the P/E boundary. The K/T boundary was described as heavily disturbed by coring but the described planktonic foraminiferal faunal succession was complete (Douglas, 1971). Abundant fragments of Inoceramus shells were recorded from the highly disturbed nannofossil ooze in the lowermost core taken (Core 47.2-14; 128.0 to 129.2 mbsf). Stable isotope analyses on single planktonic species have been performed on samples from this site and from other Shatsky Rise sites including Sites 305 and 306, and the first paleotemperature curves for the Northern Pacific were constructed for the entire Pleistocene to Cretaceous interval (Douglas and Savin, 1971, 1975). PETM excursion isotope values were identified on single specimen and multispecimen analyses of foraminifers from Hole 47.2 by Stott (1992). This interval was described as highly disturbed by drilling.

Site 1212 is the third shallowest site in the Shatsky Rise depth transect. The site is ~300 m deeper than Site 1209 (2387 m) and 230 m shallower than Site 1211, the deepest site drilled on the Southern High; Central High Site 1208 at 3346 m is ~770 m deeper than Site 1212. The objective at Site 1212 was to recover a complete and undisturbed record of the Hole 47.2 sequence with APC double coring. As part of the Shatsky Rise depth transect, drilling at Site 1212 addresses a number of leg-related objectives.

Holes 1212A and 1212B were cored with the APC (Table T1). Hole 1212A terminated at 101.6 mbsf at the uppermost chert layer in the upper Maastrichtian. In Hole 1212B, 11 chert layers were penetrated with XCB center bit drilling and a total of 207.6 mbsf was cored, reaching the upper Albian.

Summary of Results

Coring at Site 1212 recovered three lithologic units that have been separated based on sediment composition and color variation (Fig. F35). Lithologic Unit I ranges from Holocene to middle Miocene (0 to ~15.1 Ma; 0-63.0 mbsf) and consists of nannofossil ooze with clay, clayey nannofossil ooze with foraminifers, and clayey nannofossil ooze, light gray to light olive-gray in color. This unit has a higher clay content than underlying units. Two subunits are distinguished. Subunit IA (Holocene to upper Pliocene; 0 to ~3 Ma; 0-35.9 mbsf) is characterized by interbedded light gray nannofossil ooze with clay and light olive-gray clayey nannofossil ooze with foraminifers expressed as decimeter-scale light/dark alternations. The clay and foraminiferal content varies from 5% to 35% and from 0% to 32%, respectively. Minor components include siliceous microfossils (diatoms, radiolarians, silicoflagellates, and sponge spicules) and rare ash layers. Subunit IB (upper Pliocene to middle Miocene; ~3 to ~15.1 Ma; 35.9-63.0 mbsf) consists of light gray nannofossil ooze with clay and light olive-gray clayey nannofossil ooze and contains subtler decimeter-scale light/dark alternations. Siliceous microfossils are present but considerably rarer than in Subunit IA. Ash layers are absent.

An unconformity from middle Miocene to lower middle Eocene separates lithologic Unit I from Unit II (see "Biostratigraphy" in "Specialty Syntheses"). Lithologic Unit II ranges from lower middle Eocene to lowermost Paleocene (~43.6 to 65 Ma; 63.0-102.2 mbsf) and consists of alternating pale yellowish brown nannofossil ooze with clay and pale orange to grayish orange nannofossil ooze (Subunit IIA; lower middle Eocene to P/E boundary; 43.6 to 55.5 Ma; 63.0-79.9 mbsf), and very pale yellowish brown nannofossil ooze with clay interbedded with very pale orange nannofossil ooze (Subunit IIB, P/E boundary to K/T boundary; 55.5 to 65.0 Ma; 79.9-102.2 mbsf). A number of minor diastems occur in this interval as indicated by thin, darker horizons.

Lithologic Unit III ranges from uppermost Maastrichtian to upper Albian (65 to ~100 Ma; 102.2.-207.6 mbsf) and consists of soft, white nannofossil ooze and nannofossil ooze with foraminifers interbedded with chert. The unit has extremely high carbonate content and is frequently highly disturbed by drilling because of its fluid nature. Eleven chert layers were penetrated. Two significant unconformities are found toward the base of this unit, separating the upper Campanian from the Coniacian and the Coniacian from the upper Albian.

