Site 1260 represents an intermediate water depth site (current water depth = 2548.8 mbsl) of the Leg 207 Demerara Rise depth transect. Recovery of the cored interval (862.2 m) averaged 83% over the two holes (1260A and 1260B). A maximum depth of 509 mbsf was reached in Hole 1260B.
Similar to the other Demerara sites cored during Leg 207, five lithostratigraphic units were recognized at Site 1260 (Table T2; Fig. F4). The oldest unit recovered is Unit V (Albian) and is dominated by silty quartz claystone to calcareous claystone with opaque minerals and quartz interbedded with well-cemented limestones. The overlying Unit IV (Coniacian–Cenomanian) is composed of interbedded laminated organic-rich claystones and laminated limestones. The three youngest units (Campanian–Pleistocene) are pelagic and primarily composed of pervasively bioturbated nannofossil chalk or ooze. They are distinguished based on their clay, microfossil, and zeolite content. Unit III is relatively clay rich, with abundant zeolites. Unit II is relatively carbonate rich and is dominated by calcareous nannofossils and planktonic foraminifers and its lower part contains abundant siliceous microfossils. Unit I is carbonate poor and is composed of nannofossils, foraminifers, quartz, and glauconite in a clay matrix. The sediments recovered at Site 1260 include the records of several critical events including the P/E and K/T boundaries (see "Biostratigraphy"). Hiatuses correspond to the absence of middle and upper Oligocene, middle Eocene, lower Campanian, and upper Albian sediments.
Unit I was recovered only in Hole 1260A. It consists of dark greenish gray clay with 10 wt% carbonate. The top of the unit, immediately below the sediment/water interface, is an olive-brown sandy clay. There is a sharp color change to gray clay at 8 cm that likely represents a redox boundary. It overlies a 2-cm-thick glauconite-rich (20%) interval. Calcareous nannofossils are the main biogenic component in Unit I (20%). Planktonic foraminifers are common at the top of the unit, becoming increasingly rare downhole. The siliceous biogenic component consists of diatoms and a few radiolarians and is found primarily in the olive-brown layer. No bioturbation is observed, but its absence is likely an artifact of drilling disturbance.
The main mineral phases include glauconite (~5%), quartz (15%), and clay. Disseminated pyrite is present in the sediments (<3%). The boundary with Unit II is placed at the base of a white planktonic foraminifer–rich horizon at Section 207-1260A-2R-1, 10 cm.
Unit II consists of light greenish gray to greenish gray ooze and chalk with 50–80 wt% carbonate. The ooze is restricted to the upper 3 m of the unit. Bioturbation is pervasive and Zoophycos and Planolites traces are common. Chondrites are also abundant in the upper part of this unit. The upper contact of Unit II coincides with the base of the foraminifer-rich layer of Unit I. The base of Unit II is gradational and is placed at a clay bed that forms part of the P/E boundary interval. Unit II is divided into three subunits based on clay and biogenic silica content. The contact between Subunits IIA and IIB is placed at a sharp color change, whereas the contact between Subunits IIB and IIC is gradational.
Subunit IIA consists of light greenish gray nannofossil ooze with foraminifers and clay to light greenish gray foraminifer and nannofossil chalk (Fig. F5A). The top of the Subunit IIA is placed at the base of a planktonic foraminifer–rich bed of Unit I. The carbonate content of Subunit IIA is ~70 wt% and is relatively uniform throughout the sequence. Calcareous nannofossils are the major biogenic component of this subunit. Planktonic foraminifers are rare in the upper part and gradually increase in abundance from the middle to the base of the subunit. Fish debris and calcispheres are also present. Bioturbation is pervasive with Chondrites, Planolites, and Zoophycos identified. Dark material often fills the burrows. Several gradational redox color changes from light yellowish brown to light greenish gray occur in the upper part of the subunit (Sections 207-1260A-2R-1 and 2R-2 [1.10–3 mbsf]). Clay is relatively abundant (~20%) in the upper portion of the subunit and rare in the lower portion. Few zeolites and disseminated pyrite are present throughout the subunit. Uniformly inclined burrow mottles and grain fabric suggest a slump deposit of Oligocene sediment in the lower part of the subunit. The contact with Subunit IIB is a sharp color change that corresponds to an increase in siliceous microfossils (Fig. F5B).
Subunit IIB is composed of a light greenish gray calcareous biogenic (50% nannofossils) chalk containing common to abundant radiolarians (25%–30%). It is characterized by high carbonate content (70 wt%). The top and bottom boundaries of the unit are placed at the first and last consistent occurrence of siliceous microfossils in smear slides. The upper limit also coincides with a sharp color and lithologic change (Fig. F5B).
