LITHOSTRATIGRAPHY

The entire Neogene section and part of the Paleogene succession at Site 1276 was cased and not cored. Coring started at 800 mbsf following the retrieval of a wash core taken through the interval 753-800 mbsf. Beneath this wash core, recovery was excellent, approaching 100% for many cores. Five lithologic units are recognized (Fig. F6; Table T2), based primarily on proportions of sediment types, mineralogy of detrital and biogenic components, and bedding style. No attempt is made to define lithologic units that might correspond directly to units defined on the conjugate Iberia margin (Legs 149 and 173), but we do attempt in the concluding section of this chapter to compare and contrast the results from Site 1276 with those obtained from the Iberia sites (Deep Sea Drilling Project [DSDP] Site 398; ODP Legs 103, 149, and 173) and other North Atlantic drilling (DSDP Legs 43 and 76). The reader should be aware that Hole 1276A penetrated a succession that, in its lower part (e.g., in the Cretaceous interval), dips regionally oceanward at ~5° based on interpretation of seismic reflection data (see the "Leg 210 Summary" chapter). Cores preserve dips as high as ~12° relative to the core axis (see "Structural Geology"). Also, a hole deviation of 7.4° in an unknown direction was measured at ~1650 mbsf using the Tensor tool. Thus, the reported stratigraphic thicknesses in the lower part of Hole 1276A (Table T2) must exceed true thicknesses.

The description of each unit includes a detailed account of the lithologies, grouped where appropriate according to whether these are essentially background hemipelagic sediments as opposed to redeposited units. Emphasis is placed on description of the various facies, taking advantage of the potential to illustrate many features in color. Information is integrated from visual study of the split cores, smear slides (see "Site 1276 Smear Slides"), thin section microscopy (see "Site 1276 Thin Sections"), X-ray diffraction (XRD) (see Table T3), CT scans, total organic carbon (TOC) (Tables T17, T18), and inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analyses (see Table T4). Smear slide data are most useful for the study of fine-grained sediments, whereas thin sections were used to study sandstones and grainstones. XRD analysis was conducted mainly on fine-grained sediments that were systematically sampled (one per core), together with a small number of unusual lithologies. The CT scanner was used particularly to investigate depositional and diagenetic structures in whole cores, and some examples of the results are included in this report. In addition, fine-grained sediments were analyzed by ICP-AES on a routine basis (at least one per core).

The descriptive sections are followed by an interpretation section that addresses depositional processes, paleoceanography, and sediment provenance and diagenesis as appropriate. In many cases the suggested interpretations are provisional, bearing in mind the difficulty of interpreting a sedimentary basin using only a single linear stratigraphic section. Reference is also made, where useful, to structural features that are treated more fully in "Structural Geology". Details regarding individual cores are available in "Site 1276 Visual Core Descriptions."

In the initial overview of each unit, colloquial names for rock colors are used because of the large variation in the cored succession. Readers interested in quantitative color data can obtain digital imaging system (DIS) information from the Janus database, captured from the scans of individual core sections that were acquired using the DIS (www-odp.tamu.edu/database). Many sediments have borderline textures between claystone and mudstone (see "Lithostratigraphy" in the "Explanatory Notes" chapter). This sometimes makes classification a matter of personal preference, especially because the silt content cannot be easily quantified from smear slide analyses. Therefore, the informal name "mudrock" was commonly used for undifferentiated claystone and mudstone to emphasize the general similarities in the spectrum of the fine-grained facies. However, different facies of mudrocks were described separately where possible. Also, the carbonate content, as assessed using HCl and/or smear slides and reflected in the terms calcareous, marlstone, and limestone, might not be entirely faithful to CaCO₃ percentages reported in "Carbonate and Organic Carbon" in "Geochemistry" because of the typical coarse sampling interval of one carbonate analysis per core.

"Black shales" were found in lithologic Subunits 5A and 5B and, rarely, in Subunit 5C. They are recognized by their very thin lamination (<1 mm), nannofossil abundance, and high organic carbon content (>1 wt% TOC) as estimated in smear slides and inferred from the dark sediment color. These descriptions are largely corroborated by TOC analyses in the range of 2-4 wt% for this facies. However, the identification of many of the black shales was based solely on visual inspection and smear slide analysis because of the relatively small number of shipboard TOC determinations.

Interval: 210-1276A-1W, 50 cm, to 8R-5, 113 cm

Depth: top: 753.00-794.13 mbsf; base: 864.73 mbsf

Age: middle Eocene-early Oligocene

Lithology: mainly greenish brown mudstone, claystone, and grainstone, with subordinate muddy sandstone and sandy mudstone

Unit 1 consists of ~85% mudstone and claystone (hereafter referred to as mudrock), ~8% muddy sandstone and sandy mudstone, and ~7% sharp-based, graded grainstone beds. The unit is mainly a drab brownish color with shades of green and gray. The defining characteristics of Unit 1 are a high proportion of mudrock; disorganized deposits of muddy sandstone and sandy mudstone with pebble- and cobble-sized mudstone clasts; and a range of deformation features including normal faults, soft-sediment folds, zones of ductile shearing, and crenulated lamination (see "Structural Geology").

The top of Unit 1 is placed at Section 210-1276A-1W-1, 50 cm, although presumably it extends uphole through the interval that was drilled but not cored. ODP convention places the top of Unit 1 at 753.00 mbsf because this wash core was nominally collected in previously drilled material over the depth range 753.0-800.0 mbsf. However, except for 50 cm of drilling breccia at the top of Section 210-1276A-1W-1 (including inferred glacial dropstones), all of Core 1W consists of only slightly fractured lithified sediments indistinguishable in facies from those recovered in Core 2R (cored interval 800.0-809.5 mbsf). Therefore, it is believed that the 580-cm-long interval recovered in Core 210-1276A-1W originated at the base of the "washed" interval, probably below 790.0 mbsf. This sediment is uppermost Eocene to lower Oligocene, whereas Core 210-1276A-2R is upper Eocene. The base of Unit 1 is placed at a sharp color and lithologic boundary with Unit 2, at Section 210-1276A-8R-5, 113 cm. This location correlates with a middle Eocene hiatus (see "Biostratigraphy").

The mudrocks of Unit 1 are variably burrowed and represent slowly deposited hemipelagic sediments. The grainstones are normally graded and have planar lamination; they are interpreted as turbidites. The muddy sandstones and sandy mudstones form disorganized, ungraded beds with scattered floating clasts of mudrock and marlstone, and they are interpreted as debris flow deposits. Grainstones in Unit 1 include a variety of carbonate and siliciclastic grains derived from shelf depths and deeper.

The proportion of CaCO₃ determined from samples taken from Unit 1 is mostly <10 wt% (see "Carbonate and Organic Carbon" in "Geochemistry"). Point values of ~30-50 wt% are present in thin intervals of more calcareous claystone and grainstone, some of which may actually be large, transported clasts in mass-flow deposits. TOC is mostly <1 wt%, although one claystone has a TOC value of 2.35 wt% (Sample 210-1276A-5R-4, 20-21 cm).

