RESULTS

Site 1257

Site 1257 is located in 2951 m of water on a terrace above the steep (>10) northern slope >10 northwest Demerara Rise, ~400 km north of Suriname. As the second deepest location, Site 1257 serves as an intermediate member of the paleoceanographic depth transect across Demerara Rise. It was promoted to recore DSDP Site 144, which was spot cored during Leg 144 in 1970. The major objectives were the following:

  1. Core and log an Albian–Oligocene section to evaluate paleoceanographic and paleoclimatic changes during the Paleogene and Cretaceous, with emphasis on major and abrupt events during this interval such as the Eocene/Oligocene (E/O) and P/E boundaries and the Cretaceous OAEs.
  2. Reconstruct the history of the opening of the Equatorial Atlantic Gateway by obtaining benthic foraminifer proxy data. These data will help to understand changes in bottom water circulation over Demerara Rise during the gradual opening of the seaway.
  3. Define the depth of key seismic reflectors for a detailed planning of the remaining sites along the Demerara Rise depth transect.

The steep northwestern slope of Demerara Rise, where Site 1257 is located, is part of the southern transform fault that separated South America and West Africa during the rift phase of the southern Central Atlantic in the Late Jurassic–Early Cretaceous. Although spot cored, results from Site 144 suggested the presence of all the target sediments.

Operations

At Site 1257, one advanced piston corer/extended core barrel (APC/XCB) hole and two rotary core barrel (RCB) holes were cored. Hole 1257A was APC cored to 40.6 mbsf, but lithification in this part of the sediment column stopped APC coring earlier than desired. The hole was completed with XCB coring to a total depth of 284.7 mbsf (recovery = 75.9%) (Table T1). XCB coring proved slow and resulted in poor-quality cores (biscuiting), so RCB coring was used for Holes 1257B and 1257C. The upper 40 m of the succession was washed in Hole 1257B, and the section between 40 and 227.3 mbsf was RCB cored (recovery = 62.1%). In Hole 1257C, the upper 82 m was washed and the hole was RCB cored between 82 and 235.9 mbsf (recovery = 32.8%). Porcellanite, limestone, and chert beds at ~90 and 170–225 mbsf reduced recovery of both XCB and RCB coring.

Sediments at Site 1257 range in age from Miocene to Albian. A 1.5-m-thick veneer of Neogene nannofossil ooze directly overlies lower Oligocene sediment. Chalk spanning the E/O boundary was recovered in Hole 1257B. Paleogene sedimentation is cut by two hiatuses in the upper Eocene and uppermost Paleocene. In Hole 1257C, sediments spanning the P/E boundary were recovered. Good core recovery of an expanded upper Paleocene section allowed construction of a complete composite section of this interval. Foraminiferal packstone boulders intercalated in a 2- to 3-m-thick slumped sequence represent the early Paleocene. The Cretaceous sequence is cut by three hiatuses in the upper Maastrichtian and lower Campanian. The Santonian–latest Cenomanian is represented by a 44-m-thick sequence of laminated black shales that include OAEs 3 and 2. Hard limestone and chert layers in this interval prevented good core recovery. Recovery improved in the lower part of the succession, and three copies of the black shale intervals in Holes 1257A, 1257B, and 1257C will possibly allow a complete postcruise reconstruction of this interval. An unconformity underlying the black shales separates the uppermost Cenomanian from upper–middle Albian sediments.

Wireline logging was conducted with the triple combination (triple combo) and Formation MicroScanner (FMS)-sonic tool strings in Hole 1257A. The Well Seismic Tool (WST) was used to undertake a checkshot survey. Results provided accurate traveltime data for calibrating velocity logs and providing formation velocity information for time-to-depth conversion of seismic data.

Seismic Stratigraphy

Within 2.5 km northwest and 1.5 km north and northeast of the drill site the seafloor begins to fall off at a 10 angle, from the site elevation of 2951 meters below sea level (mbsl) to the abyssal plain at 4400 mbsl. The site itself is on a slight mound that appears to be an erosional remnant. Reflector "A," representing the top of this erosional unconformity, crops out at the seafloor at the site (within the resolution of the survey data). Between Reflector A (the seafloor in this case) and Reflector "B" is seismic Unit 2 (seismic Unit 1 is missing at this site). It is 173 ms thick (two-way traveltime) at this location, calculated to be 144 meters below seafloor (mbsf) by the checkshot and downhole logging velocity information. Seismic Unit 2 shows an incoherent reflection character here, indicating a disturbed sediment package or the effects of side echoes from local complex topography. A high-amplitude reflection event of short lateral duration occurs at 110 ms (90 mbsf) at the site.

Reflector "B," at 173 ms subbottom (144 mbsf), marks the top of seismic Unit 3. Unit 3 is a flat-lying sequence at this site that dips gently to the northeast at an angle of 1.7. Reflection event B is hummocky on a local scale, probably cut by channels, and is underlain by several high-amplitude reflections and then a short transparent zone to the top of Reflector B at 217 ms subbottom (181 mbsf). This section is described as Subunit 3a in Shipboard Scientific Party ("Site Survey and Underway Geophysics"), this volume. Subunit 3b underlies 3a and is the sequence between Reflectors B and Reflector "C," which occurs at 272 ms subbottom (232 mbsf).

Most acoustic energy is lost below Reflector C in the survey data, and the section is difficult to describe. A few hyperbolic reflections in this interval are visible. Industry line C2206 crosses in a northeast–southwest direction just 1 km northeast of the drill site (see Shipboard Scientific Party ["Site Survey and Underway Geophysics"], this volume). In this profile, the sequence of reflectors below Horizon C appears folded into a possible small anticline below the drill site and contacts Reflector C as an angular unconformity.

