LITHOSTRATIGRAPHY (continued)

Interval: top: 210-1276A-29R-6, 62 cm; base: 102R-1, 2 cm, through Core 104R (no recovery)

Depth: top: 1067.24 mbsf; base: 1732.12-1736.90 mbsf

Age: latest Aptian-Turonian(?)

Lithology: gray to olive-black sandy to muddy turbidites with lesser burrowed mudrocks, minor debris flows and finely laminated calcareous claystones, and black shales

Unit 5 is the thickest lithologic unit drilled during Leg 210. Its main characteristic is a high proportion of mudrock (mudstone and claystone) and marlstone, which together form 60%-70% of the unit above ~1130 mbsf and 80%-90% or more in deeper cores. There is a variable abundance of coarser-grained redeposited facies that allows a meaningful subdivision of Unit 5. Colors are mainly subdued shades of green, gray, and black. The top of Unit 5 is defined as the base of the lowest occurrence of burrowed sandy mudstone in Unit 4. It extends downhole for 664.88 m to at least the deepest recovery of a diabase sill (Subunit 5C2) at 1732.12 mbsf. The hole is deeper than this—Hole 1276A bottomed at 1736.90 mbsf—but there was no recovery below 1732.12 mbsf.

Subunit 5A is distinguished by an abundance of dark greenish gray gravity-flow deposits plus subordinate, very burrowed olive-gray hemipelagic sediment. In contrast, Subunit 5B is composed of mainly burrowed background hemipelagic sediments that are greenish gray to black plus minor olive-gray to medium gray, slightly coarser deposits from low-density turbidity currents. Subunit 5B includes very finely laminated, calcareous, carbon-rich sediments (black shales) with TOC abundances of 1.5-4.0 wt%. Subunit 5C, like Subunit 5A, is dominated by thick to very thick, commonly poorly organized olive-gray to medium gray to greenish black gravity-flow deposits, including sandy debris flow deposits and graded beds with abundant syndepositional deformation. It has little true hemipelagic sediment.

Each of Subunits 5A through 5C is summarized below and detailed in subsequent sections. An interpretation of depositional processes and sediment provenance is given after the description of each of the subunits. The clay mineralogy, sediment chemistry, diagenetic processes, and genesis of the black shales are then discussed for Unit 5 as a whole.

Some similarities exist between Subunits 5C and 5A in that most of the sandstones, silty sandstones, grainstones, and mudrocks are present in thick to very thick, graded gravity-flow deposits with planar-laminated bases, syndepositionally deformed transitions from sandstone to mudstone (including load balls, flattened recumbent folds, and sheared-out laminae), and structureless calcareous mudstone tops. Burrows are rarely present in these units and are probably restricted to the escape burrows of buried and trapped organisms. In addition to the graded units with mudstone tops, there are a number of disorganized beds of silty sandstone in Subunit 5C with scattered large sedimentary clasts and sharp bed tops. These are interpreted as sandy debris flow deposits. It is estimated that 80%-90% of Subunit 5C was emplaced by mud-laden turbidity currents or by debris flows; only ~10% is background hemipelagic sediment. One difference between Subunits 5A and 5C is that black shales are more numerous in Subunit 5A, with only rare occurrences in Subunit 5C.

Despite the similarities noted above, the three subunits are somewhat compositionally distinct. Grainstones are present only in Subunits 5A and 5B. The nannofossil content of mudrocks and the foraminifer content of marlstones generally decrease downhole. In the grainstone, sandstone, and siltstone beds of Unit 5, glauconite is much more common in Subunits 5A and 5B, whereas red algal and bryozoan fragments were observed only in Subunit 5C. Lastly, diagenetic carbonate concretions and irregular concretionary layers are present only in Subunit 5B.

Unit 5 accumulated at a relatively rapid rate of 15-100 m/m.y., increasing downhole (see "Biostratigraphy"). In the intervals dominated by gravity-flow deposits, most of the elapsed time is represented by burrowed hemipelagic mudrocks, even though they account for only a small percentage of the thickness of the succession.

CaCO₃ values determined for Unit 5 span a wide range (see "Carbonate and Organic Carbon" in "Geochemistry"), consistent with lithologies ranging from noncalcareous claystones to marls, carbonate grainstones, calcite-cemented sandstones, and calcareous black shales, plus several calcareous nodules. Burrowed mudrocks tend to have low CaCO₃ concentrations, often <1 wt%, whereas redeposited mudrocks, sandstones, and siltstones have CaCO₃ contents occasionally exceeding 50 wt%. Diagenetic nodules have ~80 wt% calculated CaCO₃ but are shown by XRD analysis to be siderite and dolomite.

TOC is highest in the calcareous black shales of Subunit 5A, in which many values exceed 6 wt%. The highest TOC value (9.83 wt%) comes from black, very finely laminated marlstone with 49 wt% CaCO₃ in Sample 210-1276A-33R-3, 91-95 cm. In Subunit 5B, TOC occasionally reaches 8 wt% in the laminated marlstones of the upper part of the succession and 6 wt% in those of the lower part.

Subunit 5A

Interval 210-1276A-29R-6, 62 cm, through 36R-2, 129 cm

Depth: 1067.24-1129.80 mbsf

Age: Cenomanian-Turonian

Lithology: medium to dark gray calcareous sandstone to mudstone turbidites with lesser burrowed mudrocks and minor black shales, including OAE 2

Subunit 5A occurs between Section 210-1276A-29R-6, 62 cm, and 36R-2, 129 cm (1067.24-1129.80 mbsf). There is a downhole change over a short interval from mainly reddish brown muddy sandstone in Unit 4 to more subdued greens and grays in Subunit 5A. The lower boundary of Subunit 5A is placed where thick, graded gravity-flow deposits pass downhole into extensively burrowed mudrocks with only thin, graded interbeds of grainstone and calcareous siltstone. This boundary is located at Section 210-1276A-36R-2, 129 cm, at the base of a prominent graded grainstone, beneath which mudrocks predominate.

The lithologic proportions in Subunit 5A are ~65% mudrock (both calcareous and essentially noncalcareous), ~20% calcareous sandstone plus minor grainstone, ~10% unlaminated marlstone, and ~5% very finely laminated ("pin-stripe laminated") calcareous claystone and marlstone (i.e., black shales). These lithologies are components of the facies discussed below. The calcareous sandstone, minor grainstone, unlaminated marlstone, and calcareous mudrock form numerous thick depositional units emplaced by mud-dominated gravity flows, and they account for ~80% of the total thickness of Subunit 5A. Individual graded gravity-flow deposits are as thick as 165 cm; many have a marlstone middle division with CaCO₃ concentrations of 30-40 wt%. These are interpreted to be the deposits of mud-laden turbidity currents. In addition, minor marlstones that have CaCO₃ concentrations of 50-60 wt% and TOC values of as high as 3-4 wt% are recognized as calcareous black shales.

Lithologies

The lithologies discussed below are listed in decreasing order of abundance.

Calcareous Mudstone and Calcareous Siltstone. There are numerous intervals of mainly calcareous mudstone that vary from grayish green (5G 4/1), moderate yellowish green (10GY 7/2), yellowish brown (10YR 5/4), greenish gray (5GY 6/1), to grayish black (N2). The mudstones form intervals as thick as several tens of centimeters that are commonly burrowed. Individual mudstone intervals are sharp-based and marked by a thin interval (several millimeters thick) of laminated siltstone and mudstone at the base, followed above by burrowed or massive mudstone. Individual laminae, where present in the siltstones/mudstones, tend to be spaced several millimeters apart, and on this basis they can be distinguished from more finely laminated black shales (see below).

In addition to dominant clay minerals, the mudstones of Subunit 5A commonly contain quartz, mica, carbonate, heavy and opaque minerals, feldspar, organic debris, rock fragments, and some pyrite. Quartz and mica are more abundant in the siltstones. Calcareous components include micrite and nannofossils.

Graded Sandstone Siltstone Mudstone (and Marlstone). Subunit 5A includes numerous continuously graded intervals of calcareous sandstone, siltstone, and calcareous mudstone that mainly range from light olive gray (5Y 6/1) to greenish gray (5G 6/1). Some of the coarse basal parts of these cycles are rich in carbonate clasts and are termed grainstones (e.g., Cores 210-1276A-32R and 33R). Also, some of the muddy tops of these graded intervals are sufficiently calcareous to be classified as marlstones (e.g., Cores 210-1276A-32R through 34R). Sandstone, siltstone, and grainstone make up ~20% of the recovery of Subunit 5A. An additional indeterminate percentage of the section forms the calcareous mudstone tops of the graded sandstone siltstone mudstone (and marlstone) depositional units. These calcareous mudstones (and marlstones) can be distinguished from the background noncalcareous mudrocks only by testing with 10% HCl.

The graded calcareous sandstone to grainstone intervals of the thick gravity-flow deposits are typically several tens of centimeters thick, reaching a maximum of ~1.35 m (Sections 210-1276A-35R-4, 127 cm, through 35R-5, 113 cm). Individual graded sandstones exhibit scoured bases and planar lamination and grade progressively upward to mudstone (Fig. F63). Wavy and convolute laminae are locally developed, especially in the medium-grained sand intervals (Fig. F64).

The graded units of calcareous sandstone siltstone mudstone (or marlstone) are generally devoid of burrows except in the upper few centimeters where burrows are filled with mudstone or claystone from units above. Many of the graded sandstone parts of these units are characterized by alternations of paler and darker laminae. The paler laminae are mainly composed of calcite-cemented siliciclastic material, whereas the dark laminae are rich in clay intraclasts that are typically flattened (Fig. F65).

The carbonate fraction of Subunit 5A marlstones is composed of nannofossils with lesser foraminifers and other microcrystalline carbonate. Locally, there are traces of organic matter and authigenic pyrite. The siliciclastic fraction consists of quartz, mica, carbonate, heavy and opaque minerals, feldspar, organic debris, rock fragments, and some pyrite.

