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RESULTS (continued)

Site 1259

Site 1259 is located at a water depth of 2354 mbsl on the gently dipping (~1°) north-facing slope of Demerara Rise, ~380 km north of Suriname. The site is located on a ridge of Paleogene sediments subcropping near the seafloor and is the second shallowest of all sites forming the intended paleoceanographic depth transect across Demerara Rise. The major objectives were the following:

  1. Core and log a Paleogene–Albian section to evaluate paleoceanographic and paleoclimatic changes, with emphasis on major and abrupt events during this interval that include the E/O boundary, the P/E boundary, and the Cretaceous OAE.
  2. Reconstruct the history of the opening of the equatorial Atlantic gateway by obtaining benthic proxy data. These data will help to understand changes in bottom water circulation over Demerara Rise during the gradual opening of the seaway.
  3. Recover continuous and expanded sediment records of the Paleogene and Cretaceous to reconstruct short- and long-term changes in greenhouse forcing.

The seismic stratigraphy established for Demerara Rise, including Horizons A, B, B', and C, has been correlated proximally to Site 1259 with line GeoB219; the closest of the three holes drilled at this site is 170 m from this line. The slope angle near the drill site is gentle (~1°) but increases downslope from the site. Reflector A, representing the top of a presumably lower Miocene erosional unconformity, outcrops at the seafloor on the nearest seismic line. The Miocene section recovered in the core is probably not present at the extrapolated site position on the seismic line.

Between Reflectors A and B, seismic Unit 2 is 465 ms thick (~405 mbsf using laboratory-measured velocity information). The topmost sequence (180 ms; ~130 m) within this seismic unit consists of incoherent reflections. Below this interval, Unit 2 is represented by a sequence of high-amplitude, parallel, coherent reflections that are relatively flat lying. This package is about 120 m thick, terminating at 300-ms subbottom (240 mbsf). Below this section to its base at Reflector B, the horizons are still parallel and coherent but lower in amplitude. The entire unit dips to the north slightly less than the seafloor, at ~0.6°. Reflector B, at 463-ms subbottom (442 m), is a high-amplitude, laterally coherent reflector that correlates with the approximate position of the K/T boundary. Seismic Unit 3, between Reflectors B and C at 571 ms, is a ~100-m-thick flat-lying sequence that dips 1° to the north. The upper part of this unit, between Reflectors B and B', is acoustically incoherent and relatively transparent. The basal part lies between Horizons B' and C (503- to 571-ms subbottom; ~495–545 mbsf) and is defined on the basis of a series of strong, parallel, coherent reflections that are laterally contiguous for several kilometers, below which is another thin (20 m) transparent zone.

Reflector C is an angular unconformity at Site 1259, with underlying reflections cropping against it at relatively shallow angles (~2°), as they appear on the strike line GeoB2219. On the industry seismic line C2206a that intersects the site in a dip profile (northwest), these low-angle reflections are not resolved and the underlying sequence appears locally conformable (a disconformity).

At Site 1259, three RCB holes were cored. Hole 1260A was cored to 558.8 mbsf, with 66.6% recovery. Hole 1259B was washed to 305 mbsf and cored to 381.9 mbsf. It was then washed to 420.5 mbsf and cored to 556.2 mbsf. Recovery was 69.2% within the cored intervals (Table T1). Hole 1259C was washed to 308.0 mbsf and cored to 373.3 mbsf. It was washed to 436 mbsf, and one core was taken between 436.0 and 445.6 mbsf to obtain a third copy of the K/T boundary interval. The following succession was washed again until 490 mbsf, where continuous coring proceeded until 553.7 mbsf. Recovery in Hole 1259C was 80% within the cored intervals.

Sandy intervals between 120 and 300 mbsf and hard layers between 380 and 400 and 490 and 520 mbsf hampered core recovery in these intervals.