Stratigraphy

Lithologic description of the cores and biostratigraphic age assignments exposed a number of hiatuses and highly condensed intervals in the sedimentary succession at Site 1257. A veneer of upper Miocene nannofossil ooze was defined as lithostratigraphic Unit I. The unit is characterized by a gradual color change downcore from pale brown to pale olive. The sediment is strongly mottled, and iron sulfide is present in discrete burrows and as mottles throughout the unit. Unit I unconformably overlies lower Oligocene–uppermost Eocene light greenish gray to greenish gray chalk and ooze. Less lithified intervals may be related to variations in clay content. Microfossil assemblages of this interval are rich in well-preserved diatoms and radiolarians. Below this section, a 20-m-thick interval of middle Eocene biosiliceous chalk disconformably overlies lower Eocene chalks with biogenic silica. The latter intervals have been grouped as lithostratigraphic Unit II, which was divided into Subunits IIA and IIB in the transition from ooze to chalk at 22 mbsf. The transition coincides with an significant increase of siliceous microfossil abundance. Sedimentation rates in the Oligocene–Eocene varied from 5 to 9 m/m.y., with increasing values in the lower Eocene.

The Paleocene comprises foraminiferal nannofossil chalks that were described as lithostratigraphic Subunit IIIA. Siliceous microfossils of the upper Paleocene were replaced by zeolite, which is abundant throughout the subunit. The presence of occasional porcellanite stringers also indicates diagenetic alteration of biogenic silica. Good recovery of an expanded upper Paleocene sequence (sedimentation rate = ~10 m/m.y.) allowed the construction of a spliced section that shows a pronounced cyclicity, possibly representing 20- to 50-k.y. periodicity.

An early–late Paleocene gravity flow deposit marks the lower boundary of Subunit IIIA. No upper Maastrichtian sediments were recovered. The subjacent lower Maastrichtian–lower Campanian zeolitic chalk (Subunit IIIB) is similar to that of the Paleocene. Well-preserved radiolarians are present in the upper Campanian but absent below. A condensed glauconite-rich horizon marks the base of Campanian sediments where there is a sharp irregular contact with underlying black shales of lithostratigraphic Unit IV (Core 207-1257A-20X).

The black shales of Unit IV primarily consist of dark olive-gray to black finely laminated calcareous claystone with carbonaceous material. Carbonate contents range from ~40 to 60 wt% in dominant lithologies and as high as 78 wt% in individual carbonate-rich layers. TOC reaches values of 16 wt%, and hydrogen index and oxygen index values indicate that the black shales contain Type II kerogen, indicating a marine source for the organic matter. The unit shows very well developed submillimeter-scale laminations and has a strong petroliferous odor, although the organic matter is thermally immature. Rhythmic color variations (dark olive gray to black) occur on a decimeter scale throughout. Lighter intervals are relatively rich in prismatic inoceramid shell material. Olive laminated calcareous porcellanite and limestone as thick as 30 cm, gray and black chert nodules, and concretions of nannofossil chalk with foraminifers are present in minor amounts. Preservation of calcareous microfossils varies between moderate to very good. Pristine glassy foraminiferal tests also are observed in this unit. Microfossil ages yield Santonian, Coniacian, and Turonian and date the base of the black shale as Cenomanian. Sedimentation rates of the Upper Cretaceous sediments vary between 4 and 6 m/m.y.

A sharp contact between the laminated black shales and the underlying pyrite-rich clayey carbonate siltstone (Unit V) was recovered in Core 207-1257C-15R. Foraminifers in a sandstone date the top of the unit in Section 207-1257A-25-CC as Cenomanian. The bottom of Hole 1257A is late–middle Albian age.

Recovery of Critical Intervals

The main objective of Leg 207 was to recover sediments with microfossils from major and abrupt events of the Paleogene and Cretaceous periods, such as the E/O and P/E boundaries and OAEs. In Hole 1257A, sediment with a continuous lower Oligocene–upper Eocene transition was recovered, presumably including the prominent stable isotope shift associated with the dramatic growth of ice sheets on Antarctica.

Sediments spanning the P/E boundary were recovered in Hole 1257C, where it comprises a 3.5-cm-thick dark greenish clay that represents carbonate dissolution associated with the P/E boundary in deep oceans. Shore-based investigations will show if the succession above the event is complete.

A 57-m-thick succession of laminated black shales, including OAEs 3 and 2, was recovered in all three holes at Site 1257. Although the recovery in the black shale interval was not as good as desired, a comparison of all three holes will probably allow for fairly complete coverage of the OAEs. A shore-based refinement of the stratigraphy including stable carbon isotope measurements will help to better define the OAE intervals. The preservation of foraminifers in this section varies between moderate and very good and has good promise for shore-based paleotemperature studies.

Geochemistry

Pore waters are characterized by the presence of a brine with maximum chlorinity of 823 mM, a ~50% increase over average seawater chlorinity. The maximum chlorinity is centered ~200 mbsf in the black shales (Unit IV), decreasing above and below the unit. The combination of the chlorinity and salinity profiles, a low-temperature anomaly recorded by the downhole logging Temperature/Acceleration/Pressure (TAP) tool, and high-porosity intervals in shales suggest that the brines are sourced externally through the black shales. Sulfate decreases downhole and is depleted by ~160 mbsf. Below 220 mbsf, sulfate increases to ~5 mM at the base of the cored interval. Methane first appears at ~108 mbsf, increasing rapidly to reach a broad maximum between 180 and 218 mbsf in black shales, largely coincident with the interval of sulfate depletion, before decreasing downhole. High methane-to-ethane ratios and the absence of measurable higher molecular weight hydrocarbons indicate that the methane was generated microbially.