Lithologies

Mudstone and Claystone (Mudrock)

The dominant lithology is mudrock; it is the "background" lithology. Mudstones are intergradational with claystones. The colors of the mudstones vary, by gradational boundaries, between greenish gray (5GY 6/1), dark greenish gray (5GY 4/1), olive gray (5Y 3/2), grayish olive (10Y 4/2), and pale brown (5YR 5/2), although the general impression is of a drab brownish color overall. Burrowing varies from absent to slight to common (e.g., Core 210-1276A-6R). Where present, the burrowing includes Chondrites. Unusual fernlike burrows (?Helicodromites) were noted in Section 210-1276A-8R-2. The mudstones are locally interburrowed with sandstone or carbonate grainstone, producing complex textures (Fig. F7). The mudstone commonly contains sand-sized carbonate and mud clasts (e.g., Section 210-1276A-1W-3). The carbonate grains in the mudstones are mainly bioclasts, including benthic and planktonic foraminifers, sponge spicules, and fragments of echinoderms, red algae, mollusks, ostracodes, and fecal pellets. Millimeter-sized mud rip-up clasts are present locally. Several small (<3 cm) carbonate concretions were noted in Section 210-1276A-6R-5.

The dominant colors of the claystones are greenish gray (5GY 6/1) to dark greenish gray (5GY 4/1). The claystone ranges from homogeneous to mottled and commonly contains white carbonate sand and silt grains. It is only weakly calcareous, except where it is associated with carbonate concretions. Where present, thin planar lamination is defined by pale silt grains.

XRD analysis of the mudrocks (mudstones and claystones) reveals abundant quartz. Plagioclase is present throughout as a minor constituent, whereas potassium feldspar is found only locally in trace amounts. Opal-A is widespread, whereas zeolites are rare. Clay minerals are mainly illite and chlorite, and they are common throughout.

The major and trace element composition of the mudrocks, as determined by ICP-AES analysis (Table T4), is within the expected range for sediments derived from mixed continental and biogenic sources. Some samples are enriched in TiO₂, Zr, Cr, and V, compared to average hemipelagic sediments. This reflects a significant terrigenous-silt input. Minor relative enrichments in SiO₂ and MnO are also observed. CaCO₃ values of the background mudrocks are <1.5 wt%, and TOC values are <1 wt%. A few samples are relatively carbon rich (as much as 2.35 wt% TOC).

Grainstone

Calcareous grainstones are a secondary component of Unit 1. The well-graded, silt-, sand-, and commonly granule-sized grainstones (e.g., Section 210-1276A-6R-3) range from olive gray (5Y 4/1) to light olive gray (5Y 5/2). Most are present as thin to medium beds (5-20 cm) with sharp, typically scoured bases; only very rarely does a grainstone grade in grain size downward into the underlying unit (e.g., Section 210-1276A-6R-5). Most grainstones are medium to fine grained. Structures are planar lamination at the base (Fig. F8), passing upward into gently inclined lamination and then to rare current-ripple cross lamination. The inclined lamination occurs in the upper parts of some graded beds. Many bed tops are penetrated by burrows that extend downward from overlying fine-grained sediments. The grainstones contain occasional <1-cm subrounded mudstone rip-up clasts (Fig. F9).

The grainstones are dominated by reworked carbonate and some biosiliceous grains, including benthic and planktonic foraminifers, sponge spicules, echinoderms, red algae, radiolarians, and ostracodes, together with occasional micritic grains and fecal pellets (Fig. F10). They also contain as much as 20% siliciclastic components, mainly quartz, feldspar, mica, and heavy minerals.

Some grainstones are unusually coarse or contain unusual structures. One bed in Section 210-1276A-6R-4 is ~95 cm thick, has a sharp scoured base, and grades from coarse sand at the base to fine sand at the top. The lower part contains abundant, small (<1 mm), elongate, flattened rip-up clasts of mudstone, claystone, and micrite. Unusual wavy lamination in some grainstones is composed of anastomosing darker and lighter seams (e.g., interval 210-1276A-1R-3, 58-65 cm). Close inspection shows that the grain size decreases smoothly across these intervals. Smear slide analysis shows that the darker laminae are mainly mudstone grains, suggesting that carbonate grains and mud particles (e.g., grain aggregates and fecal pellets) were hydrodynamically sorted during deposition.

Muddy Sandstone and Sandy Mudstone

Muddy sandstone and sandy mudstone also form a secondary component of Unit 1. Two spectacular intervals of highly disorganized muddy sandstone and sandy mudstone are present near the top and near the base of Unit 1. Sections 210-1276A-2R-1 through 2R-4, 9 cm, consist of pale yellowish brown (10YR 6/2), highly disorganized muddy sandstone and sandy mudstone (>2 m thick) containing ubiquitous biogenic clasts and large plastically deformed claystone clasts. The individual clasts exhibit subtle inferred shear laminae. Some of these clasts are larger than the core diameter; thus, some "interbeds" of mudstone in this interval could be parts of large mud clasts. The biogenic clasts include abundant benthic foraminifers. In addition, a clast of nannofossil chalk of Valanginian age, presumably transported in a gravity flow or slump, was identified in interval 210-1276A-3R-1, 13-16 cm. In Sections 210-1276A-7R-1 through 7R-4 there is a 3.5-m-thick disorganized unit that probably represents a single debris flow deposit. The matrix is structureless, poorly sorted sandy mudstone with large benthic foraminifers dispersed throughout (Fig. F11).

Diagenesis

A common form of diagenesis is color mottling and reduction halos. Precipitation of authigenic carbonate is widespread, especially in the more calcareous intervals and in some clasts. In addition, small carbonate concretions are rarely present (e.g., interval 210-1276A-8R-2, 100-110 cm).

Weathering conditions on land during the Eocene-Oligocene in the North Atlantic region favored delivery of eroded material with little chemical alteration (Chamley, 1989). The predominantly illite and chlorite composition of the clay minerals is consistent with a continental source such as the adjacent North American continent under a regime of dominantly mechanical weathering.

Some of the grainstones are partly silicified, involving precipitation of both interparticle cement and grain replacement (e.g., interval 210-1276A-3R-4, 47-49 cm). Early opaline cements and opal-CT lepispheres are present, together with later-stage replacement of some grains by finely crystalline chalcedonic quartz (Fig. F10). Opaline silica is also widespread in the mudrocks, showing that silicification is widespread throughout Unit 1. The opal-bearing sediments in Cores 210-1276A-6R and 7R contain appreciable amounts of zeolites, which reflect the availability of dissolved silica during diagenesis.

Soft-Sediment Deformation

Syndepositional deformational structures are present in the upper part of Unit 1 and are best developed in the muddy sandstone and sandy mudstone (Fig. F12). These structures include normal faults, soft-sediment folds, ductile shear zones (Fig. F13), and crenulated convolute lamination (Fig. F14). These features are common down to Section 210-1276A-3R-4 but are rare beneath this section. Small conjugate faults and reverse faults (Fig. F15) also occur rarely (see "Structural Geology"). In addition, many of the large clasts are internally deformed. For example, a small shear fold with a subhorizontal axial plane is present in mudstone in a large inferred clast in Section 210-1276A-2R-CC, together with small-scale crenulation lamination. These features resulted from deformation in a semilithified state. It is not always possible to distinguish the deformation affecting clasts from that affecting the matrix of debris flow deposits because some of the apparent interbeds in the cores might be parts of larger mud clasts.

Depositional Processes and Paleoceanography

Unit 1 accumulated in a deepwater setting below the calcite compensation depth (CCD). This setting is inferred from the noncalcareous nature of Unit 1 mudrocks, particularly the dominant burrowed mudstones that are interpreted as hemipelagic sediments (Pickering et al., 1989). The sediments show no obvious evidence of modification by bottom currents (i.e., they lack diagnostic features such as coarsening-fining cycles in bioturbated intervals [Gonthier et al., 1984] or sorted laminae or ripple lenses rich in detrital heavy minerals [Heezen and Hollister, 1971]).