Stratigraphy of Site 1259

Lithologic descriptions of the cores and biostratigraphic age assignments revealed a rather continuous sedimentary succession with only a few hiatuses. Sediments at Site 1259 range in age from Miocene to Cenomanian. The deepest unit recovered in Site 1259 did not yield any age diagnostic microfossils. A thin veneer of Holocene foraminiferal ooze at the top of the section unconformably overlies an ~30-m-thick slide of reworked lower Oligocene calcareous ooze with nannofossils and planktonic foraminifers. A disconformity separates this slide from a concordant succession of lower Miocene calcareous ooze and chalk. Another disconformity separates the Miocene from lower Oligocene to upper Eocene foraminiferal chalk. The latter is represented by a condensed interval of calcareous chalks and overlies an expanded, 235-m-thick succession of middle Eocene (planktonic foraminiferal Zone P14; calcareous nannoplankton Zone NP17) to lower Eocene nannofossil chalk with abundant and well-preserved radiolarians in the middle Eocene part of this succession. Excellent RCB recovery provided continuous core overlap between holes for the lower Eocene. The periodic variability present in the lower Eocene at Site 1259 will provide a good basis for postcruise cyclostratigraphic studies. Age control is excellent, with well-defined paleomagnetic datums present in the section (e.g., Chrons C30n and C32n). Preliminary investigation suggests the dominant periodicities of the magnetic susceptibility data are Milankovitch in nature.

In all three holes, we recovered an apparently expanded section across the P/E boundary. As at Sites 1258 and 1260, the upper Paleocene clayey nannofossil chalk sequence is relatively thick. Zeolites or locally abundant opal-CT lepispheres replace siliceous microfossils in this interval. Each hole recovered the K/T boundary with ejecta layers. The subjacent upper Maastrichtian greenish gray nannofossil chalk with foraminifers and clay and the lower Maastrichtian–upper Campanian zeolitic nannofossil claystone displays cyclic color banding between light greenish gray and greenish gray on a decimeter scale. Radiolarians in the Campanian are well preserved. The lithology becomes increasingly clay rich downhole, and carbonate contents decrease to 30%. Foraminifers and nannofossils are rare in this interval, and a significant increase in abundance of diagenetic calcite and carbonate debris is observed. Planolites, Chondrites, and Zoophycos burrows are abundant, as are barite and pyrite crystals. Average sedimentation rates in the Maastrichtian–Campanian interval were 4.5 m/m.y. The periodic variability present in the Campanian–Maastrichtian intervals at Site 1259 will provide a good basis for postcruise cyclostratigraphic studies. Age control is excellent, with well-defined paleomagnetic datums present in both sections (Chrons C30n and C32n; see "Paleomagnetism"). Preliminary investigation suggests the dominant periodicities of the magnetic susceptibility data are Milankovitch in nature.

The abundance of very dark intervals (cyclic in appearance) and glauconitic layers increase in the lower part of the clayey chalk. A condensed interval of glauconite-rich horizons and firmgrounds separates the Campanian clayey chalk from the ~50-m-thick laminated black shale sequence. The top of the black shale unit contains a 1.2-m-thin sliver of Santonian age sediments that unconformably overlies 15 m of lower Coniacian black shales with some debris flows and slumped horizons. The Turonian and OAE 2 are virtually complete and expanded, but only 15 m of Cenomanian-age black shales represents the thinnest occurrence of Cenomanian sediments of Leg 207. The preservation and abundance of calcareous microfossils is poor to good, with glassy foraminifers present in the Santonian–Cenomanian part of the black shales. The maximum TOC content in these sediments is 29 wt%. A disconformity separates the lower Cenomanian laminated black shales and limestones from the underlying silty claystone calcareous siltstone and quartz sandstone that did not yield any age diagnostic microfossils and represents the oldest sediments cored at Site 1259. Sedimentary structures and the presence of shallow-water fossils (brachiopods, oysters, and echinoderms) suggest a very shallow marine to tidal origin of these sediments.

Recovery of Critical Intervals

The main objective of Leg 207 was to recover sediments containing microfossils through major and abrupt paleoclimate events of the Paleogene and Cretaceous Periods, such as the E/O boundary, P/E boundary, and the Cretaceous OAEs.