The Site 1212 stratigraphic section shows broad similarity to the sections recovered at previously drilled Sites 1209-1211 suggesting, in general, common sedimentation histories. In particular, the critical boundaries, the P/E and K/T, show a similar sequence of lithologies. In addition, sedimentation rates throughout the Site 1212 section are generally comparable with the rates estimated at Sites 1209 and 1210, whereas they are higher than at Site 1211 in certain intervals (e.g., Paleocene-Campanian) (Figs. F31, F34). The major difference is the ~30- m.y. unconformity between lithologic Units I and II, a gap that is longer in duration than the major Neogene-Paleogene unconformity at other Southern High sites. Additional unconformities and condensed intervals in the Site 1212 section may be located in shore-based biostratigraphic investigations.

Highlights

The stratigraphic record at Site 1212 reveals differences from the records at Sites 1209, 1210, and 1211 that are not related to water depth. For example, the major Miocene-Eocene unconformity spans the lower Oligocene through middle Eocene, an interval largely preserved at the other sites, including Site 1211, which is situated 230 m deeper than Site 1212. This suggests that the missing section at Site 1212 is not a result of dissolution. The site lies near the top of a canyon and in an area of the Southern High where the stratigraphy is complex (Sliter and Brown, 1993). Thus, it is possible that mass wasting has removed part of the section in the vicinity of Site 1212.

The major highlight of coring at Site 1212 is similar to the highlights at Sites 1209, 1210, and 1211, namely the recovery of all of the critical intervals, most in both holes. One exception is the E/O boundary, which lies within the Miocene to Eocene unconformity. Although coring of Cretaceous sediments was limited, the results from Site 1212 are meaningful with regard to the geologic history of Shatsky Rise.

Recovery of Critical Events

Coring at Site 1212 recovered the PETM, the biotic event in the mid-Paleocene, the K/T boundary, and the MME. The PETM at Site 1212 is lithologically similar to Sites 1209 and 1210, corresponding to a 9-cm interval of yellowish brown clayey nannofossil ooze. This lithology has a sharp contact with underlying pale orange nannofossil ooze, and a gradational contact with overlying pale orange nannofossil ooze. Preliminary biostratigraphic investigation suggests that the sequence is similarly expanded to the Sites 1209 and 1210 PETM sections and that similar changes in assemblages and microfossil preservation occur within the event. For example, the abundance of Fasciculithus decreases sharply near the onset of the event, and excursion foraminifers (e.g., Kelly et al., 1996) have been observed. The base of the PETM event, however, appears to correspond to an unconformity. Foraminifers in Sample 198-1212B-9H-5, 77-78 cm, include Globanomalina pseudomenardii, the LO of which defines the top of Zone P4 at 55.9 Ma, ~0.4 m.y. before the onset of the event. The significance of this unconformity in terms of stratigraphy and paleoceanographic interpretation of the PETM is currently not understood.

The mid-Paleocene event was recovered in Hole 1212B. The event appears to be slightly more expanded than at other sites although the sediments in this interval are considerably lighter in color than the equivalent interval elsewhere. As at the other sites, this interval is characterized by dissolved nannofloras and planktonic foraminiferal assemblages as well as an abundance of phillipsite and fish teeth. The FO of the nannolith Heliolithus kleinpellii lies within the event that also corresponds to a nearly monogeneric planktonic foraminiferal assemblage of Igorina.

The K/T boundary interval is also lighter in color than at the other sites, and the sediments near the boundary appear to be somewhat less indurated. Ooze immediately underlying the boundary correlates to uppermost Maastrichtian nannofossil Zone CC26 based on the presence of Micula prinsii. The paleontological boundary has been bioturbated, and careful sampling of burrows of lowermost Danian sediment that extend 10 cm into the uppermost Maastrichtian yields ~100-µm-sized, amber to olive-green spherules as well as tiny planktonic foraminifers possibly representing basal Paleocene Zone P0. Planktonic foraminifers in the lowermost Danian correspond to Zone P. The identical faunal and floral changes are observed in the boundary interval as at the other sites, and the early Danian interval, in particular, appears rather expanded and remarkably undisturbed.