Thin section analysis of siltstone and sandstone from Subunit 5A indicates that these samples contain abundant quartz with rare to trace amounts of feldspar, mica, and heavy minerals. Locally, the siltstones are very micaceous. Biogenic components are dominantly foraminifers but include rare fish and organic debris as well as mollusk fragments and other bioclasts. These sediments are pervasively cemented by carbonate microspar. The more calcareous sandstones grade into grainstones that have components similar to those in the sandstones and siltstones, except that they are more enriched in calcareous intraclasts and grains.

Noncalcareous Claystone. Minor amounts of mudrock (~15%) in Subunit 5A are not parts of thick gravity-flow deposits. These mudrocks consist of dark gray (N3) to medium dark gray (N4) to grayish green (5G 3/2) burrowed claystone with variable CaCO₃ content that is mostly <2 wt%. These sediments are sporadic throughout the subunit and are interbedded with gravity-flow deposits and black shales. This type of claystone is volumetrically very subordinate to the gravity-flow units, but it is more abundant than black shales.

Laminated Calcareous Claystones to Marlstones, Including Black Shales. There are numerous but volumetrically minor (~5%) very finely laminated claystones in Subunit 5A that are sufficiently enriched in total organic matter (>1 wt%) to be termed black shales (see "Carbonate and Organic Carbon" and "Oceanic and Anoxic Events" in "Geochemistry"). One of these appears to be OAE 2. Some of the laminated intervals contain ~30 wt% carbonate and are therefore marlstones. These distinctive intervals range from several centimeters to tens of centimeters thick. Lighter and darker laminae, each submillimeter to a few millimeters thick, follow each other with extreme regularity. Individual pale laminations are rich in nannofossils, whereas darker laminae are less calcareous and are enriched in organic matter (Figs. F66, F67). Smear slides indicate the presence of terrestrial plant debris, consistent with the results from organic geochemistry (see "Carbonate and Organic Carbon" in "Geochemistry"). The siliciclastic fraction of these sediments is similar to the mudrocks described above. Some of the darker laminae include silt. The very finely laminated claystones are completely devoid of bioturbation, except where burrows extend downhole from overlying, paler mudrocks.

Oceanic Anoxic Event 2

OAE 2 was recognized in Sections 210-1276A-31R-3 and 31R-4. Section 210-1276A-31R-3 consists of interbedded dark gray laminated and greenish gray bioturbated mudrocks and minor calcareous sandstones and siltstones (Fig. F68). In this section, black shales are present in intervals 1-17 cm, 19-60 cm, and 74-104 cm. A TOC value of 7.2 wt% was recorded at 52 cm. Preliminary Rock-Eval data suggest these carbon-rich sediments may be rich in algally derived material (see "Oceanic Anoxic Events" in "Geochemistry"). These black shales are completely devoid of burrowing. One interval contains a siltstone lamina. Section 210-1276A-31R-4 is similar, with black shales occurring at 44-45 cm, 48-53 cm, 62-70 cm, and 103-108 cm. It should be noted that similar laminated, carbon-rich calcareous sediments are present in adjacent intervals of Subunit 5A, where they are interbedded with mudrocks, sandstones, and siltstones.

Depositional Processes, Paleoceanography, and Sediment Provenance

Subunit 5A is dominated by graded sandstone siltstone mudstone intervals that were rapidly emplaced from gravity flows, as suggested by the range of sedimentary structures that include normal grading, scoured bases, planar lamination, convolute lamination, and occasional soft-sediment deformation. Most of these beds grade upward into mudstone that is interpreted to be deposited from the tails of turbidity currents. Similar turbidites are present in both the western Atlantic at DSDP Sites 387 (McCave, 1979) and 534 (Robertson, 1984) and in the eastern Atlantic at DSDP Site 398 (de Graciansky and Chenet, 1979) (see "Comparisons with the Conjugate Iberia Margin and the Western Central North Atlantic").

The sandstones and siltstones were derived from a continental area that included metamorphic rocks (e.g., mica schists and quartzite), such as those exposed in Newfoundland and adjacent areas of the Atlantic continental margin (Bell and Howie, 1990). The sandstone and siltstone composition is little different from that of Units 1-4, suggesting that terrestrial provenance did not change significantly with time at Site 1276.

Sharp-based, massive, graded calcareous mudstones are also interpreted to be the deposits of mud-laden turbidity currents. A lack of structures and grading, other than occasional thin basal siltstones, makes it impossible to establish the likely density of these currents. Their calcareous nature, relative to most of the hemipelagic claystones, is attributed to initial accumulation above the CCD as hemipelagic sediments, followed by downslope redeposition to beneath the CCD. Similar mud turbidites have been described from a range of deep-sea settings (e.g., Pickering et al., 1989; Stow et al., 2001). They are also comparable to unifites, originally described by Stanley (1981) from the Mediterranean Sea and interpreted by him as the record of relatively dense turbidity currents devoid of sand and coarse silt load.

There are also relatively minor bioturbated noncalcareous to weakly calcareous mudrocks (mostly claystone) in Subunit 5A. These are interpreted as background hemipelagic sediments deposited below the CCD.

Subunit 5B

Interval: 210-1276A-36R-2, 129 cm, through 75R-3, 142 cm

Depth: 1129.80-1502.12 mbsf

Age: Albian-Cenomanian

Lithology: medium to dark gray burrowed mudrocks with minor sand- to silt-based turbidites and black shales

Subunit 5B extends from Section 210-1276A-36R-2, 129 cm, to 75R-3, 142 cm (1129.80-1502.12 mbsf). Subunit 5B is 85%-90% mudrock, which is predominantly dark-colored calcareous mudstone to mudstone in the upper part and claystone in the lower part. In addition, there are graded beds of calcareous sandstone and siltstone (~5%) that increase toward the base of the subunit, as well as 5%-10% very finely laminated to subtly planar laminated calcareous, gray-black claystone and marlstone. The base of Subunit 5B is placed at the highest occurrence of underlying (Subunit 5C) disorganized beds that are composed of poorly sorted silty sandstone rich in sedimentary clasts and interpreted as sandy debris flow deposits. Unlike Subunit 5A, the mudrocks in this subunit are almost entirely burrowed or are characterized by subtle planar lamination overprinted by a minor but persistent amount of burrowing. There are occasional diagenetic nodules and concretionary bands of siderite, dolomite, barite, and pyrite. These sediments are interpreted as hemipelagic sediments and mud turbidites rarely interrupted by thin calcareous sandy turbidites. Calcareous, carbon-rich claystones (black shales) in this interval are believed to reflect times of enhanced productivity in surface waters and deposition under anoxic seafloor conditions.

Lithologies

Mudstone and Claystone. Subunit 5B contains a high proportion (85%-90%) of mudrock of subdued colors, ranging from calcareous mudstones to generally less calcareous claystones, with colors from grayish green (5G 4/1), moderate yellowish green (10GY 7/2), and greenish gray (5GY 6/1) to grayish black (N2). Most of the mudstones are essentially homogeneous and well burrowed without much evidence of primary sedimentary structures. However, we stress that a significant proportion of these mudrocks probably is redeposited sediments. Some of the mudstone intervals are sharp-based, marked by an interval of laminated siltstone and mudstone several millimeters thick, and grade upward into mainly burrowed mudstone (Fig. F69).

There are numerous intervals, typically several centimeters to several tens of centimeters thick, composed of greenish (5G5/2) to dark gray (N3) to black (N2) claystones that are commonly carbonate poor (<2 wt%). The greenish claystones are clearly burrowed. In contrast, the dark gray to black beds either lack burrows or show only vague burrow mottling on wet split-core surfaces. These beds are very finely laminated claystones to marlstones that contain 1-10 wt% TOC, and thus they are black shales (Fig. F70) (see "Carbonate and Organic Carbon" and "Oceanic Anoxic Events" in "Geochemistry"). The beds are very similar to those described in Subunit 5A (see "Subunit 5A") and are relatively few in number. In one case, a black shale is overlain by a brightly colored oxidized interval (Fig. F71) that is comparable to the oxidized tops of Pleistocene sapropels in the Mediterranean Sea (Emeis, Robertson, Richter, et al., 1996).

The claystones and mudstones in Subunit 5B are similar in composition to those in Subunit 5A except that those in Subunit 5B contain foraminifers and slightly more mica. The calcareous mudstones are also similar to those in Subunit 5A and contain carbonate in the form of nannofossils, lithoclasts, bioclasts, micritic matrix, and authigenic cement.

Sandstone Siltstone Mudstone and Microconglomerate. There are occasional thin intervals in Subunit 5B of graded sandstone, siltstone, and mudstones as much as tens of centimeters thick (representing <5% of the recovery). Sedimentary structures include planar lamination, cross lamination (Fig. F72), and unusually common convolute lamination (Fig. F73). These intervals are very similar to beds that form a dominant component of Subunit 5A. In addition, there are rare thin (<4 cm thick) beds of sandstone with sharp bases and graded tops, overlain by parallel-laminated sandstone/siltstone. Some of these sandstones show well-developed cross lamination that can be recognized on CT scans (Fig. F74). Descending current-ripple cross lamination was observed in one bed (Fig. F75). Small-scale flame structures are rarely present (Fig. F76). There are some calcareous sandstones with a granule-grade basal division (Fig. F77), and mudstone rip-up clasts are also locally present (Figs. F78, F79). Some very thin sandstones contain subrounded matrix-supported siltstone intraclasts as wide as 0.5 cm (Fig. F80). There are a small number of thin to very thin interbeds of fine calcareous sandstone that grade through calcareous siltstone to calcareous mudstone (or marlstone); these are distributed in predominantly mudstone or claystone intervals (Fig. F81). In addition, there are examples of complex alternations of different facies, as shown in Figure F82.

Thin section studies show that the coarser siltstones and sandstones of Subunit 5B contain abundant quartz and rare to trace amounts of feldspar, mica, glauconite, and heavy minerals (Fig. F83). Locally, the siltstones are very micaceous and contain intraclasts and minor clay matrix (Figs. F84, F85). The rock fragments are dominantly metamorphic, with some sedimentary micritic carbonate grains. The latter might be intraclasts or reworked carbonate lithic fragments. Biogenic components are dominantly foraminifers, with rare fish and organic (plant) debris. Bioclast diversity increases lower in the subunit where echinoderm, ostracode, and mollusk fragments are present. The sandstones and siltstones are pervasively cemented by carbonate microspar and spar. Grainstones in Subunit 5B contain components similar to the sandstones and siltstones, but they are more enriched in calcareous intraclasts and grains and, locally, in radiolarians (Fig. F86).

