LITHOSTRATIGRAPHY

Two holes were rotary cored at Site 1213 on the southern flank of the Southern High of Shatsky Rise at a water depth of 3883 m—the deepest water depth of any site drilled during Leg 198. The drilled section totaled 494.4 m. The objectives of this site were to core through a thin Quaternary-Neogene drape to recover a Lower Cretaceous sequence inferred to be present from seismic reflection records and to penetrate and recover basaltic "basement." It was hoped that the recovery of C_org-rich horizons, which were deposited during early Aptian OAE1a, at this deeper-water site would serve for comparison with similar units obtained at Site 1207, so that the deposition of such C_org-rich units in a pelagic environment could be better documented. Basaltic basement has never been recovered during previous Deep Sea Drilling Project (DSDP) or Ocean Drilling Program (ODP) drilling at Shatsky Rise, with the exception of a basalt pebble recovered at the bottom of Hole 50. Basement recovery is essential for testing models of the origin and evolution of Shatsky Rise that suggest it is the product of a migrating hotspot. All of these objectives, to some extent, were achieved at Site 1213.

As at Site 1207 (and at Leg 32 Sites 305 and 306), core recovery was hampered by the presence of significant chert within the Cretaceous sequence. Chert was first encountered at ~64.7 mbsf in Hole 1213A, according to the driller's log, and was recovered in Section 198-1213A-7R-CC and in nearly every core thereafter to the bottom of the sedimentary section at 447.8 mbsf. In fact, chert and associated porcellanite are our main records of sedimentary environments in the Lower Cretaceous sequence. Recovery averaged ~20% at Site 1213 but was less than this throughout most of the Cretaceous section (Fig. F2). Of particular interest is the recovery of at least a partial record equivalent to the early Aptian OAE1a in Core 198-1213B-8R for which C_org contents of 2.87, 10.23, and 25.17 wt% were obtained (see "Organic Geochemistry"). Limestone and chalk, with varying carbonate content, were also recovered intermittently in the Cretaceous sequence. The record for the Berriasian through mid-Cenomanian is sufficient to conclude that deposition occurred largely above the CCD at this site through all but the interval during OAE1a. With the exception of short episodes during the early Aptian, Valanginian, and Berriasian when thin intervals of laminated, C_org-rich sediments were deposited, the recovered strata were bioturbated and C_org poor. Much of the Barremian lies within an unconformity or condensed interval (see "Biostratigraphy").

"Basement" recovery was in the form of diabase sills. Thus, we cannot ascertain the age of these units until radiometric ages are obtained. The presence of sills suggests that Shatsky Rise has had a prolonged eruptive history. Therefore, the objective of recovering normal extrusive basaltic rocks associated with the formation of Shatsky Rise volcanic edifice remains elusive.

The stratigraphic sequence at Site 1213 has been subdivided into four major lithologic units. Unit I, of Holocene to early Pliocene age, consists primarily of interbedded (cyclic) nannofossil ooze with clay and clayey nannofossil ooze. Unit II, at a depth of 56.4-85.4 mbsf, primarily consists of Santonian to Cenomanian pale orange nannofossil ooze and chert characterized by red hues. At depths of 85.4-447.8 mbsf, chert, porcellanite, and limestone primarily compose Unit III. This unit, which is early Cenomanian to Berriasian in age, is subdivided into five subunits (A-E), mainly on the basis of systematic changes in chert and porcellanite color. Unit IV comprises at least three diabase sill units totaling ~46 m.

Lithologic Unit I

Interval: 198-1213A-1R-1, 0 cm, through 6R-CC

Depth: 0.0 to 56.4 mbsf in Hole 1213A

Age: Holocene to early Pliocene

Lithologic Unit I is not subdivided into any subunits. This unit is relatively thin in comparison to the same lithology encountered at other Leg 198 sites. Unfortunately, the rotary drilling caused considerable disturbance, in places severely limiting our ability to discern primary structures and textures in the cores. The primary lithology varies from bioturbated light olive-gray (5Y 6/1) to pale yellowish brown (10YR 6/2) nannofossil ooze with clay interbedded with clayey nannofossil ooze. Carbonate contents range between 35 and 80 wt% (Fig. F2). Diatoms, foraminifers, pyrite, and ash are the most abundant accessory components with diatoms occurring only in the top 9 m. In comparison to other Leg 198 sites, no pale green "diagenetic laminae" were observed. Moderate yellowish brown (10YR 5/4) to dark yellowish brown (10YR 4/2) nannofossil clay with carbonate contents of 10-35 wt% occurs in the lowermost few meters (Fig. F3) of Unit I, just above a major unconformity between the Neogene and Upper Cretaceous.