Site 1212 is located close to Site 577, where the K/T boundary was triple cored during Leg 86 (i.e., Wright et al., 1985). Lithologically, the K/T boundaries are recovered in Holes 577, 577A, and 577B are similar although not identical to the Site 1212 record. The main difference between the sections is the degree of bioturbation and thickness of the bioturbated layer, and the thickness and color of the lowermost Paleocene foraminiferal ooze layer. Lighter color is associated with an increase in thickness of the basal layer from 10 cm in Hole 577A to more than 15 cm in Hole 577. Smear slide analysis for Site 577 showed that planktonic foraminifers are abundant (up to 35%) from basal Paleocene to ~60-70 cm above the K/T boundary, whereas above and below this interval they rarely exceed 1%. At Site 1212, abundant foraminifers are found in smear slides only in the lowermost 10-20 cm of Paleocene.

A detailed study of planktonic foraminiferal biostratigraphy and assemblages across the K/T boundary at Site 577 was conducted by Gerstel et al. (1986). These authors illustrated the same evolutionary trends and succession of events as found at Site 1212 and other Leg 198 sites. Gerstel et al. (1986) argued that the presence of Parvularugoglobigerina eugubina below the K/T boundary was evolutionary. However, we have noticed that (1) the distribution of P. eugubina below the boundary is confined to the burrows associated with tektite-like spherules and (2) it is preceded in the deepest burrows by the tiny Guembelitria assemblage reminiscent of the P0 zonal fauna. Moreover, the Site 1212 record shows that the tektite-like spherules are concentrated in the first few centimeters (2-3 cm) above the K/T boundary, and their record above is related to the intense burrowing. Nannofossil assemblages across the boundary at Site 577 were described in detail by Monechi (1985), who documented a sequence of events similar to those found at Site 1212 and other Leg 198 sites.

Cretaceous History of the Southern High

The Maastrichtian-Campanian section at Site 1212 is underlain by a thin layer of clay and glauconite-rich ooze of Coniacian age (planktonic foraminiferal Zone KS23, top of Section 198-1212B-24H-6), which in turn is underlain by ooze of late Albian age (Zones KS 16-17; base of Section 198-1212B-24H-6 to Section 27H-CC). Albian sediment has been recovered at a number of other sites on Shatsky Rise, including Sites 1207 and 1213 (see "Principal Results" in the "Site 1213" chapter), and DSDP Sites 305 and 306 (Luterbacher, 1975). Coniacian to upper Cenomanian sediment, however, is extremely rare on Shatsky Rise as noted by Sliter (1992), who also reevaluated the planktonic foraminiferal biostratigraphy of Site 305 and found an unconformity in this interval. Sliter (1992) proposed that the Coniacian to Cenomanian interval was widely unconformable as a result of deep-sea erosion or dissolution. Coring results from Site 1212 and other Leg 198 sites confirm this conclusion. The fact that the one site where the Cenomanian to Coniacian interval is partially recovered, Site 1207 on the Northern High, lies at greater depth (3101 m) than Sites 1212 and 305 (2903 m) where it is unconformable, suggests that erosion is the likely cause of the regional unconformity. In addition, seismic line TN037-17A, which crosses Sites 1209, 1210, and 1211, shows a prominent unconformity (Reflector R1) that likely correlates to the Cenomanian to Coniacian interval, where mid-Cretaceous dipping horizons are cut by horizontal uppermost Cretaceous horizons (Fig. F29; see lines 14A and 17A in oversized Figure F8 in Klaus and Sager, this volume). Thus, the stratigraphy of Site 1212 has important implications for the Cretaceous history of Shatsky Rise.

Background

Site 1213 is the southernmost and deepest site on the Shatsky Rise depth transect, located in lower bathyal (3883 m) water depth on the southern flank of the Southern High of Shatsky Rise. The site is close to the oldest part of the rise with underlying basement formed in the Tithonian (Late Jurassic). The major goals of Site 1213 drilling were to core a relatively deepwater mid- (Barremian-Cenomanian) and Lower Cretaceous sequence as well as to obtain fresh volcanic rocks from the underlying basement. The correlation of seismic lines down the southern flank of Shatsky Rise suggested that a relatively thick mid- and Lower Cretaceous section exists at relatively shallow burial depth (Fig. F36; see line 17B in oversized Figure F8 in Klaus and Sager, this volume). Basement was thought to lie between 400 and 600 mbsf, depending on velocity estimates. Thus the lower part of the Cretaceous section and basement can be recovered without time-consuming drilling through extensive chalk/ooze-chert sequences.

