Site 1259 represents one of the shallower sites (current water depth = ~2354 mbsl) of the Leg 207 Demerera Rise transect. The sediments recovered at Site 1259 range in age from early Miocene–Cenomanian (see "Biostratigraphy") and include records of several critical events, including those at the P/E and K/T boundaries. Hole 1259A was cored continuously to 559 mbsf and spot cored in Holes 1259B and 1259C to recover duplicate or triplicate copies of these critical time intervals and a complete record of the Upper Cretaceous (Cenomanian–Santonian) black shale interval.
Five lithostratigraphic units were recognized at Site 1259 (Fig. F3; Table T2). The oldest unit recovered (Unit V) consists of gray quartzose sandstone and dark-colored silty claystone and calcareous siltstone to sandstone. The superjacent unit (Unit IV) is predominantly composed of submillimeter-scale laminated calcareous claystones rich in organic matter and laminated limestones. The three youngest lithostratigraphic units (Units I–III) recovered at this site contain pelagic sediments. They are composed of calcareous chalks, with various amounts of clay and siliceous microfossils. These pelagic sediments are moderately to pervasively bioturbated
The lithostratigraphic units recognized at this site generally parallel those reported during Leg 207 for Sites 1257, 1258, 1260, and 1261, as well as those for Site 144 (Hayes, Pimm, et al., 1972), but include a >60-m-thick sequence of lower Miocene pelagic oozes and chalks (Unit I and Subunit IIA) that was not recovered elsewhere. The quartz sandstone encountered in Unit V at Site 1259 is petrographically similar to the sandstone recovered at Sites 1258, 1260, and 1261, but the silty claystones and calcareous siltstones in Unit V were only recovered at Site 1259.
Unit I consists of a foraminifer nannofossil ooze that exhibits a homogeneous texture and pale yellow color. It contains rare black mottles but discrete traces are rare to absent. Calcareous nannofossils are the dominant sediment component (up to 75%) and planktonic foraminifers constitute 25%–30% of the sediment. Clay, opaque minerals, quartz, and fragments of siliceous microfossils were observed as minor constituents in smear slides. The base of Unit I is placed at a distinct lithologic change from foraminifer nannofossil ooze to foraminifer nannofossil chalk. Unit I includes a major stratigraphic age inversion, with sediments of Oligocene age on top of lower Miocene strata, indicating a slump deposit or other type of slope failure.
Unit II consists of light greenish gray to greenish gray foraminifer nannofossil chalk, foraminifer nannofossil chalk with radiolarians, and nannofossil chalk with foraminifers, containing 50%–80% carbonate in dominant lithologies (Fig. F3). Zoophycos and Planolites trace fossils are common, and Chondrites are present but less common. The unit is divided into three subunits based on the relative abundance of radiolarians and foraminifers. Subunits IIA and IIB both have a significant foraminifer component (20%–50%), with Subunit IIB distinguished from Subunit IIA on the basis of its higher siliceous microfossil content (15%–30% radiolarians and siliceous fragments). Foraminifer abundance declines in Subunit IIC to values of ~10%–20%. The upper contact of Unit II is placed at a distinct lithologic change from nannofossil ooze to nannofossil chalk, whereas the lower unit boundary contact corresponds to an equally prominent, but more gradual, downhole increase in clay content. The lower boundary corresponds to the biostratigraphically recognized P/E boundary (see "Biostratigraphy") and is expressed as a significant drop in carbonate content and a distinct clay layer.
Subunit IIA is composed of carbonate-rich (80 wt%) (nannofossils and foraminifers) pelagic sediments with trace amounts (<5%) of clay, zeolite, and diagenetic calcite. Black mottles and streaks are found occasionally throughout the subunit and are often concentrated in burrows (Chondrites and Zoophycos trace fossils). The sediments typically range from homogeneous to slightly burrow mottled.