Lithologic Unit II

Interval: 198-1213A-7R-1, 0 cm, through 9R

Depth: 56.4 to 85.4 mbsf in Hole 1213A

Age: Santonian to early Cenomanian

In Unit II, nannofossil ooze was recovered only in Section 198-1213A-7R-1. The grayish orange (10YR 7/4) ooze in this core exhibits striking and unusual color banding of dark yellowish orange (10YR 6/6) to light brown (5YR 5/6) interposed with grayish orange ooze on a 1- to 3-cm scale (Fig. F3). The darker bands are more clay rich and somewhat indurated relative to the nannofossil ooze and are commonly underlain and/or overlain by a thin interval of very pale orange (10YR 8/2) nannofossil ooze.

Subunit IIA

Unit II is separated into two subunits, primarily on the basis of the relative abundance of chert. Subunit IIA (Core 198-1213A-7R), from 56.4 to 66.1 mbsf, consists of Santonian (possibly upper Coniacian) very pale orange (10YR 8/2) nannofossil ooze and moderate brown (5YR 3/4) to dusky brown (5YR 2/2) chert that was recovered at 64.7 mbsf, according to the driller's log.

Subunit IIB

Subunit IIA is underlain by Subunit IIB (Cores 198-1213A-8R and 9R), which extends from 66.1 to 85.4 mbsf. This subunit is early to middle Cenomanian in age and consists primarily of moderate yellowish brown (10YR 5/4), dark yellowish brown (10YR 4/2), and light brown (5YR 5/6) chert, and associated very pale orange (10YR 8/2) porcellanite.

Lithologic Unit III

Intervals: 198-1213A-10R through 21R (end of hole) and 198-1213B-1R through 27R

Depth: 85.4 mbsf in Hole 1213A to 447.8 mbsf in Hole 1213B

Age: early Cenomanian through Berriasian

Subunit IIIA

Unit III is subdivided into five subunits, mainly on the basis of chert color and changes in associated lithologies. Subunit IIIA (Cores 198-1213A-10R to 19R) extends from 85.4 to 179.6 mbsf and is early Cenomanian to late Albian in age. It consists of medium dark gray (N5) to dark gray (N4) chert associated or interbedded with light gray (N7), light greenish gray (5GY 8/1), and very pale orange (10YR 8/2) porcellanite and light greenish gray (5GY 8/1) limestone (primarily in Cores 198-1213A-12R to 19R).

Subunit IIIB

Subunit IIIB (Cores 198-1213A-20R through 21R and Cores 198-1213B-1R through 7R) encompasses the interval from 179.6 to 256.8 mbsf and is early late Albian-early late Aptian in age. Primary lithologies are light to moderate brown (5YR 5/6 to 5YR 3/4), grayish brown (5YR 3/2), moderate reddish brown (10R 4/6), and dusky red (10R 4/6) chert associated with very pale orange (10YR 6/2) to grayish orange (10YR 7/4) and pale yellowish brown (10YR 6/2) porcellanite.

Subunit IIIC

Subunit IIIC (Core 198-1213B-8R), of early Aptian age, occurs between 256.8 and 266.4 mbsf. This subunit is characterized by interbedded C_org-rich clayey porcellanite and radiolarian porcellanite, mostly olive black (5Y 2/1) to greenish black (5G 2/1) and dusky green (5G 3/2), with minor altered tuff. Although C_org contents of selected samples are high (Fig. F4), there is no clear evidence of fine lamination that would indicate deposition under an anaerobic water column (Figs. F5, F6). Bioturbation is moderate to intense in this interval.

Subunit IIID

Subunit IIID (Cores 198-1213B-9R through 23R) lies between 266.4 and 410.3 mbsf, extending from the Hauterivian to the Berriasian. Medium gray (N5) through dark gray (N3) chert, white (N9) to yellowish gray (5Y 8/1) porcellanite, and mostly light greenish gray (5GY 8/1) with some olive-gray (5Y 4/1) nannofossil chalk to clayey nannofossil chalk are the primary lithologies recovered. Incomplete recovery prohibits conclusions regarding the pervasiveness of carbonate cycles, but some longer chalk intervals indicate substantial variations in carbonate content over tens of centimeters (Fig. F7). Bioturbation is moderate to intense throughout this subunit (Fig. F8). One interval exhibits features suggesting soft-sediment deformation, possibly a small slump (Fig. F9).