Holes 1213A and 1213B were cored with the RCB (Table T1). Hole 1213A was terminated at 198.9 mbsf after the coring wireline parted. Lithology at the base of this hole consists of chert, porcellanite, and radiolarite of middle Albian age. A total of 447.8 m of sedimentary rock was penetrated in Hole 1213B with claystones near the base of the hole containing nannofossils and radiolarians of earliest Berriasian age. Core recovery in chert-bearing units was low. Underlying the sedimentary package, plutonic rocks totaling 46.6 m in thickness were penetrated and higher recovery was obtained than in the sedimentary sequence. Hole 1213B was terminated when the rate of penetration decreased significantly and the recovered material was considered sufficiently unaltered for detailed geochemical and radiometric analyses. Finally, a full suite of logs was to be collected through the whole sequence. The triple combination (triple combo) tool reached 431 mbsf but experienced several tight passes in the hole. With the approach of severe tropical storm Krosa combined with the poor hole conditions, it was considered too risky to deploy the FMS-sonic logging tool.

Summary of Results

Coring at Site 1213 recovered four lithologic units. Sedimentary units are divided based on composition and color. Lithologic Unit I (0-54.6 mbsf) consists of olive-gray to pale yellowish brown nannofossil ooze, clayey nannofossil ooze, and nannofossil clay that ranges from Holocene to lower Pliocene (0-5.0 Ma). This unit contains minor amounts of diatoms, foraminifers, and ash. Subunit IIA (54.6-66.1 mbsf) comprises pale orange nannofossil ooze and brown chert of Santonian age (84.8-85.5 Ma). Moderate to pale yellow-brown chert and light brown and pale orange porcellanite of middle to late Cenomanian age (94.1-96.8 Ma) are grouped in Subunit IIB (66.1-85.4 mbsf).

Subunit IIIA (85.4-179.6 mbsf) of early Cenomanian to middle Albian age (96.8-106 Ma) contains medium dark gray chert, gray, greenish gray, and pale orange porcellanite, and light greenish gray limestone. A mixture of chert, shades of brown and red in color, and pale orange to gray-orange and pale yellow-brown porcellanite and radiolarite with an age range from early middle Albian to late Aptian (106-119.5 Ma) are grouped in Subunit IIIB (179.6-256.8 mbsf). Subunit IIIC (256.8-266.4 mbsf) corresponds to olive-black to greenish black C_org-rich clayey porcellanite, dusky green radiolarian porcellanite, and minor altered tuff. This unit is early Aptian in age (119.5-120.5 Ma). Gray chert, white to yellowish gray porcellanite, and light greenish gray and olive-gray nannofossil chalk to clayey nannofossil chalk of Hauterivian to late Berriasian age constitute Subunit IIID (266.4-410.3 mbsf). Subunit IIIE (410.3-447.8 mbsf) of late Berriasian to earliest Berriasian or latest Tithonian age contains brown, gray, grayish red, and pale to moderate brown chert, yellowish gray and light pale orange porcellanite, and brown claystone with nannofossils. A limonitic claystone breccia is found at the base of this subunit. Throughout the sedimentary section, porcellanite, limestone, and claystone have variable amounts of radiolarians and nannofossils. In many intervals nannofossil biostratigraphy was conducted on chalk adhered to the sides of chert nodules. This sediment indicates the nature of the unrecovered intervals. Finally, Unit IV (447.8-494.4 mbsf) contains at least three separate units of dark greenish gray, sparsely phyric or fine-grained diabase with chilled basaltic margins and thin interbedded pieces of altered chert.

Preliminary investigations of the almost 500-m cored sequence at Site 1213 yields a detailed 140-m.y. history of the deep south flank of Shatsky Rise. The record from this site provides a new perspective to the evolution of the plateau. True basaltic basement was not recovered at this site; instead, we cored a sequence of diabase units with chilled margins and interbedded sediment that are interpreted as sills intruded in a widespread posteruptive plutonic event. Limonitic breccias overlie these intrusives, providing evidence for hydrothermal activity associated with sill intrusion. The earliest sediments of pelagic origin are radiolarian-rich horizons that were deposited under high-productivity surface water conditions in well-aerated deep waters that were swept by currents. These sediments have been transformed during burial to porcellanite and chert. At times, lower-productivity conditions led to deposition of nannofossil ooze, which is now chalk in the deep record.