The slow accumulation of hemipelagic sediments was interrupted episodically by turbidity currents that carried mostly carbonate detritus. These gravity flows deposited organized beds containing laminations typical of the Bouma (1962) sequence. Infrequently, more disorganized gravity flows (i.e., debris flows) deposited structureless muddy sands containing large and diverse mudstone clasts and biogenic grains including benthic foraminifers. The mudstone clasts were probably derived from Paleogene and Cretaceous submarine outcrops (e.g., the walls of gullies and canyons in the adjacent continental slope).

Gravity-induced deformation of interlayered muds and unconsolidated debris flow deposits took place after shallow burial. The cause of this postdepositional soft-sediment deformation possibly was rapid downslope gravity emplacement of younger, unsampled deposits. Seismic profiles are consistent with the presence of deformed and redeposited strata ~200 m above the depth at which coring commenced (see the "Leg 210 Summary" chapter).

Gentle-slope rather than base-of-slope setting is suggested by the paucity of contractional structures such as reverse faults (cf. Barnes and Lewis, 1991). Little oversteepening at the time of deposition could have occurred because turbidity currents are supercritical on slopes >0.5° (Komar, 1971) and under those conditions cannot form the current ripples so commonly observed in Unit 1.

Interval: 210-1276A-8R-5, 113 cm, through 15R-3, 125 cm

Depth: 864.73-929.25 mbsf

Age: late Paleocene-middle Eocene

Lithology: carbonate grainstone and marlstone with subordinate mudrock; finer-grained facies are reddish brown to greenish gray

Unit 2 contains a higher proportion of carbonate-rich lithologies than Unit 1. It consists of ~40% grainstone and calcareous sandstone, ~40% marlstone, and ~20% mudrock (claystone and mudstone). At the top, fine-grained sediments are greenish gray, whereas reddish, brownish, and greenish gray colors predominate for much of the rest of the unit. The contact between Units 1 and 2 is marked by a sharp downhole change in facies and color from the drab greenish gray to brown mudrocks of Unit 1 to light-colored (pale greenish/grayish) grainstone-dominated intervals in Unit 2. The base of Unit 2 is defined at Section 210-1276A-15R-3, 125 cm, at the base of a grainstone bed. Below this, no more grainstones are present until Section 210-1276A-18R-5, which is well within the underlying Unit 3.

Cores in Unit 2 show a high contrast between very light gray to greenish gray grainstones and much darker mudrocks that are shades of greenish gray (5GY 6/1 and 5GY 4/1) (e.g., Core 210-1276A-9R) (Fig. F16) to a variable reddish brown that appears in Core 10R and persists downhole (5YR 4/4 and 10R 4/6) (e.g., Cores 10R and 15R). Associated marlstones are darker than grainstones but lighter than mudrocks.

Graded, sharp-based beds of grainstone marlstone are dominant in Unit 2. These beds are 20-140 cm thick and are overlain by darker-colored burrowed mudrocks. Some beds have a graded but otherwise structureless base. Sedimentary structures above the base are predominantly planar and have cross lamination. Burrows rarely cut the laminated grainstones. Instead, traces of deposit feeders are restricted to bed tops in marlstone or mudrock. Physical sedimentary structures are arranged into partial Bouma sequences, supporting the interpretation of these graded beds as turbidites. However, not all sequences of sedimentary structures follow the pattern reported by Bouma (1962). Specifically, some beds show reversals and repetitions of planar-laminated and ripple-laminated divisions. The proportion of Unit 2 formed by carbonate turbidites varies from ~70% at the top (Core 210-1276A-9R) to ~25% in the middle (Core 11R) to ~65% near the base (Core 14R). Large-scale trends in bed thickness or grain size appear to be absent.

Values of CaCO₃ in Unit 2 are <5-10 wt% in dark-colored mudrocks and >30 wt% in grainstones (see "Carbonate and Organic Carbon" in "Geochemistry"). Several grainstone samples exceed 60 wt% CaCO₃, with a maximum value of ~90 wt% in cemented grainstones. TOC is mostly <1 wt%, but two samples contain significantly higher amounts: 2.42 wt% in a burrowed claystone (Sample 210-1276A-11R-6, 0-5 cm) and 4.06 wt% in a coarse grainstone that has locally visible plant debris (Sample 14R-2, 0-1 cm). Carbonate detritus in the grainstones includes foraminifers, red algae, bryozoans, echinoderms, ostracodes, and mollusks.

Lithologies

Grainstone, Calcareous Sandstone, and Minor Carbonate Granule Conglomerate Marlstone

Graded beds of grainstone (or calcareous sandstone) marlstone dominate Unit 2. Greenish to grayish (5GY 6/1-5GY 4/1) calcareous grainstones are fine and medium grained to rarely coarse grained throughout Unit 2 (Fig. F17). The thickness of individual beds ranges from several centimeters to as much as ~1 m. Most of the grainstones begin with a sharp, scoured base (Fig. F18), ornamented in one case by possible flute casts. Above the basal scours are coarse to medium, mainly carbonate grains in a rock with a minor amount of mud matrix (Fig. F19). Scattered larger grains, several millimeters in size, commonly include micritic carbonate, green glauconite, flattened claystone rip-up clasts, and benthic foraminifers. The grainstone beds grade upward from medium to fine sand, through mud-supported silt-sized carbonate, and finally into marlstone or calcareous claystone at the top (e.g., Section 210-1276A-9R-1, 30 cm, through 9R-2, 72 cm). The critical observation is that these normally graded units (grainstone marlstone calcareous mudrock) are single-event deposits that were subsequently burrowed when organisms recolonized the seabed.

The graded intervals of sand-sized sediment exhibit planar and small-scale cross lamination (Fig. F20). Typically, the cross lamination is present toward the tops of individual graded units. By contrast, especially near the base of Unit 2, some grainstones exhibit relatively sharp tops as well as bases or they are essentially ungraded (e.g., Core 210-1276A-14R). In addition, the lower part of Unit 2 includes rare sets of inclined laminae and cross laminae that are contorted and folded (interval 210-1276A-14R-2, 10-30 cm). Near the top of Unit 2, there is a compacted and therefore folded grainstone dike (Fig. F21). Other small injections are present in the unit. These cannot be confused with burrows because they extend back into the core face as now-folded, but originally planar, sheets of grainstone.

Several of the grainstones exhibit unusual features. In one instance, a single amalgamated bed of grainstone is interpreted as 10 separate depositional events (interval 210-1276A-9R-1, 30 cm, through 9R-2, 104 cm). The thickest grainstone deposited during one event in this interval is inversely graded in its lowest 7 cm from coarse sand to granules. Scattered well-rounded granules and centimeter-sized rounded to subrounded pebbles are present in its coarsest sediments. These include shale clasts, sandstone clasts, pebbles of algal limestone, rare bivalve fragments, and metamorphic fragments (Fig. F22). From the top of the inversely graded sediment, the deposit grades normally upward to very fine sand at the top, just beneath the next surface in this multievent depositional unit.

An interesting deposit of calcareous sandstones and granule-sized calcareous conglomerates is present near the base of Unit 2 in Core 210-1276A-15R. The calcareous sandstones are fine grained to very coarse grained, moderately to poorly sorted, and range from structureless to planar and current-ripple laminated. This interval represents another amalgamated bed formed of separate depositional units that range from graded stratified beds to disorganized deposits with large deformed mud clasts (Fig. F23). These deposits contain "swirled" laminae and "shear" laminae indicative of syndepositional deformation of near-fluid, water-rich sediment (Figs. F24, F25). The thinner-bedded calcareous sandstones in Core 210-1276A-15R are graded and interbedded with burrowed mudstones.