Subunit IIIE

Subunit IIIE (Cores 198-1213B-24R through 27R), of Berriasian age, is the lowermost of the sedimentary units recovered at Site 1213. This subunit encompasses the interval at 410.3-447.8 mbsf and consists of brownish gray (5YR 6/1), grayish red (10R 4/2), and pale to moderate brown (5YR 5/2 and 5YR 6/4) chert, yellowish gray (5Y 8/1) and very pale orange (10YR 8/2) porcellanite, and moderate brown (5YR 6/4) to pale brown (5YR 5/2) claystone with nannofossils. A silicified goethitic claystone breccia (Fig. F10) at the base of Core 198-1213B-27R exhibits colliform features. A thin section of this sample exhibits possible microbial structures (Fig. F11).

Lithologic Unit IV

Interval: 198-1213B-28R through 33R

Age: Unknown

Depth: 447.8-494.4 mbsf in Hole 1213B

Lithologic Unit IV consists of mafic igneous rocks, dark greenish gray (5G 4/1) diabase or dolerite (97.6%) and dark gray (N3) basalt (2.0%), with intervening metasedimentary rocks (0.4%) (Tables T2, T3). This unit is further subdivided on the basis of lithologic patterns into at least three subunits (A-C) thought to represent at least three intrusive events separated by "baked" sediment (Fig. F12). The uppermost chilled margin of Unit IV is pictured in Figure F13, and baked contacts are shown in Figures F14 and F15.

Rock names are based on hand specimen and thin section description. No bulk chemical data were collected to define the chemistry of the igneous rocks, but one bulk X-ray diffraction (XRD) of the diabase confirms the presence of clinopyroxene (diopside and/or titanium-aluminum clinopyroxene), plagioclase feldspar (calcian albite and/or sodian anorthite), and clay minerals (montmorillonite/smectite alteration products of minerals and glassy groundmass) observed in thin section. The term diabase is applied to the lighter, more phaneritic parts of the core, and basalt is applied to the darker, more aphanitic, and less crystallized rocks. Diabase crystallinity (crystal size) is not uniform through a given subunit, with the most coarsely crystalline diabase present in the middle of the subunits. The diabase is generally fine grained but locally ranges to medium grained. Where contacts between basalt and diabase are preserved, they appear to be gradational (Fig. F15). Thin section observations confirm the subtle gradation between diabase and basalt in the core (Figs. F16, F17, F18).

The diabase appears relatively fresh in hand specimen with primarily fine-grained phaneritic texture. There are few phenocrysts, with the rock mainly composed of felted groundmass of euhedral to subhedral plagioclase microlites and intervening subhedral to anhedral pyroxene, olivine, and glass. Thin sections prepared from these units show the rock to be largely hypocrystalline (mostly crystals) to nearly holocrystalline (5% glassy groundmass). These rocks exhibit subophytic (Fig. F18) to intergranular textures (Fig. F16). Plagioclase crystals are 1.8 mm long, averaging from 0.7 to 0.8 mm. Olivine and clinopyroxene crystals are generally smaller, averaging up to 0.7 mm in the subophytic diabase and up to 0.5 mm in the intergranular diabase. Additional minerals include opaque crystals of ilmenite/magnetite(?) that are locally skeletal. Both pyroxene and olivine are relatively colorless and therefore differentiated primarily on the absence/presence of cleavage. In all instances, the glassy groundmass is devitrified and/or altered to green/brown clay minerals. Plagioclase crystals often exhibit fine hairline fractures filled by birefringent clay minerals. Patches of clay and carbonate with relict cleavage in some samples suggest replacement of a mineral phase, possibly pyroxene (Fig. F15).

In hand specimen, the basalt ranges from aphyric to sparsely phyric to moderately phyric with glassy to microcrystalline groundmass and dominantly millimeter-sized phenocrysts (Figs. F13, F14, F15). Only one thin section of basalt was made to preserve this material for postcruise studies. One gradational basalt sample (198-1213B-30R-4, 39-41 cm) shows mainly intersertal (hyalophitic) texture but ranges to intergranular in some crystal-rich areas. A basalt sample with seriate texture exhibits quench textures: swallow-tail plagioclase crystals and fibrous bundles of crystallites (Sample 198-1213B-31R-1, 7-10 cm; Fig. F16). Alternatively, the latter could be devitrification products of the once glassy, now altered groundmass. This sample is cut by a smectite-filled vein and contains amygdules filled by fibrous smectite. The plagioclase crystals are moderately to extremely altered (Fig. F16) to green clay minerals (smectite?).