The top of this subunit is placed at a distinct lithologic change from nannofossil ooze to nannofossil chalk at 37.90 mbsf (Hole 1259A). Below this transition, sediments exhibit light–dark color cycles. These cycles occur to varying extents throughout Subunit IIA. The color cycles are expressed as a progression from light greenish gray chalk to 20-cm-thick bands of greenish gray chalk followed by greenish gray chalk with common bioturbation. There is a gradual change to lighter values between Sections 207-1259A-10R-5 and 10R-6 that coincides with the Oligocene/Miocene boundary (see "Biostratigraphy"). The base of Subunit IIA is placed at a marked increase in abundance of radiolarians from absent or trace amounts to abundances of 15%–25%.
Subunit IIB consists of light greenish gray foraminifer nannofossil chalk with radiolarians (Fig. F4A). The chalk contains abundant foraminifers (15%–30%) but is distinguished from the overlying and underlying lithostratigraphic Subunits IIA and IIC by high abundances of siliceous microfossils. Radiolarian contents range from 5% to 10% at the top (Core 207-1259A-15R) and base (Core 30R) of the subunit and reach peak levels (25%–30%) in Cores 18R through 20R. This subunit is mottled and is moderately to heavily bioturbated. Discrete trace fossils include Zoophycos, Chondrites, and Planolites. Distinct light–dark color cycles can be recognized in the lowermost cores (Cores 207-1259A-27R through 30R).
Subunit IIC consists of light greenish gray nannofossil chalk with foraminifers and clay, with carbonate contents of ~60 wt% (Fig. F4B). Calcareous nannofossils represent the major calcareous component (>50%), and planktonic foraminifers compose an additional 10%–35%. Bioturbation is pervasive to moderate, with white and greenish gray mottles and burrows. Discrete Planolites and Zoophycos trace fossils are common.
A distinct pattern of cyclic variations of lighter- and darker-colored sediment is observed throughout Subunit IIC. Intervals of light greenish gray sediment alternate at decimeter intervals with either greenish gray, yellowish brown, or pale olive-colored sediments. From smear slide analyses, it appears that the dark intervals have higher clay contents, whereas light intervals are rich in zeolite and calcite.
The upper boundary of this subunit is placed at the last consistent downcore occurrence of radiolarians. The transition between lithologies characteristic of Units II (relatively carbonate rich) and III (relatively clay rich) is gradual, but the base of Unit II is placed at the P/E boundary, a recognizable interval that can be correlated among holes and sites (Fig. F5). The P/E boundary is associated with a clay-rich layer in Holes 1259B and 1259C (see "Biostratigraphy") (Fig. F5B); the lack of a similar clay layer in Hole 1259A is attributed to drilling disturbance. Magnetic susceptibility records display a very prominent signal at this level in all three holes.
Unit III is composed of nannofossil chalk, calcareous siltstone, clayey nannofossil chalk, and glauconitic claystone with nannofossils. Planktonic foraminifers, calcite debris (both biogenic and diagenetic), and zeolites are minor to major constituents in parts of the unit. Carbonate contents are generally <60 wt%. Dominant colors are shades of greenish gray, although reddish brown hues are prominent for several meters above and, to a lesser extent, below the K/T boundary. Subtle to distinct color cyclicity on a decimeter scale between light and darker intervals is observed throughout this unit. Unit III is subdivided into two subunits based on differences in the abundance of foraminifers and clay content. This unit also contains the K/T boundary interval, including an apparent ejecta layer recovered in all three holes (Fig. F6B).
The sediment in Unit III has a higher clay content than Unit II, but the transition is gradational. The contact is placed at the P/E boundary. Unit III is distinguished from the underlying Unit IV by its pervasively bioturbated fabric and low organic matter content. The base of Unit III is placed at a distinct color change from green glauconite-rich claystone to dark gray claystone with organic matter. This contact is sharp in Hole 1259C (Fig. F7B) but appears gradational in Holes 1259A and 1259B because of pervasive bioturbation of green glauconitic claystone into the dark gray claystone (Fig. F7A).
Subunit IIIA is a clayey nannofossil chalk to a nannofossil chalk with clay. It is distinguished from Unit II and Subunit IIB by its relatively high clay content (with the contact placed at the P/E and K/T boundaries, respectively). Discrete Zoophycos, Chondrites, and Planolites burrows are common, and this subunit is pervasively bioturbated (Fig. F6A). Trace amounts of pyrite and occasional pyrite nodules are present in all cores.