Petrographic analysis shows that the clasts in the grainstones are dominantly fragmented to whole benthic and planktonic foraminifers with lesser (and variable) amounts of red algae, sponge spicules, radiolarians, ostracodes, phosphatic grains, and fragments of mollusks, bryozoans, and echinoderms (Figs. F26, F27, F28). The bioclasts have undergone some compaction and pressure solution, and they are locally cemented by interparticle and intraparticle carbonate. There is minor replacement of carbonate grains by chert. Noncalcareous, nonbiogenic components (<20%) are dominantly quartz with rare to trace amounts of pelletal glauconite, feldspar, mica, heavy minerals, and rock fragments including metamorphic, sedimentary, and volcanic varieties. The quartz grains range in shape from rounded to angular.

The calcareous sandstones contain a diverse suite of metamorphic and sedimentary lithic fragments and bioclastic debris. These mixed sediments also contain a significant proportion of biogenic carbonate grains, including benthic and planktonic foraminifers as well as other bioclasts. The calcareous sandstones in Core 210-1276A-15R were partly derived from felsic to mafic volcanic rocks, as inferred from the presence of large numbers of euhedral crystals that are likely to be volcanic quartz, unstrained tabular biotite, and altered volcanic glass of felsic to intermediate composition (Fig. F29). Nannofossils were extracted from small micritic carbonate clasts in the coarsest-grained calcareous sandstones. Some of these clasts were found to be contemporaneous with the Paleogene age of Unit 2, whereas other clasts were derived from Upper Cretaceous carbonate rocks.

Marlstone

Marlstones are the second most abundant component of Unit 2. The marlstones are dominantly reddish to brownish; however, moderate reddish brown (10R 4/6), brownish gray (5YR 4/1) to grayish red (10R 4/2), olive gray (5Y 4/1), light olive gray (5Y 5/2), and greenish gray (5GY 6/1) hues were recorded. The main calcareous components are nannofossils, bioclastic debris, and micrite. Noncalcareous components are mainly clay minerals plus silt- to sand-sized minerals and rock fragments similar to those in the mudrocks and grainstones.

Locally, the marlstones are moderate reddish brown (10R 4/6), contain thin laminae of grainstone (e.g., Section 210-1276A-12R-4), and exhibit a vague planar lamination defined by silt grains (e.g., Core 13R). This variety of marlstone is moderately burrowed, with local greenish burrow mottling attributed to diagenetic reduction processes. A few intervals of marlstone are strongly burrowed, with grainstone filling the burrows (e.g., interval 210-1276A-13R-1, 120-130 cm). In places (e.g., Core 210-1276A-13R), very thin bedded (<5 cm) claystones and grainstones are interbedded with the marlstone. Marlstone from interval 210-1276A-12R-1, 13-16 cm, yields nannofossils indicating an Early Cretaceous (Valanginian) age, and it must be a sedimentary clast rather than in situ sediment.

Mudrocks (Mudstone and Claystone)

Mudrocks form ~20% of the unit. They are interbedded with other lithologies on a scale of tens of centimeters or are present as longer massive or planar-laminated intervals with gradational changes to other lithologies above and below (Core 210-1276A-10R). There is considerable variation in the intensity of burrowing, from rare to common (Fig. F30). Mudrocks are dominantly greenish gray down to Core 210-1276A-9R but are mainly reddish brown below. Some mudstones contain scattered plant debris (e.g., interval 210-1276A-14R-2, 45-60 cm). Burrows filled with claystone penetrate downward into the tops of several mudstone intervals.

Claystones in Unit 2 are slightly calcareous and greenish gray (5GY 6/1). They are mostly massive to slightly burrowed with scattered white silt grains composed mainly of carbonate (e.g., Core 210-1276A-10R). In Core 210-1276A-12R, indistinct planar lamination is visible where the sediment is not too bioturbated; in Section 12R-2, there is a marked downhole change to varicolored claystone ranging from moderate reddish brown (10R 4/6) to light olive gray (5Y 6/1), with subtle color variations. Thin section examination reveals biogenic components (e.g., nannofossils), quartz, and authigenic zeolites in addition to terrigenous clay.

XRD studies of the mudrocks (mudstones, claystones, and some marlstones) reveal mainly quartz plus a variable, generally minor contribution of plagioclase and alkali feldspar. Opal-A and opal-CT are locally abundant, together with zeolite (clinoptilolite). The clay minerals comprise abundant smectite; subordinate kaolinite, chlorite, and mixed-layer clays; rare illite; and a single occurrence of possible palygorskite.

Chemical analysis of the mudrocks from Unit 2, including some marlstones and silty mudstones (Table T4), shows that most are within the compositional range expected for variably calcareous hemipelagic sediments. However, about half of the samples from the lower part of the unit are relatively enriched in Al₂O₃ (to 15.72 wt%), TiO₂ (to 1.96 wt%), MgO (to 3.26 wt%), Ni (to 125 ppm), Cr (to 107 ppm), and Zr (to 745 ppm) compared with the average composition of hemipelagic sediments. Smear slides indicate the presence of abundant silt and quartz in these intervals, whereas XRD analysis of the same levels (e.g., Sample 210-1276A-12R-3, 30-32 cm) reveals the presence of zeolite, goethite, quartz, alkali feldspar, plagioclase, and calcite, plus the clay minerals listed above. SiO₂ values reach 71.10 wt% in Sample 210-1276A-14R-1, 39-42 cm. CaCO₃ in the mudrocks is mostly >1 wt%, and the marlstones contain as much as 44.4 wt% CaCO₃. TOC values are uniformly low (<0.25 wt%) in the fine-grained sediments analyzed.

Diagenesis

Both physical and chemical diagenetic effects are present. Burrows and small shale clasts are markedly flattened as a result of compaction (Fig. F31). The coarser grainstones contain a fine calcite-spar cement (Fig. F28). Other than cementation, chemical diagenesis is mainly reflected in oxidation-reduction effects. In pale greenish gray grainstones in Core 210-1276A-13R, there are several examples of subhorizontal or anastomosing dark greenish gray (5GY 4/1) seams (several millimeters thick) defined by colors indicative of clay or reduced iron (Fig. F32). These anastomosing greenish seams can be interpreted as protostylolites that are precursors to better-developed stylolites lower in the succession. The reddish marlstones are locally reduced to a greenish color where they are in contact with grainstone (e.g., Core 210-1276A-12R). Elsewhere, in Core 210-1276A-9R, brownish claystones have been chemically reduced in irregular fashion to a greenish color. Also, two brownish black layers that may represent strong oxidation are present in interval 210-1276A-15R-5, 38-90 cm.

Structure

Unit 2 contains rare soft-sediment faults with small offsets plus other postlithification brittle structures. For example, an extensional normal fault with ~2-cm displacement is present in interval 210-1276A-9R, 96-105 cm. Rare examples of soft-sediment folding were noted in the coarser-grained facies (Fig. F33). An unusual coarse-grained calcareous sandstone to granule conglomerate exhibits small, relatively low angle (30°) normal and antithetic faults in addition to soft-sediment folding (Fig. F34).