Sediments spanning the P/E boundary were recovered in all three holes at Site 1259. The boundary interval comprises the last occurrence of benthic foraminifer Aragonia velascoensis, a species that became extinct at the P/E boundary, followed by a sharp contact between light green chalk over dark green clay. The sharp contact reflects the sudden decrease of carbonate content between the upper Paleocene and the lowermost Eocene associated with the P/E boundary. Planktonic foraminifers are absent from the base of the distinctive green clay horizon until at least 20 cm above the P/E boundary, although this interval contained a small number of benthic foraminifers and abundant, but poorly preserved, spumularian radiolarians. A sample 50 cm above the P/E boundary contains a moderately well preserved foraminiferal assemblage with abundant Morozovella allisonensis and Acarinina soldadoensis and rare Acarinina africana, Acarinina sibaiyaensis, Subbotina patagonica, and Parasubbotina varianta. Three of these species, M. allisonensis, A. africana, and A. sibaiyaensis, are associated with P/E boundary sections in other tropical and subtropical sites (Central Pacific, Egypt, Spain, New Jersey, and the Blake Plateau) and are known as the "excursion fauna" because of their abundance and near restriction to the PETM. An unusual element of these assemblages is Parasubbotina paleocenica—a clavate species that has previously been reported only from ODP Site 1220 in the equatorial Pacific and its type area in coastal Senegal. The West African and equatorial Pacific settings of these previous discoveries suggest that P. paleocenica is associated with upwelling conditions. Hence, we infer that the excursion fauna may also represent an expansion of relatively productive waters during the PETM.

The K/T boundary was recovered in all three holes cored at Site 1259, and the ejecta layer is present in each of them. A 1.9-cm-thick layer of clayey spherules overlying upper Maastrichtian chalks marks the base of the K/T boundary. Above the boundary, the lower Danian planktonic foraminiferal Zones P2 to P were distinguished. The lower Danian planktonic foraminiferal Subzone P1c to Zone P have a thickness of 8 m. The thickness of the spherule layer is similar to that at Site 1258 and 1260, suggesting that the spherule bed at both sites is a result of fallout rather than redeposition.

A ~50-m-thick Santonian–Cenomanian succession of laminated black shales and laminated foraminiferal limestones, including OAEs 3 and 2, was recovered in both holes at Site 1259. The quasiperiodic variability of the claystone and chalks/limestone comprising the black shales resulted in strong signal-to-noise ratios in both the gamma ray attenuation (GRA) bulk density and natural gamma ray data sets. These data sets, combined with good RCB recovery over a significant portion of the black shale interval, allowed for the construction of a continuous composite section from ~520 to 555 meters composite depth (mcd), the Turonian–Cenomanian interval of the black shales including OAE 2. Poor recovery from 495 to 520 mcd precluded the construction of a composite section in this upper interval of the Cretaceous black shales (OAE 3).

The transition between Campanian chalks and the underlying black shale sequence is represented by a condensed section covering the early Campanian and entire Santonian epochs. The thickness of Coniacian organic-rich sediments is comparable to other sites. OAE 2 is represented by an interval of distinctly laminated black shales. The main lithology consists of dark olive-gray to black finely laminated calcareous claystone with organic matter (black shale) and clayey chalk and limestone with organic matter. Occasional coarse-grained glauconite-rich horizons are present. The unit shows well-developed submillimeter-scale laminations and has a strong petroliferous odor. Rhythmic color variations, between dark olive gray and black, are visible on a decimeter scale. TOC values range from ~5 to 29 wt% in the black shales, with the highest values deriving from the basal upper Cenomanian part of the succession. Rock-Eval analyses indicate Type II kerogen, which is consistent with a marine origin of the organic matter. Fish scales, bone fragments, and amorphous to cryptocrystalline phosphatic nodules are common.