Sparse (<1% by volume) irregular to rounded glomeroporphyritic clusters are present throughout Unit IV (at 56 and 59 cm in Fig. F15; also see Figs. F18, F19, F20). These clusters range up to 1 cm in size and are mainly composed of fine-grained plagioclase and pyroxene. In thin section, the glomeroporphyritic clusters are composed of plagioclase feldspar crystals 2 mm long that are separated by patches of pyroxene and opaque minerals. In one instance, extinction under cross-polarized light migrated from crystal to crystal, suggesting that the crystals may be xenoliths rather than magmatic crystal aggregates.

In hand specimen, vesicles are essentially limited to the intervals of basalt, and these are often completely filled (amygdules) by calcite and/or green clay minerals (Fig. F16). The vesicles are round to irregular in shape and range up to 1.5 mm in diameter. Vesicle content, up to a few percent of the rock volume, was noted to increase toward inferred upper and lower contacts with metasedimentary rocks.

The vein systems are oriented subvertically (e.g., Subunit IVA) to subhorizontally (Fig. F21). They are filled by dark green clay minerals, white calcite, or mixtures of the two, locally with pyrite and rarely with quartz. The veins are sometimes monomineralic, but the larger ones are often zoned (Fig. F22), exhibiting crack-seal textures. The largest vein, in Section 198-1213B-33R-4, is vuggy and only partly filled by calcite. Slickensides and vein offsets suggest that there has been some microfault displacement along these features.

Two fragments of nonvolcanic rock are present within Unit IV (disregarding possible downhole contamination at the tops of some cores). These loose fragments are "sandwiched" between intervals of basalt, but with no preserved contacts (Figs. F12, F13, F14). Thin sections of these fragments show them to be silicified burrowed shale/phyllite with volcanic ash (Sample 198-1213B-30R-4, 66-69 cm) and metachert (Sample 198-1213B-32R-4, 65-74 cm). Petrographic evidence for thermal alteration of these sedimentary rocks includes a well-developed uniform extinction in the shale/phyllite (with nicols crossed) and microcrystalline (0.1 mm) subequal crystals of quartz (recrystallized) encompassing fibrous bundles of sillimanite or possibly tremolite/actinolite within the metachert (Fig. F23).

Open fractures in the core are likely drilling induced and are often localized along thin veins. Some dark seams, expansion fractures(?), were noted. These are more closely spaced in the basalt contact zones (a few millimeters apart) and become more widely spaced (a few centimeters apart) into the adjacent diabase, where they eventually disappear. This relationship suggests that their development may be associated with the rate of cooling. In thin section, these dark seams are filled with greenish brown clay minerals.

Hiatuses

Sedimentation on this part of Shatsky Rise appears to have been continuous during the Early through mid-Cretaceous (see "Biostratigraphy"). However, there are at least two intra-Cretaceous unconformities represented by the absence of upper Cenomanian through at least lower Coniacian strata between Cores 198-1213A-8R and 7R and most or all of the Barremian between Cores 1198-1213B-8R and 9R. A barren interval in the base of Core 198-1213A-6R overlies definite Santonian nannofossil ooze in Core 7R, and this, in turn, is overlain by lower Pliocene ooze in the upper part of Core 6R. The oxidized, clay-rich bands in the Santonian ooze may reflect slow deposition above the unconformity with the Cenomanian. In seismic reflection lines (see "Background and Objectives") over Site 1213, Cretaceous reflectors can be observed terminating against a more flat-lying pelagic drape that presumably reflects the Neogene-Quaternary sequence. However, it is not easily discerned whether the condensed Santonian-Coniacian interval is part of the pelagic drape. If that is the case, it would appear that the slight angular unconformity between the Santonian-Coniacian and older Cretaceous strata represents an erosional unconformity, perhaps resulting from enhanced deep currents sweeping the flanks of Shatsky Rise, rather than a dissolution horizon. This hypothesis is supported by the fact that the mid-Cretaceous sequence was present, but poorly recovered, at Site 1207 on the Northern High. However, enhanced rates of carbonate dissolution at deeper depths could have undermined the slope sediment buttress and caused slumping of sediments at shallower depths on the Southern High. Enhanced carbonate dissolution during a CCD rise in the early Aptian (see below) may also be invoked to explain the missing Barremian at Sites 1213 and 1214.