Structural Features. A synsedimentary fold with an entirely unexpected axial-planar cleavage was observed in interval 210-1276A-69R-4, 70-86 cm. Such axial-planar cleavage is normally assumed to form only as a result of the folding of lithified sedimentary rocks. However, its presence as a synsedimentary fold shows that such deformation may also occur in sediments that are not fully lithified.

Depositional Setting, Paleoceanography, and Sediment Provenance

Subunit 5B is dominated by the deposition of relatively homogeneous burrow-mottled mudrocks (mudstone and claystone) in an aerobic to dysaerobic setting. Compared with Subunit 5A, background hemipelagic sediments are relatively more abundant, whereas graded calcareous sandstone turbidites are very subordinate. Very finely laminated black shales (calcareous claystones and marlstones) accumulated under anaerobic conditions by the same processes as those in Subunit 5A. Many of the calcareous mudstones that form an important part of this subunit were apparently transported by turbidity currents from farther upslope, above the CCD, to a position beneath the CCD, much as was inferred for Subunit 5A.

Subunit 5C

Interval: top: 210-1276A-75R-3, 142 cm; base: 102R-1, 2 cm, through Core 104R (no recovery)

Depth: top: 1502.12 mbsf; base: 1732.12-1736.90 mbsf

Age: latest Aptian(?)-Albian

Lithology: dark gray to olive-black, disorganized sandy turbidites with thick tops of calcareous mudstone, green to dark gray claystone, minor conglomerate, and hydrothermally altered sediments

The upper contact of Subunit 5C is placed at the first downhole appearance of disorganized beds of silty and muddy sandstone. The base is at or below Section 210-1276A-102R-1, 2 cm. Below this, there was no core recovery down to the bottom of Hole 1276A at the base of Core 210-1276A-104R. The subunit consists of ~75% dark-colored calcareous unburrowed mudrock (mostly mudstone); ~10% burrowed mudrock; ~10% calcareous sandstone, grainstone, and silty sandstone; and ~5% siltstone. The burrowed mudrocks are a combination of calcareous mudstones and claystones that are essentially carbonate free, although a small amount of the claystone is calcareous, as determined by frequent testing with 10% HCl and smear slide analysis.

Although Subunit 5C is dominated by mudrocks, the most spectacular features of the subunit are disorganized sandy turbidites with thick tops of calcareous mudstone; these compose ~10% of the subunit. There are also very rare black shales (<1% of the recovery) and very rare mud-clast conglomerates. Hydrothermally altered sediments (mudrocks and calcareous sandstone) are localized at the contacts of two diabase sills (Subunits 5C1 and 5C2) that cut the lower part of Subunit 5C. The noncalcareous and calcareous mudrocks of Subunit 5C are similar in composition to those of Subunit 5B, except that those in Subunit 5C are relatively enriched in terrestrially derived constituents including mica and carbonized plant material. Marlstones in Subunit 5C are also similar to those in Subunits 5A and 5B, but Subunit 5C marlstones do not contain foraminifers.

Lithologies

Mudrock (Mudstone and Claystone) and Minor Marlstone. Dark greenish to grayish mudrocks compose ~85% of Subunit 5C, but a number of specific depositional facies can be recognized. Predominantly gray (N3 and N4) claystones are mainly noncalcareous and devoid of burrows. These claystones range from moderately massive (Fig. F87) to weakly laminated (Fig. F88) to finely laminated (Fig. F89). In addition, there are subordinate intervals of greenish (5G 2/1), more calcareous burrowed claystones and mudstones. Very rarely, a bluish gray (5B 5/1) variety is present; smear slide analysis indicates that this is marlstone.

There are also a number of beds of calcareous siltstone that fine upward into structureless calcareous claystone or mudstone (e.g., Sections 210-1276A-77R-4, 57 cm, through 77R-3, 62 cm; thickness = 145 cm). All of Core 210-1276A-95R is dominated by several intervals of calcareous mudstone, each as thick as ~1.5 m, that are structureless apart from occasional silt laminae. Other beds of calcareous mudrock, as thick as 50 cm, have only a few millimeters of pale laminated siltstone at the base, so >95% of the deposit is mudrock. The individual silt-rich laminae in these units contain quartz, planktonic foraminifers, and inferred fecal pellets. This variety of mudstone is interpreted as a type of gravity-flow deposit, as discussed further below.

Several thick (as much as several meters) intervals of calcareous mudstone pass uphole into claystone or marlstone, each forming an individual depositional unit. These deposits include sedimentary features indicative of deposition from gravity flows. For example, Sections 210-1276A-77R-1, 46 cm, through 77R-2, 10 cm, consist of an interval in which the lower 30 cm of contorted and folded mudstone is overlain by homogeneous claystone. These mudstones are calcareous and contain scattered flattened and elliptical sand-silt intraclasts as large as several millimeters in size. There does not appear to be a coarser-grained base to these depositional units.

In addition, distinctive mudrocks are locally present. Rare intervals of very finely laminated claystone (black shale) are observed in intervals 210-1276A-94R-5, 70-85 cm, 94R-4, 43-46 cm, and 98R-1, 87-110 cm (Fig. F90). The last example contains scattered silt grains and fine silty laminae. Several other dark-colored, nonburrowed intervals in this core show weakly developed, thin planar lamination, suggestive of incipient black shale formation.

Given the characteristic dark greenish gray colors of Subunit 5C, an isolated presence of pale reddish brown (10R 5/4) noncalcareous claystone in Section 210-1276A-89R-2 is anomalous. In this section, reddish and greenish to dark gray burrowed claystones alternate with unburrowed dark greenish marlstone on scales of a few centimeters to several tens of centimeters.

An additional feature of interest is anomalously poor consolidation of pale gray to greenish gray burrowed claystones in Core 210-1276A-96R, located between two diabase sills that intrude the lower part of Subunit 5C (see below). It is possible that the sills acted as seals that prevented pore fluid escape and thus inhibited compaction and cementation of these sediments (see "Physical Properties").

Muddy Calcareous Sandstones Mudstones Claystones. Medium to thick beds of dark olive-green (5GY 3/2) massive calcareous sandstone, grading upward into massive calcareous mudstone or claystone, form a significant part (~10%) of Subunit 5C. For example, in Core 210-1276A-81R a single depositional unit, ~195 cm thick, begins with silty sandstone (Section 210-1276A-81R-3, 135 cm) and grades upward into silty calcareous mudstone/claystone (Section 81R-2, 90 cm) and then into mudstone (Section 81R-1, 90 cm). Internally, this bed is intensely deformed, containing contorted and swirled laminae and streaked-out sandstone and mudstone intraclasts as large as several centimeters. Other calcareous silty sandstone beds reach several meters in thickness. In contrast to the graded calcareous sandstones of Subunits 5A and 5B, these sandstones have a muddy matrix. The sandstones commonly contain fine- to medium-grained sand-sized clasts mainly composed of dark mudstone but also including irregular streaked-out clasts of sandstone (Fig. F91). There are also numerous clasts of black and green claystone/mudstone that range in size from <1 mm to several centimeters (Fig. F92). The calcareous silty and muddy sandstones and the muddy siltstones locally contain scattered elongate fragments of carbonized wood, as long as several centimeters (e.g., interval 210-1276A-94R-2, 40-41 cm). In addition, there are a number of intervals of severely disrupted muddy sandstone and sandy mudstone (Fig. F93).

In Section 210-1276A-85R-4, a calcareous silty sandstone, where it passes uphole into mudstone, displays centimeter-scale recumbent folds with disconnected hinges and intricately deformed laminae (i.e., "similar folds" in structural geology nomenclature). The noses of the folds have been thickened by migration of mud into the hinge area. Sigmoidal siltstone patches in the sediment probably are the remnants of sheared-out fold limbs. Syndepositional deformation structures of this type, particularly reclined to recumbent and seemingly flattened and sheared folds, are widespread in Subunit 5C (Figs. F94, F95, F96). In interval 210-1276A-87R-3, 55-150 cm, dish structures resulting from syndepositional pore-fluid escape are associated with deformed laminae. Well-developed swirled zones in the lower part of Subunit 5C include rip-up clasts of muddy and silty sandstone, smeared-out clasts, mica grains, plant material, and flattened intraclasts (e.g., intervals 210-1276A-94R-1, 90-150 cm, and 94R-2, 0-13 cm).

In the lower part of Subunit 5C, there are several spectacular mudclast conglomerates (almost breccias) associated with intervals of structureless sandy mudstone (Figs. F97, F98). The individual mud clasts are mottled, ranging from green to black and gray. Most of the mud clasts are subangular with irregular margins, suggesting that they were reworked while they were in a plastic state. In places, the internal deformation is so intense that the original clasts and matrix cannot be distinguished in the core (Fig. F99). There are also occasional relatively thick intervals of calcareous sandy siltstone grading uphole into calcareous mudstone; this includes an entire section containing subrounded sandstone intraclasts, as large as 6 cm, and extensive soft-sediment deformation.

Petrographic study of sandstones and siltstones reveals dominantly angular quartz grains, biotite, muscovite, glauconite, rock fragments (e.g., phyllite), bioclastic grains including echinoid spines (Fig. F100), and common terrestrially derived plant material (Fig. F101). In general, the quartzose sandstones and siltstones of Subunit 5C are more feldspathic than those of Subunits 5A and 5B, but they otherwise contain similar types of mica and lithic fragments. They are not as glauconitic as those in the overlying units. Furthermore, the sandstones of Subunit 5C show enhanced diagenesis as reflected in widespread dissolution of feldspar, minor quartz overgrowths, and cementation.

Sandstone/Grainstone. There are a small number of well-sorted, thin- to medium-bedded, graded calcareous sandstones and grainstones (<5% of core recovered) in Subunit 5C. The sandstones are dominantly siliciclastic and contain a carbonate spar cement. The grainstones are also calcite cemented. The carbonate in grainstones includes numerous bioclasts. The calcareous sandstones/grainstones exhibit sharp bases, similar to the typical graded intervals of the younger Subunit 5A.