Depositional Processes, Paleoceanography, and Sediment Provenance

Unit 2 is characterized by deepwater hemipelagic deposition below the CCD, interrupted by abundant displaced calcareous and siliciclastic sediments. Calcareous turbidites dominate the upper part of the unit, whereas siliciclastic turbidites are more common in the lower part. Marlstones are interpreted as fine-grained sediment that was deposited by low-density gravity flows or from low-density tails of turbidity currents that were also carrying carbonate sand. Most of the beds in the lower part of the unit were also deposited from discrete turbidity currents, but composite units, such as those in Core 210-1276A-15R, are interpreted as a succession of deposits from repeated pulses of high-density turbidity currents. Local reverse grading indicates grain interaction during deposition as a consequence of high suspended-sediment concentrations (Middleton and Southard, 1984). Liquefaction in one case induced grainstone injection downward into underlying fine-grained sediments (interval 210-1276A-8R-5, 133-141 cm) (Fig. F21).

The bioturbated calcareous claystone tops of the turbidites formed during times of slow deposition between gravity flows. Marked variations in the color of these claystones (grayish and greenish to reddish) might indicate differences in the extent of seafloor oxidation. Occasional thin to medium beds of massive claystone are interpreted as deposits from discrete mud-laden turbidity currents. These are comparable to the "unifites" originally described by Stanley (1981) from the Mediterranean Sea.

The main sources of redeposited sediments were weakly lithified siliciclastic and carbonate sediments of late Mesozoic and contemporaneous Paleogene age, located upslope on the continental margin. Dispersed Mesozoic and Paleogene microfossils appear to have been reworked from poorly lithified intervals of the original successions. In the lower part of Unit 2, siliciclastic detritus in the turbidites is relatively more abundant than carbonate detritus. The siliciclastic components in the turbidites were derived mainly from a basement terrain, probably composed of Precambrian to Paleozoic igneous and metamorphic rocks from Newfoundland or the adjacent Atlantic continental margin (Grant and McAlpine, 1990). However, other potential source areas to the north or east cannot be ruled out. The smectite mineralogy of the fine-grained sediments as determined by XRD is consistent with derivation from the adjacent Atlantic margin at a time during the Paleocene-Eocene when the climate was relatively warm and humid (Chamley, 1989). The major and trace element composition, with relatively abundant Al, Ti, Mg, Cr, and Ni, indicates the presence of a significant terrigenous input. The local co-occurrence of opal-A, opal-CT, and zeolite (clinoptilolite), together with relative enrichment in silica, is indicative of a reactive setting of silica diagenesis and the potential onset of chert formation.

The calcareous sandstones also contain rare felsic and mafic volcanic detritus. The most likely source is the nearby continental margin, including the Cretaceous Newfoundland seamounts. The known volcanism in these seamounts is mafic (Pe-Piper et al., 1990), whereas the volcanic detritus at Site 1276 includes material of silicic composition. A possible alternate source, albeit one that would require long-distance transport, could be the Greenland-Scotland margin, which was then rifting (Saunders et al., 1997). It is also possible that an as-yet-undiscovered source of bimodal volcanism exists on the adjacent Newfoundland continental margin.

Interval: 210-1276A-15R-3, 125 cm, through 25R-5, 80 cm

Depth: 929.25-1028.00 mbsf

Age: earliest Campanian-late Paleocene

Lithology: mainly reddish mudstone and claystone with subordinate light gray grainstone and calcareous sandstone

Unit 3 is dominated by reddish mudstone and claystone (mudrocks), which together account for ~80% of the succession. Mainly grayish grainstone and calcareous sandstone, taken together, form ~10% of the unit, much less than in Unit 2. The remainder consists of minor amounts of marlstone (~5%), calcareous siltstone (~3%), and sandy mudstone (~2%). From the top to the base of Unit 3, the proportion of grainstone and marlstone increases at the expense of mudrock (Fig. F6). Grainstone constitutes ~20% of the lower part of the unit. The upper contact of Unit 3 is placed at a sharp downhole change from coarser-grained sediments above (Unit 2) into much finer-grained and muddy rocks. The lower contact of Unit 3 is placed at the base of a thick grainstone bed at Section 210-1276A-25R-5, 80 cm (Fig. F6). Below are reddish brown sandy mudstones of Unit 4.

Mudrocks in Unit 3 are predominantly reddish brown (Geological Society of America Rock Color Chart color codes and details are given in "Lithologies") (see "Site 1276 Visual Core Descriptions"). Cores 210-1276A-15R, 22R, and 23R are entirely reddish brown. Cores 210-1276A-19R, 20R, 24R, and 25R contain gray mudrocks with a definite reddish cast or a mixture of light gray and reddish brown. Cores 210-1276A-16R through 18R and 21R are the only cores that do not have a reddish hue.

Only ~15% of Unit 3 is moderately to strongly burrowed. Some mudrocks and all of the muddy sandstones are burrowed. They accumulated slowly and were effectively reworked by deposit-feeding organisms. The remainder of Unit 3 is predominantly mudrock and consists of a wide variety of graded, mud-dominated gravity-flow deposits. Most of these deposits (~75%) begin with a grainstone or calcareous sandstone base that is generally planar laminated and rests sharply or erosively on underlying mudrock. The other gravity-flow deposits (~25%) have a similar laminated basal division that is composed of coarse calcareous siltstone.

Both types of gravity-flow deposit, whether grainy or silty at the base, grade upward into a generally thick mudstone to marlstone top and, in some cases, into claystone at the very top. These graded units can be as thick as ~2 m. The thickest examples are characterized by syndepositional deformation produced by rapid accumulation, differential loading in undercompacted sediments, and shearing (see below for details). Burrowing is restricted to the uppermost few tens of centimeters of these deposits, confirming their emplacement as single, large gravity-flow deposits. Grading and tractional structures indicate that they are mud-dominated turbidites. Clearly, the thick, mud-dominated turbidites accumulated at much higher short-term rates than the intervening burrowed mudrocks. Nevertheless, the overall sedimentation rate of the unit is low (~4-6 m/m.y.) (see "Biostratigraphy"). There is an interval of very slow sedimentation in Core 210-1276A-22R (perhaps <1 m/m.y.) that is discussed separately below.

Values of CaCO₃ in Unit 3 are mostly <15 wt% (see "Carbonate and Organic Carbon" in "Geochemistry"). Peak values of 25-30 wt% occur above 970 mbsf, whereas three values in the range of 35-85 wt% occur from that point to the base of the unit. The highest values are found in marlstone and grainstone. TOC is mostly <1 wt%. The only anomalous sample, from the marlstone top of a >1-m-thick gravity-flow deposit, has 6.65 wt% TOC (interval 210-1276A-21R-2, 123-124 cm). Calcareous detritus in the grainstones and calcareous sandstones includes foraminifers, carbonate lithoclasts, and fragments of echinoderms, ostracodes, and mollusks. The terrigenous fraction includes quartz, feldspar, micas, glauconite, and heavy minerals.

Lithologies

Mudrocks

Mudrocks, estimated at 80% of the total recovery, are predominantly mudstone. There are only minor amounts of claystone, calcareous claystone, marlstone, and sandy mudstone, which are treated in this section for the sake of brevity.

The mudstone is dominantly reddish but ranges from olive black (5Y 2/1) to grayish yellow green (5GY 7/2), dark greenish gray (5GY 4/1), light olive gray (5Y 6/1), light brown (5YR 5/4), and moderate brown (5YR 3/4) (Fig. F35). It has calcareous components that are mainly nannofossils, with minor foraminifers and micrite. The silt fraction is dominated by quartz, with lesser mica, feldspar, glauconite, organic (plant) matter, rock fragments, heavy minerals, and fish debris. The brownish mudstones have rare iron oxides.