A second unconformity or condensed interval occurs between the Santonian and Neogene. This stratigraphic gap is widespread, having been encountered at Sites 1207 and 1208 on the Northern and Central Highs where most of the Paleogene and uppermost Cretaceous are missing on the basis of age calibration of seismic reflection records. It is likely that this unconformity also represents the erosive effects of deep currents sweeping the upper Shatsky Rise that waned somewhat in the Miocene. Most of the Upper Cretaceous and Paleogene were recovered, however, at all sites on the Southern High except at Site 1213. Again, carbonate dissolution could have played a role.

Carbonate Preservation and Diagenesis

Unit III encompasses the Berriasian through mid-Cenomanian. Relatively high carbonate contents persist in the nonchert lithologies recovered through this interval, suggesting that deposition occurred above the CCD except during the early Aptian (see below). We do not have a definitive indication of paleowater depth, but it is very likely that the seafloor depth was shallower than at present, particularly during the Early Cretaceous (Fig. F25). The earliest sedimentary rock above the diabase of Unit IV is red nannofossil claystone with carbonate content of 34 wt%, the lowest carbonate content of the Cretaceous with the exception of the lower Aptian C_org-rich interval. Carbonate contents generally increase upward from the basal Berriasian but vary considerably. Recovery of chalk was better in part of Subunit IIID, particularly in Cores 198-1213B-20R through 24R, and the recovered intervals reveal carbonate cycles on scales of 10-20 cm, although there is insufficient continuous material to establish a specific periodicity or pattern. These cycles may represent either variation in carbonate dissolution or productivity inasmuch as we would not expect strong variation in dilution by terrigenous material in this pelagic setting. Radiolarians appear to be concentrated in more C_org-rich intervals of some of the cycles (e.g., in Section 198-1213B-22R-1), which could indicate higher siliceous productivity; however, it is not clear that radiolarian relative abundance can be related directly to primary production (see below). Radiolarians are also associated with chert and porcellanite, which, in part, replace carbonate.

It is also interesting that, with the exception of replacement of chalks by porcellanite throughout the sequence and some limestone pieces recovered in Subunit IIIA, the diagenetic grade of carbonates did not go beyond "chalk" downhole. The lack of limestone in the pre-Cenomanian sequence must be a function of a continually shallow burial depth throughout the depositional history of this part of Shatsky Rise, possibly as a result of several erosional episodes. The presence of limestone in the lower Cenomanian Subunit IIIA remains unexplained.

Biogenic Silica Deposition and Diagenesis

Radiolarites, most occurring as porcellanite, are common throughout the Cretaceous of Site 1213. Chert is nearly ubiquitous as well, and much of the silica must have been derived from radiolarian dissolution. In the Berriasian through Hauterivian, radiolarians in chalks commonly have been replaced by calcite, and some were filled later with chalcedony. There is evidence for relatively early silica cementation because chert commonly preserves burrows without compactional flattening, whereas burrows in chalks, particularly clayey chalks, are characterized by compactional flattening. However, breakage of radiolarians in radiolarian-rich intervals can be observed in some samples. A breccia of slightly disturbed chert rip-up clasts, cemented by chalcedony, is present in Core 198-1213B-3R and suggests formation of an early "firmground," providing further support for early silica diagenesis. We interpret chert color generally to reflect the oxidation state of associated sediments. For example, red and brown chert hues accompany very pale orange to pale orange chalks and oozes and black to gray chert hues typically are associated with white to grayish green chalks and porcellanites. In some cases, however, we observed contrasting colors. For example, in Sample 198-1213B-24R-1, 115-121 cm, a very irregularly bounded reddish colored chert nodule is set in a grayish green chalk. It is difficult to understand how chert could form from postdepositional fluids that carried oxygen and only affected areas of otherwise reduced sediments where chert precipitation occurred. More likely, the quartz precipitated early, prior to postdepositional reduction of the sediment, thereby preserving a reddish, oxidized hue.

Some radiolarian-rich porcellanites exhibit signs of reworking by bottom traction currents, including ripple cross-lamination and parallel laminae to flaser structure. Such features are most common in Cores 198-1213B-4R through 6R in conjunction with oxic conditions. In general, the locally high concentrations of radiolarian tests likely indicate winnowing and concentration by currents.