A small number of the graded sandstone intervals exhibit both planar and cross lamination (Fig. F102) and then fine upward through siltstone into nearly massive mudstone with occasional very small mudstone clasts (<5 mm). These mudstones are calcareous and completely devoid of burrows. In some depositional units that are tens of centimeters or more thick, the bases are reduced to only a few millimeters of fine-grained sandstone.

Medium gray (N4) to medium light gray (N5) and yellowish gray (5Y 8/1) grainstones form as much as ~25% of several of the cores in Subunit 5C. In Core 210-1276A-80R, there are seven sharp-based graded units in which grainstone, sandstone, or siltstone pass uphole into calcareous claystone. The silty tops of individual grainstones consist of color-banded, planar-, cross-, and occasionally convolute-laminated divisions. Six of the graded intervals in Core 210-1276A-80R are succeeded by greenish gray burrowed mudstone.

Volcanogenic Sediments. Very rare thin (<5 cm thick) intervals of volcanic ash were noted. One of these, just beneath the Subunit 5B/5C boundary, appears to be a tuff in which original volcanic glass was altered to clay minerals (Fig. F103). In addition, a thin, probably volcaniclastic interval was identified in marlstone in the lower part in Subunit 5C (interval 210-1276A-89R-1, 18-20 cm). The volcanogenic sediments of both of these intervals are strongly altered.

Depositional Setting, Paleoceanography, and Sediment Provenance

Subunit 5C contains an unusually wide variety of redeposited facies, namely silty and sandy debris flow deposits, siliciclastic turbidites, carbonate-rich turbidites, and silt-mud turbidites, alternating with small amounts of background hemipelagic sediments that are mainly claystones. The greenish gray claystones are burrowed, whereas the dark gray to black mudrocks lack burrowing. The grainstones are assumed to have been derived from contemporary continental shelf or upper slope settings, whereas the more abundant siliciclastic turbidites, like those of Subunits 5A and 5B, were derived from continental source rocks. Similar intercalations of hemipelagic and redeposited sediments of carbonate to siliciclastic composition characterize the margins of other areas of the North Atlantic after continental breakup (Ogg et al., 1983).

The redeposited siliciclastic sediments are interpreted as follows:

1. The predominant silty sandstone and silty mudstone beds with (a) swirled lamination; (b) "floating" clasts of sandstone, siltstone, and mudstone; and (c) sharp tops are interpreted as debris flow deposits. These represent a variety of "slurry bed," as defined by Wood and Smith (1959) and described recently by Lowe et al. (2003). The floating sandstone lithoclasts originated by remobilization of poorly consolidated sand into viscous flows rich in silt and mud. During subsequent downslope transport the sand clasts did not fully disaggregate, and therefore the sand was not thoroughly mixed with the mud and silt fractions.
2. The subordinate graded sandstones siltstones mudstones seen throughout Subunit 5C are classic sandstone turbidites.
3. The normally graded beds of grainstone are interpreted as calcareous turbidites derived from a continental shelf or upper slope setting above the CCD.
4. The siltstone mudstone and mudstone claystone beds, reaching several meters in thickness, are interpreted as deposits from exceptionally large mud-dominated turbidity currents. These are equivalent to the "megaturbidites" described from outcrop successions (Ricci Lucchi and Valmori, 1980; Hiscott et al., 1986) and from modern abyssal plains (Weaver and Rothwell, 1987). It is envisioned that large volumes of calcareous mud were mobilized from higher on the continental slope (above the CCD) and then redeposited in the vicinity of Site 1276. These fine-grained sediments could easily be mistaken for background hemipelagic sediments. However, they can be distinguished by a slightly paler color, lack of sedimentary structures and burrowing, and a higher carbonate content compared to the dark gray and greenish hemipelagic claystones.

The predominant redeposited facies discussed above are interbedded with minor amounts of greenish to dark gray and black mudrock. Much of the mudrock is interpreted as the tops of mud turbidites, whereas the background hemipelagic sediments are mainly claystones. The greenish burrowed claystones accumulated under conditions that were at least weakly oxidizing, whereas the dark gray and black claystones, devoid of burrows, apparently accumulated under anoxic conditions (see discussion of black shales in "Implications of Sediment Chemistry"). Where background hemipelagic sediments form a significant part of the succession (e.g., Core 210-1276A-93R), sediments that accumulated under inferred oxic and anoxic conditions are intercalated, typically on a scale of tens of centimeters to meters. In addition, the few reddish (noncalcareous) claystones in the lower part of Subunit 5C are interpreted as pelagic sediments that accumulated under rare, well-oxygenated bottom waters.

Both the greenish claystones and dark gray and black claystones are effectively noncalcareous, indicating deposition beneath the CCD, as was inferred for the overlying Subunits 5A and 5B. However, relatively small amounts of both greenish and dark gray and black claystones are calcareous, as indicated by frequent testing of the cores with 10% HCl, smear slide analyses, and carbonate determinations. One explanation is that these claystones represent the low-density, clay-rich tops of the thick mud turbidites discussed above. However, some of these calcareous intervals in the upper part of Subunit 5C appear to occur independently of identified mud turbidites (e.g., Core 210-1276A-76R). In these cases, an alternative explanation is that these sediments were deposited as calcareous hemipelagic sediments above the CCD, with the implication that Hole 1276A might have been located close to the CCD during Albian time.

The very rare and very finely laminated black claystones (black shales) in the lower part of Subunit 5C are interpreted to have been deposited in a manner similar to that described for Subunits 5A and 5B.

Diagenetic Processes in Unit 5

Unit 5 as a whole is characterized by a range of compaction-related and chemical diagenetic effects. Differential compaction occurred around burrows that are composed of cemented sandstone or siltstone. The clastic sediments filling these burrows were cemented by calcite spar relatively soon after burial and later underwent little deformation, whereas the surrounding mud or clay remained soft and was molded around the lithified burrows (Fig. F104).

The rare calcareous sandstones and grainstones are well cemented by carbonate microspar. The more porous sandstone turbidites are similarly well cemented, which imparts a pale color, whereas silty sandstones are less porous and lack carbonate cement. Cementation is locally patchy, with the result that depositional structures (e.g., cross lamination) may be differentially compacted (Fig. F105). Bright spots are observed in CT scans but do not have a visual expression in corresponding core photographs of the same interval; these spots could represent areas of patchy sparry calcite cementation as much as several centimeters across (Fig. F106) or pyrite (Fig. F107).

Layers (<20 cm thick) and isolated nodules of diagenetic limestone are occasionally present in Subunit 5B. These limestones are locally rich in nannofossils but only very rarely contain foraminifers or radiolarians. In thin section, the limestones are composed mainly of microspar to micritic carbonate and they contain a mud fraction that consists of clay minerals, iron oxides, heavy minerals, quartz, and organic matter.

In some cases, the carbonate concretions and nodules do not react with 10% HCl. XRD studies showed that several samples contain dolomite and siderite (Samples 210-1276A-37R-5, 26-27 cm, and 38R-5, 96-98 cm); a few others have abundant siderite and dolomite and minor calcite (Samples 41R-4, 23-24 cm; 42R-4, 23-24 cm; 42R-4, 115-116 cm; 46R-2, 52-53 cm; and 55R-2, 21-22 cm). One nodule contains siderite plus dolomite (Sample 210-1276A-77R-2, 21-23 cm), and another is composed of mainly apatite with subsidiary dolomite and siderite.

Cryptocrystalline siderite forms occasional irregular light yellowish brown layers (<30 cm thick), especially in the less calcareous mudrocks (Fig. F108). These layers are interpreted to be concretions formed by cementation and replacement of mud or mudrocks by siderite. The laminae outside the siderite concretions are considerably more compacted than those within, suggesting that the concretions formed relatively early in diagenesis.

An unusual feature of Subunit 5B is the presence of small nodules (<5 cm in diameter) that are mainly composed of granular carbonate (e.g., interval 210-1276A-47R-7, 42-43 cm). Several nodules exhibit transitional contacts with the surrounding sediment (Fig. F109), whereas others have sharply defined margins. Thin section study of one nodule (Fig. F110) revealed a pelletal texture of circular pellets as large as 1 mm. Internally, the pellets contain clay and nannofossils. Locally, inferred primary porosity is preserved between pellets. Some pellets are partly fused, whereas others are partly or completely fused to form nearly homogeneous claystone. Elsewhere in the nodule, pellets are separated by interstitial claystone. These unusual carbonate nodules are interpreted to be remnants of an original depositional texture of fecal pellets in the mudstones of Subunit 5B and, by implication, the mudstones throughout the Albian interval. This observation is significant because it may help to explain the common occurrence of silt- to granule-sized claystone or mudstone clasts in many of the siliclastic sandstone-siltstone and grainstone turbidites. These might have been pellets rather than mud intraclasts. At the time of its redeposition, pelleted fine-grained sediment may have been sufficiently cohesive to survive transport in gravity flows.

In several adjacent intervals within the lower part of Subunit 5B, small centimeter-scale nodules or lenses with granular texture appear to have been cemented early in diagenesis and then compacted so that lamination is deflected around them (Figs. F111, F112). There are also occasional small (<2 cm) light olive-gray (5Y 6/1) dolomitic concretions, either oval or tabular in shape, in some intervals of dark gray calcareous mudstone. Nodular barite layers are present in Cores 210-1276A-43R, 44R, and 47R. A single septarian concretion was recovered (Fig. F113). This apparently formed relatively late in diagenesis because the primary lamination is only slightly deflected around the concretion.

Minor amounts of framboidal pyrite are ubiquitous throughout Unit 5. Pyrite is also present as occasional small nodules and lenses (<3 cm thick) throughout Subunit 5C, especially in the darker, more carbon rich claystones (e.g., Cores 210-1276A-89R, 92R, 95R, and 97R). These claystones occasionally contain pyritized radiolarians with well-preserved tests including spines, whereas most other radiolarians are replaced by carbonate minerals. For this reason, it is likely that the pyrite formed relatively early in diagenesis, preserving the radiolarians from later replacement by carbonate.