Planktonic foraminifers are rare in the top part of Subunit IIB and become increasingly abundant in Core 207-1260A-7R (20%). Diatoms and siliceous sponge spicules are common throughout. Bioturbation (Planolites, Zoophycos, and Chondrites) is moderate to pervasive. Burrows are often filled with black material, but white foraminifer-rich (45%) sands are found within some Planolites traces.
Mottles in the top part of the subunit (light greenish gray to pale olive) are due to pervasive bioturbation. Subtle dark–light color alternations on a scale of 20–30 cm with gradational mottled contacts are observed. At the base of the subunit, early diagenetic, millimeter-scale horizontal dark bands are present over a 20-cm interval but show no microfacies variation in smear slides. Chert as nodules and as bands with chalk inclusions also appear in the lower part of the subunit (Section 207-1260A-15R-3). Rare pyrite, both disseminated and concentrated in burrows, is found throughout Subunit IIB.
Subunit IIC consists of light greenish gray nannofossil chalk with clay and foraminifers with lower carbonate content (60 wt%) than the rest of Unit II. The upper boundary of the subunit is gradational and placed at the last consistent occurrence of radiolarians in smear slides. The base of the subunit coincides with the clay-rich layer of the P/E boundary interval. In Hole 1260B, the top of Subunit IIC was not recovered. Calcareous nannofossils represent the major component (50%), with estimates of planktonic foraminifer abundances fluctuating between 10% and 35% (from smear slide description). Bioturbation is pervasive to moderate, with discrete Planolites and Zoophycos.
Distinct cyclic color changes between light olive brownish gray bands (10 cm thick) and light greenish gray sediments over a scale of 10–50 cm occur in the upper part of the subunit (Fig. F5C). Brown material fills burrows in the darker intervals, and some of these burrows are surrounded by green redox halos. At the base of the Subunit IIC, gradational dark green-gray to light greenish gray alternations are observed on a scale of 10–20 cm. Light greenish gray intervals are rich in calcareous microfossils and calcite, whereas dark brownish and greenish gray layers have higher clay and zeolite content.
Dark green chert is present in the upper part of Subunit IIC. Estimated abundance of zeolite ranges from 5% in the upper part of the subunit to 15% in the middle part. Pyrite is rare (<2%) throughout. Clay content increases downward through the subunit (20%–30%). Diagenetic calcite is also more common at the base of the subunit (10%–20%) than elsewhere in Unit II.
Unit III is composed of nannofossil chalk, clayey nannofossil chalk, and claystone with nannofossils. Planktonic foraminifers, calcite debris (both biogenic and diagenetic), and zeolites are minor to major constituents in portions of the unit. Dominant colors are shades of greenish gray, although reddish hues are prominent for several meters above and below the K/T boundary. There are distinct to subtle alternations on a decimeter scale between light and dark intervals throughout Unit III. Unit III has higher clay content than Unit II, but the transition is gradational. The contact is placed at the base of a distinct 20-cm-thick laminated greenish gray clay layer that corresponds to the biostratigraphically recognized P/E boundary (see "Biostratigraphy"). Unit III is distinguished from Unit IV by its pervasively bioturbated fabric and low levels of organic carbon. The base of Unit III is placed at the first downhole occurrence of laminated strata. Unit III is subdivided into three subunits based on differences in the abundance of foraminifers and carbonate content. This unit contains an apparently complete sequence leading up to the P/E boundary and the record of the K/T boundary event, including an apparent ejecta layer (Fig. F6).
Subunit IIIA consists of light greenish gray to greenish gray, mottled nannofossil chalk with foraminifers and calcite to clayey nannofossil chalk with foraminifers and calcite. Olive hues are present in the lower 2–4 m of the subunit. Carbonate contents are generally between 40 and 60 wt% (Fig. F4). Zeolites, quartz, calcispheres, and/or pyrite commonly compose a small percentage of identified grains. Fish debris was observed in trace amounts in several samples. Subtle to distinct alternations between light (relatively carbonate rich) and dark (relatively clay rich) color bands occur on a decimeter scale. Burrow mottling is pervasive, and discrete traces assigned to Zoophycos (Fig. F7A), Planolites, and Chondrites are common.
Subunit IIIA is distinguished from Subunit IIIB by its generally lower and more uniform carbonate content and its lower abundance of foraminifers. The contact is placed at the top of the first downhole occurrence of light greenish gray relatively well indurated nannofossil chalk with foraminifers.