The early history of Shatsky Rise was interrupted by separate intervals of dysoxia/anoxia in the Valanginian and early Aptian that led to deposition of C_org-rich sediments. Evidence for the Valanginian event is found in southern Europe (Lini et al., 1992). To our knowledge, however, this is the first record of this event in the Pacific Ocean as well as in the pelagic realm. C_org-rich sediments deposited during the well-known OAE1a were also found at Site 1207 on the Northern High.

The Barremian is only represented by one sample, at the top of Core 198-1213B-9R, and clearly most of the stage corresponds to an unconformity (between Cores 8R and 9R). This interval was also unconformable at Site 1214 (see "Site 1214"). The CCD in the Barremian is thought to have been relatively deep (e.g., Thierstein, 1979), certainly no shallower than the paleodepths of these sites at this time (2-2.5 km). Thus, the hiatus was most likely a result of a deepwater erosional event that scoured away the southern margin of Shatsky Rise.

The remainder of the mid-Cretaceous after the early Aptian was a return to better-oxygenated conditions with variation in productivity leading to deposition of radiolarian oozes (porcellanites and chert in the record) and nannofossil ooze (limestone). A hiatus occurred in the late Cenomanian to Santonian interval, the result of a widespread erosional episode that affected a wide area on the Southern High (Sliter, 1992) (see "Site 1212"). After a short depositional episode in the Santonian, a long hiatus lasted until the late Miocene. This episode was likely a result of a combination of erosion of the exposed, deep flank of the Southern High and carbonate dissolution at frequent times when the CCD shoaled above the depth of the site.

Highlights

Anoxic Events in the Early Aptian and Valanginian

Lower Aptian C_org-rich horizons were recovered in Core 198-1213B-8R. The horizons include olive-black to greenish black clayey porcellanites and radiolarian porcellanites with minor tuff (see "Lithostratigraphy" in "Specialty Syntheses"). Three organic carbon analyses yielded contents of 2.9, 10.2, and 25.2 wt%. The sample with the highest organic carbon content is a clayey porcellanite. Gamma ray and uranium logs show that the lower Aptian C_org-rich units are ~3 m thick. Total recovery of carbonaceous and noncarbonaceous in this interval (Core 198-1213B-8R) is just over 1 m. Thus, the recovery of organic-rich rocks is <30%.

Characterization of the organic matter from the most C_org-rich lower Aptian samples indicates it is algal and bacterial in origin and that some of this organic matter was produced by haptophytes and some by cyanobacteria. The haptophyte alkenones identified are some of the oldest known records of these compounds. The character and preservation of the bacterial material are also evidence for the existence of microbial mats at the time of deposition. Finally, the excellent preservation of organic compounds indicates deposition in highly dysoxic or anoxic conditions (see "Geochemistry" in "Specialty Syntheses").

Section 198-1213B-8R-1 is mostly noncalcareous; however, a sample at the top (198-1213B-8R-1, 3 cm) contains the nannofossil Eprolithus floralis but lacks Rhagodiscus achlyostaurion and Prediscosphaera columnata and thus correlates to late Aptian Zone NC7. Sample 198-1213B-9R-1, 17 cm, lacks Hayesites irregularis, Calcicalathina oblongata, and Cruciellipsis cuvillieri and thus correlates to upper Hauterivian-Barremian Zone NC5 (see "Biostratigraphy" in the "Site 1213" chapter). Discrete intervals in Section 198-1213B-8R-1 have abundant radiolarians. Diagnostic radiolarian faunas in these samples are similar to assemblages observed in the Livello Selli in the Cismon core from the southern Alps of Italy (Premoli Silva et al., 1999). Thus, nannofossils, and particularly radiolarians, bracket the age of the carbonaceous horizons and provide a firm correlation with C_org-rich units of OAE1a from central Italy (the "Selli" level), the Italian and Swiss Alps, Sicily, France (the Goguel level), Site 641 in the eastern North Atlantic, and northern Mexico (i.e., Bréhéret, 1988; Weissert and Lini, 1991; Bralower et al., 1994; Erba, 1994; Menegatti et al., 1998; Erba et al., 1999; Bralower et al., 1999). In the Pacific, C_org-rich horizons of OAE1a age are also found at Sites 463 (Mid-Pacific Mountains) and 866 (Resolution Guyot) (Sliter, 1989; Jenkyns, 1995), and elsewhere on Shatsky Rise at Sites 305 and 1207 (see "Site 1207" for a more detailed discussion). C_org-rich lower Aptian sedimentary rocks from Sites 1207 and 1213 provide evidence for the nature of environmental changes in a truly pelagic regime during OAE1a.