Physical Properties and Logging

Core physical property (multisensor track [MST] and discrete measurements) and downhole logging physical property data show excellent agreement. Density and velocity values are uncharacteristically high for such shallow sediments but reflect their age and degree of cementation. Nonetheless, these physical property profiles demonstrate, in general, a relatively normal depth-consolidation profile that cementation appears not to have altered. On a higher-frequency scale, there are significant characteristics to these data that reflect lithologic changes and trends, and patterns of cyclicity are obvious in some intervals. The most pronounced change correlates with lithostratigraphic Unit IV, the black shales. In this case, the unit is characterized by lower density and velocity values but with a high degree of scatter. Very high peak values correlate with calcified beds. Other significant deviations from the normal consolidation trend correlate with events and hiatuses identified in the lithostratigraphy, such as the lower Eocene hiatus, the upper Paleocene hiatus, the P/E boundary, and the top and bottom of the black shale sequence. These strong physical property contrasts yield strong reflection characteristics and allow for good correlation with the seismic stratigraphy.

Depositional History

Shallow-marine Albian synrift sediments form the oldest sequence recovered at Site 1257. They are unconformably overlain by Cenomanian–Santonian laminated black shales. These organic-rich shales reflect high productivity in surface waters and low oxygen levels in the bottom water. Regional oceanic upwelling conditions are believed to be the reason for this long-lasting (~17 m.y.) phase of black shale deposition at Demerara Rise. Similar facies have been described from the Tarfaya Basin on the northwest African margin of the Atlantic and from Venezuela, Colombia, and Costa Rica.

Oxic conditions were established by the early Campanian when pelagic and open marine marls were deposited on Demerara Rise. However, the abundance of trace fossils indicating dysoxic environments points toward oxygen deficiency at the seafloor. The upper Campanian–Neogene pelagic record at Site 1257 is interrupted by several hiatuses and slump deposits reflecting the position of the site on the topographic slope of Demerara Rise. Paleobathymetric assignments of these sediments are difficult to provide, but water depths similar to those of the present were probably reached by the late Maastrichtian to Paleocene. Sedimentation and subsidence have probably kept pace since that time.

Site 1258

Site 1258 is located at a depth of 3192.2 mbsl on the gently dipping (~2) western slope of Demerara Rise, ~380 km north of Suriname. The site is located on a ridge of Paleogene sediments outcropping on the seafloor. Site 1258 is the distal and deepest end-member of the paleoceanographic depth transect across Demerara Rise. The major objectives for this site are as follows:

  1. Core and log a Paleogene–Albian section to evaluate paleoceanographic and paleoclimatic changes, with emphasis on major and abrupt events during this interval such as the E/O and P/E boundaries and Cretaceous OAEs.
  2. Reconstruct the history of the opening of the Equatorial Atlantic Gateway by obtaining benthic foraminifer proxy data. These data will help in understanding 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.

Operations

At Site 1258, three RCB holes were cored. Hole 1258A was cored to 447.5 mbsf (recovery = 83.9%), Hole 1258B was cored to 460.9 mbsf (recovery = 76.3%) (Table T1), and Hole 1258C was washed from 0 to 120.0 mbsf. To obtain a splice for a lower lower middle Eocene succession, missing because of faulting in Hole 1258A, Hole 1258C was spot cored between 120.1 and 206.1 mbsf. Spot coring the interval between 245.3 and 274.2 mbsf obtained a third copy of the K/T boundary interval. Subsequent washing to 384.8 mbsf was followed by coring of the hole to a total depth of 485.0 mbsf. The average recovery in Hole 1258C was 74.9%. Hard beds between 320 and 380 mbsf and 415 and 420 mbsf reduced core recovery in these intervals.

Seismic Stratigraphy

The seismic stratigraphy established for Demerara Rise, including Horizons A, B, B, and C, have been correlated to Site 1258 strata. Reflector A, representing the top of a presumably early Miocene erosional unconformity, crops out at the seafloor at the site (within the resolution of the survey data). Between Reflectors A and B, seismic Unit 2 is 300 ms thick (265 m using downhole logging velocity information). Seismic Unit 3, between Reflector B at 300 ms subbottom and Reflector C at 480 ms subbottom, is a 94-m-thick flat-lying sequence that dips 1.5 to the north-northwest.

Seismic Unit 2 shows an echo character of reasonably coherent but slightly contorted reflections with offsets, which probably describes a sediment sequence that has undergone mass failure and rotational displacement. Seismic Unit 3 is divided into two subunits: the basal subunit (Subunit 3b) lies between Horizons B and C (450–515 ms subbottom; 417–480 mbsf). It is defined on the basis of a series of strong, parallel, coherent reflections that are laterally contiguous and have been shown to correlate to the black shale interval.

Stratigraphy

Lithologic description of the cores and biostratigraphic age assignments revealed a rather continuous sedimentary succession with only a few hiatuses. Sediments at Site 1258 range in age from Miocene to middle Albian. An ~8-m-thick package of Miocene nannofossil ooze unconformably overlies a drape of calcareous ooze with radiolarians and diatoms of early Oligocene age. This sequence, in turn, unconformably overlies an expanded 143-m-thick succession of lower middle Eocene (planktonic foraminiferal Zone P10; calcareous nannoplankton Zone NP15) to lower Eocene nannofossil chalk. With a sedimentation rate of ~20 m/m.y., the middle–lower Eocene succession is a remarkably expanded and complete (to zonal level) section. Excellent RCB recovery provided continuous core overlap from the middle Eocene to ~10 m below the P/E boundary. The periodic variability in the Eocene magnetic susceptibility data at Site 1258 will provide a good basis for postcruise cyclostratigraphic studies. Age control is excellent, with well-defined paleomagnetic datums in the section (e.g., the top of Chron C21n and the base of Chron C22r). Preliminary investigation suggests that the dominant frequencies of the magnetic susceptibility data are likely Milankovitch periods, with significant power at ~20, 40, and 100 k.y.

We recovered an apparently expanded section across the P/E boundary in all three holes. The upper Paleocene clayey nannofossil chalk sequence is relatively thick compared to other Leg 207 sites. Zeolite or locally abundant opal-CT lepispheres replace siliceous microfossils in this interval. We recovered the K/T boundary in all three holes, and Core 207-1258B-27R contains the K/T ejecta layer. The subjacent upper Maastrichtian greenish gray nannofossil chalk with foraminifers and clay and lower Campanian–lower Maastrichtian zeolitic nannofossil claystone display cyclic color banding between light greenish gray and greenish gray on a decimeter scale. Radiolarians in the Campanian are well preserved, but planktonic foraminifers are absent or poorly preserved. The lithology becomes increasingly clay rich downhole, and carbonate contents decrease to 35 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 are 12 m/m.y.

