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RESULTS (continued)

Site 1260

Site 1260 is located at a water depth of 2549 mbsl on the gently dipping (~1°) northwest-facing slope of Demerara Rise, ~380 km north of Suriname. The site is located on a ridge of Paleogene sediments subcropping near the seafloor. Site 1260 is at an intermediate depth of 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, have been correlated to Site 1257 and 1258 strata. The seafloor in the proximity of the drill site appears hummocky in the downslope direction but reasonably flat in the contour parallel direction. Reflector A, representing the top of a presumably lower Miocene erosional unconformity, subcrops near the seafloor at the site. Between Reflectors A and B, seismic Unit 2 is 365 ms thick (~315 mbsf using laboratory-measured downhole logging and check shot velocity information). The topmost sequence within this seismic unit consists of contorted reflectors that pinch out against the seafloor within 1.5 km downslope. This sequence may represent a slumped interval. Below this slumped interval, Unit 2 is represented by a sequence of crenulated but coherent reflection horizons, separated by transparent or incoherent intervals. In the downslope direction, these reflectors are reasonably flat lying, dipping slightly less than the angle of the seafloor.

Seismic Unit 3, between Reflector B at 365-ms subbottom and Reflector C at 522 ms, is a ~170-m-thick, flat-lying sequence that dips 1.5° to the north-northwest. The basal part of this unit lies between Horizons B' and C (464- to 522-ms subbottom; ~392–485 mbsf) and is defined on the basis of a series of strong, parallel, coherent reflections that are laterally contiguous. This seismic interval has been shown to correlate to the black shale interval.

At Site 1260, two RCB holes were cored. Hole 1260A was cored to 491.9 mbsf, with 79.6% recovery (Table T1). Hole 1260B was washed to 40 mbsf and cored from 40 to 136.5 mbsf. It was then washed to 235 mbsf and cored to 509 mbsf. Recovery was 88.2% within the cored intervals. Hard beds between 130 and 154, 180 and 211, and 480 and 509 mbsf hampered core recovery in these intervals.

Stratigraphy of Site 1260

Lithologic descriptions of the cores and biostratigraphic age assignments revealed a rather continuous sedimentary succession with only a few hiatuses. Sediments at Site 1260 range in age from Oligocene to early Albian. A thin veneer of Pleistocene carbonate-poor clayey ooze with quartz and glauconite at the top of the section unconformably overlies ~30 m of heavily slumped and reworked lower Oligocene calcareous chalk with nannofossils and planktonic foraminifers. The late Eocene is represented by a condensed interval of calcareous chalks and overlies an expanded 235-m-thick succession of middle Eocene (planktonic foraminiferal Zone P13; calcareous nannoplankton Zone NP17) to lower Eocene nannofossil chalk, with abundant and well preserved radiolarians in the middle Eocene part of this succession. With the exception of a hiatus spanning the lower/middle Eocene boundary, the lower–middle Eocene succession is remarkably expanded (average sedimentation rate = ~20 m/m.y.) and complete. Excellent RCB recovery provided continuous core overlap between holes for the middle Eocene. The periodic variability present in the middle Eocene color reflectance and GRA bulk density data at Site 1260 will provide a good basis for postcruise cyclostratigraphic studies. Age control is excellent, with well-defined paleomagnetic datums present in the section (e.g., Subchrons C19n and C20n). Preliminary investigation suggests the dominant periodicities of the magnetic susceptibility data are Milankovitch in nature, with significant power at 40 k.y.

In both holes, we recovered an apparently expanded and laminated section across the P/E boundary. As at Site 1258, the upper Paleocene clayey nannofossil chalk sequence is relatively thick and sedimentation rates in the lower Eocene to Paleocene interval drop to values averaging ~12 m/m.y. Zeolite or locally abundant opal-CT lepispheres replace siliceous microfossils in this interval. In both holes, we recovered the K/T boundary with ejecta layers. The subjacent upper Maastrichtian greenish gray nannofossil chalk with foraminifers and clay and the lower Maastrichtian to upper Campanian zeolitic nannofossil claystone display cyclic color banding between light greenish gray and greenish gray on a decimeter scale. The succession is considerably expanded. Almost complete coring in both holes allowed for the construction of a splice with only one gap in the lower Maastrichtian. Radiolarians in the Campanian are well preserved. The lithology becomes increasingly clay rich downhole, and carbonate contents decrease to 30 wt%. 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 12 m/m.y.

A condensed glauconite-rich horizon in Core 207-1260A-42R separates the Campanian clayey chalk from the ~90-m-thick black shale sequence, with TOC content up to 14 wt%. This interval contains a thin Coniacian interva, and thus the lower part of OAE 3, and a virtually complete Turonian. OAE 2 overlies an expanded succession of laminated shales of Cenomanian age. The preservation and abundance of calcareous microfossils is poor to good, with glassy foraminifers present in the Turonian and Cenomanian part of the black shales. A disconformity separates the lower Cenomanian laminated black shales and limestones from the underlying silty claystone and silty limestone, which dated as late early Albian age (T. primula planktonic foraminiferal zone; Subzones NC8a–b) and represents the oldest sediments cored at Site 1260.

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 both holes at Site 1260. 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. The uppermost Paleocene chalk contains distinct laminations until 12 cm above the boundary, where the first bioturbation occurs. Pervasive bioturbation returns within about 30 cm above the boundary. Bioturbated, light green, carbonate-rich sediments are the dominant lithology of the lower Eocene.