Lithostratigraphic Subunit IIIB consists of light greenish gray to greenish gray chalk with clay to clayey chalk, although olive and reddish hues are common in the interval near the K/T boundary (see below) (Fig. F7B). Carbonate content varies from 40 to 80 wt% (Fig. F4), and this variation seems to follow light (carbonate rich) and dark (carbonate poor) color alternations (Fig. F7C) through most of the subunit. Carbonate constituents include subequal proportions of nannofossils, foraminifers, and calcareous debris. Bioturbation is pervasive with Zoophycos, Planolites, and Chondrites traces identified. The darker intervals often exhibit a subhorizontal fabric that may result from the orientation of burrows, compaction, primary lamination, or some combination of these three. Pyrite and clay increase in abundance in the lower half of the subunit. Rare barite nodules that range from a few millimeters to a centimeter in diameter are observed in the lower part of the subunit.
The top of Subunit IIIB is placed at the first downhole occurrence of relatively well indurated light-colored nannofossil chalk with foraminifers, which seems to represent the downcore initiation of pronounced alternations in carbonate content. The base of Subunit IIIB is placed at a marked increase in clay content apparent from smear slide description that corresponds to a dominance of dark greenish gray colors and approximates the level where planktonic foraminifers disappear from washed residues (see "Biostratigraphy").
An excellent record across the K/T boundary appears to be present in Subunit IIIB. The ejecta layer was best recovered in Hole 1260A, but the entire K/T interval was better recovered in Hole 1260B. Approximately 3.5 m below the boundary, the color of the Maastrichtian chalks shifts gradationally from greenish gray to dark yellowish brown, but the color returns to greenish gray ~0.5 m below the boundary. The boundary is placed at a 1.5-cm-thick graded bed composed of light green spherules (Fig. F6B). Spherules decrease in size from ~1 mm in diameter at the base of the layer to ~0.25 mm in diameter at the top. The spherule bed sits on a ~3-mm-thick gray fine-grained homogeneous layer. There are impressions of spherules on the upper surface of this gray layer. A massive green clay lies above the spherule bed, but it was disturbed by drilling in both holes at this site. About 5 cm above the spherule bed, there are burrow mottles up to 0.5 cm across and sediment color becomes progressively lighter and more mottled over the subjacent 20 cm, suggesting increased carbonate content and return to background pelagic deposition. About 30 cm above the boundary, there is a ~30-cm-thick yellowish brown interval, which grades back to a greenish gray color.
Lithostratigraphic Subunit IIIC consists of claystone with foraminifers and nannofossils to clayey nannofossil chalk. Pyrite is common and barite is present as millimeter- to centimeter-sized nodules. Light–dark alternations with a 20- to 50-cm spacing represent changes in the relative abundance of clay and carbonate (Fig. F7C). Bioturbation is heavy to pervasive, but darker intervals often have a subhorizontal fabric. Zoophycos, Planolites, and Chondrites traces are present.
This subunit is distinguished from Subunit IIIB by its higher clay content. The transition is gradational and is placed where relatively dark claystone intervals are more abundant than the relatively light clayey chalk intervals. This level approximately coincides with the disappearance of planktonic foraminifers (see "Biostratigraphy"), suggesting the relative increase in clay content downcore results in part from the dissolution of the calcareous components. Subunit IIIC is distinguished from Unit IV by the presence of burrows and low organic carbon content. The lower contact is placed at the first downhole occurrence of laminated dark olive-gray calcareous claystone (Fig. F8A).
Unit IV ranges from black, finely laminated calcareous claystone with organic matter (Fig. F8B) to laminated clayey chalks with foraminifers and organic matter and clayey foraminifer micritic limestones (Fig. F8C). The top of Unit IV is defined by the shift from bioturbated greenish gray claystone of Unit IIIC above to laminated claystones with high organic carbon contents below (Fig. F8A). The transition is gradational over ~20 cm in Hole 1260A and is characterized by a greenish gray clay with abundant glauconite, pyrite, and quartz (interval 207-1260A-42R-CC, 1–18 cm). Contacts between the major lithologies in the unit are gradational over a centimeter to decimeter scale and from light–dark color cycles that are typical for Unit IV. Color shifts correspond to significant changes in carbonate (higher in light intervals) and total organic carbon (TOC) (higher in dark intervals) content (Fig. F4), and there is a trend toward increasing carbonate content. Some intervals of Unit IV have a considerable zeolitic component (intervals 207-1260A-45R-2, 46 cm, and 46R-5, 30 cm). Phosphatic fecal pellets, fish remains, and inoceramids are present throughout the unit (Fig. F8B), as are white millimeter-scale calcite layers composed entirely of foraminifers filled with sparry calcite (Figs. F8C, F9B).
The lower portion of Unit IV (intervals 207-1260A-52R-CC, 13–22 cm, and 207-1260B-41R-5, 1–18 cm) contains massive coarse-grained decimeter-scale beds with sharp, possibly erosive, bases. These layers are primarily composed of large shell fragments, glauconite, and subangular quartz grains (Fig. F9A). Some of this lithology is found in burrows projecting up to 50 cm into the underlying laminated claystone. These beds are similar to lithologies found at the top of the black organic-rich claystones at Sites 1257, 1258, 1259, and 1261 and seem to correspond to a distinctive natural gamma ray (NGR) and magnetic susceptibility peak (see "Physical Properties"). They may represent mass flow deposits or a period of very low sedimentation and slightly higher oxygen levels, allowing for the observed bioturbation.