The abundance of radiolarian-rich strata and chert recovered from the Cretaceous section at Site 1213 might suggest high productivity of siliceous biogenic material and could be related to the near paleoequatorial location of Shatsky Rise through most of the Cretaceous. On the other hand, in the Cretaceous, radiolarian abundance could be a function of enhanced preservation in silica-rich deepwater masses as indicated by radiolarian-rich strata widespread in the Pacific and Tethyan basins.

Cretaceous Redox Variations

Orange, red, and brown hues in cherts, porcellanites, and, to some extent, chalks, are characteristic of Subunits IIIE and IIIB. Cherts in Subunits IIID and IIIA have gray hues, and the porcellanites and chalks recovered from these intervals are primarily of grayish green to olive-green hue. Subunit IIIC represents the lower Aptian C_org-rich interval for which no chert was recovered, but the porcellanites and clayey porcellanites are dark colored and C_org rich. Thus, we can outline a trend from the oxidizing conditions that prevailed in the basal Berriasian to reducing conditions in the remainder of the Berriasian through early Aptian. This was followed by oxidizing conditions in the late Aptian through middle Albian, a return to reducing conditions in the late Albian to early Cenomanian, and oxidizing conditions in the early Cenomanian and later Cretaceous, at least as far as the recovered section permits inferences to be made.

Most of the more reduced intervals are not necessarily related to lower oxygen concentrations in deep water because higher sedimentation rates can drive redox reactions by promoting initial preservation of small amounts of organic matter and limiting penetration of oxygen across the sediment/water interface. In particular, the Berriasian interval was deposited at a rapid rate (see "Sedimentation and Accumulation Rates"), as was the upper Albian through lower Cenomanian, and both intervals are characterized by chert hues reflecting reducing conditions. Most of the sequence is not particularly pyritic (megascopic observation), and C_org contents are low with the exception of one thin, irregularly laminated clayey chalk with 3.13 wt% C_org in Sample 198-1213B-19R-1, 112-113 cm. This is true of most of the recovered section above, except for a 1-cm interval of Valanginian clayey, radiolarian chalk (Sample 198-1213B-15R-1, 9-10 cm, with 2.54 wt% C_org) and the lower Aptian C_org-rich interval.

The Valanginian relatively C_org-rich layer has been heavily bioturbated and exhibits no characteristics indicative of seafloor anoxia. However, if short-term anoxia did occur, any fine lamination could have been destroyed by subsequent bioturbation. Biostratigraphic determinations place the age of the recovered interval within the interval of the Valanginian positive carbon isotope excursion (Lini et al., 1992) for which there are, to date, no known exceptionally widespread C_org-rich deposits. It is possible that we recovered only one of a number of such beds in the Valanginian at Site 1213. Apparently, the C_org-rich intervals are too thin to capture in downhole log signatures (see "Downhole Measurements"), so we cannot document the possible significance of Valanginian events here.

The lower Aptian Selli equivalent was partially sampled at Site 1207 (Core 198-1207-44R) and consists of a faintly laminated radiolarian claystone containing as much as 34 wt% C_org. In Core 198-1213B-8R, we recovered a sequence of radiolarites, porcellanites, and radiolarian claystones that contain high amounts of C_org and correspond to OAE1a (e.g., Bralower et al., 1994) or the Selli event. Several C_org determinations were made, and a maximum of 25.2 wt% was obtained. Recovery of this interval is fragmentary, and there are few continuous core pieces to examine. There are a number of individual C_org-rich units, most of which have been heavily bioturbated and homogenized such that evidence of any original fine lamination has been destroyed. The claystones and porcellanites commonly appear somewhat coarsely laminated because of abundant flattened, dark, horizontal burrows. Nonetheless, one must infer from the high contents of C_org and high pyrolysis hydrogen indices (>500) (see "Organic Geochemistry") that conditions were at least intermittently anaerobic at the seafloor. The downhole logs suggest that the interval in question was as much as 7 m thick overall, but the main interval of C_org-rich sediment occurs between 257 and 260 mbsf. This is exhibited as a double peak of higher gamma ray and resistivity values and as higher uranium and potassium values in the spectral gamma log for this interval (see "Downhole Measurements").