The mudstones are devoid of lamination apart from occasional pale silty stringers. A common pattern is a sharp-based olive-gray (5Y 4/1) or light gray (N7) calcareous siltstone, grading upward into dark greenish gray (5GY 4/1) burrowed mudstone, which is topped by pale olive (10Y 6/2) calcareous mudstone or marlstone.

In the lower part of Unit 3, as seen in Core 210-1276A-21R, the mudstones form subtly graded intervals as much as 10 cm thick, each overlying a thin basal set of sand or silt laminae. Burrowing is restricted to the upper parts of the muddy tops of these units. In Cores 210-1276A-22R and 23R, minor intervals of mudstone are planar laminated, with little or no burrowing. In places, as in Core 210-1276A-23R, the mudstones are marked by greenish gray (5GY 6/1) bands that are ~1 cm thick, inferred to be chemically reduced during diagenesis. In Cores 210-1276A-23R through 25R, massive or parallel-laminated mudstone includes numerous siltstone laminations.

Burrowing ranges from occasional mud-filled burrows to intense bioturbation. Throughout Core 210-1276A-22R, many intervals are spectacularly burrowed by Zoophycos (Figs. F36, F37). This core is unusual relative to the rest of Unit 3 both for its Zoophycos traces and because it contains a significant proportion of sandy mudstone. Careful micropaleontological work has subsequently indicated that the sedimentation rate in this core is very low. Core 210-1276A-22R likely includes the entire Maastrichtian interval (see "Biostratigraphy").

The subordinate claystones and calcareous claystones of Unit 3 are dominantly reddish, although strictly they range from moderate brown (5YR 4/4), olive black (5Y 1/2), grayish yellow green (5GY 7/2), and grayish green (5G 5/2) to light olive gray (5Y 5/2). They form a subordinate but variable part of the unit (e.g., Fig. F38). The reddish to brownish colors reflect the presence of minor iron oxides. Beneath Core 210-1276A-18R, only minor claystone is present (e.g., in Core 24R).

There are minor amounts of weakly to (locally) strongly burrowed marlstone throughout Unit 3, ranging from pale green (10G 6/2) to light brown (5YR 6/4). These sediments are typically present in the normally graded depositional units described above (Figs. F39, F40).

XRD analysis of the fine-grained sediments reveals abundant quartz and calcite, together with rare but locally abundant plagioclase and potassium feldspar (including sanidine). Sanidine is found in Core 210-1276A-23R. As in Unit 2, smectite is the dominant clay mineral, but illite and kaolinite increase in relative abundance below Core 210-1276A-20R. Opal was also detected.

ICP-AES analysis (Table T4) reveals major and trace element compositions of the mudrocks that are within the range expected for variably calcareous hemipelagic sediments. However, a few samples of claystones are relatively enriched in TiO₂ (to 1.54 wt%), Fe₂O₃ (to 9.79 wt%), MnO (to 0.29 wt%), Zr (to 586 ppm), and Cr (to 104 ppm) (e.g., Sample 210-1276A-22R-3, 91-93 cm). Smear slides of these sediments are rich in quartz, alkali feldspar, plagioclase, and calcite; thus, a correlation of this enrichment to terrigenous source rocks is likely. Several other samples exhibit relatively high values of SiO₂ (to 89.90 wt%) (e.g., Sample 210-1276A-17R-7, 24-28 cm) that may indicate silica diagenesis in these rocks, which is consistent with the XRD results. The "background" fine-grained sediments exhibit low values of CaCO₃ (<1 wt%) and low values of TOC (<1 wt%), although TOC is 1.10 wt% in one sample.

Grainstone, Calcareous Sandstone, and Calcareous Siltstone

Grainstone has a variety of colors: yellowish gray (5Y 8/1), light greenish gray (5GY 8/1), olive gray (5Y 4/1), light olive gray (5Y 6/1), white (N9), pale olive (10Y 6/2), and light brown (5YR 6/4). It forms ~10% of Unit 3. Beds of grainstone vary in abundance, thickness, and grain size from core to core. The beds are typically tens of centimeters thick, exhibit sharp scoured bases, and grade upward through planar-, cross-, or wavy-laminated divisions into mudstone (or occasionally marlstone) tops (Figs. F40, F41, F42, F43). Most grainstones are devoid of burrowing, but a few contain large cylindrical burrows that may represent the traces of organisms that attempted to escape from the turbidite immediately after its deposition (Fig. F44). Some of the grainstones are as much as several meters thick and contain unusual soft-sediment deformation and load structures. The thickest such graded bed is in Sections 210-1276A-25R-2 through 25R-4 (Fig. F45). In a thick graded bed in Sections 210-1277A-18R-5 through 18R-6, the bed grades rapidly from a sandy lower part to a thick silty/muddy top (Fig. F46). The sandy basal part of the bed is succeeded upcore by contorted laminae and zones of soft-sediment deformation ("load balls") (Fig. F47).

Based on thin section examination, the grainstones contain mainly carbonate clasts, commonly accompanied by glauconite. There is a variable siliciclastic component, especially near the base of the coarser-grained units (e.g., in Core 210-1276A-21R). Exceptionally, small rip-up clasts are present (e.g., at Section 210-1276A-24R-2, 67 cm). Carbonate components include reworked Maastrichtian (globorotalid) foraminifers (Fig. F48A) along with other mainly planktonic forms. Additional bioclasts are fragments of echinoderms, ostracodes, mollusks, sponge spicules, radiolarians, and fish debris (Fig. F48B). The percentage of reworked vs. contemporaneous bioclastic debris could not be determined. Fecal pellets and transported carbonate clasts (intraclasts) are concentrated locally. Rock fragments, where present, are mainly sedimentary (e.g., micritic limestone and claystone), with only minor metamorphic and volcanic varieties. Authigenic cements and grain replacements include carbonate, chert, and pyrite.

Throughout Unit 3, the siliciclastic component is sufficiently high (to >50%) that the coarser beds can be classified as calcareous sandstone. The calcareous sandstones are mainly gray (yellowish gray [5Y 8/1], light greenish gray [5GY 8/1], olive gray [5Y 4/1], and light olive gray [5Y 6/1]) to greenish gray (5GY 6/1), similar to the grainstones of Unit 3. Individual sandstones exhibit scoured bases and grade from medium- to fine-grained sandstone with a mudstone top (Figs. F49, F50). Most of the sandstones are unburrowed, but a few are moderately burrowed. Where present, the burrows are filled with fine-grained mudstone or fine siltstone. The medium-grained to fine-grained sandstones contain scattered coarser grains, commonly white mica or glauconite. The siliciclastic components are subrounded to angular quartz, feldspar, mica, pelletal glauconite, and heavy minerals. In addition, physical property measurements highlighted a thin interval in Section 210-1276A-15R-4 that is characterized by high magnetic susceptibility. This was found in thin section to be caused by an enrichment in magnetite grains (Fig. F51B). Volcaniclastic detritus was identified in the same interval (Fig. F51A) and in one siltstone near the base of Unit 3 (Fig. F52).