The K/T boundary was recovered in both holes cored at Site 1260, and the ejecta layer is present in Cores 207-1260A-36R and 207-1260B-23R. An 1.8-cm-thick layer of clayey spherules overlying upper Maastrichtian chalks marks the base of the K/T boundary. In the core from Hole 1260A, the spherule layer is covered by a thin (5 mm) drape of whitish nannofossil chalk of potentially reworked upper Maastrichtian sediments. The section is similar in Hole 1260B, but the spherule layer is only 1 mm thick, probably because of drilling disturbance. Above the boundary, the lower Danian planktonic foraminiferal Zones P2 to P were distinguished. The thickness of the spherule layer is similar to that at Site 1258, suggesting that the spherule bed at both sites is a result of fallout rather than redeposition.

A ~90m-thick Coniacian–Albian succession of laminated black shales and laminated foraminiferal limestones, including OAE 2, was recovered in both holes at Site 1260. Average recovery of this interval was 80%, and correlation between the holes and the high quality FMS logs will allow for a continuous reconstruction of the interval.

The transition between Campanian chalks and the underlying black shale sequence is represented by a hiatus or condensed section covering the entire Santonian. Coniacian organic-rich sediments are very thin. 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 slight petroliferous odor. Rhythmic color variations, between dark olive gray and black, are visible on a decimeter scale. TOC values range from ~5 to 14 wt% in the black shales. 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. The lower part of the black shales consists of distinctly laminated shales that are composed almost exclusively of fecal pellets. The black shale facies continues until the lower Cenomanian and thus comprises the mid-Cenomanian Event.


IW profiles at Site 1260 document reactions similar to those at earlier Leg 207 sites, with remarkably comparable profiles to Site 1258 in particular. Microbially mediated organic matter remineralization is largely centered within Unit IV, the black shales. Sulfate decreases linearly downcore to zero near the top of Unit IV at ~390 mbsf, with a corresponding increase in ammonium to values in excess of 2 mM. Below the depth of sulfate depletion, methane contents increase sharply to maximum values (~68,000 ppmv) within the black shales, then decrease below. As at Site 1258, alkalinity and calcium profiles document intervals of active carbonate diagenesis. Within the upper 400 mbsf, chloride profiles at Site 1260 are strikingly similar to the upper 400 mbsf at Site 1258, including two coincident 3% decreases at ~310 and 370 mbsf. Below 400 mbsf, the Site 1260 chloride profile only increases slightly (577–582 mM) and shows neither the presence of the brine documented at Site 1257 nor the pronounced pore water freshening that was observed at Site 1258.

Physical Properties and Logging

Index properties, P-wave velocities, and GRA densities were measured on core samples from Site 1260. Logging runs consisted of two passes with the triple combo tool string and two passes with the FMS-Sonic tool. In addition, a check shot velocity run was conducted, comprising 14 downhole stations. Hole conditions were excellent for the logging tools, and data are of extremely high quality. In general, the physical property data reflect normal consolidation down to ~328 mbsf. Anomalously high velocities and porosities in the top 40 m reflect a slump deposit within this interval. Porosity, resistivity, and FMS data show strong periodic signals superimposed on the normal consolidation trend through the Eocene interval, suggesting cyclical variations may be readily determined.

Lithologies become richer in clay below 328 mbsf. Velocity and density profiles tend to flatten with much higher scatter in magnitudes than above. All logs correlate very closely with lithologic changes identified from core descriptions. Natural gamma logs delineate the black shale facies particularly well, but all logs show distinctive changes in profile shape and a high degree of data variability in this interval. Critical intervals that are visually distinct, like the P/E and K/T boundaries, are readily distinguished with logging and physical property data as well. As a consequence, lithologic changes and these event deposits can be correlated to the seismic reflection data with a high degree of confidence.

Depositional History

Clayey quartz siltstone of lower Albian age, which was deposited in a marine marginal or epicontinental shelf setting, represents the oldest sequence recovered at Site 1260. It is unconformably overlain by a black shale sequence that dates from Cenomanian to Coniacian, but most of it is Cenomanian, with thinner Turonian and Coniacian intervals. Debris flows, wood fragments, and quartz are found at the base of the organic-rich black shale section, which indicate shallow water depths. The laminated, coarse-grained foraminiferal limestones in the succession may be related to winnowing or grain flows or may reflect changes in carbonate productivity. Continuous deepening characterizes the remaining Upper Cretaceous succession. During OAE 2, highest TOC values and very distinct laminations indicate bottom water anoxia. The top of the black shale interval is represented by a hiatus 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. The contact between upper Coniacian slumped black shales and condensed Campanian glauconite-rich chalk is erosional.

Oxic conditions were established by the 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 that reductions in bottom water oxygenation recur.

Maastrichtian- to Oligocene-age sediments at Site 1260 consist of pelagic deep marine nannofossil chalks and oozes. Sedimentation rates vary from 4.3 m/m.y. in the Maastrichtian–upper Paleocene, 12.1 m/m.y. in the upper Paleocene–lower Eocene, ~20.5 m/m.y. in the middle Eocene, and ~6.7 m/m.y. in the upper Eocene. The succession is interrupted by several hiatuses—a upper Paleocene hiatus representing ~1 m.y., an ~1.5-m.y. hiatus at the lower/middle Eocene boundary, an upper–middle Eocene hiatus of ~5 m.y., and an lower Oligocene hiatus of ~2 m.y. These hiatuses may reflect periods of slow deposition and/or erosion.

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