A few thin (several centimeters thick) phosphatic layers were observed (e.g., interval 210-1276A-92R-5, 68-72 cm). XRD analysis indicates the presence of a Ca-F apatite, quartz, and kaolinite plus chlorite.

Fine-Grained Sediment Composition of Unit 5 Based on XRD Analysis

The main detrital constituents of the mudrocks and marlstones of Unit 5, in decreasing order of abundance, are quartz, clay minerals, and feldspars. Quartz is generally the most abundant mineral even in the claystones. Plagioclase and alkali feldspar are mainly present in the coarser-grained lithologies (i.e., mudstone, sandy mudstone, and siltstone) but are rare otherwise. The main clay minerals are illite-smectite mixed-layer minerals, illite and muscovite (indistinguishable), chlorite, and kaolin-group minerals (kaolinite and possibly dickite). No attempt was made to determine the relative or absolute abundance of these minerals.

The peak spacing and amplitude of XRD diffractograms of sampled clay minerals were compared throughout Unit 5. To obtain a data set of comparable lithologies, only dark green mudrocks with variable but usually minor or trace amounts of carbonate were selected. These samples are thought to represent background hemipelagic sedimentation, which is characterized by a smaller detrital and organic matter input compared with the interbedded lithologies that are mainly turbidites.

The peak spacing and amplitude of XRD diffractograms of clay minerals were compared, and the results are summarized in Figure F114. A minor diffraction bulge at ~14 Å appears irregularly in samples down to the middle of Subunit 5B (Sample 210-1276A-47R-7, 28-29 cm; 1241.26 mbsf). This bulge is attributed to the presence of irregular illite-smectite mixed-layer minerals (IS mixed layers). These irregular minerals are characterized by random intercalation of illite and smectite layers (R = 0) (Moore and Reynolds, 1989; Reynolds, 1980). Below 1240 mbsf, the IS mixed layers are of the regular, higher-order type (R = 1 and R = 3) in all but one sample that came from a layer of altered ash (interval 210-1276A-89R-1, 20-21 cm; 1623.80 mbsf). A pronounced downhole increase in chlorite and kaolinite is detected over roughly the same interval, from 1376.68 mbsf to the bottom of the hole.

The downhole changes in IS mixed layers, together with an increasing abundance of chlorite and kaolinite, are commonly observed in deeply buried successions in other sedimentary basins, and they are common features of mid-Cretaceous sedimentary basins in the Atlantic and the western Tethyan regions (Chamley, 1979; Chamley and Debrabant, 1984; Robert, 1987; Pletsch, 1997). The conventional explanation for these trends is that they relate to burial diagenesis (Dunoyer de Segonzac, 1969). In particular, a downhole change to regular IS mixed layers can be interpreted as temperature-related diagenesis because irregular IS mixed layers become unstable at temperatures of ~100°C (Pollastro, 1993). However, this explanation is incompatible with observations at Site 1276. At this site, the transition to regular IS mixed layers is ~1240 mbsf, but two lines of evidence indicate that temperatures at this level never reached close to 100°C. First, irregular IS mixed layers are present some 384 m deeper in interval 210-1276A-89R-1, 20-21 cm, at 1624 mbsf. Second, T_max values from Rock-Eval pyrolysis of two still-deeper carbon-rich samples in Core 210-1276A-94R are as low as 440°C (see "Geochemistry"). These values correspond to maximum burial temperatures that are well below the range that should be required if the regular IS mixed layers originated diagenetically. Thus, the clay mineral assemblages at Site 1276 are interpreted to be the result of long-term variation in the input of terrigenous constituents rather than burial diagenesis.

If we consider environmental rather than burial-diagenesis effects, downhole clay mineral changes, such as we observe, could be related to waning effects of extensional tectonics during Aptian to early Albian time, combined with the influence of the first-order sea level maximum during Albian-Turonian time (Haq et al., 1987). According to this hypothesis, the abundance of kaolinite and chlorite in mudrocks in the lower part of Unit 5 could be attributed to the denudation of metamorphic rocks and deeply buried sedimentary rocks, whereas the observed uphole decrease in abundance of the minerals would be related to waning extensional tectonics in source areas of the Site 1276 sediments. However, continental breakup is assumed to have been completed on the Newfoundland margin prior to Aptian time (see "Comparisons with the Conjugate Iberia Margin and the Western Central North Atlantic"). If the tectonic hypothesis is correct, either tectonism continued beyond breakup or clastic sediments were supplied from other areas, such as the still actively rifting Labrador Sea north of Site 1276.

Another possible controlling factor is the inferred long-term sea level rise during the Albian-Turonian (Haq et al., 1987) that could have limited the input of kaolinite and chlorite. Both of these minerals usually reach the deep ocean as relatively coarse grained particles, as opposed to mostly colloidal IS mixed layers. This can lead to a preferential settling of chlorite and kaolinite in proximal shelf settings, whereas the finer-grained IS mixed layers may largely bypass the shelf and settle in the open ocean (Gibbs, 1977; Chamley, 1989). At Site 1276, chlorite and kaolinite could initially have bypassed a narrow shelf while sea level was relatively low. As sea level rose, the area of shelf deposition would have expanded, trapping chlorite and kaolinite while the IS mixed layers bypassed the shelf to reach the deep ocean.

Implications of Sediment Chemistry

Most of the mudrocks from Unit 5 fall within the range of chemical compositions typical of hemipelagic sediments with variable carbonate contents (ICP-AES geochemistry) (Table T4). Calculated carbonate and analyzed Ca values in the mudrocks drop to generally low levels below Core 210-1276A-41R, and absolute values of Al₂O₃, SiO₂, TiO₂, and other terrigenous constituents correspondingly increase. The composition of occasional sandstone turbidites lies within the range of continentally derived sandstones, but one sample (210-1276A-38R-6, 96-98 cm) contains 3.63 wt% MnO, which may relate to diagenetic cementation.

A small number of dark, laminated black shale samples were also chemically analyzed from the interval of Subunit 5A (Cores 210-1276A-32R and 33R) that was stratigraphically correlated with OAE 2 (see "Biostratigraphy"). These black shales are relatively enriched in TOC, calculated carbonate, Ca, MnO, P₂O₅, Cr, Mn, Ni, Fe, and V, whereas Al₂O₃ and TiO₂ are depleted relative to the preceding list of elements and oxides (see "Oceanic Anoxic Events" and "Trace Elements and Redox-Sensitive Metals," both in "Geochemistry"). The enrichment in carbonate and Ca in these black shales, in contrast to the adjacent background sediments (claystones and mudstones) that accumulated below the CCD, records a high preservation of biogenic carbonate. This carbonate was derived from either primary pelagic settling (calcareous nannofossils) or calcareous gravity flows (see "Origin of Finely Laminated Black Shales"). The relative enrichment of Mn in the black shales that correlate with OAE 2 could record the formation of a reduced manganese mineral that was not detected during routine XRD analysis. This further suggests that diagenetic processes played an important role in determining the present composition of the black shales.

The relative enrichment in Cr and Ni in the black shales analyzed from the Cenomanian OAE 2 interval could reflect incorporation of these elements into marine organic matter. However, Ba, commonly used as a productivity index (von Breymann et al., 1992), is not significantly enriched in the carbon-rich horizons, although low Ba levels could result from diagenetic loss of Ba and other constituents. Relative depletion of Al is observed in the Cenomanian OAE 2 interval that is most strongly enriched in TOC; this appears to contrast with some other black shales of Albian age from the North Atlantic that show a marked relative increase in terrigenous constituents (e.g., Al, Ti, K, Mg, and Fe) (Hofmann et al., 2001). Possible explanations of this discrepancy may relate to differences in the nature of the organic matter (i.e., its composition and maturity), sedimentation rate, or diagenesis. Biomarker data are needed from postcruise studies to determine the nature and maturity of the organic matter. However, it should be noted that none of the black shales analyzed appear to show anything like the strong enrichment in trace metals or the very high hydrogen indexes (HIs) (>600 mg HC/g TOC) that are typical of marine organic matter in OAE 2 elsewhere (e.g., in the Hatteras Formation of the central North Atlantic [Jansa et al., 1979; Summerhayes, 1987]). One explanation for the lack of strong trace-metal enrichment in the OAE 2 black shale interval at Site 1276 is that these carbon-rich sediments accumulated relatively rapidly and are dominated by unreactive organic matter. Under these conditions, they did not initially absorb high concentrations of trace metals (e.g., Ni and Cr) from seawater and their chemical composition was later modified during diagenesis.

There was insufficient time during Leg 210 to complete the chemical analysis of mudrocks from near the base of Subunit 5C, including several dark claystone layers (black shales) in Cores 210-1276A-94R and 98R. However, several of the black shale samples were analyzed for carbonate and were found to be strongly calcareous. Also, the Rock-Eval HIs are slightly elevated (<100 mg HC/g TOC) relative to background values (<50 mg HC/g TOC). The rare black shales in Core 210-1276A-94R near the base of Subunit 5C are inferred to be of marine origin based on the HIs (see "Oceanic Anoxic Events" in "Geochemistry" and "Biostratigraphy"), and they may correlate with OAE 1b. There are also two other thin black shale intervals in Subunit 5B that show slight elevations in HIs compared to background values, and these might conceivably correlate with OAEs (e.g., OAE 1c and OAE 1d). It should also be noted that only a few of the total number of laminated black shale intervals in Subunits 5A and 5B were chemically analyzed on the ship.

In summary, the bulk sedimentary geochemistry is consistent with a dominantly terrigenous origin of the black shales. The main exception may be the upper Cenomanian-lowermost Turonian OAE 2 interval, which was determined to have a significant amount of marine (algal) organic matter. Another exception may be OAE 1b, of early Albian age, assuming this event is confirmed by postcruise studies (see "Oceanic Anoxic Events" in "Geochemistry").