Two C_org-rich levels in the Valanginian have considerably lower organic carbon contents than the lower Aptian horizons. A level in the lowermost Valanginian has 3.1 wt% C_org, one in the mid-Valanginian contains 2.5 wt%. These horizons also contain clear evidence of bioturbation indicating that deep waters were sufficiently oxygenated to support a benthic community. The horizons do not show up on gamma ray logs, suggesting that they are thin. The mid-Valanginian horizon is significant as it appears to be broadly similar in age to C_org-rich units that had not previously been observed outside the Tethys (e.g., Lini et al., 1992). The presence of a positive carbon isotope excursion that correlates to this event suggested that it was more widespread or global in extent (e.g., Weissert and Lini, 1991). Although recovery at Site 1213 does not allow us to characterize the temporal distribution of C_org-rich units, this record extends the known geographic distribution of organic-rich sediments considerably.

Chert Record

Although poorly recovered, pieces of chert in the Site 1213 section yield valuable information on variations in redox conditions through the Cretaceous. Nodules with orange, red, and brown hues indicate deposition and diagenesis in oxidizing environments, whereas those with olive-green to black hues indicate more reducing conditions during deposition and burial (see "Lithostratigraphy" in "Specialty Syntheses"). The color stratigraphy suggests that oxygenated conditions prevailed in the early Berriasian, late Aptian through middle Albian, and, when combined with data from other Leg 198 sites, from the late Cenomanian through the Maastrichtian. Reducing conditions prevailed in the Berriasian through the early Aptian and in the late Albian to middle Cenomanian. Similar trends were observed at Sites 1207, 305, 306, and 1214 (see "Site 1214") for coeval portions of the sequence, suggesting that for much of the Cretaceous the entire Shatsky Rise experienced similar redox conditions at bathyal depths. Minor variations exist, however. At Site 1207, generally reducing conditions seem to persist until the late Aptian, with oxidizing conditions characterizing the early to middle Albian and from the late Albian through the Turonian.

Important Neocomian Reference Section in the Pacific

Site 1213 contains one of a handful of Neocomian pelagic sections in the Pacific Ocean. Although the section is generally poorly recovered, nannofossils are well preserved, and radiolarians are diverse and moderately well preserved. These groups should provide precise correlations with Neocomian sections worldwide. Planktonic foraminiferal occurrence is very rare. However, shore-based investigations have the potential to map out some of the earliest evolution of this group.

Shipboard nannofossil biostratigraphy has proven challenging because of the absence of important Neocomian marker taxa, especially the nannoconids. This group, which has a poorly understood affinity, are centerpieces of the biostratigraphy of the Jurassic/Cretaceous boundary interval in particular (e.g., Bralower et al., 1989). Their absence at Site 1213 is intriguing as they are abundant in samples from Site 463 in the Mid-Pacific Mountains (Erba, 1994). At another site, Site 167 on Magellan Rise, nannoconids are absent except in the lowermost three cores (Roth, 1973). This group is also absent in assemblages that we interpret as Berriasian in Holes 49 and 50 on the deep western flank of Shatsky Rise (Fischer, Heezen, et al., 1971). What are the major controls on the distribution of nannoconids in the Pacific? Nannoconids are generally most common in continental-margin and epicontinental locations (e.g., Roth and Bowdler, 1981; Roth and Krumbach, 1986). In the Atlantic Ocean, this group is common in sites along the margins but absent in the deep sea except where transported by turbidity currents. Nannoconids are thought to be oligotrophic (e.g., Erba, 1994), and possibly to represent calcareous dinoflagellate cysts (Busson and Noël, 1991).

The record of nannoconids in the Pacific is complex. Their abundance at relatively shallow Site 463 and at Site 167 in its early (shallow) history, and absence at deeper Site 1213, suggests that water depth may be a factor controlling their distribution. Alternatively, at Site 1213 and other sites, the abundance of radiolarians throughout the section might indicate high-productivity conditions hostile to oligotrophic organisms, including the nannoconids. However, at Site 463, radiolarians are often abundant along with nannoconids (Schaaf, 1981; Erba, 1994). Most likely, a variety of environmental parameters influenced the distribution of this group.