A condensed glauconite-rich horizon in Core 207-1258B-44R separates the Campanian clayey chalk from the ~56-m-thick black shale sequence below. This interval contains the lower part of OAE 3, a complete OAE 2, and an expanded succession of laminated shales of Cenomanian–middle Albian age. The preservation and abundance of calcareous microfossils is poor to good, with glassy foraminifers in the Cenomanian part of the black shales. A middle Albian disconformity separates the laminated black shales and limestones from the underlying TOC-rich claystones with phosphatic concretions. Although rich in predominantly marine organic matter (up to 5%), the sediments lack obvious laminations. Clay-rich beds in the latter yield some extremely well preserved microfossils. In addition, ammonites as small as a centimeter in diameter are abundant in some laminae. Rare and thin bioclastic limestone intervals are intercalated; they may represent occasional small-scale storm deposits. The base of Site 1258 is dated as late early Albian (Ticinella primula planktonic foraminiferal zone and Subzone NC8a–b).

Recovery of Critical Intervals

In each of the three holes cored at Site 1258, sediments spanning the P/E boundary were recovered. The boundary interval comprises the last occurrence of the benthic foraminifer Gavelinella beccariiformis followed by a sharp contact between light green chalk and dark green clay. The sharp contact reflects the sudden decrease of carbonate content from values of ~60 wt% in the upper Paleocene to ~13 wt% in the lowermost Eocene associated with the P/E boundary. Green clay-rich sediments prevail from the P/E boundary to ~1.90 m above the boundary. Light green carbonate-rich sediments are the dominant lithology of the lower Eocene. Site 1258 appears to provide the first highly expanded P/E boundary succession known from tropical oceans.

The K/T boundary was recovered in all the three holes cored at Site 1258, and Core 207-1258B-27R contains the K/T ejecta layer. The base of the K/T boundary is marked by a 1-mm-thick layer of clayey spherules overlying upper Maastrichtian chalks and is covered by a thin (2–3 mm) drape of whitish nannofossil chalk of potentially reworked upper Maastrichtian sediments. This white layer is overlain by a 2-cm-thick graded bed of medium to fine sand-sized green spherules, which are in turn overlain by gray clay. The Parvularogoglobigerina eugubina planktonic foraminiferal zone (P) is remarkably expanded (~3 m thick) and is overlain by planktonic foraminifers belonging to Danian foraminiferal Subzones P1a–P1c, suggesting that the recovered succession is expanded and complete (at the zone level). The recovery of the ejecta layer is, to our knowledge, the first discrete ejecta bed known in South America. Demerara Rise lies ~5000 km southeast of the impact crater in Yucatan. The distribution of documented ejecta deposits west, north, and northeast of Yucatan leads us to expect a much thinner (millimeter scale) ejecta bed than was actually recovered.

A ~56-m-thick Coniacian–Albian succession of laminated black shales and laminated limestones, including OAE 2, was recovered in all three holes at Site 1258. Average recovery of this interval was 77%, 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 shales is represented by a hiatus covering the entire Santonian. Coniacian organic-rich sediments are slumped or missing. OAE 2 is represented by an interval of distinctly laminated black shales with TOC values up to 28 wt%. 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. 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. Carbonate contents vary between 5 and 95 wt%, and the constituents include nannofossils (concentrated in fecal pellets), foraminifers, and shell fragments. TOC values range from ~5 to 28 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 black shales facies continue until the upper middle Albian, including the middle Cenomanian Event, and the lowermost black shales are dated as late middle Albian (Biticinella breggiensis planktonic foraminiferal zone). Storm or turbidity current deposits with oyster and other pelecypod fragments are present, especially toward the base of the black shale interval. Occasional layers of diagenetic calcite with a distinct bluish tint are noted. Microscopic analysis reveals that the morphology of the calcite growth resembles that of authigenic methane carbonate cements and may derive from the high methane content in the black shales. To date, no upper Albian biostratigraphic markers have been identified that would enable a stratigraphic definition of the uppermost Albian OAE 1d.

Geochemistry

Active microbial organic matter diagenesis is focused in the organic-rich black shales of Unit IV. Pore water sulfate decreases linearly throughout the overlying units to the top of the black shales at ~390 mbsf. Correspondingly, ammonium, which is produced by organic matter degradation, increases linearly through the same interval. In contrast, alkalinity, a byproduct of sulfate reduction, does not increase linearly with depth, but varies, with multiple maxima and minima. The correspondence between alkalinity and calcium profiles and intervals of increased lithification suggests alkalinity and calcium variability is controlled by carbonate diagenesis. As expected, where pore water sulfate approaches zero (approximately the top of the black shales), methane contents increase sharply to high values (~3,000–60,000 ppmv). Interstitial gas volumes and methane/ethane ratios are high at Site 1258, probably reflecting the high TOC contents of Units IV and V. Unlike Sites 1257, 1254, and 1261, however, Site 1258 pore waters are not characterized by the presence of brine. Salinity and chlorinity decrease with depth beginning at ~300 mbsf, then sharply decrease in the clayey chalks of Unit V. The minimum chloride value at the base of the hole is 465 mM, a 17% decrease from seawater chlorinity. At present, the origin of the low-salinity fluids is undetermined.