Origin of Finely Laminated Black Shales

Based on shipboard data, including the geochemical data summarized above, the occurrence of very finely laminated black shales in Subunits 5A and 5B (and very rarely in Subunit 5C) can be explained by an increase in plankton productivity or an increase in the input of terrestrial organic matter (i.e., plant debris). Six horizons marked by higher TOC contents and S₂ values were recorded. These include upper Cenomanian-lowermost Turonian OAE 2 and possibly also OAEs 1b, 1c, and 1d (see "Oceanic Anoxic Events" in "Geochemistry").

Productivity Hypothesis

One hypothesis is that black shales represent periods of enhanced primary productivity followed by pelagic settling, coupled with preferential preservation in an anoxic setting (Tucholke and Vogt 1979; Wignall, 1994; Calvert et al., 1992; Tyson, 1995). The calcareous laminae, rich in nannofossils, reflect short-term variations (i.e., blooms) in plankton productivity (Arthur et al., 1987; Hofmann et al., 2001; Tyson, 1995). For example, very finely laminated claystones and black shales in the Lower to mid-Cretaceous Hatteras Formation in the western North Atlantic, similar to those in Unit 5, were attributed to periodic surface water blooms of calcareous nannoplankton (Robertson and Bliefnick, 1983). Increased calcareous nannofossil productivity could have depressed the CCD, allowing preferential carbonate preservation. The seafloor was possibly anoxic owing to an absence of deep circulation of oxygenated bottom waters. A similar explanation might apply to the Cenomanian black shales of OAE 2.

Terrestrial-Input Hypothesis

A second hypothesis is that the very finely laminated black shales (i.e., marlstones and calcareous claystones) represent accumulations from low-density turbidity currents derived from the continental rise above the CCD (Arthur et al., 1984; Stow, 2001). Most of this organic matter would be of terrestrial origin. High accumulation rates at the final site of deposition would allow the preservation of organic carbon and transported carbonate below the CCD, as in the first hypothesis. The bottom water just above the seafloor remained oxygenated, but the pore water in the redeposited carbon-rich sediments was anoxic and inhibited burrowing. Bioturbation resumed as oxygenated water diffused into the slowly oxidizing surface sediments, prior to the arrival of the next turbidity current. This explanation may apply to the majority of the black shales cored at Site 1276 because the background green claystones are burrowed, indicating that the seafloor remained at least weakly oxidizing except during periods of carbon-rich clay deposition.

Discussion

A problematic aspect of the "productivity" hypothesis is that most of the black shales contain abundant terrigenous silt and terrestrial organic matter, suggesting that marine organic productivity was not the dominant control except possibly during formation of OAE 2 and OAE 1b. In the "terrestrial-input" hypothesis, the silt in the finely laminated black shales could have been introduced by the same low-density turbidity currents that transported the organic detritus. A problematic aspect of the terrestrial-input hypothesis is that many of the laminated black shales do not appear to be graded or have sharp bases, unlike the numerous mud turbidites in adjacent intervals. Also, in some cases the black shales form relatively thick intervals (as thick as tens of centimeters) that appear to be independent of the presence of mud turbidites.

Binocular-microscope examination reveals alternations of clay- and silt-sized particles that are not seen in the much more numerous mud turbidites. These laminae imply the occurrence of repeated pulses of sediment, with each lamina potentially deposited by a separate turbidity current. Terrestrial input therefore might have been fine-grained clay, silt, and terrestrial organic matter, which was resedimented from the continental slope above the CCD by low-density turbidity currents. These may have been very dilute "lutite flows" (McCave, 1972; Gorsline et al., 1984), perhaps originating from storm-induced resuspension or internal waves (Cacchione and Drake, 1986) that were active along the upper continental slope. In this scenario, bottom water remained relatively oxygenated most of the time, giving rise to green burrowed claystones. During intermittent times of high organic matter input, the sediments became anoxic beneath the sediment/water interface, thus inhibiting bioturbation. In addition, occasionally enhanced productivity of marine organic matter may have promoted the formation and preservation of black shales (e.g., during OAE 2 and OAE 1b).

Hydrothermally Altered Sediments

Two thick diabase sills (Subunits 5C1 and 5C2) cut the lower part of Subunit 5C. The lower sill (Subunit 5C2) is a complex that includes two smaller sills (see Fig. F130 and "Igneous and Metamorphic Petrology" for details of intervals). The upper sill is ~10 m thick, and the main lower sill is >17.50 m thick. The uncertainty of the exact thickness of the lower sill is due to the incomplete recovery of the sill. Sediments typical of Subunit 5C were affected by a range of thermal and hydrothermal alteration effects in the vicinity of these sills.

Calcareous mudstone intervals in Core 210-1276A-87R, above the upper sill, include elongate, light-colored porphyroblasts (Fig. F115) and local recrystallization of carbonate to marble (interval 210-1276A-87R-6, 5-72 cm). Spectacular alteration effects are also seen in Core 210-1276A-98R just above the lower sill that entirely occupies Core 99R. This is highly altered claystone that is interpreted to have been leached and chemically reduced. Porphyroblasts are also present at this level. The porphyroblasts in both Cores 210-1276A-87R and 98R are composed of albite, quartz, alkali feldspar, and magnesian chlorite, based on XRD analysis. As they were precipitated they produced a "spotted hornfels" texture in the two intervals (Figs. F115, F116). In interval 210-1276A-98R-3, 40-50 cm, above the lower sill (Fig. F117), the rock is composed almost entirely of crystalline albite, sudoite (magnesium chlorite), nonstoichiometric calcite (possibly Mg rich), pyrite, and zeolite, based on XRD analysis. This rock probably originated as a bed of calcareous sand (i.e., base of a turbidite). There is some evidence that these sediments were only partially consolidated at the time of intrusion of the sills (see "Physical Properties"). Before lithification, this calcareous sand would have been a porous and permeable interval compared to the interbedded muds and it is likely to have channeled fluid flow that resulted in metasomatism. Adjacent graded calcareous sands and muds were also hydrothermally altered, although primary sedimentary textures, including burrowing, can still be recognized.

Approximately 10 cm above a small sill at 1711 mbsf, Subunit 5C2 (Fig. F130), a rare interval of black shale (interval 210-1276A-98R-1, 95-104 cm) was baked by heat from the sill, as indicated by smear slide study of the organic matter in the black shale.

Finally, a small (9 cm thick) "breccia pipe" (interval 210-1276A-97R-3, 140-149 cm) was identified above the lower Subunit 5C2 diabase sill (Figs. F130, F131). This is a small diabase intrusion that shows chilled margins against mudrock and has internal diabase breccia with coarse calcite-spar cement. The diabase breccia clasts also have chilled margins. A possible explanation is that intrusion of the diabase sill (defined as Subunit 5C2) locally created gas from carbon-rich sediments and caused an initial upward, gas-driven expulsion from a gas-magma mixture that cracked and brecciated the diabase (interval 210-1276A-97R-3, 144-147 cm) (Fig. F131). As the cracks widened, more magma was intruded and chilled against the mudrock (Sections 210-1276A-97R-3, 141 cm, and 97R-3, 148 cm). After intrusion, the by-then brecciated interior was filled by calcite of possible hydrothermal origin, followed by minor deformation.

The first part of this section provides a brief comparison of the Site 1276 lithostratigraphic successions and depositional settings with sediments of the same age cored on the conjugate Iberia margin. The second part extends the comparisons to the lithostratigraphy of the central North Atlantic. The information summarized in Figure F118 and the discussion in the first part is based on information published in the "Site 398" chapter in the DSDP Leg 47B Initial Reports volume (Shipboard Scientific Party, 1979; Sibuet, Ryan, et al., 1979) and Initial Reports volumes for ODP Legs 149 (Sawyer, Whitmarsh, Klaus, et al., 1994) and 173 (Whitmarsh, Beslier, Wallace, et al., 1998). Discussion in the second part compares core recovery at Site 1276 with lithostratigraphy of the North American Basin (i.e., the western central North Atlantic) as described by Jansa et al. (1979). These publications are not cited again in the text below. Table T5 summarizes the sedimentation rates determined for the Newfoundland and Iberia conjugate margins.

The Newfoundland-Iberia Conjugate Margins

Despite the fact that 11 DSDP and ODP sites have been occupied across the Iberia margin, only at Site 398 was an almost continuous succession from the Pleistocene down to the Hauterivian penetrated and recovered. The other sites are situated over basement highs (Fig. F4 in the "Leg 210 Summary" chapter) that were progressively buried during the Cretaceous and early Paleogene, resulting in an incomplete Cretaceous to Eocene section.

In making lithostratigraphic comparisons across the Newfoundland and Iberia margins, it is important to keep in mind that interregional oceanographic controls on deposition, such as changing plankton productivity and depths of the CCD, may be modified by more local topographic and tectonic influences that change over time. The present-day setting of the sites described below is the result of tectonic and depositional history from the time of continental rifting. The locations of the drill sites on present-day abyssal plains, continental rises, and slopes are not necessarily indications of their settings during the Cretaceous and Paleogene. In particular, fault-related basement highs were draped by early postrift sediments and subsequently covered by prograding sediments across the ocean-continent transition.

All the sites except Site 398 (which is on the "continental slope" on the southern flank of Galicia Bank) are situated on present-day abyssal plains (the Newfoundland Basin and the Iberia Abyssal Plain). The westernmost Iberia sites are several tens of kilometers from the continental rise. Hence, there are three settings: abyssal plain (Sites 1070, 897-899, and 1069), margin of the abyssal plain (Sites 900, 901, 1065, 1067, 1068, and 1276), and continental slope (Site 398). Each is likely to have received different proportions of pelagic, hemipelagic, and turbiditic sediments and to have been subjected to differing amounts of mass wasting during its development.