Subunit IIIA displays subtle to distinct alternations between lighter- and darker-colored clayey nannofossil chalk on 30-cm-scale intervals throughout. Colors are dominantly shades of greenish gray but become olive to reddish brown in the lower portions of the subunit.
The lower boundary of Subunit IIIA is the base of the K/T spherule layer. The spherule layer is overlain by an interval of greenish gray clay grading upward into light greenish gray calcareous clay that grades into the reddish brown cyclic interval mentioned above. The spherule bed is graded and is in sharp contact with a 1-mm thin white layer (Fig. F6B). The spherule bed is 1.5–1.9 cm thick and varies in color from black to white into light greenish gray and dark green.
Subunit IIIB is dominantly a nannofossil chalk with varying amounts of nannofossils, calcareous debris, and clay. This subunit displays very distinct rhythmic alternations between light greenish gray and greenish gray colors at 5- to 20-cm-thick intervals. The lighter intervals have a higher percentage of nannofossils and foraminifers, whereas the darker intervals have a higher clay content. Bioturbation is moderate to pervasive with distinct black Zoophycos and Chondrites burrows (Fig. F8). Pyrite, barite, and marcasite increase in abundance downhole. Glauconite is present in the lower portion of the subunit (clayey chalk and claystone), and a greenish color marks the lowermost part of Subunit IIIB (Table T2; Fig. F8B, F8C). The top of this glauconite-rich interval can be correlated among the three holes (see also "Physical Properties" and "Composite Depths").
The boundary between Subunits IIIB and IIIA is placed at the base of the K/T boundary ejecta layer (Fig. F6B), but the lithologic transition between the subunits is gradational. On the other hand, a distinct color and lithologic change defines the boundary between Subunit IIIB (greenish gray to olive-gray clayey chalk and claystone) and Unit IV (black laminated calcareous claystone with organic matter) (Fig. F7).
Unit IV primarily consists of dark olive-gray to black, submillimeter-scale laminated calcareous claystone with organic matter (black shale) and clayey chalk and limestone with organic matter (Fig. F9). The upper contact of Unit IV lies below a distinct lithologic transition from the glauconite-rich greenish to olive-gray sediment of lower Unit III to olive-gray and black calcareous claystone with organic matter. The lower part of this transition is heavily mixed by bioturbation and includes some discrete large burrows (Fig. F7A). The lower boundary of Unit IV is defined by the first occurrence of dark greenish gray to gray, medium-grained quartz sandstone (quartz arenite) (Holes 1259A and 1259C) and dark gray calcareous siltstone and silty clay intercalations (Hole 1259B).
The uppermost part of Unit IV at Site 1259 contains a massive dark gray to black bioturbated claystone with organic matter, which is Santonian in age (Table T2). The remainder of Unit IV shows well-developed submillimeter-scale laminations and has a strong petroliferous odor. Contacts between the major lithologies in the unit are gradational over a centimeter to decimeter scale and form the rhythmic light–dark color variations that are typical for Unit IV.
Carbonate content is highly variable and ranges as high as ~95 wt% in individual carbonate-rich layers and as low as ~5 wt% in black claystones (Fig. F3). The lighter intervals have higher carbonate content largely due to a higher abundance of diagenetic calcite (see Fig. F9B). Other carbonate constituents include nannofossils (concentrated in fecal pellets), foraminifers, and shell fragments. Total organic carbon (TOC) values range from ~5 to 16 wt%, with one sample as high as 29 wt%. Rock-Eval analyses indicate Type II kerogen, which is consistent with a marine origin of the organic matter. The organic matter is clearly visible in thin sections. Fish scales and bone fragments (francolite) and amorphous to cryptocrystalline phosphatic nodules (collophane) up to 2 cm in diameter are common throughout, either parallel to bedding or concentrated in thin gravity deposits and can be seen as white to light brown blebs and streaks on core surfaces. A distinct feature in the black claystone are white millimeter-scale calcite stringers composed entirely of foraminifers filled with sparry calcite. A distinct minor lithology in Unit IV is a calcite cemented, unsorted glauconitic claystone, with phosphatic nodules, fish remains, and shells (Table T2; Figs. F15, F31). A similar lithology was found in Unit IV at Sites 1258 and 1260.