Physical Properties and Logging

Core physical property data (MST and discrete measurements) and downhole logging physical property data show excellent agreement. The downhole profiles show marked variability linked to lithologic change. In general, they exhibit a normal consolidation profile down to ~267 mbsf, with a strong negative excursion in velocity and density encompassing the P/E boundary. Below 267 mbsf, the profiles remain constant or decrease slightly, correlating with lithostratigraphic Subunit IIC to Unit V. The data in this interval also show patterns of cyclicity. The most pronounced change correlates with lithostratigraphic Unit IV, the black shales. In this case, lower density and highly variable velocity values, which reach >2600 m/s, characterize the unit. Other significant deviations from the normal consolidation trend correlate with events and hiatuses identified in lithostratigraphy, such as the P/E boundary, the K/T boundary, and the top and bottom of the black shale sequence. These pronounced physical property contrasts yield strong reflection characteristics and allow for good correlation with the seismic stratigraphy.

Depositional History

Lower–middle Albian clayey and TOC-rich sediments represent the oldest sequence recovered at Site 1258 and reflect open marine conditions in a marginal or epicontinental setting. They are unconformably overlain by middle Albian–Coniacian black shales. It appears as though the contact between these two lithologies represents Reflector C, which separates synrift from drift sediments on Demerara Rise. If so, the change between synrift and drift sedimentation at Site 1258 occurred in one planktonic foraminiferal zone, which would give the reflector a late middle Albian age (B. breggiensis planktonic foraminiferal zone). Although no major facies change is observed between the middle Albian and upper Cenomanian, the lack of late Albian index fossils point to a hiatus in this interval. The presence of possible storm deposits in the lower part of the succession might indicate a paleowater depth above storm-wave base, and continuous deepening characterizes the remaining Lower Cretaceous succession. During OAE 2, high TOC values and very distinct laminations indicate severe bottom water anoxia. Slumping and reworking of organic-rich sediments and black shales occurred during the Coniacian. The top of the black shales terminates at a hiatus covering the Santonian and lower Campanian. It is unclear whether the observed mass flows in the Coniacian are related to tectonic movements during the opening of the Equatorial Atlantic Gateway. The contact between upper Coniacian slumped black shales and condensed lower Campanian glauconite-rich chalk is erosional.

Oxic conditions were established by the early Campanian, when sedimentation on Demerara Rise changed from hemipelagic to pelagic. The abundance of radiolarians in the Campanian, however, indicates high surface water productivity. The cyclic distribution of trace fossils, indicative of oxygen deficiency, suggests recurring decreases of bottom water oxygenation. The rather continuous Maastrichtian–Eocene pelagic record at Site 1258 records the results of the K/T impact with a graded bed of spherules. The spherules are up to 2.5 mm in diameter, an exceptional size considering the distance to the proposed impact crater.

Interpretation of the seismic records, in conjunction with the observation of mass-wasting deposits and "missing" intervals between holes, indicates that a significant portion of the cored stratigraphic column has failed in a slope instability event or events. The likely scenario is a rotational slump whereby material has been displaced but not transported far and the stratigraphy is left largely intact but results in differences between holes.

Site 1259

Site 1259 is located in 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 and P/E boundaries and Cretaceous OAEs.
  2. Reconstruct the history of the opening of the Equatorial Atlantic Gateway by obtaining benthic foraminifer 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.

Operations

At Site 1259, three RCB holes were cored. Hole 1260A was cored to 558.8 mbsf (recovery = 66.6%). Hole 1259B was washed to 305 mbsf and cored to 381.9 mbsf, then washed to 420.5 mbsf and cored to 556.2 mbsf (recovery = 69.2% in the cored intervals) (Table T1). Hole 1259C was washed to 308.0 mbsf and cored to 373.3 mbsf. It was then 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 succession was washed again to 490 mbsf, where continuous coring proceeded until 553.7 mbsf (recovery = 80% in the cored intervals). Sandy intervals at 120 and 300 mbsf and hard layers at 380–400 and 490–520 mbsf hampered core recovery at these depths.

Seismic Stratigraphy

The seismic stratigraphy established for Demerara Rise, including Horizons A, B, B, and C, can be correlated 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 m thick using laboratory-measured velocity information). The topmost sequence (180 ms; ~130 m thick) in this seismic unit consists of incoherent reflections. Below this interval, Unit 2 is a sequence of high-amplitude, parallel, coherent reflections that are relatively flat lying. This package is ~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 (503–571 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).

Stratigraphy

Lithologic descriptions of the cores and biostratigraphic age assignments reveal a rather continuous sedimentary succession with only a few hiatuses. Sediments at Site 1259 are Miocene–Cenomanian in age. The deepest unit recovered at Site 1259 did not yield any age-diagnostic microfossils. A thin veneer of Holocene foraminiferal ooze at the top of the section unconformably overlies a ~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–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 in sediment properties in the lower Eocene will provide a good basis for postcruise cyclostratigraphic studies.

In all three holes, an apparently expanded section across the P/E boundary was recovered. 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 an ejecta layer. The subjacent upper Maastrichtian greenish gray nannofossil chalk with foraminifers and clay and the lower Maastrichtian–upper Campanian zeolitic nannofossil claystone display cyclic color banding between light greenish gray and greenish gray on a decimeter scale. The lithology becomes increasingly clay rich downhole, and carbonate contents decrease to 30 wt%. Radiolarians in the Campanian are well preserved, but foraminifers and nannofossils are rare in the Campanian 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. Age control is excellent, with well-defined paleomagnetic datums in both sections (Chrons C30n and C32n) (see "Paleomagnetism" in "Discussion and Conclusions"). 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 ~56-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 range from poor to good, with glassy foraminifers 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

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 contains a small number of benthic foraminifers and abundant but poorly preserved 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 "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 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 each contains the ejecta layer. A 1.9-cm-thick layer of clayey spherules overlying upper Maastrichtian chalks marks the K/T boundary. Above the boundary, the lower Danian planktonic foraminiferal Zones P2–P were distinguished, Subzone P1c to Zone P have a thickness of 8 m. The thickness of the spherule layer is similar to that at Sites 1258 and 1260, suggesting that the spherule bed at both sites is a result of fallout rather than redeposition.