In general, turbidity currents bypass the continental slope to be deposited on the abyssal plain, resulting in slope settings that are dominated by pelagic and/or hemipelagic sediments, with possible reworking by bottom currents. Such slope bypassing is likely to have been significant during the history of the Newfoundland and Iberia conjugate margins. The contrast between slope and abyssal plain sediments is well developed in the Pliocene-Pleistocene sediments drilled during Leg 149. Sediments of this age in the Iberia Abyssal Plain range in thickness from 50 m (Site 899) to 292 m (Site 897). These sediments contain many siliciclastic sand- to mud-grade turbidites that are covered by nannofossil ooze and/or nannofossil clay. The sedimentation rate at Site 897 is 92 m/m.y. In contrast, the 96-m-thick Pliocene-Pleistocene sequence at Site 900, which is situated on the edge of the abyssal plain at the foot of the continental rise, consists almost entirely of nannofossil ooze, nannofossil clay, and clay deposited at a rate of 22.5 m/m.y. The dominance of pelagic and hemipelagic deposits and the low sedimentation rate at Site 900 reflect bypassing by turbidity currents that so strongly controlled sedimentation at Site 897.

Below, each comparison of lithostratigraphy between the Newfoundland and Iberia margins begins with a brief summary of features of Site 1276 lithologic units and then relates them to intervals of the same age on the Iberia margin.

Unit 1 (Middle Eocene to Lower Oligocene)

Unit 1 at Site 1276 is the basal 118 m of an apparent abyssal fan interpreted from the seismic reflection records that cross the site. On the depth-converted seismic section, this wedge of sediment is ~800 m thick at Site 1276. Recovered sediments record dominant hemipelagic deposition that was interrupted by turbidity currents and debris flows. Clasts of Cretaceous hemipelagic and pelagic sediment in the debris flows (including a clast of Valanginian age) indicate that at times there was significant erosion of material from the adjacent continental slope.

At the Iberia Abyssal Plain sites, sedimentary rocks equivalent in age constitute lithologic Unit II. They consist of upward-fining and -darkening intervals (10-50 cm thick) of carbonate mud-dominated turbidites with thin siliciclastic sandstone bases interbedded with hemipelagic/pelagic deposits. The sediments do not show evidence of slumping, debris flows, or grainstones like those at Site 1276, and thus they do not show extensive reworking of carbonate detritus from the continental slope or shelf.

Age-equivalent lower Oligocene to middle Eocene sedimentary rocks at Site 398 on the Galicia continental slope consist of biogenic siliceous marly chalks interrupted by 1- to 5-cm-thick layers of siliciclastic and calcareous siltstone/sandstone. The calcareous sand fraction is composed dominantly of foraminifers, carbonate fragments, and a matrix that is largely nannofossils. The siltstone and sandstone layers show parallel and cross lamination, which, it is suggested in the site report for Site 398, records "geostrophic flow." In contrast, Maldonado (1979) estimates that 30%-40% of the interval is turbidites, the bulk of which are nannofossil-rich mudrocks. This interpretation, and later drilling results from Legs 149 and 173, suggests an alternate explanation that the sandstone layers were deposited by turbidity currents, although most of the entrained sediment was carried downslope to the abyssal plain below. Because the succession at Site 398 is largely composed of bioturbated nannofossil chalks, the chalks that accumulated from turbidity currents must have been deposited in relatively thin units—a conclusion that is consistent with the low sedimentation rate for this interval at Site 398 (Table T5).

From the foregoing summary it is evident that middle Eocene to Oligocene sediments both at Site 1276 and beneath the Iberia Abyssal Plain are dominated by redeposited sediments. Turbidity currents transported carbonate sediments (including nannofossil-rich sediments) from locations on and above the continental slope, as well as minor amounts of siliciclastic and carbonate material that originated from the upper slope or outer shelf. These density flows largely bypassed Site 398. There is a higher proportion of shallow-water material present in the grainstone turbidites at Site 1276 off Newfoundland. On both margins, noncarbonate bioturbated hemipelagic mudrock between the turbidites indicates deposition below the CCD of at least ~4.5 km below sea level at the time (Tucholke and Vogt, 1979). Pelagic nannofossil-rich sediments at the shallower Site 398 indicate that the seafloor there was above the CCD. Significant amounts of biogenic silica are present only at Site 398, suggesting either upwelling of nutrients along the Iberia margin or dissolution of biogenic silica at other sites.

Unit 2 (Upper Paleocene to Middle Eocene)

Lithologic Unit 2 at Site 1276 consists of ~65 m of predominantly light colored calcareous sandstones, grainstones, and marlstones, together with darker-colored mudrocks. These lithologies are arranged in upward-fining and -darkening intervals that are 20 to 140 cm thick. Unit 2 is interpreted as carbonate-rich turbidites interbedded with bioturbated hemipelagic mudrocks. The carbonates were derived from locations on the continental slope and possibly the outer shelf, and they consist of both contemporaneous sediments and material recycled from Cretaceous deposits.

At Sites 900, 1067, and 1068, carbonate turbidites (10-50 cm thick) dominate the age-equivalent upper Paleocene to middle Eocene section beneath the Iberia Abyssal Plain. The turbidites have thin, basal, mixed siliciclastic and calcareous sandstones that pass upward into nannofossil chalks and claystones capped by bioturbated dark-colored hemipelagic claystones. The hemipelagic sediments are reddish brown in the lower parts of the intervals cored at these sites. At Site 1068, reddish colors appear from basement up to the base of the Lutetian (middle Eocene), whereas at Site 900 (situated only 800 m to the east), they extend up into the upper Lutetian. This suggests that the distribution of the colors is more likely to be diagenetic than primary.

All or part of the Paleocene-Eocene succession is missing at Sites 1070, 897, and 899 because the crests of the basement ridges on which the sites were drilled still projected above parts of the abyssal plain at this time. Sediments deposited on the ridge flanks (as seen on seismic reflection profiles) were transported from the margin of Galicia Bank to the north.

At Site 398 on the Galicia slope, biogenic siliceous marly chalks with thin siliciclastic sandstones are similar to those described above from Unit 2 at Site 1276 (Fig. F118). However, silica is more common, rising to 20%-30%, and the sediments are 50%-60% pelagic carbonates. Reddish colors begin to appear about two-thirds of the way down the age-equivalent section at Site 398. The chalks are laminated in places, and some exhibit soft-sediment folds, claystone clasts, and stretched burrows accompanied by pebbly mudstones containing fine-grained carbonate clasts. These features indicate episodes of downslope movement of slumps and gravity flows (Maldonado, 1979).

Turbidity currents delivered carbonate sediments to the abyssal plains on both sides of the Atlantic during the late Paleocene to middle Eocene, but they largely bypassed Site 398. The carbonate turbidites at Site 1276 are thicker than those off Iberia and contain a greater proportion of carbonate allochems than the fine-grained nannofossil-rich sediments redeposited on the Iberia Abyssal Plain.

Sedimentation rates at Sites 1276 and 398 were similar (5-7 m/m.y.), but they were greater beneath the Iberia Abyssal Plain at Sites 900 and 1068 (~18 m/m.y.) because a greater volume of sediment was carried there by gravity flows.

Significant amounts of biogenic silica are present only at Site 398, suggesting either upwelling of nutrients above the Galicia slope or dissolution of the silica during diagenesis at the other sites.

Unit 3 (Lowermost Campanian to Upper Paleocene)

Lithologic Unit 3 at Site 1276 contains much less grainstone (carbonate sand) than Unit 2, and it is composed largely (80%-90%) of mudrocks deposited either from density flows or as background settling of hemipelagic material.

At Site 1068 on the margin of the Iberia Abyssal Plain, age-equivalent sediments have thicker turbidite intervals (as thick as a meter) and sand-sized sediment at the bases of individual turbidites is more carbonate rich. These sediments include foraminiferal sands plus peloidal and intraclastic packstones and grainstones. Siliciclastic sands are predominantly quartz, but those of granule size also contain shallow-water carbonate particles and rock fragments of green phyllite. Similar sediments appear in the Paleocene section at Site 1069 in the Iberia Abyssal Plain.

At Site 398 on the Galicia slope, red to brown marly chalks, marlstones, and claystones, with thin intervals of siliciclastic sandstone and siltstones sporadically spaced every 50 cm to 1 m, were deposited during the early Campanian to Paleocene. Reddish brown mudstones form 30%-50% of the sediments (Maldonado, 1979), similar to the lower part of Unit 3 at Site 1276. These sediments contain only small amounts of biogenic silica, suggesting that there was little upwelling compared to the overlying section. Otherwise, overall depositional processes and sedimentary settings were the same as those for the overlying upper Paleocene to middle Eocene section.

Compared to the Iberia Abyssal Plain area (Sites 900, 1067, and 1068), the influx of redeposited carbonate sediment to Site 1276 was much smaller. However, sedimentation rates were low on both margins where this interval was drilled.

Unit 4 (Turonian to Uppermost Santonian)

Lithologic Unit 4 at Site 1276 consists of reddish brown, thoroughly bioturbated sandstone, muddy sandstone, sandy mudstone, and mudstone. Pervasive bioturbation is attributed to a favorable environment for burrowing organisms (e.g., oxygenation and food supply) and a very slow sedimentation rate (~2 m/m.y. or less). Gradational coarsening-fining cycles tens of centimeters thick suggest reworking by bottom currents.

At Site 398 on the Galicia slope, an interval of reddish brown unfossiliferous claystone, contemporaneous with Unit 4 at Site 1276, is sandwiched between nannofossil-bearing rocks that provide the age control. Similar unfossiliferous sediments are present at Sites 897, 898, and 1070 in the Iberia Abyssal Plain, but their age is not well constrained by sediments above and below: the oldest sediment above is middle Eocene (Site 897), and the youngest sediment below is Aptian at all three sites. However, because the sediments are so similar to those at Sites 1276 and 398, it is tempting to infer that they are the same age. At Sites 897 and 1070, volcaniclastic material is present in sandy layers in the unfossiliferous claystones. At Sites 897 and 899, a basal interval of upward-fining, poorly sorted sandstones and conglomerates is present. The clasts are composed mainly of fine-grained limestone, chalk, and nannofossil claystone. Some of these clasts yield nannofossils of late Hauterivian, Barremian, or Aptian age.

Reddish sediment colors and low sedimentation rates on both margins suggest very limited sediment input into well-oxygenated basins. At Site 1276, the muddy sandstones deposited by gravity flows probably were reworked by bottom currents.

