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Site 1257

Site 1257 is located in 2951 m of water on a terrace above the steep northern slope >10° northwest of 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) 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 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 demonstrated the potential for presence of all the target sediments.

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 of 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 (in two-way traveltime) at this location, calculated to be 144 meters below seafloor (mbsf) by the check shot and downhole logging velocity information. Seismic Unit 2 shows an incoherent reflection character here, describing a disturbed sediment package or being affected by 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. It 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). 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. 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.

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, with 75.9% recovery (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 at Hole 1257B, and the section between 40 and 227.3 mbsf was RCB cored with 62.1% recovery. At Hole 1257C, the upper 82 m was washed and RCB cored between 82 and 235.9 mbsf, with 62.8% recovery. Porcellanite, limestone, and chert beds at ~90 mbsf and between 170 and 225 mbsf reduced recovery of both XCB and RCB coring.

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

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 of Site 1257 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 latest Cenomanian from upper to middle Albian sediments.

Recovery of Critical Intervals

The main objective of Leg 207 was the recovery of sediments with microfossils of major and abrupt events of the Paleogene and Cretaceous Periods, such as the E/O boundary, P/E boundary, and the 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. This transient global warming period at the end of the Paleocene is one of the best candidates for study of abrupt change resulting from greenhouse warming in the geologic record. A growing body of evidence implicates a massive release of greenhouse gases into the atmosphere and ocean as a cause for dramatic warming in the Southern Ocean and subtropics. The P/E boundary record in Hole 1257C 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 44-m-thick succession of laminated black shales, including OAEs 3 and 2, was recovered in all three holes at Site 1257. OAEs represent major disruptions in the ocean system, defined by massive deposition of organic carbon in marine environments. They are hypothesized to have played a major role in the evolution of Earth's climatic and biotic history. OAE 3 is an interval of organic-rich and laminated sediment, roughly defined as covering the Santonian and Coniacian epochs. They have been described from various locations around the central Atlantic and the Western Interior Basin in the USA. OAE 2 is a well-defined latest Cenomanian to earliest Turonian event that has a global distribution. It is paralleled by a set of distinctive positive stable carbon isotope excursions pointing to a severe disturbance of the global carbon cycle associated with the burial of organic-rich sediments. Although the recovery in the black shale interval at Site 1257 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.

Stratigraphy of Site 1257

Lithologic description of the cores and biostratigraphic age assignments exposed a number of hiatuses and highly condensed intervals within the sedimentary succession at Site 1257. A veneer of upper Miocene nannofossil ooze was defined as lithologic 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 mottling 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 lithologic 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 lithologic 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 rates = ~10 m/m.y.) allowed the construction of a spliced section that shows a pronounced cyclicity, possibly representing a 20- to 50-k.y. periodicity.

An early to 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 a sharp irregular contact with underlying black shales of lithologic Unit IV is present (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 up to 16 wt%, and hydrogen index and oxygen index values indicate that the black shales contain Type II kerogen, indicating a marine source of 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) are present 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 ages 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 within a sandstone date the top of the unit in Section 207-1257A-25-CC as Cenomanian. The bottom of Hole 1257A has an upper to middle Albian age.


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 within 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 data (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 lithostratigraphic changes and trends, and patterns of cyclicity are obvious in some intervals. The most pronounced change correlates with lithologic 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 are 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 the present probably were reached by the late Maastrichtian to Paleocene. Sedimentation and subsidence have probably kept pace since that time.

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