A ~56-m-thick Cenomanian–Santonian succession of laminated black shales and laminated foraminiferal limestones, including OAEs 3 and 2, was recovered in both holes at Site 1259. The quasi-periodic variability of the claystone and chalk/limestone composing 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, 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 Cenomanian–Turonian 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 in 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.

Geochemistry

Similar to the pore waters at Sites 1257 and 1261, 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 suggests that the brine is sourced laterally through these organic-rich claystones of Unit IV. Interstitial water chemistry data are dominated by the degradation of organic matter in the 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 >1 mM. Below the sulfate reduction zone, methane increases sharply to reach a maximum of 76,000 ppmv near the base of black shales before dropping to 2,000 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

Moisture and density (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 ~365 mbsf, with relatively linear profiles. Small excursions in properties appear at ~95 and 200–280 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.

Sediments 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 to the K/T boundary, and apparent cyclicity occurs 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 to organic-rich intervals. The largest peaks in velocity and density appear in quartz sandstone and siltstone below the black shales (values up to 3500 m/s and 2.4 g/cm3, respectively).

Depositional History

Probable tidal flat sediments 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–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 Eprolithus from 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 or grain flows or may 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 high surface water productivity. The cyclic pattern of trace fossil abundance suggests repeated changes in bottom water oxygenation.

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., a ~3-m.y. hiatus at the middle/upper Eocene boundary, and an upper Oligocene interval of hiatuses and condensed intervals representing ~3 m.y. These hiatuses may reflect periods of slow deposition and/or erosion.

Site 1260

Site 1260 is located in 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 and P/E boundaries and Cretaceous OAEs.
  2. Reconstruct the history of the opening of the Equatorial Atlantic Gateway by obtaining benthic foraminifer 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.

Operations

Two RCB holes were cored at Site 1260. Hole 1260A was cored to 491.9 mbsf (recovery = 79.6%) (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 = 88.2% in the cored intervals). Hard beds at 130–154, 180–211, and 480–509 mbsf hampered core recovery in these intervals.

Seismic Stratigraphy

The seismic stratigraphy established for Demerara Rise, including Horizons A, B, B, and C, has been correlated to Site 1260 strata with lines GeoB215 and 207_LIS. 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 m using laboratory-measured downhole logging and checkshot velocity information). The topmost sequence in 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 which is 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 (464–522 ms subbottom; ~392–485 mbsf) and is defined on the basis of a series of strong, parallel, coherent reflections that are laterally continuous. This seismic interval has been shown to correlate to the black shale interval.

Stratigraphy

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 from Oligocene to early Albian in age. 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 upper 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 in the middle Eocene color reflectance and GRA bulk density data will provide a good basis for postcruise cyclostratigraphic studies. Age control is excellent, with well-defined paleomagnetic datums in the section (e.g., Chrons 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, an apparently expanded and laminated section across the P/E boundary was recovered. As at Sites 1258 and 1259, the upper Paleocene clayey nannofossil chalk sequence is relatively thick and sedimentation rates in the lower Eocene–Paleocene interval drop to values averaging ~12 m/m.y. Zeolite or locally abundant opal-CT lepispheres replace siliceous microfossils in this interval. The K/T boundary with an ejecta layer was recovered in both holes. The subjacent upper Maastrichtian greenish gray nannofossil chalk with foraminifers and clay and the lower Maastrichtian–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 content decreases to 30 wt%. Foraminifers and nannofossils are rare in the Campanian, and a significant increase in the 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 ~93-m-thick black shale sequence, with TOC content up to 14 wt%. This sequence contains a thin Coniacian interval, and thus the lower part of OAE 3, and a virtually complete Turonian. OAE 2 overlies an expanded succession of Cenomanian laminated shales. The preservation and abundance of calcareous microfossils is poor to good, with glassy foraminifers 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 is dated as late early Albian (T. primula planktonic foraminiferal zone; Subzones NC8a–NC8b) and represents the oldest sediments cored at Site 1260.

Recovery of Critical Intervals

Sediments spanning the P/E boundary were recovered in both holes at Site 1260. The boundary interval includes 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 up to 12 cm above the boundary, where the first bioturbation occurs. Pervasive bioturbation returns within ~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. A 1.8-cm-thick layer of clayey spherules overlying upper Maastrichtian chalks marks the base of the K/T boundary interval. In the core from Hole 1260A, the spherule layer covers 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–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 primary fallout rather than redeposition.

A ~93-m-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. The Coniacian organic-rich interval is 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.

Geochemistry

Interstitial water 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 in 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 >2 mM. Below the depth of sulfate depletion, methane contents increase sharply to maximum values (~68,000 ppmv) in the black shales, then decrease below. As at Site 1258, alkalinity and calcium profiles document intervals of active carbonate diagenesis. In 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 and FMS-sonic tool strings. In addition, a checkshot 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 uppermost 40 m reflect a slump deposit in this interval. Porosity, resistivity, and FMS data show strong periodic signals superimposed on the normal consolidation trend through the Eocene interval, suggesting cyclic 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 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 early Albian age, which was likely deposited in a marginal marine 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. 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, high TOC values and very distinct laminations indicate bottom water anoxia. At the top of the black shale interval is a hiatus covering the Santonian and lower Campanian. Mass flow deposits in the upper Turonian and Coniacian may be a result of 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 high surface water productivity.

Maastrichtian- to Oligocene-age sediments at Site 1260 consist of pelagic deep marine nannofossil chalks and oozes. Sedimentation rates varied 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—an upper Paleocene hiatus representing ~1 m.y., a ~1.5-m.y. hiatus at the lower/middle Eocene boundary, an upper–middle Eocene hiatus of ~5 m.y., and lower Oligocene hiatus of ~2 m.y. These hiatuses may reflect periods of slow deposition and/or erosion.