Recovery of "Basement" of Shatsky Rise

Coring in Hole 1213B terminated in mafic igneous rocks on the flanks of southern Shatsky Rise. In all, 46.4 m of igneous section was cored, with 33.4 m recovered (72%). Six cores, 198-1213B-28R through 33R, recovered mostly massive diabase and basalt from Subunits IVA, IVB, and IVC, with each subunit thought to be a separate sill. The igneous rocks are predominantly hypocrystalline, fine-grained diabase (97.6%) with a small amount of sparsely phyric, aphanitic basalt (2%) at contacts. The diabase groundmass consists mainly of euhedral to subhedral plagioclase and intervening subhedral pyroxene and olivine, with minor glass. Alteration in the igneous section ranges from minor to moderate. Plagioclase and pyroxene crystals are locally altered to clay, and in thin section, glassy groundmass has been ubiquitously devitrified and/or altered to clay minerals. Basaltic material in the section is present at subunit contacts, symmetrically disposed around pieces of metasediment that mark the subunit boundaries. From the chilled contacts, the basalt grades toward more coarse-grained diabase in the unit centers, where the groundmass approaches medium grained.

The sills must be of earliest Berriasian age or younger, since this is the age of the host sediment. Paleomagnetic data show that two subunits have positive magnetic inclinations, whereas the third, basal subunit has a negative inclination, implying both normal and reversed magnetic polarities are recorded in the igneous section. This mixture indicates that the sills must have formed either before or after the long Cretaceous normal superchron (i.e., the Cretaceous Quiet Zone; 121-83 Ma). On the seismic profile along which Hole 1213B was drilled, the seismic "basement" has an odd character that may be related to the presence of intrusive, rather than extrusive, igneous rock at the sediment-igneous rock contact (see line 17B in Figure F8 in Klaus and Sager, this volume). The "basement" reflector, that being the deepest continuous seismic horizon, is weaker than elsewhere on Shatsky Rise, and other stronger reflectors occur beneath it. These deeper reflectors were not considered "basement" because they are not continuous along the profile, as is the weaker, shallower horizon. The cored section suggests that the weak "basement" horizon denotes the top of the sills, whereas the deeper reflectors may be the top of the extrusive lava pile.

Background

Site 1214 is located at 3402 m water depth on the southern flank of the Southern High of Shatsky Rise. The site is at the location of DSDP Site 306 (Larson, Moberly, et al., 1975). At Site 306, 475 m of sedimentary rocks was penetrated with RCB drilling but only continuously cored from 207.5 m to total depth. Most of the upper section (28 to 207.5 mbsf) was alternately cored and washed at ~9-m intervals. The recovery was ~7%. The recovered sedimentary rocks, mainly calcareous porcellanite and chert with minor nannofossil chalk and ooze, are late Albian to early Berriasian in age, covered by ~9 m of Holocene siliceous foraminifer-bearing nannofossil ooze.

According to the coring record from Site 306 (Larson, Moberly, et al., 1975), the uppermost Cretaceous sediments lie between Reflectors R1 and R2 of Sliter and Brown (1993). Reflector R1 (Cenomanian/Turonian boundary) crops out a few kilometers northeast of Site 306. Reflector R2 of Barremian/Aptian boundary age corresponds to the top of the porcellanite, chert, and shale section at Site 306 that rests on basement. The interval between R1 and R2 consists of chalk, chert, and porcellanite.

Site 1214 lies in the middle of the mid-Cretaceous portion of the Shatsky Rise depth transect between Site 1207 (3101 m) and Site 1213 (3883 m). The objective at Site 1214 was to recover a more complete and continuous record of the Site 306 sequence. As part of the Shatsky Rise mid-Cretaceous depth transect, drilling at Site 1214 addresses a number of leg-related objectives concerning ocean circulation and environmental change in an interval of global warmth.