The difference in present water depths of Sites 1207 (3101 m) and 1213 (3882 m) is nearly 800 m, and a larger depth difference may have characterized the early Aptian because of the younger crustal age of northern Shatsky Rise (Chron M12 at ~133 Ma vs. Chron M21 at ~147 Ma for southern Shatsky Rise). Thus, one could envision that an extensive zone of water-column anoxia prevailed in the region during the early Aptian. On the basis of plate tectonic reconstructions, the southern Shatsky Rise would have been located under the paleoequator at ~120 Ma. As a result, high rates of C_org production in surface water masses may have prevailed over the site. Sediment of Cores 198-1213B-8R and 198-1207B-44R is very low in carbonate, suggesting that the CCD rose above the depth of both sites during the event. This scenario is similar to that in the North Atlantic Basin (e.g., Arthur and Dean, 1986; Bralower et al., 1994). At the same time, radiolarians are quite abundant through this interval, suggesting high rates of siliceous production. The combination of relative lack of dilution by carbonate or clay, high C_org fluxes, and enhanced preservation under low-oxygen conditions probably produced the extraordinary C_org concentrations in this interval.

The change to oxidizing conditions in the early Aptian through middle Albian at Site 1213, following early OAE1a, can also be identified using chert coloration (Table T4) at Site 1207. Likewise, a return to reducing conditions in sediments of Cenomanian age, and post-Cenomanian oxic conditions are seen at both sites. These trends may be related to global trends in deepwater oxygen concentration but are not clearly related to paleolatitude changes inasmuch as Shatsky Rise remained in a potential region of high productivity through the Cenomanian at least (see "Background and Objectives").

Hydrothermal Alteration

Several interesting features appear in Cores 198-1213B-26R and 27R that we interpret as resulting from deformation and fluid circulation induced by diabase sills that intruded into the sedimentary sequence. The most obvious of these features is the breccia at the base of Section 198-1213B-27R-1. This breccia consists of 1- to 3-cm fragments of relatively soft goethitic clay held together by a stockwork of fine veins of chalcedony as confirmed by XRD data (Fig. F10). We suggest that this unit resulted from a clay-rich sediment that was brecciated during intrusion of one or more diabase sills then altered by warm, silica- and iron-rich fluids arising from the cooling diabase. It is possible that these fluids percolated further upward through fractures and created zones of reduction along fractures in dominantly red nannofossil claystones observed in Core 198-1213B-27R, as well as diffuse pale blue and yellowish orange alterations along fractures in chert of Core 26R. These fluids may have also nourished microbial communities within the pores of the brecciated sediment.

Diabase Sill Units

The igneous rocks recovered at Site 1213 are massive, without fragmental or pillow structures. The basalt intervals are thought to represent chilled margins of a series of intrusive dikes/sills (Subunits IVA-IVC). Gradational changes in crystallinity within the subunits, from outer chilled margins (basalt) to more coarsely crystalline and more holocrystalline interiors (diabase), are consistent with mafic magma intrusion. A fibrous mineral in the metachert fragment is likely metamorphic, given its inclusion within multiple quartz crystals within coarser (0.1 mm) mosaics associated with relict burrow mottles; burrows in Unit III cherts are often filled with more porous porcellanite and therefore would have likely been the focus of hot fluid migration during contact metamorphism. The symmetry of the basalt/diabase transition around metasedimentary rock fragments in the cores suggests that these represent country rock that hosted the intrusions. Expansion fracture patterns in the basalt, localization of vesicles near contacts, and observed quench textures in the basalts also support an intrusion interpretation.

The shallowest intrusion (Subunit IVA) is ~20 m thick, the medial intrusion (Subunit IVB) is ~14 m thick, and the basal intrusion (Subunit IVC) is, at a minimum, 7 m thick on the basis of a comparison of cored intervals vs. recovered material. There is little apparent textural or compositional difference among the three subunits, but the paleomagnetic signature of the last core (Core 198-1213B-33R) indicates that it may be much older (see "Paleomagnetism"). A thin section of the diabase from this last core (Fig. F24) shows this rock to contain large, zoned, and altered plagioclase phenocrysts unlike any observed in thin sections from Cores 198-1213B-28R to 32R. This sample also appears to be altered in general. No intrusive contact relationships were recovered in Core 198-1213B-32R, where recovery was only 70%. The presence of rubble zones and large mineralized fractures within the base of Cores 198-1213B-32R and 33R are also consistent with a fault contact interpretation.