Site 1261

Site 1261 is located in a water depth of 1899 mbsl on the gently dipping (~1), northwest-facing slope of Demerara Rise, ~350 km north of Suriname. Sites 1260 and 1258 are to the northwest and Sites 1257 and 1259 are to the north. Site 1261 is the shallowest site, forming the 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 and P/E boundaries and Cretaceous OAEs.
  2. Reconstruct the history of the opening of the Equatorial Atlantic Gateway by obtaining benthic foraminifer 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.

Operations

At Site 1261, two RCB holes were cored. Hole 1261A was spot cored to a depth of 236.9 mbsf, with cores taken at 0–22.5, 69.7–79.3, 131.4–141.1, and 189.1–198.7 mbsf. Continuous coring proceeded from 236.9 to 659.8 mbsf (recovery = 73.4% in the cored intervals) (Table T1). Hole 1261B was washed to 525.3 mbsf and cored to 674.1 mbsf (recovery = 62.2% in the cored intervals).

Seismic Stratigraphy

The seismic stratigraphy established for Demerara Rise, including Horizons A, B, B, and C, was correlated to Site 1261 with line GeoB213. Lines GeoB204 and GeoB208 are orthogonal to GeoB213 and pass within 5 km of the site to the northwest and southeast, respectively. Industry line C2206a passes orthogonal to GeoB213 as well, 8 km northwest of the drill site.

The seismic stratigraphy for Demerara Rise shows increasing sediment thickness to the south. At Site 1261, the uppermost sediment section has not been defined previously. It consists of a thin (30 ms; 24 m thick) package of parallel coherent reflections that offlap from the upslope direction (south) and truncate against the seafloor ~10 km downslope from the site. This sequence is likely Quaternary in age, and the base of it is termed Reflector "O."

What has previously been defined as seismic Unit 1 (Miocene–Pliocene) underlies Reflector O. It is largely missing at the other sites. In the immediate vicinity of Site 1261, Unit 1 comprises a well-defined set of coherent seismic reflections of varying amplitudes. The topmost reflections truncate against Reflector O, sometimes in an angular fashion. The base of the unit is Reflector A, just above which is a 50-ms-thick (~40 m) zone of incoherent reflections capped by a bright reflector. This interval appears to be a debris flow. Unit 1 is 415 ms thick (~367 m) at the drill site.

Unit 2 is below Reflector A, which is the presumed lower Miocene erosional unconformity. The base of seismic Unit 2 is correlated to Reflector B at 586 ms subbottom (550 mbsf). Unit 2 is represented by a sequence of high-amplitude, parallel, coherent reflections that are relatively flat lying, dipping 0.5 to the north. Seismic Unit 3, between Reflectors B and C, is estimated to be 150 ms thick (175 m). Much of the acoustic energy in the high-resolution site survey profile (line GeoB213) is lost in the highly reflective Unit 2. Little detail is resolved in Unit 3 as a result. It appears as an acoustically transparent package with occasional semicoherent reflectors at the top and at the very base, just above Horizon C. It is difficult to correlate Horizon B and thus to distinguish Subunits 3a and 3b because of this low reflectivity. Horizon B has been correlated to the base of the slightly coherent section near the top of Unit 3, but the tie is uncertain.

Reflector C, at the base of the section of interest, is an unconformity. No coherent subsurface data are recognizable in the survey data, but the nearby industry line C2206a shows the underlying section. At Site 1261, the unconformity appears as a disconformity and it is difficult to actually pick up Reflector C as a single event. Further below, reflections form a broad anticline, fault bounded to the southwest and folded into a syncline to the northeast.

A listric normal fault is shown on line GeoB213 5 km southeast of the drill site. This fault has offset the entire sediment column. Apparent displacement across the fault is ~30–60 ms in the deeper portion of the section (at Reflector C), whereas in the upper portion it is on the order of 200 ms. This discrepancy in offset can be accounted for only by invoking either significant rotation or slumping in the upper sediment column. The fault splays at 300 ms subbottom to the seafloor, showing significant tilting of reflections in the interval between offsets and providing further evidence of slumping.

Stratigraphy

Lithologic descriptions of the cores and biostratigraphic age assignments reveal a sedimentary succession with several hiatuses. Sediments at Site 1261 range in age from Pleistocene to Cenomanian. Approximately 13 m of Pleistocene nannofossil ooze at the top of the section unconformably overlies an apparently continuous ~300-m-thick succession of lower middle Pliocene–upper Miocene nannofossil clay. Sedimentation rates in the Neogene are extraordinarily high, with values of 65 m/m.y. A ~60-m-thick upper Miocene matrix-supported conglomerate forms the base of the Neogene at Site 1261. A disconformity separates this debris flow from a concordant succession of middle–lower Eocene calcareous chalk, porcellanite, and limestone (planktonic foraminiferal Zone P14; calcareous nannoplankton Zone NP17). A 4-m.y. hiatus covering the lower Eocene (planktonic foraminiferal Zone P6) lies above the P/E boundary, which was recovered in Hole 1261A.

As at the other Leg 207 sites, the upper Paleocene clayey nannofossil chalk contains zeolites or locally abundant opal-CT lepispheres that replaced siliceous microfossils in this interval. Foraminifers in the middle part of this succession are very well preserved. The K/T boundary layer is absent from this site because of a hiatus covering most of the lower Danian (planktonic foraminiferal Zone P1). The subjacent upper Maastrichtian greenish gray to upper Campanian claystone with nannofossils is fairly condensed and displays cyclic color banding between light greenish gray and greenish gray on a decimeter scale. Foraminifers and nannofossils are rare in the Campanian, 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 3.3 m/m.y. Magnetostratigraphic age control in this interval is excellent, with well-defined paleomagnetic datums C29r–C32r in both holes.

There are numerous very dark colored intervals, apparently cyclic in occurrence, in the lower part of the Campanian. A condensed glauconite-rich interval separates the claystone from a laminated black shale sequence. The contact between this glauconite-rich interval and the mid-Cretaceous black shales is very sharp and may be a fault plane. Site 1261 represents the most expanded black shale sequence of the Leg 207 paleoceanographic depth transect. Approximately 10 m of Santonian sediments overlie ~10 m of lower Coniacian black shales. The Turonian epoch (~30 m thick) is completely represented and underlain by 40 m of upper Cenomanian black shales. Preservation and abundance of calcareous microfossils is rated poor to moderate. The maximum TOC content in these sediments is 16 wt%. 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.