Unit 5 (Uppermost Aptian(?) to Turonian)

A thick (~700 m) sedimentary succession of late Aptian(?) to Turonian age, dominated by dark gray to black mudrocks, is present at Sites 1276 and 398. This interval was not drilled beneath the Iberia Abyssal Plain because the basement highs that were drilled were not covered by sediment until Late Cretaceous to Paleogene time. However, older sediments are clearly present between the basement highs as seen in seismic reflection profiles; the sediments form a semitransparent interval termed seismic Unit 3 by Groupe Galice (1979) and seismic Unit 5 by Wilson et al. (1996, 2001). At Site 1276, an almost identical semitransparent interval is present above the U reflection (see "Leg 210 Summary" chapter, including Fig. F12). Synthetic seismograms facilitate ties between the lithostratigraphy and the seismic units at both Sites 398 and 1276 (Bouquigny and Willm, 1979) (see "Seismic-Borehole Correlation").

Three lithologic subunits are identified at Site 1276. Subunit 5A contains abundant gravity-flow deposits consisting dominantly of mudrock and occasional marlstone above calcareous sandstone and siltstone and/or grainstone. Gravity flows were responsible for depositing ~80% of the succession, the remainder of which is either bioturbated hemipelagic mudrock or black shale. The latter may be hemipelagic and deposited when sea-bottom conditions were anoxic, or material redeposited from an oxygen-minimum zone upslope, or both. In contrast, Subunit 5B is dominated by bioturbated hemipelagic mudrocks with some carbon-rich black shale intervals and minor amounts of coarser sediment like that in Subunit 5A. The presence of siderite, dolomite, and calcite nodules is also a characteristic feature. In Subunit 5C, massive calcareous mudrocks constitute ~75% of the interval, predominantly in thick tops of turbidites. The turbidites contain basal parallel-laminated sandstone or siltstone, grading upward into disorganized fine-grained muddy siltstone with contorted and "swirled" laminae and then into mudrock tops.

At Site 398, the time-equivalent mudrock interval is also divided into three subunits (4A-4C). The oldest of these extends down into the Aptian and Barremian, in contrast to Subunit 5C at Site 1276, which is uppermost Aptian(?) to lowermost Albian. Arthur (1979) provided a succinct summary of features and environmental interpretation of subunits at Site 398, which is summarized below from the top down.

Subunit 4A at Site 398 (middle Albian to mid-Cenomanian) consists of nannofossil chalk and claystone interbedded with dark-colored claystone, with some graded silt- and sand-sized grainstone (referred to as "mudchip siltstone and sandstone") and some radiolarian sands in the upper part. Unlike sedimentary rocks of similar age at Site 1276, nannofossil-rich pelagic deposits indicate that Site 398 was just above the CCD. Otherwise, depositional processes were similar; turbidity currents transported terrigenous sand, silt and mud, contemporaneous pelagic carbonate, and older carbonates reworked from outcrops. Seafloor conditions fluctuated between slightly oxygenated and anoxic. Arthur (1979) reported an interval of dark carbon-rich mudrocks of Cenomanian age, which presumably corresponds to OAE 2, in Cores 210-1276A-32R and 33R.

Subunit 4B at Site 398 (lower to middle Albian) consists of very dark gray to black, laminated to homogeneous claystone interbedded with mudstone (including black shales) and calcareous mudstone, of which the lighter-colored varieties are bioturbated. Carbonate contents are low, organic carbon values are high because of the presence of terrigenous plant debris, and siderite nodules are common. Arthur (1979, p. 721) suggested that deposition occurred in an "outer deep sea fan" setting. Deposition is inferred to have been below the CCD, yet ammonites are relatively common. This paradox was seemingly resolved by de Graciansky and Chenet (1979), who suggested that the ammonites were transported from shallower areas by debris flows and were buried so quickly that they were not dissolved. Turbidity flows also could have been the transportation mechanism. The sedimentation rate for this subunit is ~50 m/m.y., essentially the same as the rate for the age-equivalent Subunit 5B at Site 1276 (~20-100 m/m.y.) (Table T5).

Subunit 4C at Site 398 (upper Barremian to upper Aptian) consists of dark-colored mudrock and calcareous claystone interbedded with conglomeratic debris flow deposits containing pebbles of mudrock (which were soft the time of transport) and light-colored, fine-grained limestones (including some of Tithonian age). The conglomerates are very similar to those at Site 1276 (Section 210-1276A-93R-3) (Fig. F97). Siderite concretions are less common in Subunit 4C at Site 398 than in overlying subunits, but the amount of terrestrial plant debris is considerably higher. In the lower part of the subunit, siliciclastic sandstones and siltstones occur at the bases of turbidites, as do a few graded mudchip sandstones (equivalent to sediments described as grainstones at Site 1276). Arthur (1979, p. 721) suggested that Subunit 4C accumulated probably below the CCD by "prodelta or submarine fan deposition with various sediment sources including nearby slopes and scarps from which calcareous mudchip sands and silts, carbonate-pebble conglomerates and debris flows and slump units were derived." Subunit 4C at Site 398 is lithologically very similar to Subunit 5C at Site 1276, but it is older.

Comparisons with Central North Atlantic Stratigraphy

Based on DSDP results, Jansa et al. (1979) defined six sedimentary formations that extend over much of the North American Basin (i.e., the western part of the central North Atlantic). A schematic summary of the lithology and age of these formations is shown in Figure F118. The brief discussion below highlights similarities and differences between the central North Atlantic formations and the successions drilled on the Newfoundland and Iberia conjugate margins. It also refers to the changes in the depths of the CCD charted by Tucholke and Vogt (1979) for the North American Basin.

Blake Ridge Formation (Middle Eocene to Pliocene; 200-1000 m Thick)

The Blake Ridge Formation is dominated by greenish gray and brown hemipelagic mud, a lithology that forms the background sedimentation between the turbidites of similar age at Site 1276 and beneath the Iberia Abyssal Plain. At Site 398, nannofossil-rich sediments predominate because the seafloor was above the CCD at the time of deposition. The Iberia Abyssal Plain sites were below the CCD until the middle Miocene, when nannofossil-rich sediments appeared. The sediments at Site 1276 show that it was below the CCD at least until the early Oligocene; no sediments younger than this age were cored at the site.

Bermuda Rise Formation (Lower to Middle Eocene; 130-220 m Thick)

The presence of significant amounts of biogenic silica is the defining feature of the Bermuda Rise Formation, which contains siliceous claystone and chert plus radiolarian and calcareous mudstone. The siliceous marly chalks of Unit 2 at Site 398 equate with the Bermuda Rise Formation, but no siliceous sediments of similar age were cored beneath the Iberia Abyssal Plain or at Site 1276. This difference could be caused by lack of nutrient upwelling off the Newfoundland margin and/or dissolution of biogenic silica on both margins.

All the sites on the Newfoundland and Iberia conjugate margins were below the CCD during this time interval, excepting Site 398 where pelagic carbonate sediments were deposited.

Plantagenet Formation (Upper Cenomanian to Paleocene; 30-120 m Thick)

Reddish brown noncalcareous claystone characteristic of the Plantagenet Formation is present in Unit 3 at Site 1276 and extends uphole into the background mudrocks of uppermost Paleocene to middle Eocene Unit 2. In Unit 4 at Site 1276, mudrocks are subsidiary to intensely bioturbated reddish muddy sandstones interpreted to be turbiditic sands reworked by bottom currents. Reddish brown claystones are also present at Iberia Sites 897, 899, and 1070 and in Unit 3 at Site 398. They contain volcaniclastic material at Site 1070 and lie above upward-fining conglomerates and sandstones at the other two sites. These coarser sediments were deposited by debris flows and contain pebbles of Lower Cretaceous limestone and claystone. The red claystones are Cenomanian to upper Santonian off Iberia, but red colors continue uphole into the Paleocene at Site 398. Likewise, at Sites 900 and 1068, hemipelagic mudrocks (capping carbonate turbidites) are brownish red up to the middle Eocene, but as discussed earlier, the red color disappears at different times at the two sites, suggesting that the color might be affected by diagenesis.

The change from deposition of dark-colored mudrocks and black shale facies characteristic of the underlying Hatteras Formation to the reddish sediments of the Plantagenet Formation might have been caused by enhanced bottom water circulation that led to oxygenated conditions on the seafloor. This change could relate to the separation of the Rockall Plateau from northwest Europe if it enabled cooler waters from the north to flow southward (Shipboard Scientific Party, 1979). Given that temperatures at high latitudes were still warm at this time, however, this explanation seems unlikely. It is possible that the opening of a deepwater connection between the North and South Atlantic could have led to the oxygenation of the deep North Atlantic basins near the Cenomanian/Turonian boundary (Tucholke and Vogt, 1979).

Hatteras Formation (Barremian to Cenomanian; 80-350 m Thick)

The Hatteras Formation consists of black to dark greenish gray carbonaceous clay and laminated marl, including the black shale facies characteristic of OAEs. The formation is very similar to Unit 5 at Site 1276 and Unit 4 at Site 398, except that at these sites the equivalent section is considerably thicker (~700 m).

A combination of environmental factors probably contributed to the accumulation of these dark-colored sediments under intermittently dysoxic to anoxic conditions in the North Atlantic. During the latter part of the Early Cretaceous, global sea level began to rise toward its Late Cretaceous peak (Haq et al., 1987). The climate became warm and humid, increasing weathering rates, and there was a dramatic increase in carbonate productivity (Skelton et al., 2003). Most of the coarser sediment and carbonate was sequestered on the shallow continental margins. The deep basin was below the CCD and received only fine sediment carried in suspension from the shallow margins and pelagic biogenic sediment deposited during episodes of enhanced surface water productivity. This background hemipelagic sedimentation on a poorly oxygenated seafloor is typical of the Hatteras Formation.

Off Newfoundland and Iberia, the background hemipelagic sedimentation was interrupted by numerous gravity-flow deposits derived from material that accumulated on the adjacent, shallow margins. Because the ocean basin was also relatively narrow at the time (600-1000 km) and had only restricted connections with small shallow seas to the north (Tucholke and McCoy, 1986), it probably accumulated larger-than-normal volumes of hemipelagic sediments from the margins. As a result, unusually thick sequences of Hatteras-equivalent sediments were deposited off Newfoundland and Iberia.