Unit V is composed of quartz sandstone, calcareous siltstone, and silty claystone. The top of the unit is placed at the first downhole occurrence of a dark greenish gray to gray, medium-grained quartz sandstone (quartz arenite). Sand grains are angular to subrounded and are cemented by calcite. The sandstone contains minor amounts of glauconite, pyrite, and bioclastics (echinoid fragments, bivalves, gastropods, and large agglutinated benthic foraminifers). No sedimentary structures were observed. The sandstone is 1.40 m thick in Hole 1259A, <5 cm thick in Hole 1259C, and was not recovered in Hole 1259B. At the base of the sandstone in Holes 1259A and 1259C is a sharp transition to very dark gray and brownish calcareous siltstones and silty claystones (Fig. F10). These siltstones and silty claystones contain "tepee-structures" and apparent tempestites, as well as strongly bioturbated intervals. The burrows are filled with quartz silt.
The oldest interval recovered at Site 1259 is a unit dominated by interbedded dark gray and brownish calcareous siltstones and silty claystones, as well as gray, medium-grained sandstone (Unit V) of unknown age. The lithology shows strongly bioturbated intervals and is characterized by tepee-structures and apparent tempestites. These observations suggest deposition in a shallow-marine environment, possibly including tidal flats.
Cenomanian–Santonian sediments are dominated by laminated calcareous claystones and laminated chalks/limestones of Unit IV. Organic carbon concentrations in Unit IV reach 29 wt%, with the organic matter of marine origin. High productivity and low bottom water oxygen levels resulted in the preservation of large volumes of organic and phosphatic matter in the sediments. Zeolites (clinoptilolite) and rare radiolarians suggest siliceous microfossils were a common component of Unit IV sediment. Carbonate values are highly variable (5–95 wt%) and reflect diagenetic dissolution and precipitation of calcite. Macrofossils occur frequently (mainly fish fragments and inoceramids) in the black claystones, but trace fossils are absent. The lack of bioturbation, the relatively high TOC content, and common phosphoritic layers suggest a deposition in a restricted, probably shallow, but fully marine environment.
The nonlaminated, unsorted glauconitic claystone intercalated with the TOC-rich claystones in Unit IV is interpreted as a storm deposit. If correct, this interpretation limits water depths at this site to less than storm wave base. Alternatively, a period of very slow sedimentation and slightly higher oxygen levels could allow for the bioturbation and concentration of glauconite and phophatic material. The bioturbated claystone with organic matter in the uppermost part of Unit IV may represent slightly more oxygenized conditions at the seafloor, allowing bioturbation and preventing laminae preservation during part of the Santonian.
Open marine conditions and oxic bottom waters were established by the early Campanian, as indicated by the bioturbated pelagic marls of Unit III. Variations in sedimentary fabric and minor sedimentary components in the pelagic sediments in Units III–I suggest that conditions at the seafloor and/or overlying water column fluctuated substantially during the latest Cretaceous and Paleogene interval.
The K/T boundary interval seems to be complete in all three holes at Site 1259. It consists of the uppermost Maastrichtian of Subunit IIIB, followed by a discrete graded spherule layer, and the basal Paleocene calcareous claystone of Subunit IIIA. The spherules are up to 2.5 mm in diameter, an exceptional size considering the distance to the proposed location of impact.
The P/E boundary seems to be relatively complete in Holes 1259B and 1259C, whereas its recovery was incomplete in Hole 1259A. The most extensive record appears to be the 50-cm-thick greenish and reddish brown clay layer in Hole 1259B (see "Biostratigraphy"). The lack of carbonate in this layer is consistent with sedimentary changes expected across the P/E boundary.
Similar to other Leg 207 sites, gravity flow deposits and temporal gaps are common in the uppermost pelagic record at Site 1259. The inversion of ages in Unit I (Hole 1259A), where ~20 m of Oligocene sediment overlies lower Miocene oozes, is most probably due to mass-failure displacement or faulting. Several hiatuses were recognized in Hole 1259A, with the most prominent ones occurring in the pelagic chalks of Unit IIA, at the transition between the clayey chalks and claystones of Units III, and in Unit IV (see "Sedimentation Rates").