A disconformity separates the Cenomanian-age laminated black shales and limestones from the underlying quartz sandstone and silty claystone that is upper Albian–Cenomanian in age, according to a single nannofossil datum. The oldest sediment cored at Site 1261 is a limestone with abundant oysters of the genus Aucillina.

Recovery of Critical Intervals

Sediments spanning the P/E boundary were recovered in Hole 1261A. The boundary interval comprises a sharp contact between light green chalk over dark green clay followed by a reddish clay-rich interval similar to Sites 1258 and 1259. 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 record of the K/T boundary interval was not recovered at this site.

An ~89-m-thick Santonian to upper Cenomanian succession of laminated black shales and laminated foraminiferal limestones, including OAEs 3 and 2, was recovered in both holes at Site 1261. This interval displays the most expanded record of the Santonian to Coniacian OAE 3 of Leg 207 (20 m). The variability of the claystone and chalk/limestone composing the black shales resulted in strong signal-to-noise ratios in both the 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 nearly continuous composite section with only three small gaps. If these alternations prove to be periodic, then there is a good opportunity for high-resolution age control with orbital tuning.

Geochemistry

Site 1261 is dominated by the presence of an interstitial water brine. Chloride increases in a linear fashion to the base of the black shales, where the highest chlorinity encountered on Demerara Rise is observed (907 mM; 62% greater than standard seawater). Unfortunately, it was not possible to extract pore water from the very thin interval of quartz sandstone that was recovered immediately below the shales. It could not be verified, therefore, whether the brine was sourced laterally through the black shale sequence, as was suggested by the Site 1257 chloride profile.

Unlike previous Leg 207 sites, organic matter degradation reactions occur outside the organic-rich black shales. Sulfate decreases to zero in the upper 200 mbsf in the rapidly deposited (65 m/m.y.) Pliocene nannofossil clay. Correspondingly, ammonium, a common respiration product of organic matter diagenesis, increases sharply to ~1.5 mM in the upper 140 mbsf and then slowly increases with depth to a maximum of 1.8 mM in the black shales. Methane increases modestly to ~2,000 ppmv in the upper 500 mbsf, then increases sharply near the top of the black shales to a maximum of ~110,000 ppmv. In contrast, alkalinity does not increase with depth as expected in the sulfate reduction zone, but decreases from the seafloor to a minimum at ~480 mbsf. The decreasing alkalinity corresponds to a decrease in calcium between ~250 and 375 mbsf and the interval of well-cemented Eocene chalks and limestones, indicating carbonate precipitation.

Physical Properties and Logging

Index properties, P-wave velocities, and GRA densities were measured on core samples from Site 1261. Downhole logging runs included the triple combo and the FMS-sonic tool strings, acquiring borehole caliper, acoustic velocity, formation density, porosity, electrical resistivity, and natural gamma ray emission data. The WST was run for checkshot velocities, but the tool could not be lowered past a bridge at 210 mbsf. Five WST stations were acquired in this upper interval.

Logging data in the upper ~380 mbsf are highly suspect because of the highly variable hole diameter through this interval. Consequently, correlation between laboratory-measured densities and logging densities are poor. Logging velocity data are not affected as severely by the hole diameter and correlate well with discrete core measurements in the laboratory and checkshot velocities measured with the WST tool. Physical property data show this interval is normally consolidated, with linear increases in density and velocity and a decrease in porosity. A significant perturbation in the porosity and velocity curves correlates with a major debris flow interval between ~320 and 380 mbsf.

Logging density data become stable below ~380 mbsf. They show a significant increase between 380 and 500 mbsf, concomitant with higher velocities. This interval corresponds to the Eocene sequence that has high clay and siliceous fractions relative to overlying sediments. A gradual decline in velocity and density corresponds to the P/E boundary. A subsequent sudden increase in these properties correlates with the K/T boundary and the underlying Maastrichtian chalk sequence. Absolute values show an overall drop with a high degree of scatter through the Cenomanian and Turonian black shale sequence. Velocity and density maximums in this interval correspond to limestone or coarse-grained beds, and low values in density and velocity correspond with organic-rich intervals.

Depositional History

Shallow-marine quartz sandstones with ammonite casts and limestones with oysters represent the oldest rocks recovered at Site 1261. They are overlain by a black shale sequence that dates from late Cenomanian–Santonian, but most of it is Cenomanian–Turonian with thinner Coniacian and Santonian intervals. Occasional clayey bentonite layers indicate the proximity of volcanoes. The laminated, coarse-grained foraminiferal limestones in the succession may be related to winnowing or 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 a sharp contact between the Santonian and overlying glauconite-rich upper Campanian claystones. At present, it is unclear whether the observed glauconite-rich intervals in the Turonian and Coniacian black shales reflect periods of oxygenation, condensation, or both.

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

Maastrichtian- to Pleistocene-age sediments at Site 1261 consist of pelagic deepwater marine clayey chalk, claystone, limestone, and clays. Sedimentation rates varied from 3.3 m/m.y. in the Maastrichtian, 7 m/m.y. in the Paleocene and early Eocene, to 8.9 m/m.y. in the middle Eocene. These are the lowest values observed during Leg 207 for these intervals. Average sedimentation rates for the upper Miocene–lower Pliocene reach the extremely high values of 65 m/m.y. The pelagic succession is interrupted by a few hiatuses—a lower Paleocene hiatus covering ~4 m.y., a ~2.5-m.y. hiatus in the lower Eocene, and a hiatus representing ~31 m.y. covering the upper Eocene, Oligocene, and lower Miocene. These hiatuses may reflect periods of slow deposition and/or erosion.

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