Summary of Results

Hole 1214A was cored with RCB drilling terminating at 235.9 mbsf in the Hauterivian (Table T1). Average recovery of 7% was equivalent to Site 306. The sedimentary section is divided into units based on composition and color. Lithologic Unit I (Holocene to Pleistocene; 0.0-6.9 mbsf) consists of very pale yellowish brown to moderate yellowish brown clayey foraminiferal nannofossil ooze and very pale to moderate yellowish brown clayey nannofossil ooze with foraminifers. Subunit IIA (late Albian; 6.9-34.5 mbsf) is composed of moderate yellowish brown to light olive-gray chert and very light gray to yellowish gray porcellanite, with minor white nannofossil chalk. Medium dark gray, dark gray to grayish black chert, and very light gray to light greenish gray porcellanite are grouped in Subunit IIB (late Albian to early Albian; 34.5-110.7 mbsf). Very light to light greenish gray limestone with radiolarians is a minor lithology in this subunit. Subunit IIC (early Albian to late Aptian; 110.7-207.0 mbsf) contains medium dark gray, dark gray, brownish gray, grayish brown and moderate brown chert, and yellowish gray, very light to light greenish gray porcellanite. Olive-black, greenish black and dark greenish gray claystone to clayey porcellanite is placed in Subunit IID (early Aptian; 207.0-216.6 mbsf). Finally, Subunit IIE comprises brownish gray, brownish black, and olive-black chert and light greenish gray, greenish gray, and yellowish gray porcellanite (Hauterivian; 216.6-235.9 mbsf). Throughout the section, porcellanite and limestone have variable amounts of radiolarians and nannofossils. Most of the ages were obtained from chalk adhered to the sides of chert fragments. This sediment indicates the soft and relatively unaltered nature of the unrecovered intervals.

Highlights

The results of coring at Site 1214 provide important information on the stratigraphy of the mid-Cretaceous section on Shatsky Rise. The interpreted stratigraphy of Site 1214 is different from the stratigraphy of Site 306 from Larson, Moberly, et al. (1975) and Sliter (1992). One possibility for this difference is that Site 1214 is actually located at a distance from Site 306 given the imprecision of the coordinates of the latter site. A second possibility is that the biostratigraphy of Site 306 was carried out almost exclusively using planktonic foraminifers, whereas that of Site 1214 included nannofossils and radiolarians.

Site 1214 has an expanded Albian section, ~180 m in thickness, compared to 120-130 m at Site 1213. One possible reason for this disparity is that the latter site is located at greater water depth and thus subject to greater dissolution. More significantly, the stratigraphy of Site 1214 confirms that much of the Barremian corresponds to a regional unconformity on the southern part of Shatsky Rise. In both locations, lower upper Hauterivian sedimentary rocks are found in the core below lower Aptian horizons. At Site 1213, one sample at the top of Core 198-1213B-9R contained neither Aptian nor Hauterivian markers. Even with this uncertainty, combined data from Site 1213 and 1214 suggest that a significant hiatus occurred during much of the Barremian and at least part of the upper Hauterivian. The reason for this unconformity is currently unclear. This interval was present at Site 1207, which was some 800 m shallower than Site 1213 at that time; thus, a temporary change in the CCD could have led to the hiatus. However, the Barremian in most parts of the ocean, including the Pacific, is generally regarded as an interval when the CCD was relatively deep (i.e., Thierstein, 1979). Thus, it is possible that this interval corresponded to an erosional event that removed sediment from the flanks of the Southern High and not from the top of the Northern High.

The lower Aptian section in Hole 1214A (Section 198-1214A-23R-1) contains olive-black, greenish black, and dark greenish gray claystone to clayey porcellanite. Both of these lithologies are laminated in places indicating low-oxygen conditions on the seafloor. C_org contents are low (<0.5 wt%) in the greenish gray lithologies. Four thin, discrete bentonite or tuff layers are observed within pieces of porcellanite. A sample from a piece of olive-black claystone toward the top of Section 198-1214A-23R-1 contains 1.4% C_org. Characterization of the organic matter from this sample indicates it is algal and bacterial in origin, similar to highly carbonaceous lower Aptian lithologies from Sites 1207 and 1213 (see "Geochemistry" in "Specialty Syntheses"). This interval lies in nannofossil Zone NC6, between the FOs of Eprolithus floralis and Hayesites irregularis. Most of the sedimentary rocks in Section 198-1214A-23R-1 are noncalcareous but contain abundant radiolarians in discrete layers. Diagnostic radiolarian faunas in these levels are similar to assemblages observed in the Livello Selli in the Cismon core from the southern Alps of Italy (Premoli Silva et al., 1999) and in horizons bounding organic-rich intervals in the lower Aptian at Site 1213 (see "Biostratigraphy" in "Specialty Syntheses"). Thus, combined biostratigraphic and lithostratigraphic evidence suggests that the olive-black claystone comes from within the general OAE1a interval, but certainly not from near the peak of the event as did samples with much higher C_org contents from Sites 1207 and 1213.