Similar to the pore waters of Site 1257, Site 1259 is characterized by the presence of a brine, with maximum chlorinity of 832 mM, a ~50% increase over average seawater chlorinity. The combination of the chlorinity and salinity profiles and high-porosity intervals in the black shales suggest that the brine is sourced laterally through these organic-rich claystones of Unit IV. Interstitial water (IW) chemical profiles associated with the degradation of organic matter are dominated by the presence of black shales and associated organic matter-rich sediments. Sulfate decreases downhole and is depleted by ~490 mbsf, the top of Unit IV, whereas ammonium increases to more than 1 mM. Below the sulfate reduction zone, methane increases sharply to reach a maximum of 76,000 parts per million by volume (ppmv) near the base of black shales before dropping to 2000 ppmv in Unit V. As at previous Leg 207 sites, the calcium and alkalinity profiles show the effects of carbonate diagenesis. In particular, decreases in calcium and alkalinity indicate an interval of carbonate precipitation between 300 and 490 mbsf.

Physical Properties

Index properties, compressional (P)-wave velocities, and GRA densities were measured on core samples from Site 1259. No downhole logs were run at this site because of concerns about hole conditions in the topmost part of the formation. In general, the physical property data reflect normal consolidation down to about 365 mbsf, with relatively linear profiles. Small excursions in properties appear at about 95 mbsf and between 200 and 280 mbsf, the latter correlating with a middle Eocene radiolarian-rich interval of calcareous nannofossil chalk. Porosity data show strong periodic signals superimposed on the normal consolidation trend through the lower Eocene, suggesting that cyclical variations may be readily determined.

Lithologies become richer in clay below 365 mbsf. Velocity and density profiles tend to flatten with much higher scatter in magnitudes than above. A strong increase in velocity and density corresponds with the K/T boundary, and probable cyclicity is again apparent in data through the Maastrichtian sedimentary sequence. The black shale facies is highlighted particularly well by the physical property data, with a distinct drop in average values but a high degree of scatter. Velocity and density maximums correspond with limestone or sandstone beds and low values in density and velocity correspond with organic-rich intervals. The largest peaks in velocity and density appear in the quartz sandstone and siltstone below the black shales (values up to 3500 m/s and 2.4 g/cm3).

Depositional History

Probable tidal flat deposits of unknown age (Albian?), which were deposited in a marginal marine setting, represent the oldest sequence recovered at Site 1259. They are unconformably overlain by a black shale sequence that dates from late Cenomanian to Santonian, but most of it is Turonian, with thinner Cenomanian, Coniacian, and Santonian intervals. Occasional clayey bentonite layers indicate the proximity of volcanoes. Calcareous nannofossils of the genus Ephrolitus form very dark TOC-rich shales at the base of the succession point to marginal marine environments. The laminated, coarse-grained foraminiferal limestones in the succession may be related to winnowing, grain flows, or reflect changes in carbonate productivity. Continuous deepening characterizes the remaining Upper Cretaceous succession. During OAE 2, high TOC values and very distinct laminations indicate bottom water anoxia. The top of the black shale interval is represented by a condensed interval covering the Santonian and lower Campanian. At present, it is unclear whether the observed mass flow deposits in the upper Turonian and Coniacian are part of the tectonic movements related to the opening of the equatorial Atlantic gateway.

Oxic conditions were established by the late Campanian, when sedimentation on Demerara Rise changed from hemipelagic to pelagic. The abundance of radiolarians in the Campanian, however, indicates increased surface water productivity. The cyclic pattern of trace fossil abundance suggests reduction in bottom water oxygenation recurs.

Maastrichtian- to Oligocene-age sediments at Site 1259 consist of pelagic deep marine nannofossil chalks and oozes. Sedimentation rates varied from 4.5 m/m.y. in the Maastrichtian–late Paleocene, 12 m/m.y. in the late Paleocene–late Eocene, and ~13 m/m.y. in the early Miocene. The succession is interrupted by a few hiatuses—a lower Paleocene hiatus covering ~1 m.y., an ~3-m.y. hiatus at the middle/upper Eocene boundary, and a upper Oligocene interval of hiatuses and condensed intervals representing ~3 m.y. These hiatuses may reflect periods of slow deposition and/or erosion.

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