Unit V consists of dark greenish gray silty limestones and silty calcareous claystones. The top is placed at the base of the lowest debris flow deposit in Unit IV. Unit V extends to the bottom of the hole. The limestones of Unit V display a mottled fabric but are otherwise homogeneous with no discrete burrows evident. No bedding features are discernable in the claystones, but rare white mottles (~1 mm diameter) filled with subangular quartz are present. Clay content in the claystones is estimated to be as high as 80% in smear slides, with silt-sized calcite and subangular quartz making up the remaining fraction (Fig. F9C). The claystone contains occasional pyrite nodules, and both lithologies have disseminated pyrite and rare shell debris. Some woody carbonaceous debris, often ringed with pyrite, is present in the claystones.
The oldest unit recovered at Site 1260 is Albian in age and is assigned to Unit V in the lithostratigraphic classification. It is relatively homogeneous and consists of clayey limestones with quartz and quartz silty calcareous claystone. Rare ichnofossils, pelagic microfossils, shell fragments, and fish debris are present. Terrigenous input into the basin resulted in clay-rich intervals containing subangular quartz, woody debris, and inorganic calcite. Common pyrite in the unit suggests a relatively low oxygen content of the bottom water and/or early diagenesis in pore waters. Based on these observations, Unit V appears to reflect sedimentation in a synrift proximal shallow-marine environment with high sedimentation rates.
Unit IV is Turonian–Cenomanian in age. It is an interval with high (up to 14 wt%) TOC concentrations. Rock-Eval results indicate that the organic carbon is primarily of marine origin (see "Organic Geochemistry"). The dominant lithologies in this unit are dark laminated calcareous claystone and relatively light laminated limestone. Debris flow deposits, wood fragments, and quartz grains are found at the base of the unit. The carbonate-rich deposits contain more microfossils than the organic-rich clay. This unit was likely deposited in a deeper marine environment than Unit V, and the carbonate–clay alternations may be related to fluctuations in seawater oxygen content and carbonate and organic carbon productivity, winnowing, or variations in deposition and redeposition processes. Phosphatic fecal pellets, fish remains, and inoceramids are present. The high percentage of zeolite suggests original biosiliceous fauna.
Unit III is late Paleocene–early Campanian in age and is a clayey nannofossil chalk with a wide range of carbonate content and low percentage of TOC. The lower contact is distinct and corresponds to a 20-cm-thick coarse glauconite-rich level that overlies the laminated dark calcareous claystones of Unit IV. The upper boundary is gradational and is placed at the base of the gray clay layer at the P/E boundary. Unlike Units IV and V, Unit III is pervasively bioturbated, suggesting higher benthic oxygen levels. A deepening of the calcite compensation depth (CCD) or less corrosive pore waters is suggested by better foraminifer preservation upsection. Light (carbonate rich) and dark (clay rich) cyclic color alternations indicate variations in a combination of factors (i.e., productivity, deposition, preservation, and diagenetic alteration).
The K/T and P/E boundaries are well preserved in Holes 1260A and 1260B. The K/T boundary and ejecta layer (1.5 cm thick in Hole 1260A) are present in Subunit IIIB. Sediments are reddish within a meter on either side of the K/T boundary, but immediately above and below the boundary, the sediments return to a greenish gray pelagic clayey chalk. The P/E boundary represents the boundary between Units III and II and is composed of laminated clays on either side of a distinct color change. A decrease in carbonate content at the event and evidence of reduced oxygen and benthic activity is likely related to a shoaling of the CCD (Dickens, 2000).
Unit II is early Eocene–Oligocene in age and consists of pelagic nannofossil chalk with foraminifers. Relative to Unit III, Unit II has a high carbonate content. Unit II is subdivided into three subunits based on clay content and biosiliceous abundance. Subunit IIB contains radiolarians, siliceous sponge spicules, and diatoms, whereas Subunits IIA and IIC are relatively rich in calcareous microfossils. Subunit IIC also contains more clay and zeolite than Subunits IIA and IIB. Subtle light–dark color alternations on a scale of 10–50 cm throughout Unit II are regular enough to suggest periodic forcing. A slump deposit ~13 m thick composed of pelagic Oligocene sediments is present in the upper part of Unit II. The upper Pleistocene veneer, (i.e., Unit I) is primarily composed of clay with glauconite, nannofossils, and foraminifers, indicating that this is presently a site with minimal sediment accumulation.