Siltstone and calcareous siltstone form a minor part (<5%) of Unit 3, especially toward the base. Calcareous siltstones are most commonly light olive gray (5Y 5/2) to light brown (5YR 5/4), in contrast to the slightly paler colors of the grainstones. The siltstones occur in two main forms. First, they form the middle to upper parts of discrete graded beds, confined between intervals of fine-grained, graded grainstones and sandstones beneath and calcareous mudstone above. Occasionally, the siltstone divisions of thick, graded grainstones show anastomosing laminae interpreted to have been formed by soft-sediment shearing coupled with compaction effects (Fig. F53). Second, some calcareous siltstones form discrete thin beds or laminae at the base of graded siltstone-mudstone couplets as thick as several centimeters. The thicker siltstone interbeds mainly exhibit sharp bases (e.g., Core 210-1276A-20R). Laminae are sharp based; some grade upward, typically into mudstone (e.g., Core 210-1276A-23R). Down to Core 210-1276A-20R, siltstones compose only a few percent of the succession but are relatively more abundant beneath this. In addition, there are several thin (<20 cm) intervals of sandy siltstone (e.g., Core 210-1276A-16R) to massive siltstone (e.g., Core 20R). These are mainly devoid of burrowing, especially where they form parts of graded, inferred gravity-flow deposits (see below). Some otherwise structureless siltstones and muddy siltstones contain Chondrites burrows.

Cretaceous/Tertiary Boundary Interval

From detailed micropaleontological study, the K/T boundary was identified in Section 210-1276A-21R-4, between 41 and 49 cm (see "Biostratigraphy") (Fig. F54). From a sedimentological point of view, Section 210-1276A-21R-4 is fairly typical of Unit 3 above and below. It is characterized by interbedded mudstones, marls, and graded fine-grained sandstones with planar and cross lamination, interpreted as turbidites. However, in the intervals 210-1276A-21R-4, 41-47 cm, and 57-75 cm, there are two anomalous silty mudstone intervals that coarsen upward to a sandy top. The upper interval contains scattered sand and granule grains at 41 cm, including opaline silica granules that apparently replace biogenic carbonate. Such distinctive upward-coarsening mudstones are out of sedimentary context with the adjacent rhythmically bedded hemipelagic and normally graded redeposited sediments. We are presently unable to provide a satisfactory explanation for these inversely graded units. The upper one could be related to a K/T boundary event, but the lower one seems too far removed to be related unless sedimentation rates of the intervening mudrocks were unusually fast.

Diagenesis

An obvious diagenetic feature is the local occurrence of chemically reduced zones and mottles that produce subdued greenish and grayish colors. Diagenetically formed nodules of pyrite are locally present in the black claystone of Core 210-1276A-22R. Several of the graded grainstones exhibit anastomosing, several-millimeter-thick, darker seams of reduced iron or clay minerals (e.g., Fig. F55). These seams are interpreted to partly reflect size sorting of clay-rich aggregates that were later differentially compacted. However, some of the clay-rich seams cut across the primary lamination, showing that some degree of diagenetic dissolution/precipitation of carbonate and clay minerals is likely to be involved. In addition to minor cementation by microspar, some of the grainstones of Unit 3 are characterized by incipient silicification, indicated by the occurrence of chalcedonic quartz and opal-CT lepispheres.

Structure

Rare microfaults are discussed in "Structural Geology". In addition, several examples of syndepositional folding, on a centimeter to millimeter scale, are present (Fig. F56).

Depositional Processes, Paleoceanography, and Sediment Provenance

Unit 3 is dominated by burrowed hemipelagic sediments deposited on an oxygenated seafloor below the CCD, interbedded with redeposited carbonate intervals, with an average overall sedimentation rate of ~5 m/m.y. (see "Biostratigraphy"). The presence of smectite and kaolinite indicates derivation from a warm, humid, deeply weathered landmass (Chamley, 1989). Kaolinite is known to be abundant around the K/T boundary elsewhere in the North Atlantic (Chamley et al., 1988).

Redeposited carbonate bioclasts are abundant in the upper part of the unit and are likely to represent a mixture of contemporaneous and reworked material that is partly of Cretaceous age. The bioclasts are found as isolated grains rather than in lithoclasts; thus, their most likely source was poorly consolidated granular carbonates that were originally deposited in a shelf to upper slope setting. Siliciclastic detritus is more abundant toward the base of the unit and was likely derived mainly from older sedimentary deposits, with a small contribution from igneous and metamorphic rocks. Relative enrichment in Fe, Ti, Mn, Zr, and Cr compared to average hemipelagic sediments may indicate a terrigenous or volcanogenic input, especially near the base of the unit.

The thick, graded beds are interpreted as turbidites with a silty or sandy basal division grading upward into a more calcareous or muddy upper part. Turbulent flow accounts for the grading and the organized divisions of planar and cross lamination. The restriction of burrows to the tops of graded depositional units suggests rapid emplacement. After initial deposition from turbidity currents, the top of each deposit was burrowed in an oxygenated setting. These mud-dominated deposits, particularly those with swirled laminae and load structures in an upper, finer-grained division, resemble basin-plain turbidites described by several authors (Skipper and Middleton, 1975; Pickering and Hiscott, 1985; Weaver et al., 1986). Seismic profiles show that the seafloor at Site 1276 during deposition of Unit 3 was relatively flat (see "Seismic-Borehole Correlation"). We infer, therefore, an essentially featureless basin-plain setting for the deposition of this unit in which mud-laden turbidity currents spread widely and deposited sheetlike, graded units with muddy tops. Between turbidity currents, hemipelagic deposits accumulated slowly on the basin floor.

Interval: 210-1276A-25R-5, 80 cm, through 29R-6, 62 cm

Depth: 1028.00-1067.24 mbsf

Age: Turonian-latest Santonian

Lithology: reddish brown bioturbated muddy sandstone and sandstone

Unit 4 is dominantly siliciclastic, consisting of ~40% muddy sandstone, ~25% sandstone, ~20% mudstone, ~10% sandy mudstone, and ~5% siltstone. Grainstones and marlstones are rarely present. The characteristic sediment color is reddish brown. The coarser-grained rocks—muddy sandstones, sandstones, and sandy mudstones—are thoroughly bioturbated. This characteristic alone distinguishes Unit 4 from all other sediments recovered at Site 1276. Pervasive bioturbation is attributed to favorable environmental conditions (e.g., oxygenation and food supply) and the very slow sedimentation rate of this unit, ~2 m/m.y. or less (see "Biostratigraphy"). The only unburrowed sediments are infrequent graded beds of siltstone with mudstone tops. Only the uppermost part of each mudstone is penetrated by burrows that originated from the overlying coarser deposits. The lower contact of Unit 4 is defined at the base of the lowest muddy sandstone at Section 210-1276A-29R-6, 62 cm. This contact correlates with an unconformity in seismic profiles (see "Seismic-Borehole Correlation").

Values of CaCO₃ in Unit 4 are <5 wt% above 1055 mbsf, but they are higher below that depth, with several values in the range of 25-40 wt% in rare grainstones and calcareous mudstones (see "Carbonate and Organic Carbon" in "Geochemistry"). TOC is very low, from 0 to <0.4 wt%. The siliciclastic sediments of Unit 4 were derived from terrigenous sources and include clasts of a variety of metamorphic and igneous rocks.

Lithologies

Burrowed Sandstone, Muddy Sandstone, and Sandy Mudstone

Unit 4 is predominantly strongly burrowed, fine- to medium-grained, reddish brown (5YR 4/4, 5YR 5/6, and 5YR 5/2) to grayish (N4 and N6) sandstone, muddy sandstone, and sandy mudstone. Remnants of primary lamination are only locally preserved (Fig. F57). Typically, these sediments exhibit repetitions of well-burrowed and only sparsely burrowed sediment on a decimeter scale. There are also coarsening-fining cycles of similar scale that were not observed in other units recovered in Hole 1276A. Burrow types are diverse in the sandy sediments. The muddy sandstone occasionally exhibits dewatering features (e.g., an indistinct vertical dewatering structure in interval 210-1276A-28R-1, 132-140 cm).

The burrowed muddy sandstones and sandstones have dispersed sand- to granule-sized clasts that include a variety of granitic, plutonic, metavolcanic, metasedimentary, and sedimentary lithic fragments. Monomineralic components are quartz, feldspar, muscovite, biotite, and heavy minerals including zircon and tourmaline (Figs. F58, F59). Some radiolarians are present in conjunction with opaline cement. There are rare agglutinated foraminifers (Fig. F60). The matrix mud is similar in composition to the mudstone described below.

Graded Sandstone and Siltstone

There are occasional beds of well-sorted, fine-grained sandstone and siltstone that grade upward to unburrowed mudstone; at the top of these beds is an upward transition to strongly burrowed mudstones. Two of the graded units exhibit a lenticular cross-laminated, medium- to fine-grained sandstone at the base, succeeded by siltstone above a sharp break in grain size (Fig. F61). Such ripple sets are interpreted to represent reworking of the underlying burrowed muddy sand by a turbidity current that then deposited the overlying graded unit. Burrow types are mainly restricted to Chondrites in the siltstone and mudstone tops of the graded turbidites.

Detritus in the graded sandstones and siltstones is similar to that in the burrowed sandstones and muddy sandstones, except that there are fewer rock fragments. Petrographic study revealed abundant quartz, glauconite, and other constituents shown in Figure F59. These sandstones are locally cemented by calcite spar. Although the vast majority of coarse sediments in Unit 4 are siliciclastic, there are a small number of medium-grained, glauconite-rich grainstone beds (e.g., in Section 210-1276A-28R-5). These are so minor that they do not warrant a separate heading in this report. The grainstones are white (N9) to moderate brown (5YR 3/4), graded, and contain both planar and cross lamination. In terms of facies, these strongly resemble the graded sandstones described above.

Mudstone

Grayish brown (5YR 3/2) mudstone is present as a minor component throughout Unit 4. Many mudstones form the tops of graded gravity-flow deposits above a basal division of either sandstone or siltstone. Such mudstones are finely laminated to massive. There are also several thin (<5 cm), homogeneous, sharp-based mudstone beds that are only slightly burrowed at their tops by Chondrites. Very rarely, muddy sediments are sufficiently calcareous to be termed marlstones (<1% of the recovery), as seen in Section 210-1276A-28R-4. These marlstones replace mudstones in the tops of a few of the graded beds described above. Their carbonate content is mainly nannofossils and minor foraminifers.

The main silt-sized components of the mudstones are quartz, feldspar, muscovite, mica, heavy minerals, glauconite, and organic matter including plant debris. Reddish color in the sediments is caused by the presence of fine-grained iron oxides. Carbonate is dominantly micrite.

XRD analysis shows a predominance of quartz. Calcite is abundant only in the isolated intervals containing grainstones and marlstones. Both plagioclase and alkali feldspar are present in significant amounts. Of the clay minerals, smectite is abundant in the upper part of the unit, above Section 210-1276A-27R-5, but illite (including possible muscovite) increases in relative abundance beneath this. Kaolinite and chlorite are rare.

Diagenesis

There are a few ~1-cm-thick greenish gray (5GY 6/1) bands where the sediment has been chemically reduced (e.g., Core 210-1276A-28R). In addition, diagenetically controlled mottling and irregular color banding are widespread in both the muddy sandstones and mudstones (e.g., interval 210-1276A-27R-4, 110-133 cm) (Fig. F62). Goethite is more abundant in Unit 4 than in the units above or below, in keeping with the red-brown colors. In addition, manganese minerals are found both as manganite in one concretion and as minor manganese hydroxide layers, based on XRD results.

Depositional Processes, Paleoceanography, and Sediment Provenance

The reddish sediments of Unit 4 correspond to a period of exceedingly slow sedimentation of ~2 m/m.y. or less, depending on how a sedimentation-rate curve is fitted to the biostratigraphic data (see "Biostratigraphy"). We infer that the deposition of reddish, fine-grained sediments, which are relatively enriched in fine-grained iron oxides (i.e., goethite) and rarely enriched in manganese minerals (i.e., manganese hydroxides), was favored by slow net rates of sediment accumulation and by oxidizing conditions. Slow deposition permitted intense burrowing, destroying almost all primary lamination. The uphole increase in smectite compared to illite and mica is consistent with a change to a warmer, more humid climate in the source landmass (Chamley, 1989).

The style of deposition in Unit 4 is entirely unlike that in Unit 3 (except perhaps Core 210-1276A-22R) and that in the underlying Unit 5 because the bioturbated deposits in Unit 4 are sandy mudstones, muddy sandstones, and, locally, even coarser sandstones. Section 210-1276A-25R-6 even includes dispersed granules of metamorphic and other rock types. In contrast, the finer sediments (siltstones and mudstones) generally represent graded event deposits that are free of burrows, except at their top. We envisage that most of the sand in Unit 4 was initially delivered to the seafloor in small aliquots followed by periods of nondeposition. This provided an opportunity for burrowers largely to rework and mix these sediments with muddy interlayers, so the initial depositional units rarely survived. Gravity flows occasionally deposited thicker beds (as much as tens of centimeters thick) with a sand-silt base and a laminated muddy top. These tops remained unburrowed, perhaps because they were not attractive to burrowing infauna or were only attractive to organisms that fed at shallow levels in the sediment. Locally, incoming muddy gravity flows reworked the tops of muddy sands into a single train of coarse-grained ripples before burying the seafloor with a graded silt to mud layer.

One defining characteristic of the sandy mudstones, muddy sandstones, and bioturbated sandstones in Unit 4 is the presence of gradational coarsening-fining cycles tens of centimeters thick. These strongly resemble the coarsening-fining cycles found in many modern sediment drifts that accumulate under thermohaline currents. These cycles have been explained by bottom-current velocity changes related to long-term variations in the rate of bottom water production or to changes in the paths of the currents (Gonthier et al., 1984). The deposits in Unit 4 are much coarser than most modern sediment drifts that have been studied, but there are sandy and even gravelly contourites in Rockall Trough that might be suitable analogs (Masson et al., 2002). Bottom-current redistribution of mixed sandy and silty muds might account for the cycles, and slow accumulation rates can account for the bioturbation.

Unit 4 was deposited near the time of continental breakup and the initiation of seafloor spreading in the Labrador Sea (Srivastava and Verhoef, 1992; Saunders et al., 1997). Perhaps this narrow ocean basin allowed high-latitude waters to spill into the Grand Banks-Iberia corridor, reworking the slowly accumulating sandy sediments of Unit 4. Another possibility is that the inferred bottom currents affecting Unit 4 relate to oceanographic changes as deepwater connections between the North and South Atlantic were established for the first time near the Cenomanian/Turonian boundary (see Sibuet, Ryan, et al., 1979; Tucholke and Vogt, 1979). As a result, there might have been a switch from black shale accumulation (i.e., the underlying Unit 5) in a relatively restricted ocean basin to strongly oxidizing deepwater circulation in a fully interconnected ocean.

The contact between Units 4 and 5 is an erosional unconformity (see "Seismic-Borehole Correlation") that could have been created by scouring beneath vigorous bottom currents. Perhaps strong bottom currents first scoured the seafloor and created the unconformity, and then weaker currents permitted accumulation of sandy contourites under oxygenated bottom waters. Currents appear to have waned further toward the top of Unit 4, possibly as a result of changes in paleogeography and flow pathways in the widening ocean basin.